Contrast inhomogeneity in CT angiography of the abdominal aortic aneurysm

Abstract

Background: If undetected, infrarenal Abdominal Aortic Aneurysm (AAA) growth can lead to rupture, a high-mortality complication. Some AAA patients exhibit inhomogeneous luminal contrast attenuation at first-pass CT angiography (CTA). This study assesses the association between this observation and aneurysm growth.

Methods: Sixty-seven consecutive pre-repair AAA CTAs were included in this retrospective study. The "Gravitational Gradient" (GG), defined as the ratio of the mean attenuation in a region-of-interest placed posteriorly to that in a region-of-interest placed anteriorly within the lumen of the aortic aneurysm on a single axial slice, and the maximum aneurysm diameter were measured from each CT data set. "AAA Contrast Inhomogeneity" was defined as the absolute value of the difference between the GG and 1.0. Univariate and multivariate logistic regression was used to assess the association of aneurysm growth >0.4 and >1.0 cm/year to AAA Contrast Inhomogeneity, aneurysm diameter, patient characteristics and cardiovascular co-morbidities.

Results: AAA Contrast Inhomogeneity was not correlated to aneurysm diameter (p = 0.325). In multivariable analysis that included initial aneurysm diameter and AAA Contrast Inhomogeneity, both factors were significantly associated with rapid aneurysm growth (initial diameter: p = 0.029 and 0.011, and, AAA Contrast Inhomogeneity: p = 0.045 and 0.048 for growth >0.4 cm/year and >1 cm/year respectively).

Conclusions: AAA Contrast Inhomogeneity is a common observation in first-pass CTA. It is associated with rapid aneurysm growth, independent of aneurysm diameter.

Keywords: Abdominal aortic aneurysm; Aneurysm growth; CT angiography; Contrast inhomogeneity; Risk stratification.

Figure 1

(A) Axial CTA images of a patient with aa AAA of 5.4 cm diameter and homogeneous luminal contrast attenuation (AAA Contrast Inhomogeneity = 0.03). (B) CTA of a patient with a similar-sized AAA (5.6 cm diameter) with inhomogeneous contrast attenuation and correspondingly higher AAA Contrast Inhomogeneity (0.73).

Figure 2

Distribution of the Gravitational Gradient in the study cohort.

Figure 3

Serial CTA images in one patient demonstrate luminal contrast attenuation that is (A) homogeneous at initial CTA but (B) becomes visibly inhomogeneous at 12 months (AAA Contrast Inhomogeneity=0.6) and (C) further increases at 39 months (AAA Contrast Inhomogeneity=0.86). Anterior thrombus formation and aneurysm growth at the region corresponding to the observed inhomogeneity in attenuation is noted over time.