Shannon H. Kolind

Magnetic resonance imaging of myelin.

SummaryThe ability to measure myelin in vivo has great consequences for furthering our knowledge of normal development, as well as for understanding a wide range of neurological disorders. The following review summarizes the current state of myelin imaging using MR. We consider five MR techniques that have been used to study myelin: 1) conventional MR, 2) MR spectroscopy, 3) diffusion, 4) magnetization transfer, and 5) T2 relaxation. Fundamental studies involving peripheral nerve and MR/histology comparisons have aided in the interpretation and validation of MR data. We highlight a number of important findings related to myelin development, damage, and repair, and we conclude with a critical summary of the current techniques available and their potential to image myelin in vivo.

Is diffusion anisotropy an accurate monitor of myelination? Correlation of multicomponent T2 relaxation and diffusion tensor anisotropy in human brain.

We compare T2-relaxation and diffusion tensor data from normal human brain. The relationships between myelin-water fraction (MWF) and various diffusion tensor measures [e.g., fractional anisotropy (FA), perpendicular diffusivity (ADC perpendicular) and mean diffusivity ] in white matter (WM) and gray matter (GM) structures in the brain were examined in 16 normal volunteers at 1.5 T and 6 normal subjects at 3.0 T and mean diffusivity. We found some degree of linear correlation between these measurements, but by using region of interest (ROI)-based analysis, we also observed several structures which seemed to deviate significantly from a linear relationship. From all investigated relationships between various diffusion tensor measures and myelin-water content, FA and ADC perpendicular yielded the highest correlation coefficients with MWF. However, diffusion anisotropy was also significantly influenced by factors other than myelin-water content. The less operator-dependent voxel-based analysis (VBA) between myelin-water and diffusional anisotropy measures is proposed as an innovative alternative to ROI-based analysis. We confirmed that WM structures, in general, have higher diffusional anisotropy than GM structures and also have higher myelin-water content. However, our findings suggest that in the highly organized fibre arrangement of compact WM structures such as the genu of the corpus callosum, elevated degrees of diffusional anisotropies are measured, which do not necessarily correspond to an elevated myelin content but more likely reflect the highly organized directionality of fibre bundles in these areas (low microscopic and macroscopic tortuosity) as well as strongly restricted diffusion in the interstitial space between the myelinated axons. Conversely, in structures with disorganized fibre bundles and multiple fibre crossings, such as the minor and major forceps, low FA values were measured, which does not necessarily reflect a decrease myelin-water content.

Quantitative evaluation of metal artifact reduction techniques

To develop a technique to quantify artifact, and to use it to compare the effectiveness of several approaches to metal artifact reduction, including view angle tilting and increasing the slice select and image bandwidths (BWs), in terms of metal artifact reduction, noise, and blur.

MR relaxation in multiple sclerosis.

This article provides an overview of relaxation times and their application to normal brain and brain and cord affected by multiple sclerosis. The goal is to provide readers with an intuitive understanding of what influences relaxation times, how relaxation times can be accurately measured, and how they provide specific information about the pathology of MS. The article summarizes significant results from relaxation time studies in the normal human brain and cord and from people who have multiple sclerosis. It also reports on studies that have compared relaxation time results with results from other MR techniques.

Myelin water imaging: Implementation and development at 3.0T and comparison to 1.5T measurements.

Multicomponent T2 relaxation imaging can be used to measure signal from water trapped between myelin bilayers; the ratio of myelin water signal to total water is termed the myelin water fraction (MWF). The goal of this study was to implement and develop the single‐slice T2‐imaging technique proposed by Poon and Henkelman. For refinement, scan parameters (gradient crusher height and slew rate, bandwidth, echo spacing, matrix size, repetition time, and phase rewinding) were varied in water‐based phantoms and in fixed and in vivo brain. Changes in the standard deviation of the residuals of the multiexponential fit, MWF, T2, and peak width of the intra/extracellular water were monitored to determine which scan parameters minimized artifacts. Subsequently, we compared multicomponent T2 measurements at 1.5T and 3.0T for 10 healthy volunteers, and investigated the differences in SNR, fit residuals, MWF, and T2 and peak width of the intra/extracellular water, at higher magnetic field. MWF maps were found to be qualitatively similar between field strengths. MWFs were found to be significantly higher at 3.0T than at 1.5T, but with a strongly significant correlation between measurements (R2 > 0.92, P < 0.0005). The signal‐to‐noise ratio (SNR) was nearly double at 3.0T, but the standard deviation of residuals was increased in most cases. Magn Reson Med, 2009.

Localization of the Subthalamic Nucleus: Optimization with Susceptibility-Weighted Phase MR Imaging

BACKGROUND AND PURPOSE: On clinical MR images, the subthalamic nuclei (STN) are poorly delineated from adjacent structures, impeding safe direct targeting for placement of electrodes in the treatment of Parkinson disease. Susceptibility-weighted MR phase imaging offers improved contrast and spatial resolution at reduced imaging times relative to clinically used T2-weighted spin-echo imaging for STN visualization. Our purpose was to assess STN visibility by using phase imaging, comparing phase and magnitude images obtained concurrently by using susceptibility-weighted imaging (SWI). The goal was to identify an efficient scanning protocol for high-quality phase images of STN. MATERIALS AND METHODS: Seventy-eight SWI scans were acquired at 3T by using different TEs and acceleration factors. STN visibility and delimitation from adjacent structures were scored from 0 (not interpretable) to 5 (excellent). Regression analyses assessed the relationship of STN visibility to scanning parameters RESULTS: STN were identified at all studied TEs on phase images. Visibility and delimitation of STN were consistently superior on phase images compared with magnitude images. Good visualization (score of ≥4) of STN on phase imaging occurred at a mean TE of 20.0 ms and a sensitivity encoding (SENSE) of 1.40. Scores of STN visualization on phase images were dependent on SENSE (P < .002) and TE (P < .031). Good delimitation of the STN on phase imaging occurred at a mean TE of 21.6 ms and a SENSE of 1.36. CONCLUSIONS: Visualization and delimitation of STN was superior on phase images and was achieved at 3T in <2.5 minutes. A TE of 20 ms and an acceleration factor of ≤1.5 are recommended to visualize STN by using this method,.

Reproducibility of myelin water fraction analysis: a comparison of region of interest and voxel-based analysis methods

This study compared region of interest (ROI) and voxel-based analysis (VBA) methods to determine the optimal method of myelin water fraction (MWF) analysis. Twenty healthy controls were scanned twice using a multi-echo T(2) relaxation sequence and ROIs were drawn in white and grey matter. MWF was defined as the fractional signal from 15 to 40 ms in the T(2) distribution. For ROI analysis, the mean intensity of voxels within an ROI was fit using non-negative least squares. For VBA, MWF was obtained for each voxel and the mean and median values within an ROI were calculated. There was a slightly higher correlation between Scan 1 and 2 for the VBA method (R(2)=0.98) relative to the ROI method (R(2)=0.95), and the VBA mean square difference between scans was 300% lower, indicating VBA was the most consistent between scans. For the VBA method, mean MWF was found to be more reproducible than median MWF. As the VBA method is more reproducible and gives more options for visualization and analysis of MWF, it is recommended over the ROI method of MWF analysis.

Comparison of myelin water fraction from multiecho T2 decay curve and steady-state methods.

Myelin water fraction is conventionally measured from the T2 decay curve. Recently, a steady‐state approach entitled multicomponent‐driven equilibrium single pulse observation of T1/T2 (mcDESPOT) was employed for myelin water fraction mapping. However, no direct comparison between the established multiecho T2 relaxation method and mcDESPOT has been performed.

Long T2 water in multiple sclerosis: What else can we learn from multi-echo T2 relaxation?

AbstractMulti-echo T2 measurements are invaluable in studying brain pathology in multiple sclerosis (MS). In addition to information about myelin water and total water content, the T2 distribution has the potential to detect additional water reservoirs arising from other sources such as inflammation or edema. The purpose of this study was to better define the T2 distribution in MS lesions and normal appearing white matter (NAWM) with particular emphasis on the characterisation of longer T2 components.Magnetisation transfer (MT), T1 and 48-echo T2 relaxation data were acquired in 20 MS subjects and regions of interest were drawn in lesions and NAWM. Twenty-seven out of 107 lesions exhibited signal with a markedly prolonged T2 (200–800 ms). Lesions with a Long-T2 signal also exhibited a longer geometric mean T2 (GMT2), increased water content (WC), higher T1, reduced magnetisation transfer ratio (MTR) and decreased myelin water fraction (MWF) than lesions without a Long-T2 signal. Those subjects with Long-T2 lesions had a significantly longer disease duration than subjects without this lesion subtype. A strong correlation was observed between T1 and Long-T2 fraction, while a slightly weaker relationship was found for GMT2, MTR and MWF with Long-T2 fraction. A potential source of the Long-T2 signal is an increase in extracellular water. This study supports the usefulness of increasing the data acquisition window of the multi-echo T2 relaxation sequence to better characterise the T2 decay in MS.

MR evidence of long T2 water in pathological white matter.

To describe what, if any, specific long T2‐related abnormalities occur in the white matter of subjects with either phenylketonuria (PKU) or multiple sclerosis (MS).