Dual gradient‐echo MRI of post‐contraction changes in skeletal muscle blood volume and oxygenation

Abstract

Analysis of post-contraction MRI signal intensity (SI) transients may allow noninvasive studies of microvascular reactivity and blood oxygenation recovery. The purpose of this study was to determine the physiological basis for post-contraction changes in short-echo (6 ms) and long-echo (46 ms) gradient-echo (GRE) MRI signals (S 6 and S 46 , respectively). Six healthy subjects were studied with the use of dual GRE MRI and near-infrared spectroscopy (NIRS). As the site of oxygen delivery and substrate exchange to the peripheral tissues of the body, the microcirculation has important roles in maintaining the health and function of these tissues. Because the microcirculation adapts positively to exercise training (1) and is pathologically altered in various diseases, such as diabetes (2), it is important to assess microcirculatory function in order to understand normal, adaptational, and pathological physiology. In particular there is a need for noninvasive protocols to examine microvascular function during and after a challenge, such as isometric muscle contractions.Elevated intramuscular pressure during isometric contractions compresses the vessels of the microcirculation along the fascicular lines (3). This compression causes a rapid ejection of venous blood, decreasing the total muscle and limb volumes (4), and restricts arterial inflow to the muscle for those portions of the cardiac cycle in which the intramuscular pressure exceeds the blood's hydrostatic pressure (5). Following the contraction, local, rapid increases in blood flow and volume occur in order to resupply the tissue with the oxygen that was consumed by the muscle. This reactive hyperemia has been shown to be inversely related to the degree of mechanical occlusion, and directly related to the metabolic regulators of vasodilation that are released in the tissue (6).One noninvasive technique for examining hemodynamic events in the microcirculation is near-infrared spectroscopy (NIRS), which exploits the relative ease of nearinfrared light transmission through tissues and the differential absorption of this light by oxyhemoglobin (HbO 2 ) and deoxyhemoglobin (HHb). Although one must consider the confounding influences of subcutaneous fat thickness (7,8) and the similar absorption spectrum of myoglobin when interpreting NIRS data, this technique is generally quite useful for following the changes in total hemoglobin concentration ([THb]) and HbO 2 saturation (%HbO 2 ) during and after exercise or other physiologic events. The depth of light penetration for NIRS is generally considered to be about half of the light emitter-detector spacing (9), and is about 2 cm for most NIRS devices. This depth is a limitation when perfusion or oxygen extraction heterogeneity is present (as previously demonstrated in healthy subjects (10 -12) and peripheral vascular disease patients (13)), when deep muscles are activated, and when there is an appreciable subcutaneous fat layer thickness.Functional MRI (fMRI) methods can overcome the depth-sampling limitations of NIRS. ...

Evaluation ofT2values and apparent diffusion coefficient of the masseter muscle by clenching

“…11,12 The shortening of T 2 is a result of decreased oxygen saturation and an increased proportion of deoxyhaemoglobin. 18,19 The limitation of blood flow (perfusion) has a negative effect on the increase in T 2 . Previous studies demonstrated that the increase in T 2 after exercise occurred in the presence of vascular occlusion.…”

On the correlation between T2 and tissue diffusion coefficients in exercised muscle: quantitative measurements at 3T within the tibialis anterior

“…The underlying biophysical mechanisms that cause T 2 increases in exercised muscle are becoming better understood. Possible explanations for the effect include water shifts from intra-to extra-cellular spaces, increases in extra-cellular and/or vascular fluid volumes, and/or changes within the intracellular water volume itself whereby exercise results in more "free" and less macromolecular "bound" water [5,[13][14][15][16]. In this work we hypothesized that any and all such changes would also increase the average, or trace, muscle water diffusion coefficient D [10][11][12].…”

“…those muscles associated with specific exercises performed prior to imaging. The observation has since been exploited by numerous practitioners as a means of identifying muscles involved with specific exercises [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16], a task which may prove useful in the medical and surgical management of patients. In T 2 -weighted images, the signal intensity increases that is associated with "activated" muscles following exercise is due to a prolongation of the transverse relaxation time, T 2 .…”

“…The mechanisms influencing the transverse relaxation time T 2 in skeletal muscle are highly complex and remain an intense area of investigation [2,3,[13][14][15][16]. Early nuclear magnetic resonance (NMR) studies performed on isolated excised muscles demonstrated that the T 2 relaxation decay curve of muscle is best described by multiexponential functions with up to three distinct components [17][18][19][20][21][22][23].…”

Dynamic Blood Oxygen Level-dependent MR Imaging of Muscle: Comparison of Postocclusive Reactive Hyperemia in Young Smokers and Nonsmokers

“…Moreover, as previously documented, 33 TE 45 is suitable for BOLD studies. Third, we could not compare our findings with those of near-infrared spectroscopy or MR perfusion measured by the arterial spin labeling method because these methods cannot be used simultaneously with dynamic BOLD MR imaging of calf muscle.…”