(2 + 1)D-CAIPIRINHA accelerated MR spectroscopic imaging of the brain at 7T

Abstract

PurposeTo compare a new parallel imaging (PI) method for multislice proton magnetic resonance spectroscopic imaging (1H‐MRSI), termed (2 + 1)D‐CAIPIRINHA, with two standard PI methods: 2D‐GRAPPA and 2D‐CAIPIRINHA at 7 Tesla (T).Methods(2 + 1)D‐CAIPIRINHA is a combination of 2D‐CAIPIRINHA and slice‐CAIPIRINHA. Eight healthy volunteers were measured on a 7T MR scanner using a 32‐channel head coil. The best undersampling patterns were estimated for all three PI methods. The artifact powers, g‐factors, Cramér–Rao lower bounds (CRLB), and root mean square errors (RMSE) were compared quantitatively among the three PI methods. Metabolic maps and spectra were compared qualitatively.Results(2 + 1)D‐CAIPIRINHA allows acceleration in three spatial dimensions in contrast to 2D‐GRAPPA and 2D‐CAIPIRINHA. Thus, this sequence significantly decreased the RMSE of the metabolic maps by 12.1 and 6.9%, on average, for 4 < R < 11, compared with 2D‐GRAPPA and 2D‐CAIPIRINHA, respectively. The artifact power was 22.6 and 8.4% lower, and the CRLB were 3.4 and 0.6% lower, respectively.Conclusion(2 + 1)‐CAIPIRINHA can be implemented for multislice MRSI in the brain, enabling higher accelerations than possible with two‐dimensional (2D) parallel imaging methods. An eight‐fold acceleration was still feasible in vivo with negligible PI artifacts with lipid decontamination, thus decreasing the measurement time from 120 to 15 min for a 64 × 64 × 4 matrix. Magn Reson Med 78:429–440, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine.

Density‐weighted concentric circle trajectories for high resolution brain magnetic resonance spectroscopic imaging at 7T

“…This has led to a reduction of TR, if permitted by SAR constraints 11, 43, and the development of advanced parallel imaging methods (yet restricted to accelerations of $\le$10) 15. While TR‐reduction and parallel imaging can somehow alleviate scan time problems, potentially even faster SSE approaches have not been established at $B_0\ge 7$ T due to high gradient hardware requirements 10, but are still under development.…”

“…For instance, recent FID‐MRSI studies at $B_0\ge 7$ T have reduced SNR efficiency due to both increasing g ‐factors associated with high parallel imaging acceleration 15 and the application of post‐acquisition Hamming filtering 25, 26. Our results show that DW‐CONCEPT leads to an +39.7% increased SNR efficiency even when compared to a fairly modest 5‐fold acceleration using efficient CAIPIRINHA parallel imaging and retrospective Hamming filtering.…”

“…The sampling in azimuthal direction for both CONCEPT encodings was performed as described in Ref. 22, 33.ePE: To compare both CONCEPT approaches with a gold standard, conventionally phase encoded MRSI of a circular k‐space was acquired.CAIPIRINHA‐ePE: Retrospective undersampling of the fully acquired ePE scan with a 2D‐CAIPIRINHA pattern 15 with an acceleration factor of 5 was performed to match the scan time of both CONCEPT sequences. …”

“…Both, increased spatial resolution and spectral bandwidth, lead to increased demands on gradient systems and ultimately lower signal‐to‐noise ratio (SNR) efficiency 16, 17. Parallel imaging, on the other hand, becomes more efficient at higher B 0 , but achievable accelerations are lower than for SSE ( $\le 10$) 15 and lipid contamination is aggravated, which calls for additional lipid suppression techniques 2, 18, 19. While established SSE trajectories (i.e., EPSI, spirals) can be certainly optimized for higher B 0 , the increased gradient demands at 7T due to the increased spectral bandwidth make these trajectories inefficient for high spatial resolutions.…”

Recent advances in parallel imaging for MRI

“…ACS data can be acquired by acquiring fully-sampled data at the center of k-space or by using a separate low-resolution scan. Recently, a feasibility study was performed to evaluate using CAIPIRINHA to accelerate along three spatial dimensions [147]. The method is termed (2+1)D CAIPIRINHA to denote 2D-CAIPIRINHA in-plane and MS-CAIPIRINHA along the slice dimension, resulting in a multi-slice acquisition.…”