Stress fracture in lower extremities occurs commonly among military recruits and athletes during intensive physical training. This injury has a marked impact on the health of military personnel and imposes a significant financial burden [1]. Despites advances in stress fracture studies, its pathogeneses remains poorly understood and early diagnostic tools are lacking. Focal ischemia is a potential initiator of site-specific bone remodeling and may cause stress fractures in human lower extremity [2, 3]. We hypothesize that intensive repetitive loading impairs intramedullary blood flow due to pressurization of the bone marrow cavity, leading to focal ischemia and eventual development of stress fractures. To begin to test our hypothesis, we developed and validated a quantitative, non-invasive method to measure blood flow in vivo. The approach was based on Cine Phase Contrast MRI (CPC-MRI), a dynamic motion measurement and visualization modality that was originally designed for cardiovascular studies. By measuring the phase shift that is induced by pulsatile blood flows in the magnetic resonance signal, cross-sectional images and velocity maps are acquired of the moving fluid. This technique has been adapted to study blood flows in skeletal muscles [4], intracranial flows [5], and muscle mechanics and joint kinematics [6]]. We first performed a flow phantom study to validate the reliability and accuracy of CPC-MRI in measuring flow velocity. We then quantified the effects of brief exercise on blood flows in the lower extremities of human subjects. These non-invasive measurements will help us better understand the interplay between vasculature and skeletal system in various physiological and pathological conditions.

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