R. Marc Lebel

GOCART: GOlden-angle CArtesian randomized time-resolved 3D MRI

PURPOSEnTo develop and evaluate a novel 3D Cartesian sampling scheme which is well suited for time-resolved 3D MRI using parallel imaging and compressed sensing.nnnMETHODSnThe proposed sampling scheme, termed GOlden-angle CArtesian Randomized Time-resolved (GOCART) 3D MRI, is based on golden angle (GA) Cartesian sampling, with random sampling of the ky-kz phase encode locations along each Cartesian radial spoke. This method was evaluated in conjunction with constrained reconstruction of retrospectively and prospectively undersampled in-vivo dynamic contrast enhanced (DCE) MRI data and simulated phantom data.nnnRESULTSnIn in-vivo retrospective studies and phantom simulations, images reconstructed from phase encodes defined by GOCART were equal to or superior to those with Poisson disc or GA sampling schemes. Typical GOCART sampling tables were generated in <100ms. GOCART has also been successfully utilized prospectively to produce clinically valuable whole-brain DCE-MRI images.nnnCONCLUSIONnGOCART is a practical and efficient sampling scheme for time-resolved 3D MRI. It shows great potential for highly accelerated DCE-MRI and is well suited to modern reconstruction methods such as parallel imaging and compressed sensing.

Big GABA: Edited MR spectroscopy at 24 research sites

Abstract Magnetic resonance spectroscopy (MRS) is the only biomedical imaging method that can noninvasively detect endogenous signals from the neurotransmitter &ggr;‐aminobutyric acid (GABA) in the human brain. Its increasing popularity has been aided by improvements in scanner hardware and acquisition methodology, as well as by broader access to pulse sequences that can selectively detect GABA, in particular J‐difference spectral editing sequences. Nevertheless, implementations of GABA‐edited MRS remain diverse across research sites, making comparisons between studies challenging. This large‐scale multi‐vendor, multi‐site study seeks to better understand the factors that impact measurement outcomes of GABA‐edited MRS. An international consortium of 24 research sites was formed. Data from 272 healthy adults were acquired on scanners from the three major MRI vendors and analyzed using the Gannet processing pipeline. MRS data were acquired in the medial parietal lobe with standard GABA+ and macromolecule‐ (MM‐) suppressed GABA editing. The coefficient of variation across the entire cohort was 12% for GABA+ measurements and 28% for MM‐suppressed GABA measurements. A multilevel analysis revealed that most of the variance (72%) in the GABA+ data was accounted for by differences between participants within‐site, while site‐level differences accounted for comparatively more variance (20%) than vendor‐level differences (8%). For MM‐suppressed GABA data, the variance was distributed equally between site‐ (50%) and participant‐level (50%) differences. The findings show that GABA+ measurements exhibit strong agreement when implemented with a standard protocol. There is, however, increased variability for MM‐suppressed GABA measurements that is attributed in part to differences in site‐to‐site data acquisition. This studys protocol establishes a framework for future methodological standardization of GABA‐edited MRS, while the results provide valuable benchmarks for the MRS community. HighlightsGABA‐edited MEGA‐PRESS data from 272 adults were collected from 24 sites.GABA+ data showed good agreement across vendors and sites.Variability in MM‐suppressed GABA data was attributed in part to B0 field offsets.Multi‐site studies using GABA editing are feasible using a standardized protocol.These results provide valuable benchmarks for the MRS community.

Proton spectroscopy study of the dorsolateral prefrontal cortex in youth with familial depression.

Structural, functional, and metabolic changes in the dorsolateral prefrontal cortex (DLPFC) are implicated in the pathogenesis of major depressive disorder (MDD). We used proton magnetic resonance spectroscopy (1 H‐MRS) to examine the metabolite choline (glycerophosphocholine plus phosphocholine), which is used as an index of membrane integrity in the left DLPFC, in adolescents and young adults with MDD who were treatment‐resistant and had a positive family history compared to healthy controls. Differences in the choline resonance indicate an imbalance between synthesis and degradation activity of neuronal and glia membrane phospholipids.

High-resolution whole-brain DCE-MRI using constrained reconstruction: Prospective clinical evaluation in brain tumor patients

PURPOSEnTo clinically evaluate a highly accelerated T1-weighted dynamic contrast-enhanced (DCE) MRI technique that provides high spatial resolution and whole-brain coverage via undersampling and constrained reconstruction with multiple sparsity constraints.nnnMETHODSnConventional (rate-2 SENSE) and experimental DCE-MRI (rate-30) scans were performed 20 minutes apart in 15 brain tumor patients. The conventional clinical DCE-MRI had voxel dimensions 0.9 × 1.3 × 7.0 mm(3), FOV 22 × 22 × 4.2 cm(3), and the experimental DCE-MRI had voxel dimensions 0.9 × 0.9 × 1.9 mm(3), and broader coverage 22 × 22 × 19 cm(3). Temporal resolution was 5 s for both protocols. Time-resolved images and blood-brain barrier permeability maps were qualitatively evaluated by two radiologists.nnnRESULTSnThe experimental DCE-MRI scans showed no loss of qualitative information in any of the cases, while achieving substantially higher spatial resolution and whole-brain spatial coverage. Average qualitative scores (from 0 to 3) were 2.1 for the experimental scans and 1.1 for the conventional clinical scans.nnnCONCLUSIONSnThe proposed DCE-MRI approach provides clinically superior image quality with higher spatial resolution and coverage than currently available approaches. These advantages may allow comprehensive permeability mapping in the brain, which is especially valuable in the setting of large lesions or multiple lesions spread throughout the brain.

Fast spin echo imaging of carotid artery dynamics

We propose the use of a retrospectively gated cine fast spin echo (FSE) sequence for characterization of carotid artery dynamics. The aim of this study was to compare cine FSE measures of carotid dynamics with measures obtained on prospectively gated FSE images.

A pilot study of hippocampal N-acetyl-aspartate in youth with treatment resistant major depression

BACKGROUNDnSmaller hippocampal volumes, as assessed by magnetic resonance imaging (MRI), and proton magnetic resonance spectroscopy (1H-MRS) indexed alterations in brain metabolites have been identified in adults with major depressive disorder (MDD). Our group has found similar effects in MDD youth. However, this has not been studied in youth with treatment resistant MDD (TRD), nor has the interaction between regional N-acetyl-aspartate and volume deficits. N-acetyl-aspartate is an amino acid in the synthesis pathway of glutamate, and serves a marker of neuronal viability/number.nnnMETHODSnFifteen typically developing youth (16-22 years of age; 7 males, 8 females) and eighteen youth with TRD (14-22 years of age; 8 males, 10 females) underwent 1H-MRS and MRI on a 3T scanner. A short echo PRESS protocol was used with voxels in the right and left hippocampi (6mL each). Hippocampal volume was evaluated using FreeSurfer.nnnRESULTSnCompared with the typically developing group, youth with TRD had lower concentrations of N-acetyl-aspartate in the left hippocampus (p=0.004), and a trend for smaller left hippocampal volume (p=0.067). In TRD subjects, hippocampal N-acetyl-aspartate was inversely correlated with left (r=-0.68, p=0.003) but not right hippocampal volume. Right hippocampal glutamate+glutamine was greater in TRD youth compared to typically developing controls (p=0.007).nnnCONCLUSIONSnThese results suggest a neurochemical and structural deficit in the hippocampi of youth with TRD. These findings fit with the role of N-acetyl-aspartate in glutamate neurotransmission and the effect of glutamate on brain morphology.

Multiband multi-echo imaging of simultaneous oxygenation and flow timeseries for resting state connectivity

A novel sequence has been introduced that combines multiband imaging with a multi-echo acquisition for simultaneous high spatial resolution pseudo-continuous arterial spin labeling (ASL) and blood-oxygenation-level dependent (BOLD) echo-planar imaging (MBME ASL/BOLD). Resting-state connectivity in healthy adult subjects was assessed using this sequence. Four echoes were acquired with a multiband acceleration of four, in order to increase spatial resolution, shorten repetition time, and reduce slice-timing effects on the ASL signal. In addition, by acquiring four echoes, advanced multi-echo independent component analysis (ME-ICA) denoising could be employed to increase the signal-to-noise ratio (SNR) and BOLD sensitivity. Seed-based and dual-regression approaches were utilized to analyze functional connectivity. Cerebral blood flow (CBF) and BOLD coupling was also evaluated by correlating the perfusion-weighted timeseries with the BOLD timeseries. These metrics were compared between single echo (E2), multi-echo combined (MEC), multi-echo combined and denoised (MECDN), and perfusion-weighted (PW) timeseries. Temporal SNR increased for the MECDN data compared to the MEC and E2 data. Connectivity also increased, in terms of correlation strength and network size, for the MECDN compared to the MEC and E2 datasets. CBF and BOLD coupling was increased in major resting-state networks, and that correlation was strongest for the MECDN datasets. These results indicate our novel MBME ASL/BOLD sequence, which collects simultaneous high-resolution ASL/BOLD data, could be a powerful tool for detecting functional connectivity and dynamic neurovascular coupling during the resting state. The collection of more than two echoes facilitates the use of ME-ICA denoising to greatly improve the quality of resting state functional connectivity MRI.

A comparison of inhomogeneous magnetization transfer, myelin volume fraction, and diffusion tensor imaging measures in healthy children

Abstract 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. Graphical abstract Figure. No caption available.

Direct estimation of tracer-kinetic parameter maps from highly undersampled brain dynamic contrast enhanced MRI.

The purpose of this work was to develop and evaluate a T1‐weighted dynamic contrast enhanced (DCE) MRI methodology where tracer‐kinetic (TK) parameter maps are directly estimated from undersampled (k,t)‐space data.

Transit time corrected arterial spin labeling technique aids to overcome delayed transit time effect

PurposeThis study aimed to evaluate the usefulness of transit time corrected cerebral blood flow (CBF) maps based on multi-phase arterial spin labeling MR perfusion imaging (ASL-MRP).MethodsThe Institutional Review Board of our hospital approved this retrospective study. Written informed consent was waived. Conventional and multi-phase ASL-MRPs and dynamic susceptibility contrast MR perfusion imaging (DSC-MRP) were acquired for 108 consecutive patients. Vascular territory-based volumes of interest were applied to CBF and time to peak (TTP) maps obtained from DSC-MRP and CBF maps obtained from conventional and multi-phase ASL-MRPs. The concordances between normalized CBF (nCBF) from DSC-MRP and nCBF from conventional and transition time corrected CBF maps from multi-phase ASL-MRP were evaluated using Bland-Altman analysis. In addition, the dependence of difference between nCBF (ΔnCBF) values obtained from DSC-MRP and conventional ASL-MRP (or multi-phase ASL-MRP) on TTP obtained from DSC-MRP was also analyzed using regression analysis.ResultsThe values of nCBFs from conventional and multi-phase ASL-MRPs had lower values than nCBF based on DSC-MRP (mean differences, 0.08 and 0.07, respectively). The values of ΔnCBF were dependent on TTP values from conventional ASL-MRP technique (Fu2009=u20095.5679, Pu2009=u20090.0384). No dependency of ΔnCBF on TTP values from multi-phase ASL-MRP technique was revealed (Fu2009=u20090.1433, Pu2009>u20090.05).ConclusionThe use of transit time corrected CBF maps based on multi-phase ASL-MRP technique can overcome the effect of delayed transit time on perfusion maps based on conventional ASL-MRP.