What typical CT and MRI techniques are used to image the aorta?
Computed tomographic angiography (CTA) is performed to assess diseases of the aorta and has established itself as the principal diagnostic test for aortic pathology.
Many CTA examinations include a nonenhanced acquisition from the supra-aortic vessels to the iliac and femoral arteries. The noncontrast scan can be useful for detection of aortic calcification, allowing for discrimination between calcification and contrast enhancement, as well as detection of high-attenuation intramural hematoma. This is then followed by a contrast-enhanced scan, acquired during the time when contrast enhancement is highest in the aorta (usually about 20 to 25 seconds after beginning an intravenous injection). Delayed phase imaging may be performed 1 to 5 minutes later in some cases to detect endoleaks in patients with stent-grafts, to evaluate the venous system, and to detect slow contrast extravasation. CTA scans are performed preferably using contrast agents with higher iodine concentrations in order to obtain high levels of vascular enhancement. Scanning protocols may differ from one institution to another depending on the type of scanner, data storage capacity, and resources available for patient monitoring.
CTA data acquisition can also be synchronized to the electrocardiogram (ECG) signal, allowing for effective freezing of the motion of the heart at various parts of the cardiac cycle. Two main techniques for ECG-synchronized imaging are prospective triggering and retrospective gating. Prospective triggering acquires data during only a prespecified portion of the cardiac cycle, while retrospective gating acquires data throughout the cardiac cycle. Retrospective gating allows evaluation of cardiac and valvular motion, but at the cost of increased radiation dose compared to prospective triggering.
Magnetic resonance angiography (MRA) is usually performed to include a combination of bright blood and dark blood images before and after contrast administration. As in CTA, ECG-synchronized acquisitions can be performed with prospective triggering or retrospective gating. Prospectively triggered dark blood T1-weighted or inversion recovery sequences performed in axial and oblique sagittal planes are useful for evaluating the aortic wall to look for atherosclerotic plaque and wall thickening in vasculitis.
A contrast-enhanced MR angiogram is then usually performed with a three-dimensional gradient echo acquisition obtained during a breath hold. The imaging plane and slice thickness are optimized to the anatomy of interest (e.g., an oblique sagittal acquisition for the thoracic aorta or a coronal acquisition for the abdominal aorta). Because there is no radiation dose, multiple acquisitions can be obtained at various phases of contrast enhancement as needed.
In patients with renal disease, contrast allergy, or poor venous access, noncontrast MRA techniques can be used, although image quality may not be as good as that with contrast MRA. Commonly used sequences for this approach are time-of-flight (TOF) and steady state free precession (SSFP) techniques. Additionally, phase-contrast (PC) MRA allows measurement of blood flow and can supplement the other techniques listed above.