Objectives: Early and accurate diagnosis of pulmonary embolus (PE) is crucial and the imaging modality of choice for its diagnosis is CT pulmonary angiography (CTPA). However, there are incidents when a definite answer cannot be provided due to limited vascular opacification. Since dual energy CT (DECT) has been found to improve vascular opacifications, using virtual low monoenergetic images, it can potentially salvage suboptimal PE exams. The aim of our study was to evaluate quantitively whether low monoenergetic images at 50 keV demonstrate improved quality than conventional images for imaging of the pulmonary vessels. We also wanted to evaluate the individual preferences of radiologists with regards to the use of low monoenergetic in CTPA.
Methods: A retrospective study of 150 CTPA exams conducted on a single source DECT. HU, CNR and SNR values were measured in both central and peripheral vascular locations. Comparison of these parameters on the conventional and low monoenergetic images was performed. In addition, 80 conventional and respective low monoenergetic images were randomized, anonymized and presented two separate times to three chest radiologists; for a 5-point scale quality grading. Lastly, PE visibility assessment was conducted on 41 pairs of PE slices presented on conventional and monoenergetic images side-by-side by 9 radiologists: chest specialists, non- chest specialists and residents. Paired samples t-tests, a-parametric Wilcoxon test, McNemar test and kappa statistics were used for statistical analysis.
Results: Attenuation of the pulmonary arteries were significantly higher in the monoenergetic images compared to the conventional images with a 2.09 - 2.26 ratio range (P<0.05). The SNR and CNR showed significant increase in the low monoenergetic images as well (p<0.05). All 10 (100%) suboptimal exams on the conventional images were salvaged on the monoenergetic images. Image quality grading showed great inter and intra-reader variability between the 3 chest radiologists with average score of 4.26 and 3.83, 4.03 and 4.11, 4.57 and 4.76 for the conventional and monoenergetic images, respectively, in the first reading and 4.07 and 3.6, 4.11 and 4.66, 3.98 and 3.52 for the second reading (kappa=0.34, 0.301, -0.01, respectively). There was clear preference of the monoenergetic images among the non-chest radiologists and residents while the chest radiologists, showed only a slight trend that was not significant.
Conclusions: There is significant and consistent increase in CNR and SNR in the 50KeV monoenergetic images compared to the conventional ones. Non chest radiologists clearly preferred these images while the more experienced chest radiologists show only a trend towards using low monenergentic images.
