Samuel, T. Jake

ABSTRACT Purpose To simultaneously measure skeletal muscle energetics and blood flow (BF) before, during, and after dynamic plantar flexion exercise (PFE). Methods Non‐localized pulse‐acquire phosphorus‐31 magnetic resonance spectroscopy ( 31 P MRS) and phase contrast flow magnetic resonance imaging ( 1 H MRI) using golden‐angle rotated spiral readouts were acquired in an interleaved fashion. Data were collected at rest, during PFE to volitional fatigue, and post‐exercise recovery in 10 healthy adults. Interleaved popliteal artery BF was validated against conventional cine phase‐contrast measures at rest and post‐exercise recovery, while calf muscle energetics were validated by comparing to conventional 31 P MRS‐only measures at rest. To minimize motion artifacts during dynamic PFE, flow data were resolved in 3 dimensions (cardiac phase, pedal position, and exercise stage). Results Resting phosphocreatine (PCr) and inorganic phosphate concentrations were similar between both techniques (both p > 0.05). During PFE, PCr declined to 46% ± 10% of rest. PCr recovery time was 43 ± 15 s. Resting BF was 132 ± 43 mL/min and increased up to four‐fold during PFE depending on exercise stage and pedal position. Notably, there was excellent agreement between the interleaved and conventional BF measures ( r 2 = 0.95, slope = 0.977, p < 0.0001) with minimal bias (12.62 mL/min) over a wide range of physiologically relevant BF values (from ∼60 to 600 mL/min). Conclusion The feasibility of simultaneous assessment of skeletal muscle energetics and BF during dynamic exercise by interleaved 31 P MRS/ 1 H flow MRI was demonstrated along with good agreement between interleaved and conventional techniques during rest and recovery. One can now simultaneously quantify critical BF‐metabolism relationships and use this to probe disease‐related adaptations.

Abstract Purpose To investigate the feasibility of rapid CEST MRI acquisition for evaluating oxidative phosphorylation (OXPHOS) in human skeletal muscle at 3T, utilizing ultrafast Z‐spectroscopy (UFZ) combined with MRI and the Polynomial and Lorentzian line‐shape Fitting (PLOF) technique. Methods UFZ MRI on muscle was evaluated with turbo spin echo (TSE) and 3D EPI readouts. Five healthy subjects performed in‐magnet plantar flexion exercise (PFE) and subsequent changes of amide, PCr, and partial PCr mixed Cr (Cr + ) CEST dynamic signals post‐exercise were enabled by PLOF fitting. PCr/Cr CEST signal was further refined through pH correction by using the ratios between PCr/Cr and amide signals, named PCAR/CAR, respectively. Results UFZ MRI with TSE readout significantly reduces acquisition time, achieving a temporal resolution of <50 s for collecting high‐resolution Z‐spectra. Following PFE, the recovery/decay times (τ) for both PCr and Cr in the gastrocnemius muscle of the calf were notably longer when determined using PCr/Cr CEST compared to those after pH correction with amideCEST, namely = 87.1 ± 15.8 s and = 98.1 ± 20.4 s versus = 32.9 ± 19.7 s and = 43.0 ± 13.0 s, respectively. obtained via 31 P MRS ( = 50.3 ± 6.2 s) closely resemble those obtained from pH‐corrected PCr/Cr CEST signals. Conclusion The outcomes suggest potential of UFZ MRI as a robust tool for non‐invasive assessment of mitochondrial function in skeletal muscles. pH correction is critical for the reliable OXPHOS measurement by CEST.