TM43.7mr the signal gain is 1 to 7% (average 3.3%).* MRS signal loss due to cerebtal motion is mostly caused by the combination of signal averaging and phase dispersion.* Quantitative MRS with STEAM sequences of long Th4 without phase carecting single FID's prone to Serious errors.For nonquantitative MRS, substantial loss of SNR results.* With shart TM, TE and standard gradients.signal loss due to motion is less of a problem, if sufficient signal averaging is performed (Caveat.now that stronger gradients are available!)* Since particularly errors introduced by phase dispersion are most prominent if only a few FJWs are averaged, this effect is critical for refexem scans of the unsuppressed water signal, even with short TM.* Most of the signal lost by sbaight averaging can be regained by retmgating.Even mom importantly, the large spread in resulting signal is prevented.Thi guarantees higher reproducibility.* Re-gating is advantageous with respect to phasing single water-suppnased mD's.because it is independent of SNR of single acquisitions.* Respiration-related brain motion also leads to signal loss.but its effect is smaller.It can be eliminated by patient controlled respiration triggering.
Skeletal muscle voluntary contractions (VC) and electrical stimulations (ES) were compared in eight healthy men. High-energy phosphates and myoglobin oxygenation were simultaneously monitored in the quadriceps by interleaved 1 H- and 31 P-NMR spectroscopy. For the VC protocol, subjects performed five or six bouts of 5 min with a workload increment of 10% of maximal voluntary torque (MVT) at each step. The ES protocol consisted of a 13-min exercise with a load corresponding to 10% MVT. For both protocols, exercise consisted of 6-s isometric contractions and 6-s rest cycles. For an identical mechanical level (10% MVT), ES induced larger changes than VC in the P i -to-phosphocreatine ratio [1.38 ± 1.14 (ES) vs. 0.13 ± 0.04 (VC)], pH [6.69 ± 0.11 (ES) vs. 7.04 ± 0.07 (VC)] and myoglobin desaturation [43 ± 15.9 (ES) vs. 6.1 ± 4.6% (VC)]. ES activated the muscle facing the NMR coil to a greater extent than did VCs when evaluated under identical technical conditions. This metabolic pattern can be interpreted in terms of specific temporal and spatial muscle cell recruitment. Furthermore, at identical levels of energy charge, the muscle was more acidotic and cytoplasm appeared more oxygenated during ES than during VC. These results are in accordance with a preferential recruitment of type II fibers and a relative muscle hyperperfusion during ES.
The contribution due to fast-chemical exchange processes in off-resonance relaxation measurement is studied both theoretically and experimentally. The relaxation rates depend explicitly on the effective field amplitude in the rotating frame, and therefore on the rf field strength and offset. This gives access to a larger domain of slow internal motion (characteristic times between 0·1 µs and 10 μs) in comparison to on-resonance relaxation measurement, avoiding problems of HOHAHA coherence transfer or angular dispersion. Different methods to detect and measure fast-chemical exchange processes using off-resonance ROESY and local effective correlation time measurement are described. They are essentially based on a variation of the rf field amplitude at constant angle 0 between the effective field and the static magnetic field. As an illustration, the chemical shift difference between the two conformations of cyclohexane at room temperature is measured.
PURPOSE: We sought to examine the potential role of oxidative stress on skeletal muscle function with advancing age. METHODS: Nuclear magnetic resonance (NMR) was employed to simultaneously assess muscle perfusion (arterial spin labeling, ASL) and energetics (31P NMR spectroscopy) in the lower leg of young (26 ± 5 yrs, n = 6) and older (70 ± 5 yrs, n = 6) healthy volunteers following consumption of either placebo (PL) or an oral antioxidant (AO) cocktail (Vitamins C, E, and alpha lipoic acid) previously documented to decrease plasma free radical concentration. NMR measurements were made during and after 5-min of moderate intensity (≈5W) plantar flexion exercise. RESULTS: AO administration significantly improved end-exercise perfusion (50 ± 5 ml·100g-1·min-1, AO; 43 ± 4 ml·100g-1·min-1, PL) and post-exercise perfusion area-under-the-curve (1286 ± 236 ml·100g-1, AO; 866 ± 144 ml·100g-1, PL) in older subjects, while AO administration did not alter hemodynamics in the young group. Concomitantly, muscle oxidative capacity (time constant of PCr recovery) was improved following AO in the older (43 ± 1s, AO; 51 ± 7s, PL) but not the young (54 ± 5s, AO; 48 ± 7s, PL) group. CONCLUSIONS: These findings support the concept that oxidative stress is partially responsible for the age-related decline in skeletal muscle perfusion. Acute antioxidant administration restored perfusion toward that of the young and consequently improved oxidative capacity in the elderly, demonstrating the functional consequence of free-radical-mediated hypoperfusion on skeletal muscle energetics in this cohort.
A totally noninvasive set-up was developed for comprehensive NMR evaluation of mouse skeletal muscle function in vivo. Dynamic pulsed arterial spin labeling-NMRI perfusion and blood oxygenation level-dependent (BOLD) signal measurements were interleaved with (31)P NMRS to measure both vascular response and oxidative capacities during stimulated exercise and subsequent recovery. Force output was recorded with a dedicated ergometer. Twelve exercise bouts were performed. The perfusion, BOLD signal, pH and force-time integral were obtained from mouse legs for each exercise. All reached a steady state after the second exercise, justifying the pointwise summation of the last 10 exercises to compensate for the limited (31)P signal. In this way, a high temporal resolution of 2.5 s was achieved to provide a time constant for phosphocreatine (PCr) recovery (τ(PCr)). The higher signal-to-noise ratio improved the precision of τ(PCr) measurement [coefficient of variation (CV) = 16.5% vs CV = 49.2% for a single exercise at a resolution of 30 s]. Inter-animal summation confirmed that τ(PCr) was stable at steady state, but shorter (89.3 ± 8.6 s) than after the first exercise (148 s, p < 0.05). This novel experimental approach provides an assessment of muscle vascular response simultaneously to energetic function in vivo. Its pertinence was illustrated by observing the establishment of a metabolic steady state. This comprehensive tool offers new perspectives for the study of muscle pathology in mice models.
