Once a pressure test says something is wrong, the next question is where and how bad — and that needs a picture. There are three ways to make one, they are not interchangeable, and the differences come down to your kidneys, your implants and what the picture is for.
Imaging comes second
Pressures first, pictures after. An ankle-brachial index or toe pressure tells you whether there is a significant problem and roughly how severe. Imaging tells you where it sits and what could be done about it, which only matters once treatment is being considered. Being sent for a scan before anyone has measured a pressure is worth asking about.
The three methods
| Duplex ultrasound | CT angiography | MR angiography | |
|---|---|---|---|
| What it is | Sound waves showing both the vessel and the flow inside it | X-ray scan with iodine contrast injected into a vein | Magnetic scan, usually with gadolinium contrast, sometimes without |
| Radiation | None | Yes | None |
| Contrast | None, or a bubble agent occasionally | Iodine — the main consideration for kidneys | Gadolinium — used with care in advanced kidney disease |
| Time | 30 to 60 minutes, operator-dependent | Minutes | 30 to 60 minutes, in an enclosed scanner |
| Best at | Veins, first-line arterial mapping, follow-up of grafts and stents, repeatable as often as needed | Fast, detailed arterial maps including the pelvis and abdomen | Detail without radiation; good below the knee |
| Limited by | Body habitus, bowel gas, heavily calcified vessels, and the skill of the person scanning | Heavy calcium can obscure small vessels; kidney function | Pacemakers and some implants, claustrophobia, tends to overstate narrowing |
| Usual role | The starting point and the follow-up tool | The planning scan for most procedures | The alternative when contrast or radiation is a problem |
Catheter angiography
The fourth option is the oldest: a catheter threaded into the artery and contrast injected directly. It gives the clearest picture of all, particularly in the small vessels below the knee, and its distinguishing feature is that treatment can happen in the same sitting — the balloon or stent goes in while the catheter is already there.
Because it is invasive, with a puncture site and a small risk of bleeding, vessel injury or contrast affecting the kidneys, it is generally reserved for when treatment is expected rather than used purely to look. If you are booked for one, it is reasonable to ask whether the plan is diagnosis, treatment, or both.
The kidney question
Iodine contrast for CT can worsen kidney function in people whose kidneys are already impaired. This matters in vascular disease more than in most fields, because kidney disease and arterial disease travel together. It is managed rather than avoided — checking kidney function beforehand, using the smallest dose that answers the question, giving fluids, and reviewing medicines such as metformin around the scan — and for many people ultrasound or MR is chosen instead.
Tell whoever orders the scan if you have kidney disease, are on dialysis, have had a contrast reaction before, take metformin, or might be pregnant. None of those necessarily stops the test; all of them change how it is done.
What to expect
- Ultrasound. No preparation for leg arteries or veins; an abdominal or pelvic scan may ask you to fast. Warm gel, firm pressure from the probe, and a lot of quiet while the sonographer works. The pressure on a tender leg can be uncomfortable — say so.
- CT angiography. Usually a few hours without food, a cannula in the arm, and a warm flushing sensation as the contrast goes in that feels like you have wet yourself. You have not. It passes in seconds.
- MR angiography. Remove everything metal, expect loud knocking noises and a narrow tube. Say in advance if you are claustrophobic — this is common and there are ways to manage it. Declare any implant, clip, pump or stimulator.
- Catheter angiography. Fasting, a puncture usually at the groin, local anesthetic and sedation, then lying flat for a period afterward while the site seals. Someone drives you home.
- Afterward, ask when and how you will get the result, and who is responsible for acting on it. Scans sitting unread on a system is a real failure mode, and one phone call prevents it.
The report is yours
Ask for a copy, and ask for the images on a disc or portal link if a second opinion is likely. If you are ever told nothing can be done for your leg, having the imaging in hand is what makes a second opinion possible without repeating everything. Why that second opinion sometimes matters.
Common questions
Which scan is best?
There is no single best. Ultrasound is the usual starting point because it is safe, repeatable and free of contrast. CT gives the fastest detailed map for planning. MR avoids radiation and contrast issues. The right one depends on your kidneys, your implants, what part of the body is in question, and what the answer will be used for.
Is the contrast dangerous?
For most people it is well tolerated. The two groups who need care are people with reduced kidney function and people who have reacted to contrast before. Both are managed with precautions or by choosing a different test — which is why declaring them beforehand matters more than worrying about them.
Why does my report say the narrowing is worse than my symptoms suggest?
Images show anatomy; symptoms reflect function. A long-standing narrowing with good collateral vessels around it can cause remarkably little trouble, while a shorter one that developed quickly causes a lot. MR in particular tends to overstate the degree of narrowing. Treatment decisions are made on symptoms and pressures together with the pictures, not on the pictures alone.
Do I need imaging if my ABI is abnormal?
Not necessarily. If you have claudication being managed with exercise and medicines, an abnormal ABI does not by itself require a scan. Imaging is for when a procedure is being considered, when symptoms are severe or progressing, or when there is a wound or rest pain. Otherwise it answers a question nobody is going to act on.
Where to go next
Sources
- National Heart, Lung, and Blood Institute. Peripheral Artery Disease — Diagnosis.
- MedlinePlus, National Library of Medicine. Angiography.
- National Institute of Diabetes and Digestive and Kidney Diseases. Chronic Kidney Disease.
- Society for Vascular Surgery. Peripheral Artery Disease.
Written by Tom Biernacki, DPM, FACFAS. Published 21 September 2026. General education, not medical advice.