Peripheral Arterial Disease & CTA Runoff
- Peripheral arterial disease (PAD) is plumbing trouble: atherosclerosis narrows the arteries feeding the legs, so the muscles run short on blood when they ask for more.
- The classic symptom is claudication — cramping leg pain that shows up with walking and quits with rest, like a muscle that keeps hitting its allowance.
- CTA runoff is a contrast CT that maps the arteries from the aorta down to the feet, so we can see where the pipe is narrowed and how badly.
- The make-or-break job on a runoff study is calling the difference between a stenosis (narrowed but flowing) and an occlusion (fully blocked), and finding the collaterals that reroute around it.
- The scary end of the spectrum is critical limb ischemia — rest pain, ulcers, or tissue death — where the leg is genuinely at risk.
Imagine the arteries in your legs as a garden hose, and atherosclerosis as decades of gunk slowly caking the inside. On a lazy day, when your leg muscles are just sitting there, even a half-clogged hose delivers enough. But the moment you start walking, those muscles demand a firehose of blood — and the clogged pipe simply can't keep up. The muscle starves, cramps, and forces you to stop. Sit for a minute, demand drops, the cramp fades. That, in one paragraph, is claudication, and it's the everyday face of peripheral arterial disease (PAD).
What's actually happening in the pipe
PAD is just atherosclerosis — the same cholesterol-and-calcium plaque that menaces coronaries and carotids — except it's set up shop in the arteries of the pelvis and legs. The narrowing builds slowly, which is why the body often has time to grow collaterals: little detour vessels that sprout around the blockage like a side road that opens up when the highway jams. Collaterals are a clue, not a cure — when you see a fat tangle of them, it tells you the main road has been bad for a while.
The severity ladder runs roughly: no symptoms → claudication (pain only with exertion) → critical limb ischemia, where the blood supply is so poor the leg hurts at rest, won't heal its ulcers, or starts to die. That last rung is a limb emergency, not a lifestyle complaint.
Where the blockage sits predicts where the pain shows up. Aortoiliac disease tends to cramp the buttock and thigh; femoropopliteal disease (the usual suspect, at the superficial femoral artery) cramps the calf. As a rule of thumb the pain sits one level below the blockage, because that's the muscle left out in the cold.
Enter the CTA runoff
When we need a map rather than a guess, we order a CT angiogram (CTA) runoff — sometimes called a "lower extremity runoff." It's a CT scan timed so a bolus of iodinated contrast lights up the arteries just as the scanner sweeps from the abdominal aorta all the way down to the toes. The result is a glowing white road map of every artery feeding the leg. If you want the nuts-and-bolts of how that timing works, the CTA/MRA protocol page is the place to geek out.
The catch — and there's always a catch — is timing. The scanner has to chase the contrast down the leg at just the right speed. Too fast and it outruns the bolus, so the feet look empty even though they're fine. Too slow and the veins fill in and clutter the picture. Getting a clean runoff is genuinely a bit of an art.
Stenosis vs occlusion — the call that matters
The whole point of reading a runoff is to grade the disease at each segment. Two words do most of the work:
| Finding | What it means | The tell on CTA |
|---|---|---|
| Stenosis | Narrowed but still flowing | The contrast column pinches in but stays continuous through the segment. |
| Occlusion | Completely blocked | The contrast column stops dead, a dark gap, then often re-lights ("reconstitutes") further down via collaterals. |
The reconstitution point is gold for the vascular surgeon or interventionalist: it tells them where a bypass graft would need to land, or where a wire would have to re-enter the true channel.
Heavy calcium is the runoff's great liar. Densely calcified plaque "blooms" — it looks bigger and brighter than it really is — and can hide the actual flowing channel inside it, making a so-so stenosis look like a total occlusion. Below-the-knee vessels in long-standing diabetes are especially calcified and especially deceptive. When calcium is fooling the eye, ultrasound or catheter angiography may be needed to get the real answer.
Why we bother getting it right
Calling the level and severity correctly is what turns a scan into a plan. A short, focal stenosis might be opened with a balloon or stent; a long occlusion might need a surgical bypass; a foot full of dead-end vessels might mean the leg can't be saved no matter what. The imaging doesn't just describe the disease — it picks the door the patient walks through next.
Always eyeball both the inflow (aorta and iliacs) and the outflow (the tibial vessels at the ankle) before you sign off. A beautiful bypass to a good target is useless if the inflow above it is choked or the runoff vessels below it are all blocked — the new pipe has to connect a working source to a working destination.
If you remember one thing: PAD is a slow plumbing problem, and the CTA runoff is the map that shows exactly where the pipe failed and whether there's still somewhere for the blood to go.