808 research outputs found
On Nature of the Gradient Echo MR Signal and Its Application to Monitoring Multiple Sclerosis
Multiple Sclerosis is a common disease, affecting 2.5 million people world-wide. The clinical course is heterogeneous, ranging from benign disease in which patients live an almost normal life to severe and devastating disease that may shorten life. Despite much research, a fully effective treatment for MS is still unavailable and diagnostic techniques for monitoring MS disease evolution are much needed.
As a non-invasive tool, Magnetic resonance imaging: MRI) plays a key role in MS diagnosis. Numerous MRI techniques have been proposed over the years. Among most widely used are conventional T1-weighted: T1W), T2-weighted: T2W) and FLuid Attenuated Inversion Recovery: FLAIR) imaging techniques. However their results do not correlate well with neurological findings. Several advanced MRI techniques are also used as research tools to study MS. Among them are magnetization transfer contrast imaging: MT), MR spectroscopy: MRS), and Diffusion Tensor Imaging: DTI) but they have not penetrated to clinical arena yet.
Gradient Echo Plural Contrast Imaging: GEPCI) developed in our laboratory is a post processing technique based on multi-echo gradient echo sequence. It offers basic contrasts such as T1W images and T2* maps obtained from magnitude of GEPCI signal, and frequency maps obtained from GEPCI signal phase.
Phase information of Gradient Echo MR signal has recently attracted much attention of the MR community since it manifests superior gray matter/ white matter contrast and sub-cortical contrast, especially at high field: 7 T) MRI. However the nature of this contrast is under intense debates. Our group proposed a theoretical framework - Generalized Lorentzian Approach - which emphasizes that, contrary to a common-sense intuition, phase contrast in brain tissue is not directly proportional to the tissue bulk magnetic susceptibility but is rather determined by the geometrical arrangement of brain tissue components: lipids, proteins, iron, etc.) at the cellular and sub-cellular levels - brain tissue magnetic architecture . In this thesis we have provide first direct prove of this hypothesis by measurement of phase contrast in isolated optic nerve. We have also provided first quantitative measurements of the contribution to phase contrast from the water-macromolecule exchange effect. Based on our measurement in protein solutions, we demonstrated that the magnitude of exchange effect is 1/2 of susceptibility effect and to the opposite sign.
GEPCI technique also offers a scoring method for monitoring Multiple Sclerosis based on the quantitative T2* maps generated from magnitude information of gradient echo signal. Herein we demonstrated a strong agreement between GEPCI quantitative scores and traditional lesion load assessment. We also established a correlation between GEPCI scores and clinical tests for MS patients. We showed that this correlation is stronger than that found between traditional lesion load and clinical tests. Such studies will be carried out for longer period and on MS subjects with broader range of disease severity in the future.
We have also demonstrated that the magnitude and phase information available from GEPCI experiment can be combined in multiple ways to generate novel contrasts that can help with visualization of neurological brain abnormalities beyond Multiple Sclerosis.
In summary, in this study, we 1) propose novel contrasts for GEPCI from its basic images; 2) investigate the biophysical mechanisms behind phase contrast; 3) evaluate the benefits of quantitative T2* map offered by GEPCI in monitoring disease of Multiple Sclerosis by comparing GEPCI results to clinical standard techniques; 4) apply our theoretical framework - Generalized Lorentzian Approach - to better understand phase contrast in MS lesions
Quantification of Localized Brain Iron Sources Using Magnetic Resonance Phase
Brain microbleeds (BMB), often present in cerebrovascular and neurodegenerative diseases and neurotrauma, are associated with both chronic and acute illness of significant social and economic impact. Because BMB present a source of potentially cyctotoxic iron to the brain proportional to the amount of extravasated blood, non-invasive quantification of this iron pool is potentially valuable both to assess tissue risk and as a biomarker to monitor disease progression, treatment efficacy, and inform treatment. Past efforts to quantify brain iron have focused on distributed (e.g., anatomical) brain regions. However, BMB represent localized sources of iron deposition. In addition, conventional magnitude MR images have significant limitations, especially for localized iron quantification. Moreover, due to susceptibility effects, the localized bypointensities in gradient recalled T2* magnitude images associated with BMB typically appear larger than the actual tissue lesion (the blooming effect) and obscure the true dimensions of an iron susceptibility source. In the present research, we proposed a family of techniques that use magnetic resonance phase images (instead of magnitude images) to quantify the iron content and dimensions of localized iron sources such as BMB. The techniques were tested in four systems: 1) magnetic resonance agarose phantom and 2) postmortem rat brain, using a ferric iron oxy0hydroxide mimic for hemosiderin, 3) the living rat brain, using collagenase-induced bleeds, and 4) with actual BMB in postmortem cerebral amyloid angiopathy brain. Measurements of geometric features in phase images were related to source iron content and diameter using mathematical models. Iron samples and BMB lesions were assayed for iron content using atomic absorption spectrometry. Results from experiments 1 and 3 in particular showed very good agreement with predictions of the theory underlying the techniques, providing validation for the methods and demonstrating that prominent phase image features can potentially be used to measure localized iron content including iron in real BMB. Our methods potentially allow the calculation of brain iron load indices based on BMB iron content as well as classification of BMB by size unobscured by the blooming effect. These results represent significant steps toward the use of similar localized iron quantification methods in experimental and clinical settings
Assessing the Impact of Flip Angle and on Image Quality and reliability of Ernst Angle optimization across Varied Conditions in Magnetic Resonance Imaging
This study investigates the significance of flip angle, an imaging parameter,
in enhancing Magnetic Resonance image quality under various imaging conditions.
It specifically explores the extent to which the Ernst angle, an optimal flip
angle, optimizes image quality under different imaging parameters. The
investigation begins with a theoretical derivation of the Ernst angle, assuming
steady state imaging conditions. Then multiple studies that examine the effect
of flip angle on signal-to-noise ratio (SNR), a key indicator of image quality,
in different areas of the human body (blood, liver, and brain), are analysed.
The study compares the results of these studies and compares their respective
optimal flip angles with the Ernst angle. The findings reveal that flip angle
plays a crucial role in enhancing SNR and image quality. However, the Ernst
angle only optimizes SNR under steady state conditions and when using a spoiled
gradient echo (GRE) sequence. Therefore, further investigations are necessary
to determine the optimal flip angle under different imaging conditions to
optimize SNR and enhance overall image quality.Comment: 23 pages, 5152 word
WHOLE BODY AND UPPER EXTREMITY ULTRA-HIGH FIELD MAGNETIC RESONANCE IMAGING: COIL DEVELOPMENT AND CLINICAL IMPLEMENTATION
Since Magnetic Resonance Imaging (MRI)’s introduction into the clinical imaging application arena, MRI has become one of the most promising non-invasive methods for evaluating and identifying body organs in normal and diseased conditions. In the last two decades, a few research groups have been working on addressing the challenges to Ultra-High Field (UHF) imaging (≥ 7 Tesla), such as magnetic field inhomogeneities and elevated Radiofrequency (RF) power absorption through technological developments. In recent years, imaging at 7 Tesla has shown an inherent ability to improve scan time and anatomical resolution.
To address the current challenges associated with UHF imaging, this thesis presents the development of innovative whole body and extremity RF coil systems for 7 Tesla imaging. For body imaging, the transmit (Tx) coil is based on the innovative Tic-Tac-Toe (TTT) design, which possesses a load insensitive characteristic in terms of magnetic and electric field distributions. 7 Tesla homogenous whole-body in-vivo imaging with and without a receive (Rx) only insert array is demonstrated showing excellent anatomical detail.
As a part of upper extremity imaging, we have developed a transverse electromagnetic (TEM) coil as a transmitter in conjunction with an eight channel receive only insert for 7 Tesla hand/forearm imaging. We have acquired a wide variety of different sequences and used post-processing methods to extract specific anatomy from high resolution scans (i.e. nerve and vessels), which in turn has helped in exploring new clinical applications, such as arm transplantation, and has added knowledge to existing ones.
The developed RF coil systems and methodologies not only enhance the fundamental scientific knowledge of RF coil design approaches at high frequencies but they also add to the realm of clinical applications of UHF human imaging
An investigation of fluid transport in porous solids using nuclear magnetic resonance
A commercially available NMR spectrometer has been used to investigate fluid transport within porous solids. Two water-wet porous solids were investigated. The first was a sample of Fontainebleau sandstone, and the second was an idealised porous solid made from a random packing of glass beads. The samples were saturated with two immiscible phases, i.e. an oil and water phase. Pulsed field gradient (PFG) NMR measurements of one- and two-dimensional displacement probability distributions are reported, for steady-state flow and diffusion, within two phase saturated porous solids. Measurements were made with the porous solids prepare in different steady-state saturations. NMR relaxation measurements are also reported.
Using the NMR data it was possible to evaluate the physical importance of parameters such as wettability and phase saturation on transport phenomena in two phase saturated porous solids. Various computer simulations were developed to model the experimental data
Textural Features in Medical Magnetic Resonance Image Analysis of the Brain and Thigh Muscles
Magnetic resonance imaging (MRI) provides high-quality images with excellent contrast detail of soft tissues and anatomic structures. MR images contain a large amount of detailed information – some of which is invisible to the human eye. Detailed information can be analysed with computer-assisted texture analysis (TA), which is based on features describing the grey level relationships between image pixels. The aim of this thesis was to assess the information content of textural features based on the image histogram, grey level co-occurrence matrix, and grey level run-length matrix. The strengths and limitations of the various textural features in medical MR image analysis were evaluated. The study was conducted by analysing different clinical data with TA in the clinical environment, and the results of the learning process were then gathered in this thesis. Our results indicated that all features have limitations in terms of their discrimination capacity in medical MR images and their dependence on the size of the region of interest and MR imaging parameters. By considering these limitations, TA may help in various MR imaging applications by revealing textural information of the images of various human organs. <br/
Imaging the subthalamic nucleus in Parkinson’s disease
This thesis is comprised of a set of work that aims to visualize and quantify the anatomy, structural variability, and connectivity of the subthalamic nucleus (STN) with optimized neuroimaging methods. The study populations include both healthy cohorts and individuals living with Parkinson's disease (PD). PD was chosen specifically due to the involvement of the STN in the pathophysiology of the disease. Optimized neuroimaging methods were primarily obtained using ultra-high field (UHF) magnetic resonance imaging (MRI). An additional component of this thesis was to determine to what extent UHF-MRI can be used in a clinical setting, specifically for pre-operative planning of deep brain stimulation (DBS) of the STN for patients with advanced PD. The thesis collectively demonstrates that i, MRI research, and clinical applications must account for the different anatomical and structural changes that occur in the STN with both age and PD. ii, Anatomical connections involved in preparatory motor control, response inhibition, and decision-making may be compromised in PD. iii. The accuracy of visualizing and quantifying the STN strongly depends on the type of MR contrast and voxel size. iv, MRI at a field strength of 3 Tesla (T) can under certain circumstances be optimized to produce results similar to that of 7 T at the expense of increased acquisition time
- …