Over the past few years, several studies have evaluated the role of CT in the diagnosis of TAR yielding controversial results. Before the introduction of helical CT in clinical practice, CT diagnosis of aortic rupture relied primar-
ily on the detection of mediastinal hematoma as an indirect sign rather than on the detection of direct signs of aortic injury. In most trauma centers, the next diagnostic step included the performance of aortography to confirm or exclude the aortic origin of the mediastinal bleeding; if hematoma was not present, aortography was not carried out. As pointed out by Raptopoulos in a commentary35, the characteristics of mediastinal hematoma on CT scans have not been described in detail in the majority of reports. Brooks et al.36 describe the CT appearance of a mediastinal hematoma as the presence of material of soft tissue attenuation or slightly higher in the mediastinum. In interpreting the mediastinum for the detection of hemorrhage, false positive findings may occur as a result of thymic tissue, periaortic atelectasis of the left lower lobe, volume averaging of the pulmonary artery with mediastinal fat, and left medial pleural effusion. The hematoma secondary to aortic rupture is mostly periaor-tic and may extend along the descending aorta. In blunt chest trauma patients, a mediastinal hemorrhage may be present for other reasons such as bleeding from small mediastinal vessels (arteries or veins) often in association with fractures of the thoracic cage. Furthermore, if the adventitia is intact, aortic rupture may occur without hemorrhage (Figure 15.2).
For these reasons, the interpretation of a positive CT scan based only on the presence of mediastinal hematoma results in a large number of negative aortograms and a resulting low specificity. To increase the specificity of CT, direct signs of aortic rupture must be considered. These signs include aortic pseudoaneurysm, an abrupt change in the aortic contour, intimal tear, intramural hematoma, extravasation of contrast material from the aorta (Figure 15.3), diminished caliber of the descending aorta (pseudocoarctation), and double aortic lumen37. Problems in the evaluation of these direct findings may arise from artefacts due to respiratory and voluntary movement of the traumatized patients, from cardiovascular motion, and from streak artefacts caused
by nasogastric tubes, external leads, and other devices; presence of effusion in the upper pericardial recess can mimic a double lumen.
In a large study including 104 patients who underwent both CT and aortography, Miller et al.38 reported a CT sensitivity and a specificity of 55% and 65%, respectively, with five missed major thoracic arterial injuries. Two of these false negative CT scans were due to aortic rupture and the other three were found to be branch injuries.
With conventional CT, the detection of subtle aortic injuries still represents a problem since a small intimal tear that extends on the axial plane may be obscured by volume averaging with the normal aortic lumen. The advent of helical CT overcomes most of these limitations, and it is particularly useful in critically injured patients with suspected associated neurological, visceral, or retroperitoneal lesions, some of which may be more critical than an aortic injury. Helical scanning has the great advantage of providing a better direct evaluation of the aorta with an acquisition time of about 25-30 seconds for the evaluation of the thoracic aorta. Gavant et al.39 used helical CT exclusively to screen 1,518 patients with nontrivial blunt chest trauma, 127 of whom presented abnormal findings at CT and underwent aortography. Helical CT was found to be more sensitive than aortography (100% vs. 94.4%) but less specific (81.7% vs. 96.3%, respectively) in detecting aortic lesions. False positives are reported in cases of prominent mediastinal vessels adjacent to the aorta (such as a right bronchial artery) or in cases of volume averaging from the left brachiocephalic or left superior intercostal veins. The authors concluded that negative findings on helical CT of the aorta, even in the presence of mediastinal hematoma, are sufficient to exclude aortic rupture. In a subsequent study, Gavant et al.40 evaluated a larger number of patients with the addition of two-dimensional and three-dimensional reconstructions including shaded-surface display and maximum intensity projection volume-rendering techniques. The retrospective reconstruction of additional axial images with a 50% overlap proved to be critical for detecting subtle aortic injury. Even if two-dimensional and three-dimensional reconstructions do not produce improvement in terms of diagnosis, they are useful supplements to the axial helical CT examination allowing depiction of important anatomical details such as relationships between the aortic lesion and the major branch vessels.
Since it provides high-quality images with a drastic reduction of acquisition time and motion artefacts, helical CT represents a method of great diagnostic value, potentially the method of first choice in the acute phase, in the evaluation of blunt chest trauma patients with suspected aortic injury, particularly in polytraumatized patients with other associated lesions41.
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