Browsing by Author "Lebel, Robert Marc"
Now showing 1 - 5 of 5
Results Per Page
Sort Options
Item Open Access Accelerated Quantitative Magnetization Transfer (qMT) Imaging(2018-10-24) Mclean, Melany Ann; Pike, G. Bruce; Forkert, Nils Daniel; Lebel, Robert MarcQuantitative magnetization transfer (qMT) is an advanced magnetic resonance imaging (MRI) technique with enhanced specificity to myelin. The acquisition of many images with unique magnetization transfer (MT) saturation results in a signal response curve known as the z-spectrum. The two-pool tissue model, which describes properties of nuclei with free and restricted motion, can be fit to the z-spectrum to provide details of macromolecular tissue content (including myelin) beyond what can be seen from conventional single saturation approaches (e.g. MT ratio). Widespread use of qMT has been hindered by long acquisition times inherent to z-spectrum-based imaging techniques including qMT and chemical exchange saturation transfer (CEST). This thesis uses sparseSENSE, a combined parallel imaging and compressed sensing technique, to accelerate MT-weighted images. In this thesis, sparsifying reconstruction algorithms are shown to enable high-quality image reconstruction from 4D qMT datasets, retrospectively under-sampled by factors of up to 32. MT-weighted images demonstrate exceptional image quality at high acceleration factors, which is shown to translate well to accelerated z-spectra. However, qMT parametric maps produced from accelerated z-spectra are shown to be sensitive to acceleration artifacts and can only be accelerated by a factor of 4 with minimal loss of image quality. Nonetheless, this acceleration can yield a significant acquisition time savings when applied to prospectively under-sampled data. In addition, time savings created by acceleration can be used to increase spatial resolution or collect more MT-weighted images, enabling even higher acceleration factors. Long acquisition times have often been cited as a limitation of the qMT method. This work has addressed that limitation, making qMT protocols more feasible for in vivo research studies, particularly in youth and patient populations.Item Open Access Combined Segmentation and Hemodynamic Analysis of Cerebrovascular Structures using Arterial Spin Labeling(2020-01-17) Phellan Aro, Renzo; Forkert, Nils Daniel; Frayne, Richard; Walker, Richard E. A.; Lebel, Robert Marc; Far, Behrouz H.; Duong, LucSpatiotemporal arterial spin labeling magnetic resonance angiography (4D ASL MRA) is a non-invasive imaging modality used to acquire dynamic images of cerebrovascular structures. It can achieve high spatial and temporal resolution, while capturing morphological and blood flow data. Recent scientific studies have used 4D ASL MRA to analyze the cerebrovascular system for characterization, diagnosis, and post-treatment assessment of different cerebrovascular diseases, such as aneurysms, arteriovenous malformations, and moyamoya disease. However, this image sequence generates a considerable amount of data, which can be tedious to analyze by simple visual inspection, a problem also present with other 4D imaging methods. In this case, medical image processing methods can be used to extract the morphological and blood flow data contained in 4D ASL MRA datasets and present it in a more useful format to clinicians and researchers. The aim of this work was to develop and evaluate novel image processing methods for advanced analysis of 4D MRA datasets. The overreaching idea for the development of the corresponding methods is to use blood flow information for improving the vessel segmentation while the vessel segmentation is used to improve the results of the hemodynamic analysis. It was hypothesized that this combined analysis improves the vessel segmentation and results the hemodynamic analysis at the same time. The methods were developed and evaluated using 15 datasets of healthy volunteers, flow phantom measurements, and two datasets of patients with a stenosis. The findings of this work indicate that the proposed combined segmentation and hemodynamic analysis can improve the overall accuracy of the segmentation and blood flow parameter estimation while first experiments also show that the proposed methods can be applied to patients with a cerebrovascular disease. The methods developed in this work could help translating 4D ASL MRA datasets into clinical practice and support clinical research of various cerebrovascular diseases using 4D ASL MRA while the developed methods have also the potential to be useful for other 4D imaging sequences.Item Open Access A comparison of inhomogeneous magnetization transfer, myelin volume fraction, and diffusion tensor imaging measures in healthy children(2017-09-12) Geeraert, Bryce L.; Lebel, Robert Marc; Mah, Alyssa C.; Deoni, Sean C. L.; Alsop, David C.; Varma, Gopal; Lebel, Catherine A.Sensitive and specific biomarkers of myelin can help define baseline brain health and development, identify and monitor disease pathology, and evaluate response to treatment where myelin content is affected. Diffusion measures such as radial diffusivity (RD) are commonly used to assess myelin content, but are not specific to myelin. Inhomogeneous magnetization transfer (ihMT) and multicomponent driven equilibrium single-pulse observation of T1 and T2 (mcDESPOT) offer quantitative parameters (qihMT and myelin volume fraction/VFm, respectively) which are suggested to have improved sensitivity to myelin. We compared RD, qihMT, and VFm in a cohort of 23 healthy children aged 8-13 years to evaluate the similarities and differences across these measures. All 3 measures were significantly related across brain voxels, but VFm and qihMT were significantly more strongly correlated (qihMT-VFm r = 0.89) than either measure was with RD (RD-qihMT r = -0.66, RD-VFm r = -0.74; all p < 0.001). Mean parameters differed in several regions, especially in subcortical gray matter. These differences can likely be explained by unique sensitivities of each measure to non-myelin factors, such as crossing fiber geometry, axonal packing, fiber orientation, glial density, or magnetization transfer effects in a voxel. We also observed an orientation dependence of qihMT in white matter, such that qihMT decreased as fiber orientation went from parallel to perpendicular to B0. All measures appear to be sensitive to myelin content, though qihMT and VFm appear to be more specific to it than RD. Scan time, noise tolerance, and resolution requirements may inform researchers of the appropriate measure to choose for a specific application.Item Open Access Image quality impact of randomized sampling trajectories in MRI: Implications for compressed sensing(2017) Jones, Melissa; Lebel, Robert Marc; Frayne, RichardMagnetic resonance imaging provides exceptional soft tissue contrast but it is limited by long scan times. Compressed sensing (CS) is a novel technique that leverages the underlying transform sparsity of medical images, randomized under-sampling of k-space, and nonlinear reconstruction to recover images from accelerated scans. However, in some cases image quality of prospectively implemented CS is not as good as predicted by retrospectively under-sampled data. This dissertation investigates the source of decreased image quality in CS. Our findings demonstrate that random k-space trajectories are specifically susceptible to encoding errors between repetition periods. Sources of errors may be due to eddy currents or motion, and errors have implications for accelerated reconstructions. CS is sensitive to data inconsistencies from random trajectories as acceleration rates increase, but minimizing trajectory length in random under-sampled trajectories can reduce data inconsistencies. Solving these issues will improve image quality and help realize the potential of CS.Item Open Access Method for Optimizing Quantitative Temporal Lobe Epilepsy MR Imaging(2018-01-17) Basiri, Reza; Lebel, Robert Marc; Federico, Paolo; Agha-Khani, Yahya; Takele Zewdie, Ephrem; Frayne, Richard; Sotero Díaz, Roberto C.Many neurological disorders such as epilepsy rely on MRI for detection of structural abnormalities. However, the current clinical MRI methods are insufficient and insensitive in detection of subtle abnormalities. MRI quantitative T2 mapping is a promising quantitative medical imaging technique as it is highly sensitive to tissue composition. The conventional approach for T2 mapping assumes mono-exponential signal decay; however, this is rarely observed due to transmit field inhomogeneity and miscalibration at high field MRIs. The nonexponentially results in poor fits and a systematic bias in estimated decay rates. A recently proposed fitting method, called stimulated echo correction, uses the same input data but estimates the major confounds associated with mono-exponential fitting. Optimal accuracy and non-optimal precision is achieved in this method. My first aim was to develop a stimulated echo correction based method with fewer parameters and higher precision relative to the original one. The second aim was to implement this new method in order to better identify abnormal brain regions in temporal lobe epilepsy that were poorly visualized on standard images. I hypothesized that my improved stimulated echo correction with fewer parameters would provide more accurate and reliable transverse relaxometry imaging than does conventional or the original stimulated echo correction fitting methods, and would improve our ability to detect subtle irregularities associated with epilepsy. The new method was evaluated with simulated and in-vivo data, in which up to 27% reduction in variance in the new method compared to the original stimulated echo correction was observed. Moreover, the new method had greater reliability in categorizing abnormalities in hippocampal regions when compared with exponential and stimulated echo correction methods. I concluded that the new method is able to reduce the variance in T2 relaxometry from multi-echo spin echo sequences; therefore, this method can potentially help in detection of those lesser obvious hippocampus abnormalities.