In this project, the investigators propose to use high-resolution magnetic resonance imaging (MRI) to measure blood perfusion in different groups of calf muscle. This imaging approach is standard of care for evaluation of coronary artery disease (CAD), and has never been successfully applied to PAD. Preliminary results show distinct patterns of muscle perfusion between healthy and PAD patients, and thus great promise of the technique. The investigators will first verify the reproducibility of the technique, and then compare the calf muscle perfusion measures in PAD patients against healthy age-matched controls. This comparison will test the feasibility of detecting functional abnormality in PAD patients. After the baseline scans, the PAD patients will opt to undergo a 12-week supervised exercise therapy, and then a post-therapy MRI scan. Comparison of the pre- and post-therapy measurements will indicate how the therapy improves the calf-muscle perfusion, and how this perfusion change correlates with increases in patient's walking ability.

The long term goals of this project are to develop an improved diagnostic test for patients with PAD to predict who will benefit from therapeutic intervention. The MRI perfusion studies of calf muscle can be performed in conjunction with routine peripheral MR angiography to assess the functional significance of vascular stenosis.

Inclusion Criteria:

PAD patients with

• Intermittent claudication
• Ankle-brachial blood-pressure index (ABI) < 0.9

Exclusion Criteria:

• Estimated glomerular filtration rate (eGFR) < 30 ml/min/1.73m2
• Chronic pulmonary diseases
• Arthritis or other joint ailments