Aortic Imaging

1

What is the normal imaging appearance of the aorta on computed tomography (CT) and magnetic resonance imaging (MRI)?

The normal aorta on imaging can be divided into the following five parts: the aortic root, the ascending aorta, the proximal (anterior) aortic arch, the distal (posterior) aortic arch, and the descending thoracic aorta. The proximal segment of the ascending aorta is the aortic root, which begins at the upper part of the left ventricle and is approximately 3 cm in diameter in adults. The aortic root and most of the ascending aorta (approximately 5 cm long) is contained within the pericardium. The aortic root has three cusps, which define three spaces called the sinuses of Valsalva: the right, left, and noncoronary sinuses. The right coronary artery normally arises from the right cusp, and the left coronary artery arises from the left cusp. The ascending aorta is located along the right mediastinal border and becomes the aortic arch at the origin of the innominate artery. The aortic arch also gives rise to the left common carotid and left subclavian arteries.

The descending aorta begins where the aortic arch ends, immediately beyond the origin of the left subclavian artery. The aorta descends within the thorax adjacent to the vertebral column after crossing superiorly and then posteriorly over the root of the left lung. After crossing the diaphragmatic hiatus, it becomes the abdominal aorta, which is approximately 2 cm in diameter. Eventually, the abdominal aorta ends by dividing into right and left common iliac arteries, slightly to the left of the midplane at the lower border of the fourth lumbar vertebra. The descending aorta is the most posterior structure adjacent to the spine.

On CT or MRI, the normal aortic lumen will appear round in cross-section with a thin wall.

2

What typical CT and MRI protocols/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.

3

What are aneurysms and pseudoaneurysms, and how do they differ?

An aneurysm (also known as a true aneurysm) is dilation of a vessel in which all three layers of the vessel wall remain intact. A pseudoaneurysm (also known as a false aneurysm) is a contained rupture and implies disruption of at least part of the aortic wall (i.e., the intima, media, or both), with intact adventitia. Pseudoaneurysms are potentially highly unstable lesions. They are often post-traumatic or iatrogenic in nature ( Figure 13-1 ).

4

What is a mycotic aneurysm? Which imaging findings help suggest the diagnosis?

One might think mycotic means “related to fungus,” but mycotic aneurysm refers to an aneurysm caused by any type of infection, usually bacterial. Imaging features that may suggest the diagnosis are rapid change in size, adjacent inflammation, perivascular gas, eccentric or saccular configuration, or lack of atherosclerotic calcification.

5

What is an inflammatory abdominal aortic aneurysm (AAA)?

Inflammatory AAAs are aortic aneurysms associated with retroperitoneal fibrosis. These aneurysms are surrounded by extensive inflammation and fibrosis that often lead to complications such as ureteral obstruction. Some authorities believe that the surrounding fibrosis is a result of microscopic leakage of the aneurysm causing a severe secondary inflammatory response, whereas others espouse a viral or immune reaction–related etiology. Inflammatory aneurysms are much less common than atherosclerotic aneurysms.

6

What are aortoenteric and aortocaval fistulas, and how are they diagnosed?

Aortic aneurysms, usually when inflamed or infected, can erode into adjacent structures, such as bowel or the inferior vena cava, leading to connections between the aortic lumen and the lumina of these structures. The transverse portion of the duodenum and the inferior vena cava (IVC) are common locations for such fistulas as the expanding aneurysmal abdominal aorta exerts pressure on these adjacent structures. An aortoenteric fistula is suggested when there is a loss of fat plane between the enlarged aorta and the involved bowel loop, with inflammatory stranding in that region. Sometimes, air can be seen in the aortic lumen. If an aortoenteric or aortocaval fistula is suspected, imaging should first be performed without oral or intravenous contrast material, so that high attenuation in the affected bowel or IVC lumen on postcontrast scans can be correctly attributed to the fistula rather than from normal opacification by the administered contrast material ( Figure 13-2 ).

7

What is an aortic dissection, and how is it different from aortic transection?

Aortic dissection is caused by a tear of the intima, creating a false channel or lumen within the wall of the aorta. Flowing blood dissects along the course of the aorta, extending the false lumen longitudinally and into branch vessels. The classic clinical presentation is “ripping” back or chest pain.

Aortic transection (perhaps better referred to as traumatic aortic injury) occurs in trauma patients. The entire aortic wall is injured or transected, leading to hemorrhage or pseudoaneurysm formation. Transection is usually a focal injury, usually in the chest. Dissections extend for some distance and require more extensive imaging of the entire aorta and its branch vessels.

8

In the setting of blunt trauma, where do aortic injuries most commonly occur?

Most injuries seen by radiologists occur at the aortic isthmus, which is just distal to the takeoff of the left subclavian artery. Most authorities believe this is due to asymmetric aortic fixation at this location, leading to shear stress. Radiologists less commonly see injuries to the ascending aorta because many patients with this type of injury die before reaching the hospital.

9

How are traumatic aortic injuries diagnosed?

Clinical suspicion or chest radiographic findings should be enough to send a patient with an unstable condition to surgery. If the patient's condition is relatively stable, most authorities now recommend CTA, preferably with a multidetector computed tomography (MDCT) scanner. Catheter angiography, previously the mainstay of diagnosis, is now most useful for problem solving when CTA is nondiagnostic or equivocal.

10

Name the chest radiographic findings that are suggestive of thoracic aortic pathology.

Primary signs include an enlarged or enlarging aorta, an indistinct aortic contour, or displacement of intimal atherosclerotic calcifications (in aortic dissection). Secondary signs, indicating mass effect from an aneurysm or hematoma, include mediastinal widening; displacement of adjacent structures such as the trachea, the left main stem bronchus, or a nasogastric tube in the esophagus; an “apical cap,” or mediastinal hematoma extending above the lung apex; or pleural or pericardial effusion (which can be hemorrhagic, but is more commonly due to exudative fluid secondary to the adjacent acute aortic process). Many of these findings are not specific for aortic pathologic conditions. Even mediastinal hematoma in the setting of trauma is not specific because it may be secondary to venous rather than arterial injury.