Interleaved ³¹P MRS/¹H ASL for analysis of metabolic and functional heterogeneity along human lower leg muscles at 7T

Abstract

Purpose MR offers the unique possibility to noninvasively investigate cellular energy metabolism via 31P MRS, while blood perfusion, which provides oxygen and substrates to the tissue, is accessible by arterial spin labeling (ASL) 1H MRI. Because metabolic and hemodynamic parameters are linked, it would be desirable to study them simultaneously. A 3D‐resolved method is presented that allows such measurements with high spatiotemporal resolution and has the potential to discern differences along an exercising muscle. Methods Multi‐voxel localized 31P MRS was temporally interleaved with multi‐slice pASL 1H MRI. Phosphorus spectra were collected from two adjacent positions in gastrocnemius medialis (GM) during rest, submaximal plantar flexion exercise and recovery, while perfusion and T2*‐weighted axial images were acquired at the same time. Seventeen healthy volunteers (9 f / 8 m) were studied at 7 T. Results An increase of postexercise perfusion and T2*‐weighted signal in GM positively correlated with end‐exercise PCr depletion and pH drop. At proximal positions functional and metabolic activity was higher than distally, that is, perfusion increase and peak T2*‐weighted signal, end‐exercise PCr depletion, end‐exercise pH, and PCr recovery time constant were significantly different. An NOE‐induced SNR increase of approximately 20 % (P < .001), at rest, was found in interleaved 31P spectra, when comparing to 31P‐only acquisitions. Conclusions A technique for fast, simultaneous imaging of muscle functional heterogeneity in ASL, T2* and acquisition of time‐resolved 31P MRS data is presented. These single exercise recovery experiments can be used to investigate local variations during disease progression in patients suffering from vascular or muscular diseases.

Post‐exercise intramuscular O2 supply is tightly coupled with a higher proximal‐to‐distal ATP synthesis rate in human tibialis anterior

“…Intramuscular differences in k PCr have also been observed in the gastrocnemius muscle (Niess et al . 2020). In contrast to our study, PCr recovery after exercise was faster distally than proximally and the difference was much smaller (1.25 min −1 vs .…”

“…2010; Niess et al . 2020). This is reflected in larger NIRS changes (deoxygenation) in the distal part of the vastus lateralis and gastrocnemius (Miura et al .…”

“…Furthermore, an MRI study using arterial spin label (ASL) showed a lower muscle perfusion more distally in the gastrocnemius (Niess et al . 2020). Direct comparison of these studies with our work are problematic as we investigated spatial variations along nearly the whole TA, covering 20 cm, while the coverage of the NIRS and ASL studies was 10 cm and 8 cm, respectively.…”

Simultaneous Measurement of Perfusion and T2* in Calf Muscle at 7T with Submaximal Exercise using Radial Acquisition

“…Our maximum TE affected T 2 * quantification with a maximum error <4%, which was validated in phantom experiment. Also, some recent works showed that muscle metabolic and oxidative capacity varies along the length of the muscle even within the same muscle group 39 , which was also observed as a gradient in T 2 * weighted signals between two distal slice locations following exercise 23 . This suggests that even though the T 2 * slice should have similar kind of T 2 * response as the perfusion slice, but there might still be a gradient between the actual numbers.…”

“…This pulse sequence and exercise protocol provides a unique ability to investigate different aspects of muscle metabolic function (i.e., regional perfusion and tissue blood oxygenation). Although some studies reported simultaneous measurement of perfusion and T 2 * weighted signal at 7T 22,23 , but none of them measured quantitative T 2 *. T 2 * weighted signal, which can also be termed as BOLD signal 24,25 , is dependent on the change in T 2 * as well as the initial signal intensity, which is influenced by parameters such as inflow, baseline drifts and changes in T 1 25 .…”