Evaluation of virtual monochromatic imaging using portal-venous phase dual-energy CT for detection of pulmonary embolism.

Background: As CT pulmonary angiography (CTPA) is not performed in routine contrast-enhanced CT, it is important to improve the accuracy of embolism detection from the portal-venous phase images that are generally obtained. Purpose: To assess the diagnostic performance of virtual monochromatic images (VMIs) obtained from portal phase dual-energy CT (PP-DECT) by qualitative and quantitative evaluation of contrast-to-noise ratio (CNR) and signal-to-noise ratio (SNR) compared with those of CTPA. Material and Methods: Included were 45 oncology patients who had been diagnosed with pulmonary embolism based on CTPA. Two radiologists evaluated conventional PP-DECT images and VMIs created from PP-DECT at 40, 55, 70, and 85 keV with the window width and level fixed at 400/70. The energy with the highest diagnostic accuracy was determined, and the image was re-evaluated under conditions where the window could be freely set. Quantitative evaluation was based on CNR and SNR. Results: Under the condition of fixed window width and level, the energy with the highest accuracy was 55 keV for both radiologists. When the window was set freely in the 55 keV image, accuracy rates were 96.0% and 96.3%; and were 98.5% and 98.1% when limited to the main pulmonary artery or lobar level. CNR and SNR were highest at 40 keV, and were similar to those of CTPA. Conclusion: Compared to original PP-DECT images, 55 keV images created from PP-DECT appear more useful for detecting pulmonary embolism.