Chest: Aortic Aneurysm  Dissection Evaluation

The role of CT in the evaluation of known or suspected aneurysm has become one of the major applications of CT angiography. The use of a non-invasive imaging technique that also allows specific measurements of an aneurysm and its relationships to normal vessel is critical in this era of endovascular stent design and placement . Development of single detector spiral CT and then dual slice spiral CT was very useful for thoracic aortic imaging. Mesurolle et al. found that "helical CT of the thoracic aorta provides a minimally invasive and rapid assessment of aortic lumen and branch vessels, the aortic wall, and the surrounding tissues." Similarly, Dyer et al. found that CT was equivalent to angiography in cases of suspected aortic injury and could be used to exclude aortic injury. She found that:

" Of the 802 patients who were examined with CT, 382 underwent follow-up aortography. In this group, there were 10 true-positive and no false-negative CT scans. CT had 100% sensitivity and a 100% negative predictive value for the detection of traumatic aortic injury.

Our experience has been that MDCT is ideal for the evaluation of suspected thoracic aneurysm and/or dissection. We routinely will use 1.25 mm slice thickness (from 4 x 1 mm detectors) in order to get the best detail possible when looking at the great vessels arising off the aortic arch. In cases where the patient is unable to cooperate or in cases where we are doing both the thoracic and abdominal aorta we will use 3mm slice thickness (from 4 x 2.5 mm detectors). In both cases we typically reconstruct the data a 1 mm intervals to get the best data sets possible for 3D imaging.

Several other technical pearls are that in the evaluation of the thoracic aorta we will scan in a caudalcranial direction beginning from the bottom of the diaphragm and scanning through the lung apices. We have found that this result in less artifact from the contrast bolus in the axillary zone or off the superior vena cava. Some have also suggested a lower contrast concentration although I still prefer the higher concentration (Omnipaque-350 rather than Omnipaque-240) but with a decreased volume of 100 cc (120 cc if the abdominal aorta is to be evaluated as well). In select cases of borderline renal function, I have been successful using 70-80 cc of contrast. In fact, with faster scanners there is little doubt that contrast volumes in the 70-90 cc range may be reasonable for nearly any patient under 180 pounds.

The presence of an aortic aneurysm and/or dissection as well as its extent must be provided to the referring physician. We have found that while classic axial CT is adequate for simple diameter measurements of an aneurysm as well to monitor any change over time, for other information it may not be ideal. The use of multiplanar reconstruction in coronal, sagittal or oblique mode may be helpful but the ideal presentation mode is as a 3D-CT angiogram. Using a 3D display combines the advantages of classic angiography with the advantages of a 3D real-time rendered CT dataset. The ability to pick any plane or perspective is ideal for the evaluation of the orientation and extent of a dissection especially in regards to the great vessels. Using virtual Endoscopic techniques is also helpful in these cases by defining the orientation of the flap to the left subclavian artery. Unique display perspectives are also of value in patients with congenital variations like an aberrant right subclavian especially when it is aneurysmally dilated.

One of the collateral advantages of CT is the ability to detect serendipitous findings which are not primarily vascular in nature. We have had several patients evaluated for proposed aneurysm repair where an occult lung cancer was detected. Similarly findings consistent with pulmonary embolism or interstitial lung fibrosis have also been detected.

Finally, a practical advantage of CT angiography that is especially important in this era of cost-containment in medicine is that a CT angiogram is far lower in cost than a conventional catheter-based angiogram. Although no cost analysis has been done for the thoracic aorta it is safe to assume that the savings will be similar to the results shown by Rubin et al. in evaluating CT vs. classic angiography for abdominal aortic aneurysms where he found that "With 95% confidence, intraarterial DSA cost 3.2-3.7 times more than CT angiography for the assessment of AAA."

Purpose: To determine whether chest CT can be used to exclude aortic injury?
Conclusion: Chest CT can be used to exclude aortic injury. Can Chest CT Be Used to Exclude Aortic Injury ?
Dyer DS et al.
Radiology 1999; 213:195-202

"Of the 802 patients who were examined with CT, 382 underwent follow-up aortography. In this group, there were 10 true-positive and no false-negative CT scans. CT had 100% sensitivity and a 100% negative predictive value for the detection of traumatic aortic injury." Can Chest CT Be Used to Exclude Aortic Injury ?
Dyer DS et al.
Radiology 1999; 213:195-202


"Assuming equal diagnostic utility and procedure related morbidity, institutions may have substantial cost savings whenever CT angiography can replace intraarterial DSA for imaging AAA's." Cost Identification of Abdominal Aortic Aneurysm Imaging by Using Time and Motion Analyses
Rubin GD et al.
Radiology 2000; 215:63-70


"The mean total direct cost of intraarterial DSA was $1,052 +/- 71, and that of CT angiography was $300+/- 30, which are significantly different. With 95% confidence, intraarterial DSA cost 3.2-3.7 times more than CT angiography for the assessment of AAA." Cost Identification of Abdominal Aortic Aneurysm Imaging by Using Time and Motion Analyses
Rubin GD et al.
Radiology 2000; 215:63-70