Abstract
PurposeProton MR spectroscopic imaging (MRSI) benefits from B 0 ≥ 7T and multichannel receive coils, promising substantial resolution improvements. However, MRSI acquisition with high spatial resolution requires efficient acceleration and coil combination. To speed up the already‐fast sampling via concentric rings, we implemented additional, non‐Cartesian, hybrid through‐time/through‐k‐space (tt/tk)‐generalized autocalibrating partially parallel acquisition (GRAPPA). A new multipurpose interleaved calibration scan (interleaved MUSICAL) acquires reference data for both coil combination and PI. This renders the reconstruction process (especially PI) less sensitive to instabilities.MethodsSix healthy volunteers were scanned at 7T. Three calibration datasets for coil combination and PI were recorded: a) iMUSICAL, b) static MUSICAL as prescan, c) moved MUSICAL as prescan with misaligned head position. The coil combination performance, including motion sensitivity, of iMUSICAL was compared to MUSICAL for single‐slice free induction decay (FID)‐MRSI. Through‐time/through‐k‐space‐GRAPPA with constant/variable‐density undersampling was evaluated on the same data, comparing the three calibration datasets. Additionally, the proposed method was successfully applied to 3D whole‐brain FID‐MRSI.ResultsUsing iMUSICAL for coil combination yielded the highest signal‐to‐noise ratio (SNR) (+9%) and lowest Cramer‐Rao lower bounds (CRLBs) (‐6%) compared to both MUSICAL approaches, with similar metabolic map quality. Also, excellent mean g‐factors of 1.07 and low residual lipid aliasing were obtained when using iMUSICAL as calibration data for two‐fold, variable‐density undersampling, while significantly degraded metabolic maps were obtained using the misaligned MUSICAL calibration data.ConclusionThrough‐time/through‐k‐space‐GRAPPA can accelerate already time‐efficient non‐Cartesian spatial‐spectral 2D/3D‐MRSI encoding even further. Particularly promising results have been achieved using iMUSICAL as a robust, interleaved multipurpose calibration for MRSI reconstruction, without extra calibration prescan.
Highlights
Proton MRSI is a powerful noninvasive tool that enables in vivo imaging of local biochemical changes in healthy and diseased tissue.[1,2] The main obstacle for its clinical application is a low SNR, which limits the maximum achievable spatial resolution and minimum scan times.The development of improved hardware (e.g., ≥7T MR scanners and multichannel receive coils)[3-7] and more efficient data acquisition[8-10] increase the SNR substantially
We developed an interleaved water reference scan that is used as calibration data for both coil combination and PI reconstruction, which capitalizes on all the benefits of MUSICAL but requires no prescan and is more general and, could be tailored to arbitrary spatial‐spectral encoding (SSE) trajectories
While increases of 17% and 18% in total creatine (tCr) CRLB were obtained for two‐fold variable‐density undersampling for “static” and “moved” aMUSICAL, respectively, compared to iMUSICAL (p < .05), up to 53% and 69% increases in total choline (tCho) CRLB were found for four‐fold constant undersampling (p < .05)
Summary
The main obstacle for its clinical application is a low SNR, which limits the maximum achievable spatial resolution and minimum scan times. The development of improved hardware (e.g., ≥7T MR scanners and multichannel receive coils)[3-7] and more efficient data acquisition (e.g., short TE and direct FID detection)[8-10] increase the SNR substantially. This SNR boost can be traded for impoved spatial resolution[8,11] or acquisition speed.[12-14]. Several recent MRSI studies provide evidence that high‐resolution 2D‐MRSI can be accelerated efficiently via PI at ≥7T,12-14 and show that 3D‐MRSI – ideally whole‐ brain – will be difficult to realize within clinically acceptable scan times without the use of faster spatial‐spectral encoding (SSE) techniques.[15]. While non‐water‐suppressed MRSI techniques, such as metabolic cycling[17] or SPICE methods,[18] directly provide a water reference image, MRSI methods using water suppression require an additional – possibly interleaved – prescan
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