Estimating the accuracy of dual energy chest radiography for coronary calcium detection with lateral or anteroposterior orientations.

PURPOSE: Coronary artery calcium (CAC) scoring with CT has been studied as a risk stratification tool for cardiovascular disease. However, concerns remain from the radiation dose, economic expense, and incidental findings associated with this exam. Dual energy chest X-ray (DE CXR) has been proposed as an alternative, but validation of this technique remains limited. The purpose of this work was twofold: first, to estimate the sensitivity and specificity of DE CXR using simulation of patient datasets in a CAC screening cohort; second, to assess if sensitivity and specificity could be improved using a lateral instead of an anteroposterior (AP) orientation.
METHODS: We started from a cohort of 73 CAC scoring CT exams after exclusions for metal wires, data truncation, or with age outside 40-75 years. The fraction of CT CAC scores in the validation set of 0, 1-99, 100-299, and 300+ were 36, 25, 14, and 26%, respectively. CT datasets were decomposed on a voxel-by-voxel basis into mixtures of water and calcium according to CT number. DE CXR images were simulated using polyenergetic forward projection with scatter estimated from Monte Carlo. We assumed a technique of 60 and 120 kVp for the dual energy acquisition. The tube current was scaled such that the estimated radiation dose from DE CXR was 10 times less than CAC scoring CT. Patient motion was not simulated. Two readers read the validation set in a blinded, randomized fashion, and estimated the amount of CAC in each DE CXR image using a semiquantitative 4-point scale. Although patients present on a spectrum of CAC severity, in the primary analysis, sensitivity and specificity were calculated by dichotomizing patients into two categories of CT CAC (Agatston) scores of either 0-99 or 100+.
RESULTS: From the lateral orientation, average sensitivity between two readers was 69% (range, 69-69%), specificity was 85% (range, 84-86%), and area under the curve (AUC) was 0.81 (range, 0.80-0.81). From the AP orientation, average sensitivity was 35% (range, 31-38%), average specificity was 70% (range, 66-73%), and AUC was 0.54 (range, 0.53-0.55). Reader DE CXR scores agreed within 1 point of the 4-point scale on 97% of ratings from the lateral orientation and 80% from the AP orientation. From the lateral orientation, AUC increased when considering higher CT CAC score thresholds as disease positive; for thresholds of 1+, 300+, and 1000+, average AUC was 0.72, 0.81, and 0.92, respectively. From the AP orientation, AUC was 0.57, 0.55, and 0.61, respectively.
CONCLUSIONS: DE CXR for CAC scoring may have higher diagnostic accuracy when acquired from the lateral orientation. The sensitivity and specificity of lateral DE CXR, when combined with its modest cost and radiation dose, suggest a possible role for this technique in screening coronary calcium in lower risk individuals. These estimates of diagnostic accuracy are derived from simulation of patient datasets and have not been corroborated with experimental or clinical images.