Quantifying 18F-sodium fluoride uptake in abdominal aortic aneurysms

Background: Aortic microcalcification activity is a recently described method of measuring aortic 18F-sodium fluoride uptake in the thoracic aorta on positron emission tomography . In this study, we aimed to compare and to modify this method for use within the infrarenal aorta of patients with abdominal aortic aneurysms.Methods: Twenty-five patients with abdominal aortic aneurysms underwent an 18F-sodium fluoride positron emission tomography and computed tomography scan. Maximum and mean tissue to background ratios (TBR) and abdominal aortic microcalcification activity were determined following application of a thresholding and variable radius method to correct for vertebral 18F-sodium fluoride signal spill-over and the non-linear changes in aortic diameter respectively. Agreement between the methods, and repeatability of these approaches were assessed.Results: The aortic microcalcification activity method was three-fold quicker to perform than the TBR method (14 versus 40 min, p<0.001). There was moderate to good agreement between TBR and aortic microcalcification activity measurements for maximum (interclass correlation co-efficient, 0.67) and mean (interclass correlation co-efficient, 0.88) values. These correlations 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 optimized method had good intra-observer (mean 1.57 ± 0.42, bias 0.08, co-efficient 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.Conclusions: Aortic microcalcification activity is a quick and simple method which demonstrates good intra-observer and inter-observer repeatabilities and provides measures of 18F-sodium fluoride uptake that are comparable to established methods.