Imaging of abdominal aortic aneurysm disease activity and implications for endovascular aneurysm repair

Abstract

BACKGROUND: Abdominal aortic aneurysm treatment pathways are initiated when an aortic size threshold of 55 mm is reached as this signifies a high annual risk of rupture due to presumed active aneurysm disease. Endovascular aneurysm repair (EVAR) is one treatment modality which reduces the procedural morbidity and mortality associated with open surgical repair. However, EVAR patients can develop complications such as leaks behind their stent graft (endoleak) which reduces the long-term benefit of EVAR. Here, we explore the use of two potential imaging biomarkers, periaortic adipose tissue (on computed tomography) and sodium [18F]fluoride positron emission tomography, in different abdominal aortic aneurysm disease states to better understand the disease process and to assess its implications in EVAR.

METHODS: First, we assessed periaortic adipose tissue attenuation in 70 patients with untreated abdominal aortic aneurysm disease (asymptomatic, symptomatic and rupture patients) and 18 control subjects (Chapter 3). Then, in 25 patients with an abdominal aortic aneurysm, we developed a method of quantifying sodium [18F]fluoride uptake on positron emission tomography, which we termed aortic microcalcification activity (AMA) (Chapter 4). We then considered sodium [18F]fluoride uptake in 10 patients before and after their aneurysm was treated with EVAR (Chapter 5). Following this, we assessed sodium [18F]fluoride uptake in 37 patients whose aneurysm had been treated with EVAR, in 15 of whom this had been complicated by endoleak formation (Chapter 6).

RESULTS: There were no differences in the periaortic adipose tissue attenuation in aneurysmal and non-aneurysmal segments of the aorta in asymptomatic patients (-81.4±7 versus -75.4±8 Hounsfield units, HU) and comparable segments in control subjects (-83.3±9 versus -78.8±6 HU, p>0.05 for all comparisons). However, symptomatic patients demonstrated higher periaortic adipose tissue attenuation in both aneurysmal (-57.9±7 HU, p<0.0001) and non-aneurysmal segments (-58.2±8 HU, p<0.0001).

There was moderate-to-good agreement between mean tissue-to-background ratio and aortic microcalcification activity (AMA) measurements (intraclass correlation co-efficient, 0.88). These, sequentially improved with the application of thresholding (intraclass correlation coefficient 0.93, 95% confidence interval 0.89–0.95) and variable diameter (intraclass correlation coefficient 0.97, 95% confidence interval 0.94–0.99) techniques. The optimised method had good intra-observer (mean 1.57 ± 0.42, bias 0.08, coefficient of repeatability 0.36 and limits of agreement -0.43 to 0.43) and inter-observer (mean 1.57 ± 0.42, bias 0.08, co-efficient of repeatability 0.47 and limits of agreement -0.53 to 0.53) repeatability.

We found that following EVAR, sodium [18F]fluoride uptake was markedly reduced in the suprarenal (0.62 reduction, p=0.03), neck (0.72 reduction, p=0.02) and body of the aneurysm (0.69 reduction, p=0.02) while it remained unchanged in the thoracic aorta (0.11 reduction, p=0.41).

When compared to those without an endoleak, patients with an endoleak had higher AMA in the thoracic (1.22±0.2 versus 1.07±0.2, p<0.01), suprarenal (1.58±0.3 versus 1.36±0.2, p<0.05) and neck (1.40±0.3 versus 1.14±0.3, p<0.05) regions of the aorta but not within the aneurysm body (0.94±0.3 versus 1.08±0.3, p>0.05).

CONCLUSION: Periaortic adipose tissue attenuation is not increased in stable abdominal aortic aneurysm disease. The generalised increase in patients with symptomatic disease likely reflects the systemic consequences of acute rupture. Aortic microcalcification activity (AMA) provides repeatable measures of sodium [18F]fluoride uptake that are comparable to established methods. EVAR is associated with a reduction in AMA within the stented aortic segment, whilst endoleaks after EVAR are associated with higher AMA in aortic regions outside the aneurysm. These findings suggest that whilst EVAR can modify aortic disease activity within the treated aneurysm, aortic degeneration appears to continue beyond the aneurysm. Aortic sodium [18F]fluoride uptake is a promising non-invasive measure of aneurysm disease activity that may inform treatment strategies and provide additional prognostic information.