Abdominal Aortic Aneurysms with High Thrombus Signal Intensity on Magnetic Resonance Imaging are Associated with High Growth Rate

OBJECTIVES: A layer of intraluminal thrombus is commonly observed in abdominal aortic aneurysms (AAAs). The purpose of this study was to investigate whether AAAs with high thrombus signal intensity (SI) at T1-weighted (T1w) magnetic resonance imaging (MRI) exhibit a faster aneurysm growth rate. METHODS: This was a prospective follow-up study. Patients with a small AAA underwent MRI examinations at 6 month intervals. Aneurysm thrombus and psoas muscle SI at the point of maximal diameter on T1w images were measured and expressed as a ratio (thrombus SI/muscle SI). Based on these measurements, patients were categorized into three groups: AAA with relative thrombus SI above (group A) and below (group B) the mean relative thrombus SI of 1.20. Patients with AAA without thrombus constituted group C. Eight patients were scanned twice within 2 weeks to investigate scan-rescan reproducibility. Aneurysm growth rates were expressed as the change in maximal cross sectional area (cm2). RESULTS: A total of 35 patients (m/f: 26/9; age 72 +/- 7 years; AAA maximal diameter 4.9 +/- 0.5 cm) were included. Mean aneurysm growth rate for patients in group A (n = 11, 1.87 cm2/0.5 year) was two-fold higher than group B (n = 17, 0.78 cm2/0.5 year, p = .005) and eight-fold higher than group C (n = 7, 0.23 cm2/0.5 years, p = .004) at 6 months' follow-up. At 12 months' follow-up, the mean aneurysm growth rate remained significantly higher in group A (n = 7, 3.03 cm2/year) than groups B (n = 10, 1.63 cm2/year, p = .03) and C (n = 7, 0.73 cm2/year, p = .004). The reproducibility for thrombus SI measurements was found to be high with a coefficient of variation of 6.2%. Aneurysm maximal cross-sectional area at baseline was not significantly different for the three groups. CONCLUSIONS: Abdominal aortic aneurysms with high thrombus SI on T1w MR images are associated with higher aneurysm growth rates.