Evaluating The Diameter Of The Dissected Aorta

The dissected aorta has the tendency to enlarge, and aortic dilatation may be present upon initial presentation or develop on subsequent follow up examinations. Sueyoshi et al. evaluated the growth rate of classic type B aortic dissection44. This study included 62 patients with spontaneous aortic dissection and excluded cases with traumatic dissection and Marfan syndrome; 81% of patients in this study had a history of hypertension. Growth rate was 2.2 ± 6.9 mm per year for the aortic arch, 2.2 ± 10.1 mm per year for the descending thoracic aorta, 1.0 ± 5.8 mm per year for the suprarenal abdominal aorta, and 1.0 ± 2.2 mm per year for the infrarenal abdominal aorta. Mean growth rate of the iliac arteries was - 0.4 ± 4.7 mm per year.

Aneurysmal dilatation of the dissected aorta increases the risk of complications, particularly the risk of aortic rupture45,46. As outlined above, an expanding false lumen may cause compression of the true lumen with subsequent ischemic complications. Thus, the aortic diameter and its increase or decrease on subsequent follow-up examinations is an important risk factor considered in the decision whether to perform surgery47-50.

Although measurement of the aortic diameter on contrast-enhanced CT images seems to be a straightforward task, there is relatively high interobserver variability51-53. Furthermore, measurements on axial images may result in overestimation of the aortic diameter, particularly if the aorta is tortuous and what appears to be a measurement across the aortic lumen is really an oblique measurement. Multiplanar reformatted images allow for measurements perpendicular to the blood flow and improve interobserver agreement, as shown by Sprouse et al.52 Furthermore, consistency of repeated measurements will be further improved by software that automatically determines the centerline of the aorta using 3D data (Advanced Vessel Analysis™, GE Healthcare Technologies, Waukesha, Wisconsin, USA). However, although measurements on multiplanar reformatted images are more accurate, Dillavou et al. found a high correlation between the short axis diameters measured on axial CT slices and those measured perpendicular to the blood flow on multiplanar reformatted images and postulate that short axis measurements on axial slices are sufficient in most clinical situations54.

Independent of how the aortic diameter is measured, it is important to keep in mind the substantial interobserver variability and never compare with reported measurements without comparing the actual images when assessing aortic growth on follow-up examinations.

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