Dissertations / Theses: 'MRI physics'

1

Hiltunen, S. (Sami). "Puun lahoamisprosessin seuraaminen NMR- ja MRI-menetelmillä." Bachelor's thesis, University of Oulu, 2018. http://urn.fi/URN:NBN:fi:oulu-201801201103.

Full text

Abstract:

Tutkielma käsittelee NMR-spektroskopian ja magneettikuvauksen menetelmiä, joilla lahoavaa puuta voidaan mahdollisesti kuvata ja tutkia. Lisäksi tutkielmassa esitetään tärkeimpiä teoreettisia lähtökohtia puun lahoamiselle, NMR-spektroskopialle sekä magneettikuvaukselle.

APA, Harvard, Vancouver, ISO, and other styles

2

Seeber, Derek A. "Toward MRI microimaging of single biological cells /." The Ohio State University, 2001. http://rave.ohiolink.edu/etdc/view?acc_num=osu1486398528559205.

Full text

APA, Harvard, Vancouver, ISO, and other styles

3

Kidane, Tesfaye Kebede. "Toward Faster and Quieter MRI." Case Western Reserve University School of Graduate Studies / OhioLINK, 2005. http://rave.ohiolink.edu/etdc/view?acc_num=case1118850517.

Full text

APA, Harvard, Vancouver, ISO, and other styles

4

Anblagan, Devasuda. "MRI of foetal development." Thesis, University of Nottingham, 2012. http://eprints.nottingham.ac.uk/30592/.

Full text

Abstract:

Foetal MRI represents a non-invasive imaging technique that allows detailed visualisation of foetus in utero and the maternal structure. This thesis outlines the quantitative imaging techniques used to investigate the effect of maternal diabetes and maternal smoking on foetal development at 1.5 Tesla. The effect of maternal diabetes on placental blood flow and foetal growth was studied. The placental images were acquired using Echo Planar Imaging and blood flow was measured using Intra Voxel Incoherent Motion. The results indicate that peak blood flow in the basal plate and chorionic plate increases across gestation in both normal and diabetic pregnancies. Conversely, diffusion in the whole placenta decreases across gestation, with a more pronounced decrease in diabetic placentae. Following this, a method was developed to use a Tl weighted fat suppressed MRI scan to quantify foetal fat images in-utero. In addition, HAlf Fourier Single-shot Turbo spin Echo (HASTE) and balanced Fast Field Echo (bFFE) were used to acquire images encompassing the whole foetus in three orthogonal planes. These scans were used to measure foetal volume, foetal length and shoulder width. The data shows that foetal fat volume and intra-abdominal fat were increased in foetuses of diabetic mothers at third trimester. The HASTE and bFFE sequences were also used to study the effect of maternal smoking on foetal development. Here, foetal organ volumes, foetal and placental volume, shoulder width and foetal length were measured using a semiautomatic approach based on the concept of edge detection and a stereological method, the Cavalieri technique. The data shows that maternal smoking has significant negative effect on foetal organ growth and foetal growth, predominantly foetal kidney and foetal volume. The work described here certainly has a great potential in non-invasive assessment of abnormal placental function and can be used to study foetal development.

APA, Harvard, Vancouver, ISO, and other styles

5

Yao, Zhen. "OPTIMIZING RF AND GRADIENT COILS IN MRI." Case Western Reserve University School of Graduate Studies / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=case1402058570.

Full text

APA, Harvard, Vancouver, ISO, and other styles

6

Soltaninejad, Mohammadreza. "Supervised learning-based multimodal MRI brain image analysis." Thesis, University of Lincoln, 2017. http://eprints.lincoln.ac.uk/30883/.

Full text

Abstract:

Medical imaging plays an important role in clinical procedures related to cancer, such as diagnosis, treatment selection, and therapy response evaluation. Magnetic resonance imaging (MRI) is one of the most popular acquisition modalities which is widely used in brain tumour analysis and can be acquired with different acquisition protocols, e.g. conventional and advanced. Automated segmentation of brain tumours in MR images is a difficult task due to their high variation in size, shape and appearance. Although many studies have been conducted, it still remains a challenging task and improving accuracy of tumour segmentation is an ongoing field. The aim of this thesis is to develop a fully automated method for detection and segmentation of the abnormal tissue associated with brain tumour (tumour core and oedema) from multimodal MRI images. In this thesis, firstly, the whole brain tumour is segmented from fluid attenuated inversion recovery (FLAIR) MRI, which is commonly acquired in clinics. The segmentation is achieved using region-wise classification, in which regions are derived from superpixels. Several image features including intensity-based, Gabor textons, fractal analysis and curvatures are calculated from each superpixel within the entire brain area in FLAIR MRI to ensure a robust classification. Extremely randomised trees (ERT) classifies each superpixel into tumour and non-tumour. Secondly, the method is extended to 3D supervoxel based learning for segmentation and classification of tumour tissue subtypes in multimodal MRI brain images. Supervoxels are generated using the information across the multimodal MRI data set. This is then followed by a random forests (RF) classifier to classify each supervoxel into tumour core, oedema or healthy brain tissue. The information from the advanced protocols of diffusion tensor imaging (DTI), i.e. isotropic (p) and anisotropic (q) components is also incorporated to the conventional MRI to improve segmentation accuracy. Thirdly, to further improve the segmentation of tumour tissue subtypes, the machine-learned features from fully convolutional neural network (FCN) are investigated and combined with hand-designed texton features to encode global information and local dependencies into feature representation. The score map with pixel-wise predictions is used as a feature map which is learned from multimodal MRI training dataset using the FCN. The machine-learned features, along with hand-designed texton features are then applied to random forests to classify each MRI image voxel into normal brain tissues and different parts of tumour. The methods are evaluated on two datasets: 1) clinical dataset, and 2) publicly available Multimodal Brain Tumour Image Segmentation Benchmark (BRATS) 2013 and 2017 dataset. The experimental results demonstrate the high detection and segmentation performance of the III single modal (FLAIR) method. The average detection sensitivity, balanced error rate (BER) and the Dice overlap measure for the segmented tumour against the ground truth for the clinical data are 89.48%, 6% and 0.91, respectively; whilst, for the BRATS dataset, the corresponding evaluation results are 88.09%, 6% and 0.88, respectively. The corresponding results for the tumour (including tumour core and oedema) in the case of multimodal MRI method are 86%, 7%, 0.84, for the clinical dataset and 96%, 2% and 0.89 for the BRATS 2013 dataset. The results of the FCN based method show that the application of the RF classifier to multimodal MRI images using machine-learned features based on FCN and hand-designed features based on textons provides promising segmentations. The Dice overlap measure for automatic brain tumor segmentation against ground truth for the BRATS 2013 dataset is 0.88, 0.80 and 0.73 for complete tumor, core and enhancing tumor, respectively, which is competitive to the state-of-the-art methods. The corresponding results for BRATS 2017 dataset are 0.86, 0.78 and 0.66 respectively. The methods demonstrate promising results in the segmentation of brain tumours. This provides a close match to expert delineation across all grades of glioma, leading to a faster and more reproducible method of brain tumour detection and delineation to aid patient management. In the experiments, texton has demonstrated its advantages of providing significant information to distinguish various patterns in both 2D and 3D spaces. The segmentation accuracy has also been largely increased by fusing information from multimodal MRI images. Moreover, a unified framework is present which complementarily integrates hand-designed features with machine-learned features to produce more accurate segmentation. The hand-designed features from shallow network (with designable filters) encode the prior-knowledge and context while the machine-learned features from a deep network (with trainable filters) learn the intrinsic features. Both global and local information are combined using these two types of networks that improve the segmentation accuracy.

APA, Harvard, Vancouver, ISO, and other styles

7

Ball, Iain Keith. "Functional pulmonary MRI using hyperpolarised 3He." Thesis, University of Nottingham, 2011. http://eprints.nottingham.ac.uk/12207/.

Full text

Abstract:

The microstructure of the lung is complex, containing many branching airways and alveolar sacs for optimal gas exchange. Lung diseases such as cystic fibrosis (CF), asthma, and emphysema lead to a destruction of this microstructure. As such, there is a growing interest in the early identification and assessment of lung disease using non invasive imaging techniques. Pulmonary function tests such as spirometry and plethysmography are currently used for this purpose but can only provide quantitative lung function measurements rather than direct measurements of lung physiology and disease. Computed tomography (CT) has also been used but due to risk of cell damage and mutation from the ionising radiation, long term monitoring of the lungs is severely constrained. Recently, new methods based on magnetic resonance imaging (MRI) have been developed to provide diagnostic imaging of the lung. Conventional MRI is not very well suited for lung imaging due to the very low proton density of the pulmonary airspaces. This problem can be overcome by making the patient inspire noble gases such as 3He whose polarisations have been vastly increased through optical pumping. Therefore 3He MRI permits a non-invasive determination of lung function. The high diffusion coefficient of 3He can be exploited to probe the microstructure of the lung. By measuring how fast 3He diffuses within the lung, the size of the lung microstructure can be assessed. Normally, the airspace walls impede the diffusion of the gas but for diseased lungs where microstructure has been destroyed, diffusion is less restricted and a higher apparent diffusion coefficient (ADC) is observed. The research conducted for this thesis focused on the measurement of ADC using three different MRI pulse sequences with each sequence being designed to assess the peripheral airspaces over different length scales. These sequences were then implemented on three different subject study groups.

APA, Harvard, Vancouver, ISO, and other styles

8

Massey, Jermane E. (Jermane Edward) 1972. "Line narrowing of fiber coupled laser diode array and ³He lung MRI." Thesis, Massachusetts Institute of Technology, 1999. http://hdl.handle.net/1721.1/85356.

Full text

APA, Harvard, Vancouver, ISO, and other styles

9

Thorpe, James. "Lung mechanics and hyperpolarised gas MRI." Thesis, University of Nottingham, 2018. http://eprints.nottingham.ac.uk/53283/.

Full text

Abstract:

Lung diseases affect the lives of millions of people across the UK and result in the thousands of deaths every year. It is therefore vitally important to continue to develop a wide range of diagnostic techniques to improve our understanding of lung diseases and how they can be treated. This thesis provides an overview of the main methods of assessing lung condition before focussing on developments in two specific areas: Forced Oscillation Technique (FOT) and Hyperpolarised (HP) gas MRI. FOT is an inexpensive, non-invasive lung function test that measures the acoustic impedance of the airways by applying an oscillating waveform via a mouthpiece. FOT cannot be used to image the lung but instead provides information on a variety of other physiological parameters. Two FOT studies are presented in this thesis: a multi-site phantom study and a patient based study. The phantom study confirmed the validity of the Nottingham FOT system used in the patient study and investigated the effects of lung stiffness and airway obstruction on measured FOT parameters using a 3D printed lung phantom, as well as comparing phantom results between three different FOT devices (an in-house device from the University of Nottingham, an Erich Jaeger Master-Screen IOS and a tremoFlo C-100 airwave oscillometry system) at two sites (the University of Nottingham and Glenfield Hospital, Leicester). It was found that changes in lung stiffness and airway obstruction are observable in the reactive and resistive (respectively) components of measured impedance. A difference was seen between the Jaeger IOS system and the other two devices. The patient based study was undertaken to investigate the efficacy of FOT, in comparison to spirometry, in differentiating between three patient groups, healthy, asthmatic and chronic obstructive pulmonary disease (COPD), with a particular focus on investigating the effect of a bronchodilator on measured FOT parameters. It was found that both FOT and Spirometry were effective at differentiating between the patient groups, however, they provided different information about patient response to bronchodilator thus demonstrating that both techniques should be performed to obtain the maximum information about a patient's disease state. HP gas MRI uses isotopes of noble gases, such as 3He and 129Xe, to either image the lungs or perform non-imaging measurements of parameters such as the Apparent Diffusion Coeffcient (ADC). A short study using 3He was performed comparing ADC measurements at two different time scales between two sites (the University of Nottingham, 13ms, and the University of Sheffield, 2ms) with a secondary aim of investigating the effect of age on ADC. A study on HP 129Xe MRI is presented covering developments that have been made in various imaging techniques including breathing protocols and scanning techniques. The objective of this study is to establish a reliable scanning protocol using healthy volunteers before expanding the study to investigate different disease states including COPD and idiopathic pulmonary Fibrosis (IPF). Although progress has been made in testing the validity of various imaging techniques, with ventilation images at 25mm and 10mm slice thicknesses obtained, more development is still needed to improve the quality of the images in order for them to be useful in a clinical setting.

APA, Harvard, Vancouver, ISO, and other styles

10

Sulikowska, Aleksandra. "Motion correction in high-field MRI." Thesis, University of Nottingham, 2016. http://eprints.nottingham.ac.uk/33674/.

Full text

Abstract:

The work described in this thesis was conducted at the University of Nottingham in the Sir Peter Mansfield Imaging Centre, between September 2011 and 2014. Subject motion in high- resolution magnetic resonance imaging (MRI) is a major source of image artefacts. It is a very complex problem, due to variety of physical motion types, imaging techniques, or k-space trajectories. Many techniques have been proposed over the years to correct images for motion, all looking for the best practical solution in clinical scanning, which would give cost- effective, robust and high accuracy correction, without decreasing patient comfort or prolonging the scan time. Moreover, if the susceptibility induced field changes due to head rotation are large enough, they will compromise motion correction methods. In this work a method for prospective correction of head motion for MR brain imaging at 7 T was proposed. It would employ innovative NMR tracking devices not presented in literature before. The device presented in this thesis is characterized by a high accuracy of position measurements (0.06 ± 0.04 mm), is considered very practical, and stands the chance to be used in routine imaging in the future. This study also investigated the significance of the field changes induced by the susceptibility in human brain due to small head rotations (±10 deg). The size and location of these field changes were characterized, and then the effects of the changes on the image were simulated. The results have shown that the field shift may be as large as |-18.3| Hz/deg. For standard Gradient Echo sequence at 7 T and a typical head movement, the simulated image distortions were on average equal to 0.5%, and not larger than 15% of the brightest voxel. This is not likely to compromise motion correction, but may be significant in some imaging sequences.

APA, Harvard, Vancouver, ISO, and other styles

11

Fu, Luke. "Automated analysis of multi-site MRI data for the NIHPD." Thesis, McGill University, 2006. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=101124.

Full text

Abstract:

In a large multi-center study where many groups collaborate to produce data, the conditions under which the data are collected can vary significantly. In the case of the NIH funded MRI study of normal brain development (NIHPD), 6 centers across the US collect MRI scans of the developing child brain. In all, four different models of scanners from 2 manufacturers are used. The subject of this thesis is to quantify differences in acquired data that are due to scanner differences and thus enable improved biological study.
The aim of this study is to analyze the scanner-induced differences across sites using a specialized phantom recommended by the American College of Radiology (ACR). Together with the 7 tests listed by the ACR MRI accreditation manual, phantom data from each site can be analyzed for parameters that are characteristic of image quality. The measurements are done automatically using programs written at the MNI, that use MINC tools and the MNI autoreg package to compute the necessary parameters.
The results demonstrate that, in general, all data obtained complied with the 7 tests of the ACR MRI manual. Statistically significant variations over time and between sites were found in the data. However, the magnitude of the variations is of the order of 1%-2% indicating stable data acquisitions and comparability of data between sites.

APA, Harvard, Vancouver, ISO, and other styles

12

Shah, Simon Michael. "Magnetisation transfer effects at ultra high field MRI." Thesis, University of Nottingham, 2017. http://eprints.nottingham.ac.uk/39398/.

Full text

Abstract:

Increased signal to noise ratio in ultra high field Magnetic Resonance Imaging (MRI) has allowed the development of quantitative imaging techniques and new contrast mechanisms, such as Chemical Exchange Saturation Transfer (CEST) to be probed. The development of CEST contrast imaging has involved overcoming a number of technical challenges associated with ultra high field MRI. The B1 transmit field was, and still is, a major challenge. Presented in this thesis, the B1 transmit field in regions of low B1 are improved with the use of dielectric pads and a simulation study shows that the overall B1 transmit field homogeneity is significantly improved when multi-transmit slice-selective RF spokes pulse sequences are used. Multiple methods have been developed to quantify the chemical exchange from slow exchanging proton pools seen in CEST contrast imaging. However, magnetisation transfer (MT) from the macromolecular bound pool contaminates current quantification methods, and presented in this thesis is a method whereby the CEST and MT are simultaneously saturated using dual frequency saturation pulses, allowing the CEST contrast in z-spectra to be separated from the MT and to enhance visualisation of the CEST effects. Despite the considerable interest in CEST, only one study has probed the CEST effects in blood, and interestingly high levels of CEST signals can be observed from the superior sagittal sinus. To investigate these effects, z-spectra from ex vivo blood samples considering the effects of oxygenation, haematocrit levels and cell structure were quantified. Quantification shows that the main source of the CEST signals was from the cells within the blood.

APA, Harvard, Vancouver, ISO, and other styles

13

Bagwell, Scott G. "A high order finite element coupled multi-physics approach to MRI scanner design." Thesis, Swansea University, 2018. https://cronfa.swan.ac.uk/Record/cronfa40797.

Full text

Abstract:

Magnetic Resonance Imaging (MRI) scanners are becoming increasingly popular with many clinical experts for use in both medical research and clinical imaging of patients, due to their ability to perform high-resolution non-intrusive imaging examinations. Recently, however, there has been an increasing demand for higher resolution scanners that are capable of performing quicker scans with increased patient comfort. With this demand for more advanced MRI systems, there also follows a number of challenges facing designers. Understanding the physical phenomena behind MRI is crucial in the development of scanners that are capable of producing accurate images of the patient with maximum comfort and minimal noise signatures. MRI scanners utilise strong static magnetic fields coupled with rapidly time varying gradient magnetic fields to generate images of the patient. In the presence of these time varying fields, the conducting components of MRI scanners generate eddy currents, which give rise to Lorentz forces and cause the conductors to vibrate. These vibrations cause acoustic waves to form that propagate through the air and result in audible noise which can cause significant discomfort for the patient. They also generate Lorentz currents which feedback into the electromagnetic field and this process results in a fully coupled non-linear acousto-magneto-mechanical system. The determination of the coupling mechanisms involved in such a system is a nontrivial task and so, in order to understand the behaviour of MRI systems during operation, advanced computational tools and techniques are required. Moreover, there exists certain small scale physical phenomena that arise in the coupled system which require high resolutions to obtain accurate results. In this thesis, a new computational framework for the treatment of acoustomagneto-mechanical coupling that arises in low-frequency electro-magneto-mechanical systems, such as MRI scanners, is proposed. The transient Newton-Raphson strategy involves the solution of a monolithic system, obtained from the linearisation of the coupled system of equations and two approaches are considered: (i) the linearised approach and (ii) the non-linear approach. In (i), physically motivated by the excitation from static and time varying current sources of MRI scanners, the fields may be split into a dominant static component and a much smaller dynamic component. The resulting linearised system is obtained by performing the linearisation of the fields about this dominant static component. This approach permits solutions in the frequency domain, for understanding the response of MRI systems under various excitations, and provides a computationally efficient way to solve this challenging problem, as it allows the tangent stiffness matrix to be inverted independently of time or frequency. In (ii), there is no approximation from a physical standpoint and the linearization is performed about the current solution. This approach requires that solutions are obtained in the time domain and thus the focus is then put on transient solutions to the coupled system of equations to address the following two important questions: 1) How good is the agreement between the computationally efficient linearised approach compared with the intensive non-linear approach?; and 2) Over what range of MRI operating conditions can the linearised approach be expected to provide acceptable results for MRI scanner design? Motivated by the need to solve industrial problems rapidly, solutions will be restricted to problems consisting of axisymmetric geometries and current sources. This treatment also discusses, in detail, the computational requirements for the solution of these coupled problems on unbounded domains and the accurate discretisation of the fields using hp-finite elements. A set of academic and industrially relevant examples are studied to benchmark and illustrate both approaches, in a hp- finite element context, as well as performing rigorous comparisons between the approaches.

APA, Harvard, Vancouver, ISO, and other styles

14

Alsameen, Maryam. "Functional MRI Study of Sleep Restriction in Adolescents." University of Cincinnati / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1602152924202332.

Full text

APA, Harvard, Vancouver, ISO, and other styles

15

Lebois, Alice. "Brain microstructure mapping using quantitative and diffsusion MRI." Phd thesis, Université Paris Sud - Paris XI, 2014. http://tel.archives-ouvertes.fr/tel-01063198.

Full text

Abstract:

This thesis is focused on the human brain microstructure mapping using quantitative and diffusion MRI. The T1/T2 quantitative imaging relies on sequences dedicated to the mapping of T1 and T2 relaxation times. Their variations within the tissue are linked to the presence of different water compartments defined by a specific organization of the tissue at the cell scale. Measuring these parameters can help, therefore, to better characterize the brain microstructure. The dMRI, on the other hand, explores the brownian motion of water molecules in the brain tissue, where the water molecules' movement is constrained by natural barriers, such as cell membranes. Thus, the information on their displacement carried by the dMRI signal gives access to the underlying cytoarchitecture. Combination of these two modalities is, therefore, a promising way to probe the brain tissue microstructure. The main goal of the present thesis is to set up the methodology to study the microstructure of the white matter of the human brain in vivo. The first part includes the acquisition of a unique MRI database of 79 healthy subjects (the Archi/CONNECT), which includes anatomical high resolution data, relaxometry data, diffusion-weighted data at high spatio-angular resolution and functional data. This database has allowed us to build the first atlas of the anatomical connectivity of the healthy brain through the automatic segmentation of the major white matter bundles, providing an appropriate anatomical reference for the white matter to study individually the quantitative parameters along each fascicle, characterizing its microstructure organization. Emphasis was placed on the construction of the first atlas of the T1/T2 profiles along the major white matter pathways. The profiles of the T1 and T2 relaxation times were then correlated to the quantitative profiles computed from the diffusion MRI data (fractional anisotropy, radial and longitudinal diffusivities, apparent diffusion coefficient), in order to better understand their relations and to explain the observed variability along the fascicles and the interhemispheric asymmetries. The second part was focused on the brain tissue modeling at the cell scale to extract the quantitative parameters characterizing the geometry of the cellular membranes, such as the axonal diameter and the axonal density. A diffusion MRI sequence was developed on the 3 Teslas and 7 Teslas Siemens clinical systems of NeuroSpin which is able to apply any kind of gradient waveforms to fall within an approach where the gradient waveform results from an optimization under the hypothesis of a geometrical tissue model, hardware and time constraints induced by clinical applications. This sequence was applied in the study of fourteen healthy subjects in order to build the first quantitative atlas of the axonal diameter and the local axonal density at 7T. We also proposed a new geometrical model to model the axon, dividing the axonal compartment, usually modelled using a simple cylinder, into two compartments: one being near the membranes with low diffusivity and one farer from the membranes, less restricted and with higher diffusivity. We conducted a theoretical study showing that under clinical conditions, this new model allows, in part, to overcome the bias induced by the simple cylindrical model leading to a systematic overestimation of the smallest diameters. Finally, in the aim of going further in the physiopathology of the autism, we added to the current 3T imaging protocol the dMRI sequence developed in the framework of this thesis in order to map the axonal diameter and density. This study is ongoing and should validate shortly the contribution of these new quantitative measures of the microstructure in the comprehension of the atrophies of the corpus callosum, initially observed using less specific diffusion parameters such as the generalized fractional anisotropy. There will be other clinical applications in the future.

APA, Harvard, Vancouver, ISO, and other styles

16

Abeykoon, Sumeda B. "Quantification of Myocardial Perfusion Based on Signal Intensity of Flow Sensitized MRI." University of Cincinnati / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1352403722.

Full text

APA, Harvard, Vancouver, ISO, and other styles

17

BAIG, TANVIR N. "NEW DIRECTIONS IN THE DESIGN OF MRI GRADIENT COILS." Case Western Reserve University School of Graduate Studies / OhioLINK, 2007. http://rave.ohiolink.edu/etdc/view?acc_num=case1169749200.

Full text

APA, Harvard, Vancouver, ISO, and other styles

18

Che, Ahmad Azlan. "Dynamic contrast-enhanced MRI of breast cancer at 3T." Thesis, University of Aberdeen, 2011. http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=165831.

Full text

Abstract:

3T MRI provides higher signal-to-noise ratio images compared to lower field machines. However, a major drawback of 3T MRI is a higher B1 transmission-field inhomogeneity across the field-of-view compared to imaging at lower fields. B1-field mapping was performed on volunteers using a Philips 3.0T MR scanner and a typical head-first prone patient positioning technique. The B1-field transmitted in the breasts was found to be reduced towards the right side of the body. In some volunteers, the B1-field was reduced to about one-half of the nominal field in the right breast. To minimize the B1 inhomogeneity artefacts, a saturation recovery snapshot FLASH (SRSF) imaging sequence was proposed. Different saturation techniques were assessed. The best saturation efficiency was produced by Hoffmann’s saturation method. By using Hoffmann’s SRSF sequence, the error in the enhancement ratio (ER) can be reduced to about one half compared to imaging obtained using typical FLASH sequence in the presence of a 50% B1-field reduction. Other techniques i.e. bilateral power optimization and a dedicated patient support system were also tested. Both of these approaches produced substantial reductions of the B1 inhomogeneity seen with the standard technique. To address the effects of the native T1 (T10) of different tissues on DCE-MRI, novel enhancement factor indices calculated using SRSF sequence images were introduced and assessed. Computer simulations and gel phantom experiments showed that less error was observed in the indices calculated compared to the ER calculated using the conventional and widely used FLASH sequence. Furthermore, the effect of B1-field inhomogeneity on the novel indices is also reduced. One of the indices proposed is directly related to the contrast agent concentration. The theory and results presented show that the SRSF pulse sequence and the quantification techniques proposed have the potential to improve the accuracy of breast DCE-MRI at 3T.

APA, Harvard, Vancouver, ISO, and other styles

19

Brodeur, Marylène. "Verification of IMRT beam delivery with a ferrous sulfate gel dosimeter and MRI." Thesis, McGill University, 2003. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=78331.

Full text

Abstract:

Intensity modulated photon beam radiation therapy often results in dynamically delivered beams with small field sizes and steep dose gradients. This defines a need for an integrating, tissue-equivalent, high resolution dosimeter. 3D ferrous sulfate gel based dosimetry involves the use of magnetic resonance (MR) images of radiosensitive paramagnetic gels. The goal of this work is to create a patient specific quality assurance (QA) procedure that links measured dosimetnc information to clinical goals.
The gel dosimeter system is tested through a set of simple experiments which characterize and confirm the system as a valid QA tool for conformal and intensity modulated radiation therapy.
For this work, dynamic photon beams are created on a commercially available inverse treatment planning system and the treatment is delivered to a gel filled acrylic mold. Software has been developed to quantify dose from the QA MR images, and to register this information to the planning computed tomography (CT) scan. The software displays the measured dose on the planning CT, and calculates dose-volume histograms for the registered measured data and contoured patient structures. This work reveals good agreement between planned and measured dose distributions, with less than 5% difference in the mean doses of the contoured patient structures.

APA, Harvard, Vancouver, ISO, and other styles

20

Wheeler-Kingshott, Claudia A. "High speed MRI : analysis of new approaches to fast imaging using Burst-based sequences." Thesis, University of Surrey, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.268089.

Full text

APA, Harvard, Vancouver, ISO, and other styles

21

Shou, Xingxian. "The Suppression of Selected Acoustic Noise Frequencies in MRI." Case Western Reserve University School of Graduate Studies / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=case1281404517.

Full text

APA, Harvard, Vancouver, ISO, and other styles

22

Poole, Charles Randall. "QUENCH PROTECTION STUDIES OF MAGNESIUM DIBORIDE SUPERCONDUCTING MAGNETS FOR MRI APPLICATIONS." Case Western Reserve University School of Graduate Studies / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=case1523017967730333.

Full text

APA, Harvard, Vancouver, ISO, and other styles

23

Jonsson, Joakim H., Magnus G. Karlsson, Mikael Karlsson, and Tufve Nyholm. "Treatment planning using MRI data: an analysis of the dose calculation accuracy for different treatment regions." Umeå universitet, Radiofysik, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-35610.

Full text

Abstract:

BACKGROUND: Because of superior soft tissue contrast, the use of magnetic resonance imaging (MRI) as a complement to computed tomography (CT) in the target definition procedure for radiotherapy is increasing. To keep the workflow simple and cost effective and to reduce patient dose, it is natural to strive for a treatment planning procedure based entirely on MRI. In the present study, we investigate the dose calculation accuracy for different treatment regions when using bulk density assignments on MRI data and compare it to treatment planning that uses CT data. METHODS: MR and CT data were collected retrospectively for 40 patients with prostate, lung, head and neck, or brain cancers. Comparisons were made between calculations on CT data with and without inhomogeneity corrections and on MRI or CT data with bulk density assignments. The bulk densities were assigned using manual segmentation of tissue, bone, lung, and air cavities. RESULTS: The deviations between calculations on CT data with inhomogeneity correction and on bulk density assigned MR data were small. The maximum difference in the number of monitor units required to reach the prescribed dose was 1.6%. This result also includes effects of possible geometrical distortions. CONCLUSIONS: The dose calculation accuracy at the investigated treatment sites is not significantly compromised when using MRI data when adequate bulk density assignments are made. With respect to treatment planning, MRI can replace CT in all steps of the treatment workflow, reducing the radiation exposure to the patient, removing any systematic registration errors that may occur when combining MR and CT, and decreasing time and cost for the extra CT investigation.

APA, Harvard, Vancouver, ISO, and other styles

24

Segerdahl, Tony. "MRI Safety, Test Methods and Construction of a Database." Thesis, Stockholm University, Medical Radiation Physics (together with KI), 2007. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-6968.

Full text

Abstract:

Magnetic Resonance Imaging, MRI, is a diagnostic tool in progress which has been available at major hospitals since the mid eighties. Today almost all hospitals world wide may depict the human body with their own MRI scanner. MRI is dependent on a uniform magnetic field inside the scanner tunnel and Radio frequent (RF) waves used for excitation of the magnetic dipole moments in the body. These properties along with the magnetic field surrounding the scanner are associated with dangerous effects - when interacting with medical implants made of metals. These dangerous effects are twisting forces or torques, heating and translational forces respectively. A database containing information about known implants behaviour regarding these effects among with earlier documentation and information concerning MRI patient safety at Karolinska hospital, Huddinge was constructed.

Also a phantom used for heating effect measurements was constructed and heating effect measurements were performed at a SPC4129 locking titanium Peritoneal Dialysis (PD) catheter adapter and a Deep Brain Stimulator (DBS) in order to test the phantom and confirm the theory about RF induced heating on medical implants. Evidence for heating effects caused by the implants was found.

A torque measurement apparatus was constructed and measurements were performed. All measurements where performed in order to investigate the functionality of the apparatus and also the theory behind dangerous magnetically induced torques (twisting movements). Substantial torque were measured on the ferromagnetic device used for the test.

The heating phantom and torque measurement apparatus is slightly modified models of those proposed by ASTM (American Society for Testing and Materials).

APA, Harvard, Vancouver, ISO, and other styles

25

Fan, Mingdong. "THREE INITIATIVES ADDRESSING MRI PROBLEMS." Case Western Reserve University School of Graduate Studies / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=case1585863940821908.

Full text

APA, Harvard, Vancouver, ISO, and other styles

26

Sinozic, Tanja. "Learning in clinical practice : findings from CT, MRI and PACS." Thesis, University of Sussex, 2014. http://sro.sussex.ac.uk/id/eprint/49367/.

Full text

Abstract:

This thesis explores learning in clinical practice in the cases of CT, MRI and PACS in UK hospitals. It asks the questions of how and why certain evolutionary features of technology condition learning and change in medical contexts. Using an evolutionary perspective of cognitive and social aspects of technological change, this thesis explores the relationships between technology and organisational learning processes of intuition, interpretation, integration and institutionalisation. Technological regimes are manifested in routines, skills and artefacts, and dynamically evolve with knowledge accumulation processes at the individual, group and organisational levels. Technological change increases the uncertainty and complexity of organisational learning, making organisational outcomes partially unpredictable. Systemic and emergent properties of medical devices such as CT and MRI make learning context-specific and experimental. Negotiation processes between different social groups shape the role and function of an artefact in an organisational context. Technological systems connect artefacts to other parts of society, mediating values, velocity and directionality of change. Practice communities affect how organisations deal with this complexity and learn. These views are used to explore the accumulation of knowledge in clinical practices in CT, MRI and PACS. This thesis develops contextualised theory using a case-study approach to gather novel empirical data from over 40 interviews with clinical, technical, managerial and administrative staff in five NHS hospitals. It uses clinical practice (such as processes, procedures, tasks, rules, interpretations and routines) as a unit of analysis and CT, MRI and PACS technology areas as cases. Results are generalised to evolutionary aspects of technological learning and change provided by the framework, using processes for qualitative analysis such as ordering and coding. When analysed using an evolutionary perspective of technology, the findings in this thesis suggest that learning in clinical practice is diverse, cumulative and incremental, and shaped by complex processes of mediation, by issues such as disease complexity, values, external rules and choice restrictions from different regimes, and by interdisciplinary problem-solving in operational routines.

APA, Harvard, Vancouver, ISO, and other styles

27

Hoffman, David. "Hybrid PET/MRI Nanoparticle Development and Multi-Modal Imaging." VCU Scholars Compass, 2013. http://scholarscompass.vcu.edu/etd/3253.

Full text

Abstract:

The development of hybrid PET/MRI imaging systems needs to be paralleled with the development of a hybrid intrinsic PET/MRI probes. The aim of this work was to develop and validate a novel radio-superparamagnetic nanoparticle (r-SPNP) for hybrid PET/MRI imaging. This was achieved with the synthesis of superparamagnetic iron oxide nanoparticles (SPIONs) that intrinsically incorporated 59Fe and manganese iron oxide nanoparticles (MIONs) that intrinsically incorporated 52Mn. Both [59Fe]-SPIONs and [52Mn]-MIONs were produced through thermal decomposition synthesis. The physiochemical characteristics of the r-SPNPs were assessed with TEM, DLS, and zeta-potential measurements, as well as in imaging phantom studies. The [59Fe]-SPIONs were evaluated in vivo with biodistribution and MR imaging studies. The biodistrubution studies of [59Fe]-SPIONs showed uptake in the liver. This corresponded with major MR signal contrast measured in the liver. 52Mn was produced on natural chromium through the 52Cr(p,n)52Mn reaction. The manganese radionuclides were separated from the target material through a liquid-liquid extraction. The αVβ3 integrin binding of [52Mn]-MION-cRGDs was evaluated with αVβ3 integrin solid phase assays, and the expression of αVβ3 integrin in U87MG xenograft tumors was characterized with fluorescence flow cytometry. [52Mn]-MION-cRGDs were used for in vivo PET and MR imaging of U87MG xenograft tumor bearing mice. PET data showed increased [52Mn]-MION-cRGD uptake compared with untargeted [52Mn]-MIONs. ROI analysis of PET and MRI data showed that MR contrasted corresponded with PET signal. Future work will utilize [52Mn]-MION-cRGDs in other tumor models and with hybrid PET/MRI imaging systems.

APA, Harvard, Vancouver, ISO, and other styles

28

Eichner, Cornelius. "Slice-Accelerated Magnetic Resonance Imaging." Doctoral thesis, Universitätsbibliothek Leipzig, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-184944.

Full text

Abstract:

This dissertation describes the development and implementation of advanced slice-accelerated (SMS) MRI methods for imaging blood perfusion and water diffusion in the human brain. Since its introduction in 1977, Echo-Planar Imaging (EPI) paved the way toward a detailed assessment of the structural and functional properties of the human brain. Currently, EPI is one of the most important MRI techniques for neuroscientific studies and clinical applications. Despite its high prevalence in modern medical imaging, EPI still suffers from sub-optimal time efficiency - especially when high isotropic resolutions are required to adequately resolve sophisticated structures as the human brain. The utilization of novel slice-acceleration methods can help to overcome issues related to low temporal efficiency of EPI acquisitions. The aim of the four studies outlining this thesis is to overcome current limitations of EPI by developing methods for slice-accelerated MRI. The first experimental work of this thesis describes the development of a slice-accelerated MRI sequence for dynamic susceptibility contrast imaging. This method for assessing blood perfusion is commonly employed for brain tumor classifications in clinical practice. Following up, the second project of this thesis aims to extend SMS imaging to diffusion MRI at 7 Tesla. Here, a specialized acquisition method was developed employing various methods to overcome problems related to increased energy deposition and strong image distortion. The increased energy depositions for slice-accelerated diffusion MRI are due to specific radiofrequency (RF) excitation pulses. High energy depositions can limit the acquisition speed of SMS imaging, if high slice-acceleration factors are employed. Therefore, the third project of this thesis aimed at developing a specialized RF pulse to reduce the amount of energy deposition. The increased temporal efficiency of SMS imaging can be employed to acquire higher amounts of imaging data for signal averaging and more stable model fits. This is especially true for diffusion MRI measurements, which suffer from intrinsically low signal-to-noise ratios. However, the typically acquired magnitude MRI data introduce a noise bias in diffusion images with low signal-to-noise ratio. Therefore, the last project of this thesis aimed to resolve the pressing issue of noise bias in diffusion MRI. This was achieved by transforming the diffusion magnitude data into a real-valued data representation without noise bias. In combination, the developed methods enable rapid MRI measurements with high temporal efficiency. The diminished noise bias widens the scope of applications of slice- accelerated MRI with high temporal efficiency by enabling true signal averaging and unbiased model fits. Slice-accelerated imaging for the assessment of water diffusion and blood perfusion represents a major step in the field of neuroimaging. It demonstrates that cur- rent limitations regarding temporal efficiency of EPI can be overcome by utilizing modern data acquisition and reconstruction strategies.

APA, Harvard, Vancouver, ISO, and other styles

29

Ceccarelli, Mattia. "Optimization and applications of deep learning algorithms for super-resolution in MRI." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2020. http://amslaurea.unibo.it/21694/.

Full text

Abstract:

The increasing amount of data produced by modern infrastructures requires instruments of analysis more and more precise, quick, and efficient. For these reasons in the last decades, Machine Learning (ML) and Deep Learning (DL) techniques saw exponential growth in publications and research from the scientific community. In this work are proposed two new frameworks for Deep Learning: Byron written in C++, for fast analysis in a parallelized CPU environment, and NumPyNet written in Python, which provides a clear and understandable interface on deep learning tailored around readability. Byron will be tested on the field of Single Image Super-Resolution for NMR imaging of brains (Nuclear Magnetic Resonance) using pre-trained models for x2 and x4 upscaling which exhibit greater performance than most common non-learning-based algorithms. The work will show that the reconstruction ability of DL models surpasses the interpolation of a bicubic algorithm even with images totally different from the dataset in which they were trained, indicating that the generalization abilities of those deep learning models can be sufficient to perform well even on biomedical data, which contain particular shapes and textures. Ulterior studies will focus on how the same algorithms perform with different conditions for the input, showing a large variance between results.

APA, Harvard, Vancouver, ISO, and other styles

30

Theilmann, Rebecca Jean. "High-resolution diffusion imaging with DIFRAD-FSE (diffusion-weighted radial acquisition with fast spin echo) MRI." Diss., The University of Arizona, 2001. http://hdl.handle.net/10150/284332.

Full text

Abstract:

A novel MRI method, DIFRAD-FSE (D̲i̲f̲fusion with R̲adial A̲cquisition of D̲ata with F̲ast S̲pin-E̲cho) is presented that enables rapid, high-resolution, multi-shot diffusion-weighted MRI without significant artifacts due to motion. Following a diffusion-weighted spin-echo preparation, multiple radial lines of Fourier data are acquired using spin-echo refocusing. Data can be acquired in either 2D or 3D Fourier space. Motion correction is accomplished via one of four correction techniques: phase correction, shift correction, a combination of the phase and shift correction, or magnitude. Images from a radial data set are reconstructed with filtered back projection reconstruction. Results from human brain imaging will demonstrate the ability of DIFRAD-FSE to acquire high-resolution images without significant artifacts due to motion in both 2D and 3D. Results from liver and heart imaging demonstrate the versatility of the 2D DIFRAD-FSE.

APA, Harvard, Vancouver, ISO, and other styles

31

Donnelly, Cara. "Shearing waves and the MRI dynamo in stratified accretion discs." Thesis, University of Cambridge, 2014. https://www.repository.cam.ac.uk/handle/1810/246452.

Full text

Abstract:

Accretion discs efficiently transport angular momentum by a wide variety of as yet imperfectly understood mechanisms, with profound implications for the disc lifetime and planet formation. We discuss two different methods of angular momentum transport: first, generation of acoustic waves by mixing of inertial waves, and second, the generation of a self-sustaining magnetic field via the magnetorotational instability (MRI) which would be a source of dissipative turbulence. Previous local simulations of the MRI have shown that the dynamo changes character on addition of vertical stratification. We investigate numerically 3D hydrodynamic shearing waves with a conserved Hermitian form in an isothermal disc with vertical gravity, and describe the associated symplectic structure. We continue with a numerical investigation into the linear evolution of the MRI and the undular magnetic buoyancy instability in isolated flux regions and characterise the resultant quasi-linear EMFs as a function of height above the midplane. We combine this with an analytic description of the linear modes under an assumption of a poloidal-toroidal scale separation. Finally, we use RAMSES to perform full MHD simulations in a zero net flux shearing box, followed by spatial and a novel temporal averaging to reveal the essential structure of the dynamo. We find that inertial modes may be efficiently converted into acoustic modes for "bending waves", despite a fundamental ambiguity in the inertial mode structure. With our linear MRI and the undular magnetic buoyancy modes we find the localisation of the instability high in the atmosphere becomes determined by magnetic buoyancy rather than field strength for small enough azimuthal wavenumber, and that the critical Alfven speed below which the dynamo can operate increases with increasing distance from the midplane. We calculate analytically quasi-linear EMFs which predict both a vertical propagation of toroidal field and a method for creation of radial field. From our fully nonlinear calculations we find an electromotive force in phase with the toroidal field, which is itself 3π/2 out of phase with the radial (sheared) field at the midplane, and good agreement with our quasi-linear analytics. We have identified an efficient mechanism for generating acoustic waves in a disc. In our investigation of the accretion disc dynamo, we have reproduced analytically the EMFs calculated in our simulations, given arguments based on the phase of relevant quantities, several correlation integrals and the scalings suggested by our analytic work. Our analysis contributes significantly to an explanation for the dynamo in an accretion disc.

APA, Harvard, Vancouver, ISO, and other styles

32

Gerardin, Emilie. "Morphometry of the human hippocampus from MRI and conventional MRI high field." Phd thesis, Université Paris Sud - Paris XI, 2012. http://tel.archives-ouvertes.fr/tel-00856589.

Full text

Abstract:

The hippocampus is a gray matter structure in the temporal lobe that plays a key role in memory processes and in many diseases (Alzheimer's disease, epilepsy, depression ...).The development of morphometric models is essential for the study of the functional anatomy and structure alterations associated with different pathologies. The objective of this thesis is to develop and validate methods for morphometry of the hippocampus in two contexts: the study of the external shape of the hippocampus from conventional MRI (1.5T or 3T) with millimeter resolution, and the study of its internal structure from 7T MRI with high spatial resolution. These two settings correspond to the two main parts of the thesis.In the first part, we propose a method for the automatic classification of patients from shape descriptors. This method is based on a spherical harmonic decomposition which is combined with a support vector machine classifier (SVM). The method is evaluated in the context of automatic classification of patients with Alzheimer's disease (AD) patients, mild cognitive impairment (MCI) patients and healthy elderly subjects. It is also compared to other approaches and a more comprehensive validation is available in a population of 509 subjects from the ADNI database. Finally, we present another application of morphometry to study structural alterations associated with the syndrome of Gilles de la Tourette.The second part of the thesis is devoted to the morphometry of the internal structure of the hippocampus from MRI at 7 Tesla. Indeed, the internal structure of the hippocampus is rich and complex but inaccessible to conventional MRI. We first propose an atlas of the internal structure of the hippocampus from postmortem data acquired at 9.4T. Then, we propose to model the Ammon's horn and the subiculum as a skeleton and a local measure thickness. To do this, we introduce a variational method using original Hilbert spaces reproducing kernels. The method is validated on the postmortem atlas and evaluated on in vivo data from healthy subjects and patients with epilepsy acquired at 7T.

APA, Harvard, Vancouver, ISO, and other styles

33

Svanholm, Ulrika. "Evaluation of measurements of pulsating flow under controlled conditions using phase contrast MRI." Thesis, Stockholm University, Medical Radiation Physics (together with KI), 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-7222.

Full text

Abstract:

The accuracy and precision of measurements of pulsating flow obtained with phase contrast magnetic resonance imaging (PC MRI) was studied. Measurements were carried out using known flow rates through a phantom connected to a pump that created pulsation in the flow. Repeated measurements were made in both the negative and positive encoding direction, using both breath-hold and non breath hold sequences. The obtained data was analyzed using code written in MATLAB and also using the FLOW software that is offered by the manufacturer of the MRI system.

A range of different flow velocities was scanned, and results show that the overall accuracy of the measurements is relatively good, with an average error of between 1.2% to 5.7% using the clinically employed flow calculation software. There is however indication of a systematic phase offset in the data that influences the measurements. The effect of the offset on the results depends on the direction of flow and the sequence used. The results also show the importance of properly selecting the area over which the flow rate is calculated.

APA, Harvard, Vancouver, ISO, and other styles

34

Kara, Danielle Christine. "Production of a Viable Product in Magnetic Resonance Imaging Using MgB2." Case Western Reserve University School of Graduate Studies / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=case1386343733.

Full text

APA, Harvard, Vancouver, ISO, and other styles

35

Newton, Hayley Louise. "Hyperpolarised xenon production via Rb and Cs optical pumping applied to functional lung MRI." Thesis, University of Nottingham, 2014. http://eprints.nottingham.ac.uk/14339/.

Full text

Abstract:

Hyperpolarisation encompasses a multitude of methods to increase the species' spin polarisation for nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) applications. Hyperpolarised 129Xe is produced via spin-exchange optical-pumping (SEOP). Firstly, electronic spins of alkali metal vapour are polarised via absorption of circularly polarised light. Alkali metal polarisation is subsequently transferred to noble gas nuclei via collisions. Within this thesis, the SEOP process is examined by probing the kinetics of the 129Xe polarisation build up. A combination of diagnostic techniques are used including low field NMR to measure 129Xe polarisation (PXe) at different spatial positions, near-IR optical absorption to give a global estimate of the alkali metal polarisation, and in situ Raman spectroscopy to spatially monitor the energy transport processes by detecting the internal gas temperatures (TN2). TN2 values were found to be dramatically elevated above oven thermocouple readings, with observations of up to 1000 K for an oven heated to only 400 K. Internal gas temperatures are presented for the first time along the length of the optical cell, showing spatial temperature and PXe variations during steady state and rubidium runaway conditions. Two contrasting methods of Raman spectroscopy are examined: a conventional orthogonal arrangement of detection and excitation optics, where intrinsic spatial filtering of the probe laser is utilised; and a newly designed inline module with all components in the same optical plane. Optical filtering is used to reduce the Rayleigh scattering and the probe laser line. This new inline device is presented herein and has a 23 fold improvement in signal to noise enabling increased accuracy and precision of `real-time' temperature monitoring. Rubidium, caesium and a rubidium/caesium hybrid are compared as the alkali metal of choice in the SEOP process. Caesium has a higher spin-exchange cross-section with 129Xe, thus a system is envisaged where current Rb D1 lasers in many polarisers can be utilised with a Rb/Cs hybrid to gain improvements in polarisation rates or levels. Xenon polarisations are shown up to 50% for a hybrid cell. Finally, preparatory experiments crucial to the imminent lung imaging study are presented, including measurements of PXe at low and high magnetic fields. In addition, polariser technology is examined including the current Nottingham device and an open-source consortium polariser.

APA, Harvard, Vancouver, ISO, and other styles

36

Zhao, Xuandong. "A study of Quantification of Aortic Compliance in Mice using Radial Acquisition Phase Contrast MRI." University of Cincinnati / OhioLINK, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1273001921.

Full text

APA, Harvard, Vancouver, ISO, and other styles

37

Barbosa, Leonardo da Silva. "Inclusão de MRI e informação multigrid a priori para inferência bayesiana de fontes de M/EEG." Universidade de São Paulo, 2011. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-30092011-141308/.

Full text

Abstract:

A Neuroimagem Funcional evoluiu muito nos últimos anos com o aparecimento de técnicas como Positron Emission Tomography ou PET (Tomógrafo por Emissão de Pósitrons) e Functional Magnetic Ressonance Image ou fMRI (Imagem de RessonÂncia Magnética Funcional) [Belliveau et al., 1991]. Elas permitem a observação de atividade no cérebro com uma resolução de alguns milímetros, e devido a natureza do sinal medido, com uma resolução temporal da ordem de 5 segundos [Kim et al., 1997]. Magnetoencefalografia e Eletroencefalografia (M/EEG), por outro lado, possuem uma resoluçao temporal da ordem de milissegundos, já que o sinal é produzido pela movimentação do íons através das membranas celulares [Nunez and Srinivasan, 2006]. Porém a sua resoluçeo espacial é muito baixa jé que tipicamente são problemas mal postos, com muito mais variáveis do que dados. Um equipamento de M/EEG de alta resolução possui da ordem de O(200) canais, que permitem medidas do campo magnético (para o MEG) ou do potencial elétrico (para o EEG) em O(200) posições em torno da cabeça. Para uma escala com resolução de ordem l existem (L /l )3 variáveis, onde L = aprox. 15cm. Neste trabalho procuramos estudar métodos para aumentar a resolução espacial das técnicas de EEG, pois o mapeamento funcional do cérebro humano esta intimamente relacionado à localização da atividade no espaço bem como no tempo [Friston, 2009] (muitas relativo ao momento de um estímulo externo). Todo o trabalho de localização de fontes para EEG pode ser facilmente estendido para MEG. Métodos Bayesianos são o cenário natural para lidar com problemas mal postos [Wipf and Nagarajan, 2009]. Existem, essencialmente, duas direções nas quais os algoritmos Bayesianos podem ser melhoradas, através da construção de uma melhor verossimilhança ou uma distribuição a Priori. Embora reconheçamos que avanços importantes podem ser feitos no direção anterior, aqui nos concentramos na segunda. Neste trabalho nós introduzimos um método multiescala para construir uma melhor distribuição a Priori. Uma idéia similar foi estudada dentro do contexto mais simples de fMRI [Amaral et al., 2004]. Muitos novos problemas aparecem ao lidar com o caráter vetorial do EEG. O mais importante, é a construção de um conjunto de superfícies renormalizadas que aproximam a região cortical onde a fonte de atividade esta localizada e o problema relacionado de de nir as variáveis relevantes para representar o cérebro em uma escala com menor resolução. A validação do novo algoritmo é sempre um problema essencial. Nós apresentamos resultados que sugerem, em dados simulados, que nosso método pode ser uma alternativa válida para os atuais algoritmos, julgando ambos pela taxa de erros na localização de fontes bem como pelo tempo que eles levam para convergir.
Functional Neuroimaging has evolved in the last few decades with the introduction of techniques such as Positron Emission Tomography or PET and Functional Magnetic Ressonance Image or fMRI [Belliveau et al., 1991]. These allow observing brain activity with a resolution of a few millimeters and, due to the nature of the signal, a time resolution of the order of 5 seconds [Kim et al., 1997]. M/EEG, on the other hand, have a millisecond time resolution, since the signal is produced by the transport of ions through cell membranes [Nunez and Srinivasan, 2006]. However their space resolution is much lower since these are typically ill posed problems with many more unknowns than data points. A high resolution M/EEG has of the order of O(200) data channels, which allow measuring the magnetic or electric field at O(200) positions around the head. For a resolution scale of order l there are O(L l )3 variables, where L = 15cm. In this work we aim at studying methods to increase the spatial resolution of EEG techniques, since functional mapping of the human brain is intimately related to the localization of the activity in space as well as in time [Friston, 2009] (often relative to the time of external stimuli). Any advance in the inverse problem of source localization for EEG can rather easily be extended to deal with MEG. Bayesian methods are the natural setting to deal with ill posed problems [Wipf and Nagarajan, 2009]. There are essentially two directions in which Bayesian algorithms can be improved, by building a better likelihood or a prior distribution. While we recognize that important advances can be done in the former direction we here concentrate in the latter. In this work we introduce a multiscale method to build an improved prior distribution. A similar idea has been studied within an easier context of fMRI [Amaral et al., 2004]. Several new problems appear in dealing with the vectorial character of EEG. The most important, is the construction of a set of renormalized lattices that approximate the cortex region where the source activity is located and the related problem of de ning the relevant variables in coarser scale representation of the cortex. Validation of a new algorithm is always an essential problem. We present results which suggest on simulated data, that our method might be a valid alternative to current algorithms, judged both by the rate of errors in source localization as well as by the time it takes to converge.

APA, Harvard, Vancouver, ISO, and other styles

38

Marklund, Sandra. "Feasibility Study of Phase Measurements of the Arterial Input Function in Dynamic Contrast Enhanced MRI." Thesis, Umeå University, Radiation Physics, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-23226.

Full text

Abstract:

Acquired data from dynamic contrast enhanced MRI measurements can be used to non-invasively assess tumour vascular characteristics through pharmacokinetic modelling. The modelling requires an arterial input function which is the concentration of contrast agent in the blood reaching the volume of interest as a function of time. The aim of this work is testing and optimizing a turboFLASH sequence to appraise its suitability for measuring the arterial input function by measuring phase.

Contrast concentration measurements in a phantom were done with both phase and relaxivity techniques. The results were compared to simulations of the experiment conditions to compare the conformance. The results using the phase technique were promising, and the method was carried on to in-vivo testing. The in-vivo data displayed a large signal loss which motivated a new phantom experiment to examine the cause of this signal reduction. Dynamic measurements were made in a phantom with pulsatile flow to mimic a blood vessel with a somewhat modified turboFLASH sequence. The conclusions drawn from analyzing the data were used to further improve the sequence and this modified turboFLASH sequence was tested in an in-vivo experiment. The obtained concentration curve showed significant improvement and was deemed to be a good representation of the true blood concentration.

The conclusion is that phase measurements can be recommended over relaxivity based measurements. This recommendation holds for using a slice selective saturation recovery turboFLASH sequence and measuring the arterial input function in the neck. Other areas of application need more thorough testing.

APA, Harvard, Vancouver, ISO, and other styles

39

Öhman, Tuva. "The Influence of the Reference Measurement in MRI Image Reconstruction Using Sensitivity Encoding (SENSE)." Thesis, Stockholm University, Medical Radiation Physics (together with KI), 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-7217.

Full text

Abstract:

The use of MRI for patient examinations has constantly increased as technical development has lead to faster image acquisitions and higher image quality. Nevertheless, an MR-examination still takes relatively long time and yet another way of speeding up the process is to employ parallel imaging. In this thesis, one of these parallel imaging techniques, called SENSE, is described and examined more closely.

When SENSE is employed, the number of spatial encoding steps can be reduced thanks to the use of several receiving coils. A reduction of the number of phase encoding steps not only leads to faster image acquisition, but also to superimposed pixel values in image space. In order to be able to separate the aliased pixels, knowledge about the spatial sensitivity of the coils is required.

There are several different alternatives to how and when information about the sensitivities of the coils should be collected, but in this thesis, focus is on the method of performing a reference measurement before the actual scan. The reference measurement consists of a fast, low-resolution sequence which either is collected with both the body coil and the parallel imaging coil or only with the parallel imaging coil. A comparison of these two methods by simulations in program written MATLAB leads to the conclusion that even if the scan time of the reference measurement is doubled it seems like there are numerous advantages of also collecting data with the body coil:

• the images are more homogeneous which facilitates the establishment of a diagnose

• the noise levels in the reconstructed images are somewhat lower

• images collected with a reduced sampling density show better agreement with those collected without reduction.

Furthermore, it is shown that the reference measurement preferably should be a 3D sequence covering all the volume of interest. If a 2D sequence is used it is absolutely necessary that it can be performed in any plane and it has to be repeated for every plane that is imaged.

APA, Harvard, Vancouver, ISO, and other styles

40

Russell, Gregory. "Understanding the Effects of Diffusion and Relaxation in Magnetic Resonance Imaging using Computational Modeling." Diss., The University of Arizona, 2014. http://hdl.handle.net/10150/333215.

Full text

Abstract:

The work described in this dissertation was motivated by a desire to better understand the cellular pathology of ischemic stroke. Two of the three bodies of research presented herein address and issue directly related to the investigation of ischemic stroke through the use of diffusion weighted magnetic resonance imaging (DWMRI) methods. The first topic concerns the development of a computationally efficient finite difference method, designed to evaluate the impact of microscopic tissue properties on the formation of DWMRI signal. For the second body of work, the effect of changing the intrinsic diffusion coefficient of a restricted sample on clinical DWMRI experiments is explored. The final body of work, while motivated by the desire to understand stroke, addresses the issue of acquiring large amounts of MRI data well suited for quantitative analysis in reduced scan time. In theory, the method could be used to generate quantitative parametric maps, including those depicting information gleaned through the use of DWMRI methods. Chapter 1 provides an introduction to several topics. A description of the use of DWMRI methods in the study of ischemic stroke is covered. An introduction to the fundamental physical principles at work in MRI is also provided. In this section the means by which magnetization is created in MRI experiments, how MRI signal is induced, as well as the influence of spin-spin and spin-lattice relaxation are discussed. Attention is also given to describing how MRI measurements can be sensitized to diffusion through the use of qualitative and quantitative descriptions of the process. Finally, the reader is given a brief introduction to the use of numerical methods for solving partial differential equations. In Chapters 2, 3 and 4, three related bodies of research are presented in terms of research papers. In Chapter 2, a novel computational method is described. The method reduces the computation resources required to simulate DWMRI experiments. In Chapter 3, a detailed study on how changes in the intrinsic intracellular diffusion coefficient may influence clinical DWMRI experiments is described. In Chapter 4, a novel, non-steady state quantitative MRI method is described.

APA, Harvard, Vancouver, ISO, and other styles

41

Shereen, Ahmed D. "Diffusion Tensor Magnetic Resonance Imaging Applications to Neurological Disease." University of Cincinnati / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1300393032.

Full text

APA, Harvard, Vancouver, ISO, and other styles

42

Adjeiwaah, Mary. "Quality assurance for magnetic resonance imaging (MRI) in radiotherapy." Licentiate thesis, Umeå universitet, Institutionen för strålningsvetenskaper, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-142603.

Full text

Abstract:

Magnetic resonance imaging (MRI) utilizes the magnetic properties of tissues to generate image-forming signals. MRI has exquisite soft-tissue contrast and since tumors are mainly soft-tissues, it offers improved delineation of the target volume and nearby organs at risk. The proposed Magnetic Resonance-only Radiotherapy (MR-only RT) work flow allows for the use of MRI as the sole imaging modality in the radiotherapy (RT) treatment planning of cancer. There are, however, issues with geometric distortions inherent with MR image acquisition processes. These distortions result from imperfections in the main magnetic field, nonlinear gradients, as well as field disturbances introduced by the imaged object. In this thesis, we quantified the effect of system related and patient-induced susceptibility geometric distortions on dose distributions for prostate as well as head and neck cancers. Methods to mitigate these distortions were also studied. In Study I, mean worst system related residual distortions of 3.19, 2.52 and 2.08 mm at bandwidths (BW) of 122, 244 and 488 Hz/pixel up to a radial distance of 25 cm from a 3T PET/MR scanner was measured with a large field of view (FoV) phantom. Subsequently, we estimated maximum shifts of 5.8, 2.9 and 1.5 mm due to patient-induced susceptibility distortions. VMAT-optimized treatment plans initially performed on distorted CT (dCT) images and recalculated on real CT datasets resulted in a dose difference of less than 0.5%. The magnetic susceptibility differences at tissue-metallic,-air and -bone interfaces result in local B0 magnetic field inhomogeneities. The distortion shifts caused by these field inhomogeneities can be reduced by shimming. Study II aimed to investigate the use of shimming to improve the homogeneity of local B0 magnetic field which will be beneficial for radiotherapy applications. A shimming simulation based on spherical harmonics modeling was developed. The spinal cord, an organ at risk is surrounded by bone and in close proximity to the lungs may have high susceptibility differences. In this region, mean pixel shifts caused by local B0 field inhomogeneities were reduced from 3.47±1.22 mm to 1.35±0.44 mm and 0.99±0.30 mm using first and second order shimming respectively. This was for a bandwidth of 122 Hz/pixel and an in-plane voxel size of 1×1 mm2. Also examined in Study II as in Study I was the dosimetric effect of geometric distortions on 21 Head and Neck cancer treatment plans. The dose difference in D50 at the PTV between distorted CT and real CT plans was less than 1.0%. In conclusion, the effect of MR geometric distortions on dose plans was small. Generally, we found patient-induced susceptibility distortions were larger compared with residual system distortions at all delineated structures except the external contour. This information will be relevant when setting margins for treatment volumes and organs at risk. The current practice of characterizing MR geometric distortions utilizing spatial accuracy phantoms alone may not be enough for an MR-only radiotherapy workflow. Therefore, measures to mitigate patient-induced susceptibility effects in clinical practice such as patient-specific correction algorithms are needed to complement existing distortion reduction methods such as high acquisition bandwidth and shimming.

APA, Harvard, Vancouver, ISO, and other styles

43

McComb, Christie. "In vivo assessment of the performance of strain-encoded MRI (DENSE) in healthy subjects and patients with myocardial infarction." Thesis, University of Glasgow, 2014. http://theses.gla.ac.uk/5157/.

Full text

Abstract:

Introduction: In patients with myocardial infarction (MI), regional left ventricular contractile function has important prognostic value. Displacement ENcoding with Stimulated Echoes (DENSE) is an MRI technique which has been developed to allow quantitative assessment of myocardial strain. To date, much of the research performed with DENSE has been methods development, and its application in a routine clinical setting has been incompletely investigated. The purpose of the research presented within this thesis was to investigate variations in strain within the healthy heart, and then to assess the in vivo performance of DENSE strain imaging in acute and chronic myocardial infarction (MI). Methods: 80 healthy subjects (M:F = 40:40, age 43 +/- 17 years) were recruited from the community. 50 male patients (age 56 +/- 10 years) were recruited from the clinical service and scanned within 7 days of myocardial infarction (“acute MI”), and invited to return for a follow-up scan after 6 months (“chronic MI”). MR imaging was performed on a 1.5T Siemens Avanto scanner, using an imaging protocol which included DENSE, cine, late gadolinium enhancement (LGE, patients only) and T2-weighted imaging acquired from left ventricular (LV) short-axis slices in both basal and mid-ventricular positions, which were divided into 6 segments for analysis. The percentage of LGE and T2 hyperenhancement within each segment were calculated, and the area at risk (acute MI) and myocardial salvage index (chronic MI) were determined. DENSE images were analysed to obtain values for strain parameters relating to circumferential strain (Ecc). Strain measurements obtained from healthy subjects were used to investigate the variations in Ecc with age, gender, slice position and myocardial segment. Strain measurements obtained from MI patients were used to investigate the relationships between Ecc and the extent of myocardial infarction, area at risk and/or salvage, and to determine whether DENSE strain measurements are informative in acute and chronic MI. Results: Comparison of DENSE strain measurements in healthy subjects revealed statistically significant differences between males and females, and between measurements obtained from basal and mid-ventricular short-axis slice positions. These differences must be taken into account to allow appropriate analysis of DENSE data in patients. DENSE was found to be informative in both acute and chronic MI. At both time points, strain measurements can be used to distinguish between myocardial segments with 0%, <50% and >50% infarction. There is the potential for the development of reference ranges which could be applied to strain measurements from future MI patients to allow assessment of the extent of infarction. In acute MI, four additional applications were identified: i) comparison with references ranges, established from strain measurements in healthy subjects, can be used to identify the presence of infarction with high specificity and moderate to high sensitivity, ii) peak Ecc can be used to distinguish between segments categorised as remote and adjacent, iii) strain measurements in the acute setting may provide prognostic information relating to the potential progression or recovery of contractile abnormalities in the chronic setting, iv) peak Ecc may allow a preliminary assessment of LV ejection fraction. Sensitivity for the detection of injured but non-infarcted segments was low. In chronic MI, two additional applications were identified: i) strain recovery can be detected in infarcted myocardial segments, and also in non-infarcted segments which are located adjacent to infarcted segments, which could improve identification of changes in contractile function compared to conventional qualitative analysis of cine imaging, ii) strain measurements can be used to distinguish between segments in which the extent of infarction has increased and those in which it has decreased. The relationships with myocardial salvage index were not found to be informative. Conclusions: DENSE images were successfully acquired and analysed from both healthy subjects and patients with myocardial infarction, which indicates that the technique is feasible in different clinical settings. DENSE strain measurements were found to be informative in both acute and chronic MI, and can provide insight into the presence and extent of infarction and the progression or recovery of contractile abnormalities.

APA, Harvard, Vancouver, ISO, and other styles

44

Jayatilake, Mohan L. "Optimization and construction of passive shim coils for human brain at high field MRI." University of Cincinnati / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1313772791.

Full text

APA, Harvard, Vancouver, ISO, and other styles

45

Derakhshan, Jamal Jon. "Innovations Involving Balanced Steady State Free Precession MRI." Cleveland, Ohio : Case Western Reserve University, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=case1247256364.

Full text

Abstract:

Thesis (Ph.D.)--Case Western Reserve University, 2009
Title from PDF (viewed on 19 August 2009) Department of Biomedical Engineering Includes abstract Includes bibliographical references Available online via the OhioLINK ETD Center

APA, Harvard, Vancouver, ISO, and other styles

46

Vaidya, Manushka. "Steering Electromagnetic Fields in MRI| Investigating Radiofrequency Field Interactions with Endogenous and External Dielectric Materials for Improved Coil Performance at High Field." Thesis, New York University, 2017. http://pqdtopen.proquest.com/#viewpdf?dispub=10261392.

Full text

Abstract:

Although 1.5 and 3 Tesla (T) magnetic resonance (MR) systems remain the clinical standard, the number of 7 T MR systems has increased over the past decade because of the promise of higher signal-to-noise ratio (SNR), which can translate to images with higher resolution, improved image quality and faster acquisition times. However, there are a number of technical challenges that have prevented exploiting the full potential of ultra-high field (≥ 7 T) MR imaging (MRI), such as the inhomogeneous distribution of the radiofrequency (RF) electromagnetic field and specific energy absorption rate (SAR), which can compromise image quality and patient safety.

To better understand the origin of these issues, we first investigated the dependence of the spatial distribution of the magnetic field associated with a surface RF coil on the operating frequency and electrical properties of the sample. Our results demonstrated that the asymmetries between the transmit (B₁⁺) and receive (B₁^–) circularly polarized components of the magnetic field, which are in part responsible for RF inhomogeneity, depend on the electric conductivity of the sample. On the other hand, when sample conductivity is low, a high relative permittivity can result in an inhomogeneous RF field distribution, due to significant constructive and destructive interference patterns between forward and reflected propagating magnetic field within the sample.

We then investigated the use of high permittivity materials (HPMs) as a method to alter the field distribution and improve transmit and receive coil performance in MRI. We showed that HPM placed at a distance from an RF loop coil can passively shape the field within the sample. Our results showed improvement in transmit and receive sensitivity overlap, extension of coil field-of-view, and enhancement in transmit/receive efficiency. We demonstrated the utility of this concept by employing HPM to improve performance of an existing commercial head coil for the inferior regions of the brain, where the specific coil’s imaging efficiency was inherently poor. Results showed a gain in SNR, while the maximum local and head SAR values remained below the prescribed limits. We showed that increasing coil performance with HPM could improve detection of functional MR activation during a motor-based task for whole brain fMRI.

Finally, to gain an intuitive understanding of how HPM improves coil performance, we investigated how HPM separately affects signal and noise sensitivity to improve SNR. For this purpose, we employed a theoretical model based on dyadic Green’s functions to compare the characteristics of current patterns, i.e. the optimal spatial distribution of coil conductors, that would either maximize SNR (ideal current patterns), maximize signal reception (signal-only optimal current patterns), or minimize sample noise (dark mode current patterns). Our results demonstrated that the presence of a lossless HPM changed the relative balance of signal-only optimal and dark mode current patterns. For a given relative permittivity, increasing the thickness of the HPM altered the magnitude of the currents required to optimize signal sensitivity at the voxel of interest as well as decreased the net electric field in the sample, which is associated, via reciprocity, to the noise received from the sample. Our results also suggested that signal-only current patterns could be used to identify HPM configurations that lead to high SNR gain for RF coil arrays. We anticipate that physical insights from this work could be utilized to build the next generation of high performing RF coils integrated with HPM.

APA, Harvard, Vancouver, ISO, and other styles

47

Amin, Abdullah Al. "MULTISCALE MULTIPHYSICS THERMO-MECHANICAL MODELING OF AN MGB₂ BASED CONDUCTION COOLED MRI MAGNET SYSTEM." Case Western Reserve University School of Graduate Studies / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=case151385068164148.

Full text

APA, Harvard, Vancouver, ISO, and other styles

48

Heilman, Jeremiah A. "Multi-Dimensional Excitation in Magnetic Resonance Imaging for Homogeneity Correction in the Presence of Dielectric Media." Cleveland, Ohio : Case Western Reserve University, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=case1238442193.

Full text

APA, Harvard, Vancouver, ISO, and other styles

49

Cassidy, Maja. "Hyperpolarized Silicon Particles as In-vivo Imaging Agents." Thesis, Harvard University, 2012. http://dissertations.umi.com/gsas.harvard:10649.

Full text

Abstract:

This thesis describes the development of hyperpolarized silicon particles as a new type of magnetic resonance imaging (MRI) agent. Silicon particles are inexpensive, non-toxic, biodegradable, targetable, and have unique physical properties that lead to extremely long nuclear polarization times. The \(^{29}Si\) nuclei are hyperpolarized by low temperature dynamic nuclear polarization using naturally occurring defects at the particle surface and directly imaged using \(^{29}Si\) MRI. The imaging window achievable is several orders of magnitude longer than other hyperpolarized imaging agents. The technique requires no additional imaging agent to be incorporated into the silicon, and so toxicity complications are reduced. The construction of a system for low temperature dynamic nuclear polarization and a NMR spectrometer for studying the nuclear polarization dynamics in silicon particles is described. Room temperature nuclear spin relaxation \((T_1)\) times are investigated for a variety of silicon particles spanning five orders of magnitude in mean diameter, from 10nm nanoparticles to mm-scale granules. The nuclear \(T_1\) times of all Si particles are found to be long, ranging from many minutes to several hours at room temperature. \(T_1\) is found to be a function of particle size, dopant concentration, synthesis method and crystallinity. A core-shell model to describe the electron and nuclear spin dynamics in the particles is developed. The decay in nuclear hyperpolarization is studied as a function of ambient magnetic field and temperature, demonstrating that the long spin relaxation times persist despite changing environmental conditions. A new technique is reported for enhancing the dynamic nuclear polarization in silicon particles using modulated microwave irradiation. A theoretical model for understanding this enhanced polarization process is developed. As well as providing an efficient mechanism for polarizing the \(^{29}Si\) nuclei within the particle, the surface defects are also found to be efficient at polarizing \(^1H\) nuclei in frozen solutions surrounding the particles. Several in-vivo applications of hyperpolarized \(^{29}Si\) MRI are demonstrated, including gastrointestinal imaging, intravenous imaging and mapping blood flow in a tumor. The spin relaxation rates are found to be unaffected by surface functionalization, the particles tumbling in solution, or the in-vivo environment.
Engineering and Applied Sciences

APA, Harvard, Vancouver, ISO, and other styles

50

Shokouhi, Mahsa. "Assessment of the potentials and limitations of cortical-based analysis for the integration of structure and function in normal and pathological brains using MRI." Thesis, University of Glasgow, 2012. http://theses.gla.ac.uk/3210/.

Full text

Abstract:

The software package Brainvisa (www.brainvisa.tnfo) offers a wide range of possibilities for cortical analysis using its automatic sulci recognition feature. Automated sulci identification is an attractive feature as the manual labelling of the cortical sulci is often challenging even for the experienced neuro-radiologists. This can also be of interest in fMRI studies of individual subjects where activated regions of the cortex can simply be identified using sulcal labels without the need for normalization to an atlas. As it will be explained later in this thesis, normalization to atlas can especially be problematic for pathologic brains. In addition, Brainvisa allows for sulcal morphometry from structural MR images by estimating a wide range of sulcal properties such as size, coordinates, direction, and pattern. Morphometry of abnormal brains has gained huge interest and has been widely used in finding the biomarkers of several neurological diseases or psychiatric disorders. However mainly because of its complexity, only a limited use of sulcal morphometry has been reported so far. With a wide range of possibilities for sulcal morphometry offered by Brainvisa, it is possible to thoroughly investigate the sulcal changes due to the abnormality. However, as any other automated method, Brainvisa can be susceptible to limitations associated with image quality. Factors such as noise, spatial resolution, and so on, can have an impact on the detection of the cortical folds and estimation of their attributes. Hence the robustness of Brainvisa needs to be assessed. This can be done by estimating the reliability and reproducibility of results as well as exploring the changes in results caused by other factors. This thesis is an attempt to investigate the possible benefits of sulci identification and sulcal morphometry for functional and structural MRI studies as well as the limitations of Brainvisa. In addition, the possibility of improvement of activation localization with functional MRI studies is further investigated. This investigation was motivated by a review of other cortical-based analysis methods, namely the cortical surface-based methods, which are discussed in the literature review chapter of this thesis. The application of these approaches in functional MRI data analysis and their potential benefits is used in this investigation.

APA, Harvard, Vancouver, ISO, and other styles

Dissertations / Theses on the topic 'MRI physics'

Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles