Imaging of muscle blood volume changes during exercise with Vascular Space Occupancy (VASO) MRI

Abstract

Large, rapid changes in perfusion accompany exercise, mediated in part by a local vasodilatory response. Impaired vasodilation occurs in diabetes, obesity, and other conditions. VASO MRI was developed to measure blood volume changes in the brain by nulling the signal of blood; vasodilation thus causes a loss of MRI signal intensity. The purpose of this work was to implement VASO for skeletal muscle. Eighteen subjects participated in the optimization of this method. Subjects were placed supine in a Philips 3.0T scanner with their foot on a force transducer, a knee coil around the calf, and an occlusion cuff around the thigh. Anatomical and VASO images were acquired (Fig 1).Maximal plantarflexion contractions were performed for 1–20 seconds, six minutes apart. The cuff was inflated to 240 mm Hg for five minutes, occluding arterial flow; VASO response was monitored for up to fifteen minutes after cuff release. NIRS spectroscopy was also performed to relate observed VASO signal changes to blood oxygenation and volume changes. VASO signal follows a similar time course to NIRS post occlusion.The decrease in VASO signal following a series of three ten second maximal plantarflexion contractions, as shown in Fig 2, should be proportional to increased blood volume demonstrating the promise of this new imaging paradigm to study blood volume changes in exercising muscle. This work was supported by NIH/NIAMS R01 AR050101.