Intra-session and inter-subject variability of 3D-FID-MRSI using single-echo volumetric EPI navigators at 3T

Abstract

Purpose: In this study, we demonstrate the first combination of 3D FID proton MRSI and spatial encoding via concentric-ring trajectories (CRTs) at 3T. FID-MRSI has many benefits including high detection sensitivity, in particular for J-coupled metabolites (e.g., glutamate/glutamine). This makes it highly attractive, not only for clinical, but also for, potentially, functional MRSI. However, this requires excellent reliability and temporal stability. We have, therefore, augmented this 3D-FID-MRSI sequence with single-echo, imaging-based volumetric navigators (se-vNavs) for real-time motion/shim-correction (SHMOCO), which is 2× quicker than the original double-echo navigators (de-vNavs), hence allowing more efficient integration also in short-TR sequences.

Methods: The tracking accuracy (position and B₀ -field) of our proposed se-vNavs was compared to the original de-vNavs in phantoms (rest and translation) and in vivo (voluntary head rotation). Finally, the intra-session stability of a 5:40 min 3D-FID-MRSI scan was evaluated with SHMOCO and no correction (NOCO) in 5 resting subjects. Intra/inter-subject coefficients of variation (CV) and intra-class correlations (ICC) over the whole 3D volume and in selected regions of interest ROI were assessed.

Results: Phantom and in vivo scans showed highly consistent tracking performance for se-vNavs compared to the original de-vNavs, but lower frequency drift. Up to ~30% better intra-subject CVs were obtained for SHMOCO (P < 0.05), with values of 9.3/6.9/6.5/7.8% over the full VOI for Glx/tNAA/tCho/m-Ins ratios to tCr. ICCs were good-to-high (91% for Glx/tCr in motor cortex), whereas the inter-subject variability was ~11-19%.

Conclusion: Real-time motion/shim corrected 3D-FID-MRSI with time-efficient CRT-sampling at 3T allows reliable, high-resolution metabolic imaging that is fast enough for clinical use and even, potentially, for functional MRSI.

Keywords: concentric rings; dynamic functional magnetic resonance spectroscopic imaging; intra-subject reproducibility; real time motion correction; reliability.

Figure 1

(A) Schematic diagram of the navigated 3D‐FID‐MRSI sequence: volumetric navigators (vNavs), iMUSICAL coil combination pre‐scan, water suppression enhanced through T₁ effects (WET), outer‐volume‐suppression (OVS), and the 3D‐FID‐MRSI sequence with concentric‐ring readout (acquisition delay = 0.8 ms, TR = 850 ms). (B) Positioning of the MRSI volume including the placement of OVS slabs below the VOI. (C) Online reconstruction pipeline for our proposed single‐echo navigators and the original double‐echo navigators. The pipelines differ only by how the B₀‐maps are created: for the single‐echo approach, the pre‐calculated phase offsets are subtracted from the single phase images, whereas for the double‐echo approach, B₀‐maps are generated by subtracting the phase images from the 2 TEs

Figure 2

Comparison of the tracking performance of our proposed single‐echo navigators (se‐vNavs) and the original double‐echo navigators (de‐vNavs). Translation, rotation, frequency, and 1st‐order shim logs are shown for both navigator approaches. For the in vivo measurement, a MR‐trained volunteer was acoustically instructed to perform a predefined head rotation pattern

Figure 3

Representative spectra for volunteer 2 from 4 different ROIs (motor cortex, visual cortex, dorsolateral prefrontal cortex [DLPFC], and auditory cortex). For every ROI, the voxel position is marked on T₁‐weighted images. The spectra are 1st‐order phase corrected for display

Figure 4

Metabolic ratio maps (Glx/tCr) for volunteer 3 depicted in 3 adjacent slices for the 4 scans with real‐time motion/shim correction turned on (SHMOCO); TR = 850 ms, acquisition delay = 0.8 ms, 50 × 50 × 21 matrix, nominal voxel size = 0.12 mL; TA = 5:40 min