Optimal monochromatic energy levels in spectral CT pulmonary angiography for the evaluation of pulmonary embolism

Abstract

Background: The aim of this study was to determine the optimal monochromatic spectral CT pulmonary angiography (sCTPA) levels to obtain the highest image quality and diagnostic confidence for pulmonary embolism detection.

Methods: The Institutional Review Board of the Shanghai Jiao Tong University School of Medicine approved this study, and written informed consent was obtained from all participating patients. Seventy-two patients with pulmonary embolism were scanned with spectral CT mode in the arterial phase. One hundred and one sets of virtual monochromatic spectral (VMS) images were generated ranging from 40 keV to 140 keV. Image noise, clot diameter and clot to artery contrast-to-noise ratio (CNR) from seven sets of VMS images at selected monochromatic levels in sCTPA were measured and compared. Subjective image quality and diagnostic confidence for these images were also assessed and compared. Data were analyzed by paired t test and Wilcoxon rank sum test.

Results: The lowest noise and the highest image quality score for the VMS images were obtained at 65 keV. The VMS images at 65 keV also had the second highest CNR value behind that of 50 keV VMS images. There was no difference in the mean noise and CNR between the 65 keV and 70 keV VMS images. The apparent clot diameter correlated with the keV levels.

Conclusions: The optimal energy level for detecting pulmonary embolism using dual-energy spectral CT pulmonary angiography was 65-70 keV. Virtual monochromatic spectral images at approximately 65-70 keV yielded the lowest image noise, high CNR and highest diagnostic confidence for the detection of pulmonary embolism.

Figure 1. Enhanced axial image at pulmonary arterial phase from a patient with pulmonary embolism.

The marked regions of interest are the endoluminal clot (diameter >5 mm) within the adjacent pulmonary artery.

Figure 2. Comparison of subjective imaging scores.

A, 50 keV; B, 55 keV; C, 60 keV; D, 65 keV; E, 70 keV; F, 75 keV; G, 80 keV. The 65 keV monochromatic image shows less image noise than 50, 55, 60 keV and high conspicuity when compared with the 75 and 80 KeV images. However, there was no significant difference in the subjective scores of the CTPA between the 70 and 65 keV images. H, oblique coronal CTPA image shows the endoluminal clot within the left inferior pulmonary artery.