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Optimization of selective inversion recovery magnetization transfer imaging for macromolecular content mapping in the human brain.

ABSTRACT: PURPOSE:To optimize a selective inversion recovery (SIR) sequence for macromolecular content mapping in the human brain at 3.0T. THEORY AND METHODS:SIR is a quantitative method for measuring magnetization transfer (qMT) that uses a low-power, on-resonance inversion pulse. This results in a biexponential recovery of free water signal that can be sampled at various inversion/predelay times (tI/ tD ) to estimate a subset of qMT parameters, including the macromolecular-to-free pool-size-ratio (PSR), the R1 of free water (R1f ), and the rate of MT exchange (kmf ). The adoption of SIR has been limited by long acquisition times (?4?min/slice). Here, we use Cramér-Rao lower bound theory and data reduction strategies to select optimal tI /tD combinations to reduce imaging times. The schemes were experimentally validated in phantoms, and tested in healthy volunteers (N?=?4) and a multiple sclerosis patient. RESULTS:Two optimal sampling schemes were determined: (i) a 5-point scheme (kmf estimated) and (ii) a 4-point scheme (kmf assumed). In phantoms, the 5/4-point schemes yielded parameter estimates with similar SNRs as our previous 16-point scheme, but with 4.1/6.1-fold shorter scan times. Pair-wise comparisons between schemes did not detect significant differences for any scheme/parameter. In humans, parameter values were consistent with published values, and similar levels of precision were obtained from all schemes. Furthermore, fixing kmf reduced the sensitivity of PSR to partial-volume averaging, yielding more consistent estimates throughout the brain. CONCLUSIONS:qMT parameters can be robustly estimated in ?1?min/slice (without independent measures of ?B0 , B1+, and T1 ) when optimized tI -tD combinations are selected.

SUBMITTER: Dortch RD

PROVIDER: S-EPMC6107392 | biostudies-literature | 2018 Nov

REPOSITORIES: biostudies-literature

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Optimization of selective inversion recovery magnetization transfer imaging for macromolecular content mapping in the human brain.

Dortch Richard D RD Bagnato Francesca F Gochberg Daniel F DF Gore John C JC Smith Seth A SA

Magnetic resonance in medicine 20180324 5

<h4>Purpose</h4>To optimize a selective inversion recovery (SIR) sequence for macromolecular content mapping in the human brain at 3.0T.<h4>Theory and methods</h4>SIR is a quantitative method for measuring magnetization transfer (qMT) that uses a low-power, on-resonance inversion pulse. This results in a biexponential recovery of free water signal that can be sampled at various inversion/predelay times (t<sub>I/</sub> t<sub>D</sub> ) to estimate a subset of qMT parameters, including the macromol ...[more]

PMID: 29573356

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Project description:PurposeThe aim of this study was to optimize the 3-dimensional (3D) fluid attenuated inversion recovery (FLAIR) pulse sequence for isotropic high-spatial-resolution imaging of white matter (WM) and cortical lesions at 7 T.Materials and methodsWe added a magnetization-prepared (MP) FLAIR module to a Cube 3D fast spin echo sequence and optimized the refocusing flip angle train using extended phase graph simulations, taking into account image contrast, specific absorption rate (SAR), and signal-to-noise ratio (SNR) as well as T1/T2 values of the different species of interest (WM, grey matter, lesions) at 7 T. We also effected improved preparation homogeneity at 7 T by redesigning the refocusing pulse used in the MP segments. Two sets of refocusing flip angle trains-(a) an SNR-optimal and (b) a contrast-optimal set-were derived and used to scan 7 patients with Alzheimer disease/cognitive impairment and 7 patients with multiple sclerosis. Conventional constant refocusing flip MP-FLAIR images were also acquired for comparison. Lesion SNR, contrast, and lesion count were compared between the 2 optimized and the standard FLAIR sequences.ResultsWhole brain coverage with 0.8 mm isotropic spatial resolution in ∼5-minute scan times was achieved using the optimized 3D FLAIR sequences at clinically acceptable SAR levels. The SNR efficiency of the SNR-optimal sequence was significantly better than that of conventional constant refocusing flip MP-FLAIR sequence, whereas the scan time was reduced more than 2-fold (∼5 vs >10 minutes). The contrast efficiency of the contrast-optimal sequence was comparable with that of the constant refocusing flip sequence. Lesion load ascertained by lesion counting was not significantly different among the sequences.ConclusionMagnetization-prepared FLAIR-Cube with refocusing flip angle trains optimized for SNR and contrast can be used to characterize WM and cortical lesions at 7 T with 0.8 mm isotropic resolution in short scan times and without SAR penalty.

Dataset Information

Optimization of selective inversion recovery magnetization transfer imaging for macromolecular content mapping in the human brain.

Publications

Optimization of selective inversion recovery magnetization transfer imaging for macromolecular content mapping in the human brain.

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