The influence of spatial resolution on the spectral quality and quantification accuracy of whole‐brain MRSI at 1.5T, 3T, 7T, and 9.4T

Abstract

Purpose Inhomogeneities in the static magnetic field ( B 0 ) deteriorate MRSI data quality by lowering the spectral resolution and SNR. MRSI with low spatial resolution is also prone to lipid bleeding. These problems are increasingly problematic at ultra‐high fields. An approach to tackling these challenges independent of B 0 ‐shim hardware is to increase the spatial resolution. Therefore, we investigated the effect of improved spatial resolution on spectral quality and quantification at 4 field strengths. Methods Whole‐brain MRSI data was simulated for 3 spatial resolutions and 4 B 0 s based on experimentally acquired MRI data and simulated free induction decay signals of metabolites and lipids. To compare the spectral quality and quantification, we derived SNR normalized to the voxel size (nSNR), linewidth and metabolite concentration ratios, their Cramer‐Rao‐lower‐bounds (CRLBs), and the absolute percentage error (APE) of estimated concentrations compared to the gold standard for the whole‐brain and 8 brain regions. Results At 7T, we found up to a 3.4‐fold improved nSNR (in the frontal lobe) and a 2.8‐fold reduced linewidth (in the temporal lobe) for 1 cm 3 versus 0.25 cm 3 resolution. This effect was much more pronounced at higher and less homogenous B 0 (1.6‐fold improved nSNR and 1.8‐fold improved linewidth in the parietal lobe at 3T). This had direct implications for quantification: the volume of reliably quantified spectra increased with resolution by 1.2‐fold and 1.5‐fold (when thresholded by CRLBs or APE, respectively). Conclusion MRSI data quality benefits from increased spatial resolution particularly at higher B 0 , and leads to more reliable metabolite quantification. In conjunction with the development of better B 0 shimming hardware, this will enable robust whole‐brain MRSI at ultra‐high field.

An integrated RF-receive/B0-shim array coil boosts performance of whole-brain MR spectroscopic imaging at 7 T

“…AC/DC shimming provided a significant reduction (29%) in spectral linewidth and an increase (31%) in SNR, which resulted in improved accuracy for metabolite quantification by smaller (22%) Cramer-Rao lower bounds. While local homogeneity might be improved by smaller voxels with high resolution MRSI 58 , such acquisitions may not always be possible due to SNR constraints for low concentration metabolites (GABA, GSH), and better shimming may also improve high resolution data. On the other hand, challenges for water suppression and spectral editing due to global inhomogeneity do not change with image resolution and cannot be corrected with post-processing B 0 correction methods 24 , hence requiring better shimming.…”

Whole‐Slab 3D MR Spectroscopic Imaging of the Human Brain With Spiral‐Out‐In Sampling at 7T

“…Signal excitation and slab selection were performed in the same way for both SO and SOI readouts and included the following components (Fig. 1): 1) fat suppression with inversion recovery using an asymmetric adiabatic hypergeometric (HGSB) pulse 22 and 270 msec delay for lipid nulling; 2) adiabatic spin‐echo sequence (ASE) 22 with an excitation hyperbolic secant adiabatic half passage pulse (HS8 modulation; duration = 4 msec; bandwidth = 5 kHz; B 1+,max = 6 μT) and a pair of gradient offset‐independent adiabatic refocusing pulses (GOIA‐W 16,4 ) 23 modulation (duration = 5 msec; bandwidth = 20 kHz; B 1,max = 16 μT); and 3) water suppression with a four‐pulse water suppression enhanced through T 1 effects (WET) module of 160 msec total duration which was optimized for 7T.…”

“…The WET module used Gauss pulses of 150 Hz bandwidth, flip angles of 83.6°, 99.7°, 74.7°, and 160°, and 40‐msec interpulse delays. Frequency and slice selection profiles of the HGSB, AHP‐HS8, and GOIA‐W 16,4 pulses are shown in Fig. 2(a‐c).…”

“…( b ) Excitation profile of the hyperbolic secant (HS8) adiabatic half passage pulse for 4 msec duration, 5 kHz bandwidth, 6 μT B 1+ amplitude; dashed lines indicate the spectral range of interest. ( c ) Slice selection profile achieved by the gradient offset independent adiabatic refocusing GOIA‐W 16,4 pulses of 5 msec duration, 20 kHz bandwidth, 16 μT B 1+ amplitude; dashed lines indicate the transition band and the chemical shift displacement error for ±1 ppm. ( d ) Spiral‐out (SO) ( k x , k y ) trajectories.…”

“…PROTON ( 1 H) MAGNETIC RESONANCE SPECTROSCOPIC IMAGING (MRSI) can noninvasively probe the in vivo spatial distribution of the brain's neurochemistry 1,2 . Clinically available 7T magnetic resonance imaging (MRI) systems have increased flexibility for trade‐offs between the signal‐to‐noise ratio (SNR), spatial resolution, and acquisition time 3,4 compared to lower field MRI systems. A major goal of 7T MRSI acquisitions is to develop higher spatial/spectral resolution and faster acquisition to facilitate the clinical translation of in vivo neurochemical imaging 5,6 .…”