Contrast ultrasound is a technique that can quantify blood flow in the tissues of the body by ultrasound detection of microbubble contrast agents that behave in the circulation similar to red blood cells. In this study, the investigators hypothesize that contrast ultrasound of blood flow in the leg (thigh and calf) at rest and during stress produced by medications that mimic exercise (vasodilator stress) can provide information on the location and severity of peripheral vascular disease (blockages of the blood vessels in the leg). The investigators will also determine whether symptom improvement after revascularization (procedures to open up or bypass the blockages) is directly related to the improvement in blood flow.

Inclusion Criteria:

1. Known or suspected PAD
2. Lower extremity angiography or MRA planned or performed within 6 months

Exclusion Criteria:

1. Age <19 y.o.
2. Pregnant or lactating females
3. Hypersensitivity to ultrasound contrast agent, dipyridamole, or regadenoson
4. Severe reactive airways disease
5. Evidence right-to-left shunt (identified on screening echo)
6. NYHA Class III or IV heart failure.
7. Planned amputation
8. Unstable coronary artery disease or severe aortic stenosis

Patients with peripheral arterial disease who are referred for diagnostic angiography and/or revascularization procedures