Accelerated MR spectroscopic imaging—a review of current and emerging techniques

Abstract

Over more than 30 years in vivo MR spectroscopic imaging (MRSI) has undergone an enormous evolution from theoretical concepts in the early 1980s to the robust imaging technique that it is today. The development of both fast and efficient sampling and reconstruction techniques has played a fundamental role in this process. Stateof-the-art MRSI has grown from a slow purely phase-encoded acquisition technique to a method that today combines the benefits of different acceleration techniques.These include shortening of repetition times, spatial-spectral encoding, undersampling of k-space and time domain, and use of spatial-spectral prior knowledge in the reconstruction. In this way in vivo MRSI has considerably advanced in terms of spatial coverage, spatial resolution, acquisition speed, artifact suppression, number of detectable metabolites and quantification precision. Acceleration not only Abbreviations: B0, static magnetic field strength; B1 + , transmit RF field; CAIPIRINHA, controlled aliasing in parallel imaging results in higher acceleration; CRT, concentric ring trajectory; CS, compressed sensing; CSDE, chemical shift displacement error; EPI, echo planar imaging; EPSI, echo-planar spectroscopic imaging; FID, free induction decay; FIDLOVS, FID acquisition localized by outer volume suppression; GRAPPA, generalized autocalibrating partially parallel acquisition; GSLIM, generalized series approach to MR spectroscopic imaging; IDEAL, iterative decomposition of water and fat with echo asymmetry and least-

Minimum Reporting Standards for in vivo Magnetic Resonance Spectroscopy (MRSinMRS): Experts' consensus recommendations

“…If any frequency offset is used, it should also be described.For MRSI methods, the necessary details include the FOV and matrix size so that the nominal volume of MRS voxels can be determined. Acceleration methods (such as parallel imaging, compressed sensing, or spatial‐spectral encoding) can be used to reduce the scan times required for MRSI methods and should be described with details of the method and parameters used 26 . Similarly, k ‐space weighting of the acquisition should also be described, such as whether full or elliptical k ‐space sampling is used or retrospective filters used (eg Hamming), and any k ‐space zero‐filling factors applied.…”

High-resolution metabolic imaging of high-grade gliomas using 7T-CRT-FID-MRSI

“…Over the course of the last decade, a different approach for MRSI acquisition, based on the direct acquisition of the free-induction-decay (FID) signal, was proposed ( Bogner et al, 2020 , Bogner et al, 2012 , Gruber et al, 2017 , Henning et al, 2009 ) that is less affected by these issues. The most important feature of FID-MRSI is the outstanding boost in SNR for J-coupled metabolites, such as Gln, Glu, my-Inositol (mIns), and taurine (Tau).…”

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

“…In recent studies, the spatial resolution of brain MRSI has been pushed at ultrahigh field using free induction decay (FID) sequences with low flip angle, short TRs, short acquisition delay, undersampling, and/or spatial‐spectral encoding 5,6 . Spin echo MRSI sequences have been demonstrated at ultrahigh field with lower spatial resolution mainly due to longer TRs associated with the higher SAR of refocusing pulses 10,14,24 .…”

“…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 . Higher resolution and faster acquisition can be achieved with non‐Cartesian readouts; however, implementing these trajectories for MRSI sequences at 7T is associated with several technical challenges, including gradient hardware constraints, larger spectral bandwidth, and imperfections of the B 0 field 3 .…”