15.1 CT Angiography & Vascular Procedures — CTA, CTV, Runoff, Dissection & PE
Key Takeaways
- ARRT's 'Additional Procedures' focus theme names vascular CTA/CTV explicitly, and it cross-cuts every Procedures subcategory, so vascular protocols are tested against head/neck, chest, and abdomen/pelvis anatomy alike.
- Bolus tracking for arterial studies typically triggers at 100 HU above baseline, with ROI placement in the vessel of interest (ascending aorta or main pulmonary artery for aortic/PE studies, descending aorta for peripheral runoff).
- Runoff CT angiography uses a bolus-chase (stepping-table) technique because contrast transit time slows progressively toward the diseased periphery — a fixed table speed will outrun the bolus in stenotic legs.
- Indirect CT venography for suspected DVT is acquired on a delay of roughly 180 seconds after a single contrast injection so both the pulmonary arteries and deep leg veins opacify from one bolus.
- Distinguishing active extravasation from a pseudoaneurysm, and true lumen from false lumen in dissection, depends on how a focal contrast collection behaves between arterial and delayed phases, not on its appearance in a single phase.
Why Vascular CT Gets Its Own Focus Theme
ARRT's CT content specifications call out "Additional Procedures" — vascular CT angiography (CTA) and CT venography (CTV), image-guided biopsy/drainage/aspiration, trauma, and surgical planning — as a focus theme that spans all three Procedures subcategories (Head/Spine/MSK, Neck/Chest, Abdomen/Pelvis) rather than living in one place. Practically, that means a vascular question can be dressed up as a neuro case (carotid CTA for stroke workup), a chest case (pulmonary embolism), or an abdominal case (aortic aneurysm) and you are expected to apply the same timing and protocol logic in each anatomic context. This section consolidates that logic so you are not relearning bolus physics for every organ system in Chapters 10-14.
CTA vs. CTV: Timing Is the Only Real Difference
A CT angiogram (CTA) and a CT venogram (CTV) can use the identical scanner, contrast dose, and anatomic coverage — what changes is the scan delay relative to injection, because you are chasing a different phase of contrast transit.
| Study | Target Vessels | Typical Trigger/Delay | Peak Enhancement Pattern |
|---|---|---|---|
| CTA (arterial) | Aorta, carotids, pulmonary arteries, renal/mesenteric arteries, runoff | Bolus tracking, ROI in target artery, trigger ~100 HU above baseline | Earlier, brighter arterial peak |
| CTV (venous) | Deep leg veins, portal vein, IVC, dural sinuses | Fixed delay, commonly ~180 sec after a single injection for indirect lower-extremity CTV | Later, more gradual venous peak |
| Combined CTA-CTV | PE + DVT ("one-stop" venous thromboembolism workup) | Pulmonary arterial acquisition first, then a delayed pass through the pelvis/thighs/calves off the same bolus | Two acquisitions, one injection |
Bolus tracking places a monitoring ROI directly in the vessel you need to opacify and fires the diagnostic acquisition once attenuation crosses a preset threshold — commonly 100 HU above baseline blood-pool density, with the scan starting after a short (5-8 second) post-trigger delay to allow the table/gantry to reach diagnostic speed. Because true peak enhancement (400-600 HU) is much higher than the 100 HU trigger, the threshold is intentionally conservative: the system is predicting where the bolus will be a few seconds from now, not scanning at the instant of triggering.
CT Pulmonary Angiography (CTPA)
For suspected pulmonary embolism (PE), the bolus-tracking ROI sits in the main pulmonary artery, with a 100 HU trigger and a short fixed delay of roughly 5 seconds. A common pitfall is a threshold set too low, which triggers on right-heart transit before the bolus reaches the pulmonary arteries, or too high, which risks venous contamination from recirculation. On the images, a PE appears as a filling defect — a dark, low-attenuation clot outlined by bright, opacified blood — most dramatically as a saddle embolus straddling the main pulmonary artery bifurcation.
Runoff CTA: Chasing a Slowing Bolus
Runoff CT angiography images the arterial tree from the celiac axis/renal arteries down to the pedal arch to evaluate peripheral arterial disease, claudication, or critical limb ischemia. Because the legs have longer transit times than the aorta — and stenotic or occluded segments slow flow even further — a single fixed table speed cannot keep pace with the bolus over that much anatomy. The solution is a bolus-chase (stepping-table) technique: the scanner tracks the leading edge of the contrast column and advances the table caudally at a speed matched to the (decelerating) bolus front, rather than at a constant rate. In patients with markedly asymmetric disease (for example, a chronically occluded superficial femoral artery on one side), technologists may use a dual-injection technique or adjusted per-leg table speed so the slower limb still gets adequate opacification without over-scanning the faster limb.
Aortic Dissection and Aneurysm
Aortic CTA for suspected dissection or aneurysm requires coverage from the aortic root through the femoral arteries, because dissection flaps and aneurysms are not confined to one segment.
- Dissection: An intimal tear creates two lumens separated by a flap — the true lumen is usually smaller, has faster/brighter flow, and is continuous with the undissected aorta; the false lumen is often larger, enhances later or less intensely, and may show a slower "cobweb" sign or thrombus. Full aortic coverage (root to femorals) is required because dissection flaps propagate and branch-vessel involvement (renal, mesenteric, iliac) changes surgical planning.
- Aneurysm: Measured as the maximum outer-wall-to-outer-wall diameter perpendicular to the vessel's long axis on a reformatted image (not on an oblique axial slice, which overestimates size). A crescent sign — a crescentic area of high attenuation within the aortic wall or thrombus — suggests intramural hematoma and impending rupture and is a critical finding to flag immediately.
Common Traps
- Treating CTA and CTV as needing different contrast volumes when the real difference is timing, not dose.
- Using a fixed table speed for runoff studies instead of bolus-chase tracking, which produces poor distal opacification in diseased limbs.
- Confusing a pseudoaneurysm (a focal, well-circumscribed contrast collection that enhances similarly on arterial and delayed phases, matching blood-pool density) with active extravasation (an irregular contrast blush that grows larger and denser on delayed images as it continues to leak) — the distinction depends on comparing phases, not on a single image.
- Measuring aneurysm diameter on a non-perpendicular axial slice, which inflates the apparent size and can trigger unnecessary surgical referral.
During a CT pulmonary angiogram, the bolus-tracking region of interest is placed in the main pulmonary artery with a trigger threshold of 100 HU. What is the primary reason the diagnostic scan does not begin the instant the ROI reaches 100 HU?
A runoff CT angiogram is ordered for a patient with a chronic superficial femoral artery occlusion on the left and normal flow on the right. Using a single, constant table speed for the entire study is most likely to produce which problem?