Diagnostic Performance of a Prototype Dual-Energy Chest Imaging System: ROC Analysis

Abstract

To assess the performance of an experimental prototype dual-energy (DE) chest imaging system in comparison to digital radiography (DR) in detection and characterization of lung lesions using receiver-operating characteristic (ROC) tests. A cohort of 129 patients (80 M, 49 F; mean age, 64.8 years) was drawn from a trial of patients referred for percutaneous biopsy of a lung lesion. DR and DE images were acquired of each patient (posteroanterior view) before biopsy using a prototype system developed in our laboratory. The system incorporated a flat-panel detector and previously reported imaging techniques optimized such that the total dose for the DE image was equivalent to that of a DR acquisition. Each DE image was decomposed to three components (soft-tissue, bone, and composite "equivalent radiograph") by log subtraction with optimized noise reduction techniques. ROC tests were performed to evaluate the diagnostic performance of DR imaging in comparison to DE for nodule detection, with 258 left/right "half-chest" images derived from the 129 cases to give a roughly equal number of disease and normal cases. Five chest radiologists scored 258 half-chest DE and 258 half-chest DR (516 in total) images on a 5-point scale, and results (including ROC and area under the curve [AUC]) were analyzed using the ROCkit toolkit. Statistical significance in the observed differences was evaluated in terms of P values determined by a z test. Performance was analyzed for all cases pooled (258 DE vs. 258 DR images) and by retrospective stratification of the data according to nodule size, density, gender, lung region, and chest thickness. For results pooled over the entire cohort, there was no significant difference in ROC performance between DE and DR (AUC(DE) = 0.795 AUC(DR) = 0.789; P = .696). This finding is believed to be due to a large portion of lesions that were fairly conspicuous in either modality. In retrospective analysis of subgroups, a significant advantage was measured for DE imaging of small nodules (<1 cm diameter; AUC(DE) = 0.778; AUC(DR) = 0.706; P = .056), for nodules located in the right upper lobe (AUC(DE) = 0.836; AUC(DR) = 0.779; P = .003), and nodules located in right lower lobe (AUC(DE) = 0.804; AUC(DR) = 0.752; P = .054). DE imaging provided a clinically significant differential diagnosis in approximately one third of patients (49/158) (ie, disease cases in which the lesion was correctly identified in DE [(ROC rating > or =3], but missed in DR [ROC rating < or =2]). DE imaging also appeared to provide more definitive diagnosis (ie, a greater proportion of ROC ratings = 5 and 1 for identification of disease and normal cases, respectively), which presumably translates to increased confidence and a steeper ROC curve (even if the AUC are the same). DE imaging at dose equivalent to DR exhibited similar overall ROC performance to DR, although the radiologists noted qualitatively improved visualization (eg, improved characterization of lesion margins, visibility of calcifications and rib fractures). DE imaging demonstrated significant improvement in diagnostic performance for specific subgroups, including subcentimeter lung lesions and lesions in the right upper lobe, each of which is a potentially important factor in detecting early-stage malignancy.