Ashish A. Tamhane

k-space undersampling in PROPELLER imaging

PROPELLER MRI (periodically rotated overlapping parallel lines with enhanced reconstruction) provides images with significantly fewer B0‐related artifacts than echo‐planar imaging (EPI), as well as reduced sensitivity to motion compared to conventional multiple‐shot fast spin‐echo (FSE). However, the minimum imaging time in PROPELLER is markedly longer than in EPI and 50% longer than in conventional multiple‐shot FSE. Often in MRI, imaging time is reduced by undersampling k‐space. In the present study, the effects of undersampling on PROPELLER images were evaluated using simulated and in vivo data sets. Undersampling using PROPELLER patterns with reduced number of samples per line, number of lines per blade, or number of blades per acquisition, while maintaining the same k‐space field of view (FOVk) and uniform sampling at the edges of FOVk, reduced imaging time but led to severe image artifacts. In contrast, undersampling by means of removing whole blades from a PROPELLER sampling pattern that sufficiently samples k‐space produced only minimal image artifacts, mainly manifested as blurring in directions parallel to the blades removed, even when reducing imaging time by as much as 50%. Finally, undersampling using asymmetric blades and taking advantage of Hermitian symmetries to fill‐in the missing data significantly reduced imaging time without causing image artifacts. Magn Reson Med 53:675–683, 2005.

Motion correction in periodically‐rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) and turboprop MRI

Periodically‐rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) and Turboprop MRI are characterized by greatly reduced sensitivity to motion, compared to their predecessors, fast spin‐echo (FSE) and gradient and spin‐echo (GRASE), respectively. This is due to the inherent self‐navigation and motion correction of PROPELLER‐based techniques. However, it is unknown how various acquisition parameters that determine k‐space sampling affect the accuracy of motion correction in PROPELLER and Turboprop MRI. The goal of this work was to evaluate the accuracy of motion correction in both techniques, to identify an optimal rotation correction approach, and determine acquisition strategies for optimal motion correction. It was demonstrated that blades with multiple lines allow more accurate estimation of motion than blades with fewer lines. Also, it was shown that Turboprop MRI is less sensitive to motion than PROPELLER. Furthermore, it was demonstrated that the number of blades does not significantly affect motion correction. Finally, clinically appropriate acquisition strategies that optimize motion correction are discussed for PROPELLER and Turboprop MRI. Magn Reson Med, 2009.

Quantitative magnetization transfer imaging in human brain at 3 T via selective inversion recovery.

Quantitative magnetization transfer imaging yields indices describing the interactions between free water protons and immobile, macromolecular protons—including the macromolecular to free pool size ratio (PSR) and the rate of magnetization transfer between pools kmf. This study describes the first implementation of the selective inversion recovery quantitative magnetization transfer method on a clinical 3.0‐T scanner in human brain in vivo. Selective inversion recovery data were acquired at 16 different inversion times in nine healthy subjects and two patients with relapsing remitting multiple sclerosis. Data were collected using a fast spin‐echo readout and reduced repetition time, resulting in an acquisition time of 4 min for a single slice. In healthy subjects, excellent intersubject and intrasubject reproducibilities (assessed via repeated measures) were demonstrated. Furthermore, PSR values in white (mean ± SD = 11.4 ± 1.2%) and gray matter (7.5 ± 0.7%) were consistent with previously reported values, while kmf values were approximately 2‐fold slower in both white (11 ± 2 s–1) and gray matter (15 ± 6 s–1). In relapsing remitting multiple sclerosis patients, quantitative magnetization transfer indices were sensitive to pathological changes in lesions and in normal appearing white matter. Magn Reson Med, 2011.

Investigating the Medial Temporal Lobe in Alzheimer’s Disease and Mild Cognitive Impairment, with Turboprop Diffusion Tensor Imaging, MRI-volumetry, and T2-relaxometry

The first neuropathological alterations in Alzheimer’s disease (AD) occur in the medial temporal lobes (MTLs), in the entorhinal cortex (EC), perforant pathway (PP), and hippocampus. The purpose of this study was to investigate the microstructural integrity, size, and T2-relaxation times of MTL structures in patients with AD and mild cognitive impairment (MCI), using MRI techniques that are immune to magnetic field inhomogeneities. Turboprop-DTI and high-resolution anatomical MRI data were obtained on AD and MCI patients, and healthy controls. Significant AD-related changes were detected in more MTL structures of AD patients by means of Turboprop-DTI than with any other technique used. Mean diffusivity and T2-relaxation times of specific MTL structures increased in both the MCI and AD cohorts when compared to normal controls. Therefore, Turboprop-DTI and T2-relaxometry may be valuable non-invasive tools in the investigation of the early AD-related neuropathological alterations.

Contribution of cardiac‐induced brain pulsation to the noise of the diffusion tensor in Turboprop diffusion tensor imaging (DTI)

To assess the effects of cardiac‐induced brain pulsation on the noise of the diffusion tensor in Turboprop (a form of periodically rotated overlapping parallel lines with enhanced reconstruction [PROPELLER] imaging) diffusion tensor imaging (DTI).

Rapid PROPELLER-MRI: a combination of iterative reconstruction and under-sampling.

To develop a technique that is able to reduce acquisition time and remove uneven blurring in reconstructed image for PROPELLER MRI. By using under‐sampling and iterative reconstruction, this proposed technique will be less sensitive to subject motion.

Iterative image reconstruction for PROPELLER-MRI using the nonuniform fast fourier transform

To investigate an iterative image reconstruction algorithm using the nonuniform fast Fourier transform (NUFFT) for PROPELLER (Periodically Rotated Overlapping ParallEL Lines with Enhanced Reconstruction) MRI.

Ex-vivo quantitative susceptibility mapping of human brain hemispheres

Ex-vivo brain quantitative susceptibility mapping (QSM) allows investigation of brain characteristics at essentially the same point in time as histopathologic examination, and therefore has the potential to become an important tool for determining the role of QSM as a diagnostic and monitoring tool of age-related neuropathologies. In order to be able to translate the ex-vivo QSM findings to in-vivo, it is crucial to understand the effects of death and chemical fixation on brain magnetic susceptibility measurements collected ex-vivo. Thus, the objective of this work was twofold: a) to assess the behavior of magnetic susceptibility in both gray and white matter of human brain hemispheres as a function of time postmortem, and b) to establish the relationship between in-vivo and ex-vivo gray matter susceptibility measurements on the same hemispheres. Five brain hemispheres from community-dwelling older adults were imaged ex-vivo with QSM on a weekly basis for six weeks postmortem, and the longitudinal behavior of ex-vivo magnetic susceptibility in both gray and white matter was assessed. The relationship between in-vivo and ex-vivo gray matter susceptibility measurements was investigated using QSM data from eleven older adults imaged both antemortem and postmortem. No systematic change in ex-vivo magnetic susceptibility of gray or white matter was observed over time postmortem. Additionally, it was demonstrated that, gray matter magnetic susceptibility measured ex-vivo may be well modeled as a linear function of susceptibility measured in-vivo. In conclusion, magnetic susceptibility in gray and white matter measured ex-vivo with QSM does not systematically change in the first six weeks after death. This information is important for future cross-sectional ex-vivo QSM studies of hemispheres imaged at different postmortem intervals. Furthermore, the linear relationship between in-vivo and ex-vivo gray matter magnetic susceptibility suggests that ex-vivo QSM captures information linked to antemortem gray matter magnetic susceptibility, which is important for translation of ex-vivo QSM findings to in-vivo.

A BIOMARKER FOR ARTERIOLAR SCLEROSIS BASED ON MRI-DERIVED FEATURES

using the AUC. For thickness and volume, the most important features (occurrence in CV-Lasso > 5) were used to select a subset of significant features in combined models. Results: Figure 1 reports the results obtained when each feature group is taken individually. Adding APOE4 status to BF only yields a marginal gain (Fig-1B). Adding either the Ab status, volume or thickness leads to a larger AUC increase. After feature selections, the combined volume and thickness (#features1⁄417) yields the highest AUC1⁄40.8260.13 comparable to the BF+Ab (Fig-1C). Figure 2 reports the results of the BF + features combined. BF+Ab+APOE4 model leads to no increase in the AUC compared to BF+Ab. The highest accuracy is obtained when combining MRI volume, thickness and Ab status (AUC1⁄40.8660.11). Conclusions: Combining regional brain volumes, cortical thickness and basic baseline characteristics appears to be a powerful tool for predicting MCI conversion to AD. Determination of thresholds based on these baseline values may help identify rapid decliners, who would be the ideal population for a clinical trial aiming at delaying disease onset, as it would help decrease sample size and study cost. P3-428 A BIOMARKER FOR ARTERIOLAR

NEUROPATHOLOGIC CORRELATES OF ENLARGED PERIVASCULAR SPACES IN A COMMUNITY COHORT OF OLDER ADULTS

Background:Enlarged perivascular spaces (EPVS), also known as Virchow-Robin spaces, are cerebrospinal fluid-filled spaces surrounding blood vessels. EPVS have been associated with aging, small-vessel disease and cognitive decline. However, the relationship of EPVS with age-related neuropathologies has not been thoroughly investigated, and published studies suffer from small sample-size, lack of thorough neuropathological assessment, and/or long intervals between in-vivo imaging and autopsy. Therefore, the purpose of this research was to assess the neuropathologic correlates of EPVS burden using a study design that addresses the above shortcomings by combining ex-vivo MRI and pathology on a large community cohort of older adults. Methods: Cerebral hemispheres were obtained from 281 deceased participants of the Rush Memory and Aging Project and the Religious Orders Study, two longitudinal, epidemiologic clinical-pathologic cohort studies of aging. All hemispheres were imaged ex-vivo on a 3T clinical MRI scanner, while immersed in 4% formaldehyde solution, using a 2D fast spin-echo sequence with multiple echo-times (TEs). Following ex-vivo MRI, hemispheres underwent neuropathologic examination. Demographic, clinical and neuropathologic information is included in Figure 1. EPVS burden was manually assessed using a four-level scale. Examples are shown in Figures 2-5. Intra-