Nagel, A. M. | Search bioRxiv

bioRxiv2022

Capturing early human memory consolidation utilizing the higher functional specificity of 7T compared to 3T-fMRI

Functional magnetic resonance imaging (fMRI) visualizes brain structures at increasingly higher resolution and better signal-to-noise ratio (SNR) as field strength increases. Yet, mapping the BOLD response to distinct neuronal processes continues to be challenging. Here, we performed 3T and 7T-fMRI analysis of motor-task activation and resting-state connectivity with adjusted SNR. We then applied graph theory to analyze resting-state neuronal networks detected by fMRI after a simple motor task. Despite adjusted SNR, 7T achieved a higher functional specificity of the BOLD response than 3T-fMRI. Following the motor task, 7T-fMRI therefore enabled the detection of an offline replay that was directly linked to brain regions associated with memory consolidation. These findings reveal how memory processing is initiated even after simple motor tasks and begins earlier than previously shown. Thus, the superior capability of 7T-fMRI to detect subtle functional dynamics promises to improve diagnostics and therapeutic assessment of neurological diseases.

neuroscience↗

bioRxiv2020

Fast in vivo 23Na imaging and T2* mapping using accelerated 2D-FID magnetic resonance spectroscopic imaging at 3 T: Proof of concept and reliability study

PurposeTo implement an accelerated MR-acquisition method allowing to map sodium T2* relaxation and absolute concentration within skeletal muscles at 3T. MethodsA fast-2D density-weighted concentric-ring-trajectory 23Na-MRSI technique was used to acquire 64 time-points of FID with a spectral bandwidth of 312.5 Hz from a 2.5 x 2.5 mm2 in-plane resolution within about 15 minutes. The fast relaxing 23Na signal was localized with a single-shot, inversion-recovery based, non-echo (SIRENE) OVS method. The sequence was verified using simulation and phantom studies before implementing it in human calf muscles. Within two same-day sessions, 2D-SIRENE-MRSI (UTE = 0.55 ms) and 3D-MRI (UTE = 0.3 ms) data were acquired. The T2* values were fitted voxel-by-voxel using a bi-exponential model for the 2D-MRSI data. Within-subject coefficients of variation were estimated for both acquisition methods. ResultsThe MRSI-FID data allowed for fast and slow T2* mapping of the calf muscles in vivo with minimal sensitivity reduction. The spatial-distributions of 23Na concentration for both in vivo MRSI and 3D-MRI acquisitions were significantly correlated (r = 0.7, P<0.001). The test-retest results rendered high reliability for both MRSI (CV = 5%) and 3D MRI (CV = 6%). The mean [Formula] in calf muscles was 0.7 {+/-} 0.1 (contribution fraction = 37%), while [Formula] was 13.2 {+/-} 0.2 ms (63%). The mean absolute muscle 23Na concentration calculated from the T2*-corrected data was 28.6 {+/-} 3.3 mM. ConclusionThe proposed MRSI technique is a reliable technique to map sodiums absolute concentration and T2* within a clinically acceptable scan time at 3T.

biophysics↗