Density‐weighted concentric rings k‐space trajectory for ¹H magnetic resonance spectroscopic imaging at 7 T

Abstract

It has been shown that density‐weighted (DW) k‐space sampling with spiral and conventional phase encoding trajectories reduces spatial side lobes in magnetic resonance spectroscopic imaging (MRSI). In this study, we propose a new concentric ring trajectory (CRT) for DW‐MRSI that samples k‐space with a density that is proportional to a spatial, isotropic Hanning window. The properties of two different DW‐CRTs were compared against a radially equidistant (RE) CRT and an echo‐planar spectroscopic imaging (EPSI) trajectory in simulations, phantoms and in vivo experiments. These experiments, conducted at 7 T with a fixed nominal voxel size and matched acquisition times, revealed that the two DW‐CRT designs improved the shape of the spatial response function by suppressing side lobes, also resulting in improved signal‐to‐noise ratio (SNR). High‐quality spectra were acquired for all trajectories from a specific region of interest in the motor cortex with an in‐plane resolution of 7.5 × 7.5 mm2 in 8 min 3 s. Due to hardware limitations, high‐spatial‐resolution spectra with an in‐plane resolution of 5 × 5 mm2 and an acquisition time of 12 min 48 s were acquired only for the RE and one of the DW‐CRT trajectories and not for EPSI. For all phantom and in vivo experiments, DW‐CRTs resulted in the highest SNR. The achieved in vivo spectral quality of the DW‐CRT method allowed for reliable metabolic mapping of eight metabolites including N‐acetylaspartylglutamate, γ‐aminobutyric acid and glutathione with Cramér‐Rao lower bounds below 50%, using an LCModel analysis. Finally, high‐quality metabolic mapping of a whole brain slice using DW‐CRT was achieved with a high in‐plane resolution of 5 × 5 mm2 in a healthy subject. These findings demonstrate that our DW‐CRT MRSI technique can perform robustly on MRI systems and within a clinically feasible acquisition time.

Density-Weighted Concentric Ring Trajectory using simultaneous multi-band acceleration: 3D Metabolite-cycled Magnetic Resonance Spectroscopy Imaging at 3 T

“…Compared to the EPI base acceleration techniques, DW-CRT has been demonstrated to offer a substantial improvement in SNR (12). The present study further accelerates the DW-CRT with simultaneous multi-slice excitation combined with SENSE parallel imaging reconstruction.…”

“…The intent of this work, therefore, is to develop a non-water-suppressed MB MRSI using a density weighted concentric ring trajectory (DW-CRT) acquisition. Sampling k-space using a DW pattern to shape the spatial response function improve side lobe artefacts and signal-to-noise ratio (SNR) (12). A non-water-suppressed metabolite-cycling is proposed for simultaneous detection of the metabolites and water signals at short echo time (TE = 14 ms) using stimulated echo acquisition mode (STEAM) localization (13).…”

High‐resolution echo‐planar spectroscopic imaging at ultra‐high field

“…Nevertheless, as the lower-bandwidth excitation pulse produced a larger CSDE, the three pairs of OVS pulses applied in the slice selection dimension could not be removed. Accelerated acquisition sequences have been recently proposed and compared for high-resolution MRSI at 7 T.. 36 These include encoding efficient sequences based on spiral readouts 37 and concentrically circular echo-planar trajectories 38,39 capable of acquiring large matrix sizes with the bandwidth required at ultra-high fields. However, non-Cartesian reconstruction may introduce undesired artifacts that are difficult to correct, especially in the presence of B 0 inhomogeneity.…”

High-Resolution Metabolic Mapping of the Cerebellum Using a Zoomed Magnetic Resonance Spectroscopic Imaging

“…To cover the 48 x 48 grid in a rFOV, we have modified our 2D-density weighted concentric ring trajectory (DW-CRT) design (20). Each ring in DW-CRT consisting of 64 points (number of points per ring, N p_ring = 64) was collected with an ADC bandwidth of 40 kHz.…”