Three‐dimensional, 2.5‐minute, 7T phosphorus magnetic resonance spectroscopic imaging of the human heart using concentric rings

Abstract

A three-dimensional (3D), density-weighted, concentric rings trajectory (CRT) magnetic resonance spectroscopic imaging (MRSI) sequence is implemented for cardiac phosphorus ( 31 P)-MRS at 7 T. The point-by-point k-space sampling of traditional phase-encoded chemical shift imaging (CSI) sequences severely restricts the minimum scan time at higher spatial resolutions. Our proposed CRT sequence implements a stack of concentric rings, with a variable number of rings and planes spaced to optimise the density of k-space weighting. This creates flexibility in acquisition time, allowing acquisitions substantially faster than traditional phase-encoded CSI sequences, while retaining high signal-to-noise ratio (SNR). We first characterise the SNR and point-spread function of the CRT sequence in phantoms. We then evaluate it at five different acquisition times and spatial resolutions in the hearts of five healthy participants at 7 T. These different sequence durations are compared with existing published 3D acquisition-weighted CSI sequences with matched acquisition times and spatial resolutions. To minimise the effect of noise on the short acquisitions, low-rank denoising of the spatiotemporal data was also performed after acquisition. The proposed sequence measures 3D localised phosphocreatine to adenosine triphosphate (PCr/ATP) ratios of the human myocardium in 2.5 min, 2.6 times faster than the minimum scan time for acquisition-weighted phase-encoded CSI. Alternatively, in the same scan time, a 1.7-times smaller nominal voxel volume can be achieved. Low-rank Abbreviations used: 2,3-DPG, 2,3-diphosphoglycerate; 31 P-MRS, phosphorus magnetic resonance spectroscopy; AMARES, advanced method for accurate, robust and efficient spectral fitting; ATP, adenosine triphosphate; BISTRO, B 1 -insensitive train to obliterate signal; CRT, concentric rings trajectory; CSI, chemical shift imaging; FLASH, fast low angle shot; FWHM, full width at half maximum; ISIS, image-selected in vivo spectroscopy; MUSICAL, multichannel spectroscopic data combined by matching image calibration data; MRSI, magnetic resonance spectroscopic imaging; NUFFT, nonuniform fast Fourier transform; PCr, phosphocreatine; PCr/ATP, ratio of phosphocreatine to adenosine triphosphate; PDE, phosphodiester; PSF, point-spread function; SNR, signalto-noise ratio; WSVD, whitened singular value decomposition.Ladislav Valkovič and Christopher T. Rodgers made an equal contribution to this study.

Scaling the mountains: what lies above 7 Tesla magnetic resonance?

“…Implications of these gains in sensitivity and speed include the promise of sodium MR of the heart with a sub-millimetre spatial resolution in 5-10 min scan time, and the potential for probing cardiac metabolism with 31 P MR spectroscopy in clinically acceptable examination times. This is in stark contrast to scan durations of approximately 1-2 h available today for the same sub-millimetre spatial resolution [45][46][47]. MR of nuclei such as 35 Cl and 39 K will greatly benefit from 14 and 20 T. Arguably, 39 K MR remains quite challenging because the sensitivity is about six orders of magnitude less than that of 1 H MR [48].…”

Multi-site Ultrashort Echo Time 3D Phosphorous MRSI repeatability using novel Rosette Trajectory (PETALUTE)

“…A significant barrier against the wide use of 31 P MRSI in clinical practice is the need for a prolonged acquisition to compensate for the lower abundance and lower sensitivity (correspondingly low SNR) of 31 P. Accelerated acquisition trajectories have been developed 18 and are in place to reduce the acquisition time with advanced reconstruction methods 22 , including the compressed sensing developed at 3T 20 . However, these techniques require additional temporal and/or spatial interleaves, resulting in a longer acquisition duration to reach the required broad spectral dispersion of the spectrum due to the lower gyromagnetic ratio of 31 P. Alternatively, a limited spectral range has been used to eliminate the temporal leaves, resulting in missing essential features of the 31 P spectrum 19,21 . Furthermore, some of these efforts have been implemented at UHF to compensate for the SNR deficiency but still require the additional cost of temporal interleaves and/or limited spectral range to fulfill the broad spectral dispersion requirement of the 31 P 19,21 .…”

Reproducibility of 3D MRSI for imaging human brain glucose metabolism using direct (2H) and indirect (1H) detection of deuterium labeled compounds at 7T and clinical 3T

“…Physiological variance of glucose metabolism between repeated sessions could additionally affect the results. Using spatial spectral non-cartesian sampling of the k-space (47,48), could potentially increase the spatial resolution without prolonging scan time, but puts more stress on the gradient system due to the 6.5 times lower gyromagnetic ratio of 2 H compared to 1 H as higher gradient amplitudes are required and additionally smaller voxel yield lower SNR (49). These challenges need to be addressed in future DMI studies.…”