Vascular Chest Emergencies

Anatomy

The thoracic aorta begins at the level of the aortic valve and extends to the level of the diaphragmatic hiatus ( Fig. 10-1 ). The aortic root is said to be made of three outpouchings or sinuses of Valsalva and the aortic valve with its annulus. The tubular ascending aorta begins just above the sinuses at a waist known as the sinotubular junction . At the level of the innominate or brachiocephalic artery the ascending artery gives way to the arch. Just distal to the left subclavian artery the descending aorta begins. The most proximal portion of the descending aorta is referred to as the aortic isthmus . A majority of the thoracic aorta is extrapericardial in its location. Only the first 2 to 3 cm of the ascending aorta resides within the pericardial sac.

The aortic wall has three layers. Although not distinguishable by computed tomography (CT) or magnetic resonance imaging (MRI), these layers include the intima, media, and adventitia. The media is the muscular layer in which many of the acute aortic syndromes may propagate. The inner two thirds of the media has concentric fibers that are orthogonal to the outer one third consisting of longitudinal muscle fibers. The net effect is a natural cleavage plane. If blood can access the space within the media, it can easily travel along the course of the media as either a false lumen or intramural hematoma (IMH).

Imaging

Computed tomography has remained the workhorse for aortic imaging in the ED. Its ubiquitous use comes from the proximity of many scanners to the ED, often with 24-hour technologist staffing and high speed of image acquisition. In addition, many of our surgical colleagues are quite comfortable with CT image review. The advent of multidetector computed tomography (MDCT) has only strengthened the role of CT in the evaluation of acute aortic pathologic conditions. With increased detector rows, thinner section images can be obtained at faster speeds. These thinner sections allow for higher quality multiplanar and three-dimensional reconstructions. In the modern era CT has close to 100% accuracy in depicting acute aortic pathologic conditions. Neither echocardiography nor MRI is more accurate.

Our nontraumatic aortic CT protocol begins with a noncontrast acquisition. The noncontrast images are helpful for looking for high attenuation within the wall, indicative of IMH. The IMH should be higher in attenuation than the blood pool, but not as high as calcium. It usually involves only a portion of the wall, not 360 degrees. The images are acquired as 3-mm thick images with a 3-mm reconstruction interval to avoid any gap. The noncontrast images are followed by a CT angiogram using bolus tracking software and a rapid injection rate of 4 to 5 mL/sec. These images are reconstructed as 1- to 2-mm thick images with 1- to 2-mm reconstruction intervals. A noncontrast study should not be used to exclude acute aortic pathologic conditions. If the patient cannot receive intravenous contrast, a noncontrast study can be performed to exclude aortic rupture, but it should be followed by another test, either echocardiography or MRI.

Although MDCT allows for electrocardiogram (ECG) gating, it is not routinely used in our ED for assessment of the aorta. With fewer than 4% of these studies being positive for aortic pathologic conditions, the potential for increased radiation and the added technical prowess of ECG gating have not been justified in our practice. Instead, we have relied on ECG gating as a problem-solving tool once an aortic pathologic condition is encountered. If a question arises as to whether the ascending aorta is truly involved by a process, the patient may be rebolused and rescanned using ECG gating ( Fig. 10-2 ). With the advent of dual-source CT, we have also begun to rely on ultrahigh-pitch imaging (pitch of 3.2) for the evaluation of the ascending aorta. With its lower radiation dose profile, this technique has shown greater promise in evaluating the ascending aorta on a more regular basis.

Oblique sagittal reconstructions are created directly at the scanner for all aortic CTs. The candy cane reconstruction allows for a close approximation of an angiogram view and allows for clear delineation in the z-axis of any aortic process. Volume-rendered and shaded surface images are rarely performed in real time. Although the images can be very artistic, they rarely add to the diagnosis.

Magnetic resonance imaging is increasingly being used as a secondary study when the patient is unable to receive iodinated contrast material or if a question arises from the CT. With technologic improvements and improved MR angiography techniques, MRI offers a nonradiation alternative. Unfortunately, distance from the ED and longer examination times have kept MRI as a second-line test at many institutions. Although gadolinium may allow for images that appear more similar to CT, contrast material is not required for diagnosis of acute aortic conditions. This allows for the potential of noncontrast MRI in the evaluation of the aorta in patients in renal failure. An added benefit of MRI is the use of black-blood sequences and fat-suppressed T2-weighted or short tau inversion recovery (STIR) images. These sequences can be useful for the evaluation of increased signal within the aortic wall, which may be indicative of an IMH and may help clarify a subtle CT finding.

No discussion of aortic imaging in the ED would be complete without discussing conventional radiography and angiography. Conventional radiography is useful when findings are positive because it allows the team to mobilize while support lines are being placed before CT. Radiography findings include widening of the mediastinum, effacement of mediastinal lines, and mass effect on the trachea and the left mainstem bronchus. A majority of the time the radiography signs can be subtle. The most useful finding is change in the mediastinal contour compared to a prior radiograph. A negative radiograph should not be used to exclude acute aortic pathologic conditions.

Angiography was once considered the gold standard for the evaluation of the acute nontraumatic aorta. In the current era, angiography is mainly used in the deployment of endovascular stents.

Mimics and Pitfalls on Computed Tomography

Artifacts and mimics of aortic disease are more commonly encountered than real aortic pathologic conditions. To avoid sending a patient for needless surgery, one must be comfortable with some of the more common mimics of acute disease. The most commonly encountered artifact is pulsation of the aorta at the root ( Fig. 10-3 ). This pulsation artifact can simulate a dissection flap at the root. Despite advances in CT technology, pulsation is still encountered. Clues to pulsation artifact include visualization of a flap in the pulmonary artery at the same level, lack of pericardial effusion or aortic wall thickening, and an ill-defined craniocaudal extension on multiplanar imaging. Although pulsation can usually be excluded, sometimes an echocardiogram or a repeat CT with ECG gating or ultrahigh-pitch imaging is required.

Other artifacts may stem from other diseases, such as a vasculitis or an adjacent neoplasm that may simulate aortic wall thickening or hematoma. The presence of lymphadenopathy can be helpful in suggesting these other entities.

Aortic Aneurysms

An aneurysm can be defined as failure of tapering of the aorta or even dilatation of the distal aorta. In the ascending aorta 4 cm is often used as the minimal size of an ascending aortic aneurysm; in the descending aorta, 3 cm is the minimal size. Measurements must be performed orthogonal to the long axis of the aorta. Occasionally this can be performed on transverse images, but usually these measurements require multiplanar reconstructions. Although patients with known aneurysms are often followed to determine a rate of change (if any) over time, in the ED the radiologist is not often afforded the luxury of a prior study for comparison. It becomes imperative then that certain imaging signs are used to determine when a thoracic aneurysm is unstable.

One of the most important findings is the shape of the aneurysm. Most aneurysms can be described as fusiform (diffuse concentric enlargement) or saccular (eccentric outpouching). Although fusiform aneurysms can become unstable, saccular outpouchings are more often indicative of a pseudoaneurysm that has the potential of being unstable. Most saccular aortic aneurysms are indicative of infection, traumatic injury, or a postoperative complication, such as dehiscence of a suture line. The one major exception is the pseudoaneurysm from a penetrating atherosclerotic ulcer (PAU; see later discussion).

Other imaging findings of an unstable aneurysm include high attenuation within the wall (sometimes referred to as the impending rupture sign ) ( Fig. 10-4 ), irregular shape, calcium that is somewhat tangential to the arc of the aorta, and periaortic stranding. All of these signs require careful attention. Usually more than one of these are present. The impending rupture sign and tangential calcium may be easier to see on noncontrast images.

Rarely, high attenuation indicative of blood may be seen in the mediastinum and in the pleural space (hemothorax). More commonly, mild fat stranding is seen with a low-attenuating pleural effusion. This transudate is felt to represent a sympathetic effusion. In fact, it is very unusual to see an unstable aorta without a left pleural effusion. Care must be taken to not mistake the concomitant left-sided atelectasis, which often briskly enhances, for contrast extravasation. In the past the visualization of blood would result in termination of the scan and immediate contact with the surgeon. In the era of endoluminal therapy the presence of blood in the mediastinum or pleura results in immediate contact with the surgeon, but often the postcontrast images are continued because they are key in planning appropriate therapy. Even more rarely, extravasation of intravenous contrast may be seen. Of all the signs, this one has the gravest prognosis.

Acute Aortic Syndromes

Over the past decade the term acute aortic syndrome has come into use as an aortic equivalent of the acute coronary syndrome as an explanation for an aortic cause of chest pain. Although some authors include unstable aneurysms in this grouping, most reserve the term for the spectrum of aortic medial pathologic conditions, namely, aortic dissection, IMH, and PAU. These entities are rare but can be lethal and require a high index of suspicion. Because CT is critical in the diagnoses of these entities, every radiologist working in the ED must be familiar with their imaging appearances.