Intervention: administration of ultrasound contrast agent (drug) to assess muscle perfusion. Normal healthy subjects (n=10) will be used for optimization of dose and acoustic settings for performing real-time contrast ultrasound perfusion imaging of the limb.

Intervention: administration of ultrasound contrast agent (drug) to assess muscle perfusion. Patients with moderate to severe PAD (ABI 0.4 to 0.6) will be evaluated with contrast ultrasound perfusion imaging using methods optimized in the first Arm to determine whether symptoms better correlate with perfusion imaging than other measures of PAD severity.

Intervention: administration of ultrasound contrast agent (drug) to assess change in muscle perfusion produced by revascularization (surgical or percutaneous procedure). Patients with symptomatic PAD will be evaluated with contrast ultrasound perfusion imaging using methods optimized in the first Arm before and within 1 month of revascularization to determine whether improvement in symptoms correlate with perfusion imaging data.

Skeletal muscle perfusion assessment of the thigh and calf skeletal muscle will be performed with contrast ultrasound of leg muscle with Sonazoid while at rest and during exercise stress