Computed Tomographic Angiography

Michael L. Martin

As with catheter angiography, prior to imaging, it is essential to evaluate the patient to determine the appropriateness of the examination, address any clinical issues that might impact the study, select the area to be imaged, and assess how cooperative the patient is likely to be. Technologists not familiar with the regions of coverage or bolus timing issues germane to vascular studies will need guidance from the radiologist.

Choose the Appropriate Modality

1. Use CTA instead of MRA when (1)

a. There is greater local expertise with computed tomographic angiography (CTA).

b. Patient is claustrophobic.

c. Implants are contraindicated in magnetic resonance (MR) (pacemaker, cochlear implant, etc.).

d. In-stent lumen evaluation is required.

e. Depiction of arterial calcification is desired (e.g., assessment for bypass graft site).

2. Use MRA instead of CTA when

a. There is greater local expertise with magnetic resonance angiography (MRA).

b. Patient is allergic to iodinated contrast.

c. There is a desire to avoid ionizing radiation.

d. Excessive vessel calcification, especially small vessels, is observed or anticipated.

e. Endoleak is suspected, and the patient has an MR-compatible stent graft (2). Consider MRA when a questioned endoleak is occult to other imaging modalities.

Preprocedure Preparation

1. Contrast-induced nephropathy (CIN): Vasculopaths may have multiple risk factors for CIN; therefore, determination of calculated glomerular filtration rate (GFR) and avoidance of dehydration are essential. Screening guidelines and interventions to mitigate CIN are dealt with in Chapter 65.

a. If the GFR <30 mL per minute, consideration should be given to noncontrast MRA, CO₂ angiography, or a combination of techniques. CTA can be performed after intra-arterial injection of contrast diluted 1:10 or more with normal saline, resulting in considerably lower contrast volumes (3). When possible, patients on dialysis should have their CTA scheduled immediately prior to a dialysis treatment session.

2. Clothing: The patient should remove all clothing with metallic components from the area to be examined.

3. Sedation: Sedation is rarely necessary, but if given, sedated patients need a responsible adult escort.

4. Intravenous (IV) lines: A right arm IV access is preferred when evaluation of the thoracic aorta is part of the study (including carotid CTA); otherwise, the site of puncture and size of cannula are chosen to allow an injection rate of at least 4 mL per minute. A 20-gauge cannula in the antecubital fossa is commonly used.

Procedure

This chapter describes general principles; imaging protocols are scanner-specific and will need to be developed within an institution for each examination (4). CTA of all vascular beds can be performed with any modern computed tomography (CT) scanner without additional hardware. Automated tube current modulation is recommended to individualize radiation dose and decrease image noise (5). Submillimeter (typically 0.6 mm) collimation should be utilized for all vascular territories to ensure adequate z-axis resolution. Pulsation artifact in the ascending aorta can be largely eliminated through the use of cardiac-gating techniques. This may increase the diagnostic accuracy in the depiction of subtle ascending aortic disease though currently cardiac-gating is rarely used outside of this limited indication.

1. Scanning protocol

a. Scout sequence: A digital radiograph topogram is performed to define scanning area and to choose a level for contrast detection for bolus-triggering.

b. Occasionally, an optional nonenhanced acquisition is performed. Nonenhanced imaging is indicated in assessment for acute thoracic aortic syndromes, gastrointestinal (GI) bleeding, and endoleaks in patients following endovascular aneurysm repair.

c. Test-bolus, if bolus-triggering is not used

d. CTA acquisition series

e. An optional “delayed phase” acquisition in the event of nonopacification of distal vessels or expectation of delayed opacification (e.g., endoleak study, GI bleed studies)

2. Contrast injection

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