The radiographic quantitation of aortic valve calcification: implications for assessing bioprosthetic valve calcification in vitro

Calcification of natural aortic and bioprosthetic heart valves is a poorly understood phenomenon that results in valvular obstruction and tissue failu re. We describe a non-destructive quantitative computed microtomographic (Q CT) technique for determining both calcium content and local calcium distri bution within explanted valves. As a reference standard, a dual-energy x-ra y absorptiometry (DEXA) system with an accuracy demonstrated to be within 1 % of the true calcium mass of test material was used to obtain the total ca lcium content of 24 human aortic valve cusps recovered at autopsy from pati ents aged 51-80 years. These cusps were then scanned using our unique volum e QCT scanner, with multiple x-ray projections acquired by rotating the exp lanted tissue through a single axis of rotation. A three-dimensional cross- sectional map was reconstructed for each cusp. Voxel size was 0.003 mm(3) a nd a calibration phantom was used to calculate calcium content. The minimum detection limit for calcium mass was 1 mg within the whole cusp. The DEXA and QCT scans were compared with respect to total calcium content, which ra nged from 0 to 15 mg. An excellent correlation between the two independent techniques was demonstrated with an r(2) value of 0.94 (p < 0.001). Non-des tructive microtomographic CT scanning provided excellent volumetric density measurements, with quantitative 3D images permitting an assessment of any individual area of the cusp for calcium content and spatial distribution. T his new approach to valve tissue analysis allows for subsequent histologic assessment.