Intra-patient comparison of pulmonary nodule enhancement in subtraction CT and dual-energy CT

PURPOSE/OBJECTIVES: Subtraction CT (SCT) is a new software-based imaging modality that has the same diagnostic performance as dual-energy CT (DECT) in pulmonary embolism (PE) detection, but its performance for other applications is not investigated yet. In this study we aimed to determine the capability of SCT in depicting benign and malignant pulmonary nodule enhancement by comparing it to that of DECT. METHODS AND MATERIALS: We identified 29 patients with solid pulmonary nodules (>= 4 mm) in a cohort of 295 consecutive patients recruited for a prospective study on PE. Patients were worked up or followed for at least 20 months to obtain a standard of reference. In total, 96 nodules were investigated (72 malignant, 24 benign). Patients in this study had received a pre-contrast CT at 100 kV prior to a dual-energy CT angiography (CTA) at 100 and 140 kV (Siemens Definition Flash, Germany), with 60 ml of iodinated contrast (300 mg I/ml) followed by 40 ml saline flush injected at 5 ml/s. Scan acquisition was 5 s after reaching a relative threshold enhancement (60 HU) in the pulmonary trunk. For SCT, iodine maps were obtained by subtracting the pre contrast from the contrast-enhanced scan after non-rigid registration (Canon Medical Systems, Japan). Nodules were annotated and volumetrically segmented on the CTA on a lung screening workstation (CIRRUS Lung Screening, The Netherlands) and mean enhancement calculated. Student's t-test was used for statistical testing. RESULTS: Mean enhancement of the nodules was significantly higher for SCT than for DECT (34.6+-12.7 vs 25.7+-20.8 HU; p<0.001). Mean enhancement in malignant and benign nodules was 35.5+-11.7 HU and 31.9+-15.4 HU on SCT (p=0.24), and 26.6+-20.7 HU and 22.8+-21.4 HU on DECT (p=0.44), respectively. Dose-length-product was slightly lower for SCT than DECT (156 and 163 mGy*cm; p<0.001). CONCLUSION: Subtraction CT shows higher nodule enhancement compared to dual-energy CT. As expected, due to the use of early acquisition, CTA for PE cannot be used to characterize incidental nodules as benign or malignant. However, previous studies have shown the feasibility of nodule characterization with DECT with late acquisition. Therefore, given that it results in better enhancement depiction, it is expected that, SCT with optimized timing could be a promising development for improved nodule characterization at a comparable radiation dose to DECT, without the need for dedicated additional hardware.