Computed tomography angiography (CTA)
CT is the primary imaging modality for vascular assessment due to its availability, speed, and high spatial resolution. The scanner generates isotropic submillimeter cross-sectional images that are easily converted into 3D models. Multiplanar reconstructions (MPR), maximum intensity projections (MIP), and volume rendering techniques (VRT) form the basis of image interpretation.
The key challenge in CTA is timing (contrast synchronization). Data acquisition must take place precisely during the peak arterial contrast phase, before the venous phase begins. This is particularly difficult in patients with reduced cardiac output. Two methods are used to ensure precise timing:
- Test bolus (test bolus): preliminary measurement of contrast circulation time from the antecubital vein to the target region.
- Bolus tracking (bolus tracking): automated monitoring of X-ray attenuation within a large artery, which triggers the scan once a threshold is reached.
Although single-phase imaging is standard, multiphasic scanning is necessary to differentiate contrast medium from fresh thrombi and vascular calcifications.
Stenting complications: endoleaks
Following endovascular aneurysm repair (EVAR), blood leakage into the aneurysm sac—endoleaks—may occur. Multiphasic CT angiography allows for the detection of delayed filling of defects. There are 5 types of leaks:
- Type I: secondary leak due to an incomplete seal at the proximal or distal attachment sites of the stent-graft. Rare (up to 10%), but requires mandatory intervention.
- Type II: retrograde filling of the sac via collateral branches (e.g., lumbar arteries, inferior mesenteric artery, or internal iliac arteries). The most frequent type (up to 25% of cases).
- Type III: mechanical failure of the graft itself or modular disconnection.
- Type IV: transgraft seroma or plasma seepage through the graft porosity. Resolves spontaneously within a few days.
- Type V (Endotension): expansion of the aneurysm sac without a visible source of endoleak on CT.
Imaging of aortic pathologies
Acute aortic dissection. CTA is the modality of choice for unstable patients. It rapidly and accurately demonstrates the extent of the intimal dissection flap. MRA provides a similar picture and additionally assesses cardiac function, but is only applicable to stable patients. If the clinical presentation is typical but the flap is not visible, MRI can detect an intramural hematoma.
Thoracic or abdominal aortic coarctation. Both methods accurately map the anatomy. The typical location of thoracic coarctation is immediately distal to the origin of the left subclavian artery. In the abdominal variant, the origins of the visceral arteries (celiac trunk, superior mesenteric artery) are frequently involved, leading to the development of prominent collaterals, such as the hypertrophied arc of Riolan. The advantage of MRI here lies in the use of phase-contrast techniques. These allow measurement of peak blood flow velocity (V) and calculation of the pressure gradient across the stenosis using the modified Bernoulli equation: $\Delta P = 4V^2$. This is a crucial parameter for monitoring patient dynamics.
Briefly: answers to topical questions
What are the contraindications for CT angiography?
Limitations of CT angiography include conditions where the administration of iodinated contrast media may be contraindicated or undesirable.
- Renal failure — contrast media are often contraindicated depending on the severity of renal impairment; to minimize the risk of contrast-induced nephropathy, the minimum possible dose of contrast is used.
- Allergy to iodinated contrast media may provide a rationale for choosing MRI instead of contrast-enhanced CT.
Risk groups for contrast-induced nephropathy also include patients with kidney disease, diabetes mellitus, and those taking metformin, aminoglycosides, or NSAIDs.
What complications can occur during angiography and stenting, aside from endoleaks?
Angiography and stenting procedures may lead to various complications related to contrast administration and procedural techniques.
- Allergic reactions — a primary complication of contrast administration.
- Contrast-induced nephropathy (CIN) — the main risk following allergic reactions.
- Suboptimal stent positioning — a technical complication during elective intervention.
- Stent thrombosis — the risk of its development increases in the presence of other percutaneous coronary intervention complications.
What collateral pathways develop in abdominal aortic coarctation?
In abdominal aortic coarctation, the arc of Riolan may be visualized as a collateral pathway connecting the superior and inferior mesenteric arteries. With prolonged blood flow restriction in the superior mesenteric artery territory, the arc of Riolan can become dilated and hypertrophied.
Which specific agents are used as radiocontrast media in CT angiography?
Low-osmolar and iso-osmolar iodinated agents are used as radiocontrast media in CT angiography.
- Iodixanol (iodixanol) — an iso-osmolar contrast agent with lower nephrotoxicity.
- Iohexol — a non-ionic low-osmolar agent that causes a greater increase in creatinine levels compared to iso-osmolar alternatives.
Administration of these agents is performed using an automated power injector to ensure optimal image quality.
How to remember endoleak types: 1 — ends (top/bottom of the graft), 2 — branches (from the side), 3 — stent itself (fracture/graft failure), 4 — pores (material), 5 — mystery (growth without apparent cause).
Class and exam questions
Why is multi-phase scanning often required in CT angiography?
A single-phase scan alone does not reliably differentiate contrast medium in the lumen from a fresh thrombus or calcifications. Multi-phase scanning is also required to detect late-filling endoleaks or slow gastrointestinal bleeding.
Which diagnostic imaging method is preferred when acute aortic dissection is suspected?
CT angiography is the modality of choice. It takes only a few seconds, is suitable for hemodynamically unstable patients, and clearly visualizes the intimal flap.
What is the advantage of MRI over CT when evaluating a patient with aortic coarctation?
MRI utilizing phase-contrast techniques allows not only for anatomical evaluation but also for measuring peak blood flow velocity across the stenotic segment to precisely calculate the pressure gradient.
What else to study on this topic
- Features of non-contrast MR angiography
- CT diagnosis of acute and chronic mesenteric ischemia
- CT protocols for gastrointestinal bleeding
- Imaging in pulmonary embolism (PE)
- Subclavian Steal Syndrome
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