Diffusion-tensor magnetic resonance imaging (DT-MRI) offers great potential for understanding structure-function relationships in human skeletal muscles. The purposes of this study were to demonstrate the feasibility of using in vivo human DT-MRI fiber tracking data for making pennation angle measurements and to test the hypothesis that heterogeneity in the orientation of the tibialis anterior (TA) muscle's aponeurosis would lead to heterogeneity in pennation angle. Eight healthy subjects (5 male) were studied. T(1)-weighted anatomical MRI and DT-MRI data were acquired of the TA muscle. Fibers were tracked from the TA's aponeurosis by following the principal eigenvector. The orientations of the aponeurosis and muscle fiber tracts in the laboratory frame of reference and the orientation of the fiber tracts with respect to the aponeurosis [i.e., the pennation angle (theta)] were determined. The muscle fiber orientations, when expressed relative to the laboratory frame of reference, did not change as functions of superior-to-inferior position. The sagittal and coronal orientations of the aponeurosis did not change in practically significant manners either, but the aponeurosis' axial orientation changed by approximately 40 degrees . As a result, the mean value for theta decreased from 16.3 (SD 6.9) to 11.4 degrees (SD 5.0) along the muscle's superior-to-inferior direction. The mean value of theta was greater in the deep than in the superficial compartment. We conclude that pennation angle measurements of human muscle made using DT-MRI muscle fiber tracking are feasible and reveal that in the foot-head direction, there is heterogeneity in the pennation properties of the human TA muscle.

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. ...

The time course of exercise-induced T 2 -weighted signal intensity (SI) changes contains an initial rise, early dip, and secondary rise. The purposes of this study were to test the hypothesis that the secondary rise occurs earlier during more intense contractions, and to determine the contribution of BOLD contrast to the SI changes. Eight subjects performed 90-s isometric dorsiflexion contractions at 30%

It has previously been observed that during isometric dorsiflexion exercise, the time course of T2-weighted signal intensity (SI) changes is spatially heterogeneous. The purpose of this study was to test the hypothesis that this spatial heterogeneity would increase at higher contraction intensities. Eight subjects performed 90-s isometric dorsiflexion contractions at 30% and 60% of maximum voluntary contraction (MVC) while T2-weighted (repetition time/echo time=4000/35 ms) images were acquired. SI was measured before, during and after the contractions in regions of interest (ROIs) in the extensor digitorum longus (EDL) muscle and the deep and superficial compartments of the tibialis anterior (D-TA and S-TA, respectively). For all ROIs at 30% MVC, SI changes were similar. The maximum postcontraction SI was greater than the SI during exercise. At 60% MVC, SI changes during contraction were greater in the S-TA than in the D-TA and EDL. For the EDL and D-TA, the maximum postcontraction SI was greater than those during exercise. For the S-TA, the maximum postcontraction change was greater than the changes at t=8, 20 and 56 s but not the end-exercise value. We conclude that spatial heterogeneity increases during more intense dorsiflexion contractions, possibly reflecting regional differences in perfusion or neural activation of the muscle.

No abstract

No abstract

No abstract