Defining an Upper Hounsfield Unit Threshold for Acute Pulmonary Embolism in Computed Tomography Pulmonary Angiography

PURPOSE: To measure the computed tomography (CT) attenuation of acute pulmonary embolism (PE) and establish an upper Hounsfield unit (HU) threshold above which it is unlikely to occur. MATERIALS AND METHODS: This retrospective study analyzed CT pulmonary angiography (CTPA) exams in patients with acute PE (2018-2023) who had a prior negative CTPA in the preceding 2 weeks. Clot and artifact attenuation were measured on the positive and prior negative CTPA respectively. Analysis of variance with post hoc tests and linear regression analysis were performed to determine the effects of technical and other parameters on clot and artifact attenuation. RESULTS: A total of 60 patients were included, each with 1 record or a pair of CTPA exams, yielding 105 clots and 41 artifacts. Mean attenuation for acute PE was 50±40 HU, and for artifact was 110±146 HU. A threshold of 125 HU achieved 97% (102/105) sensitivity for acute PE below it and the same specificity for artifact above it. Specificity for acute PE below this threshold was 37% (15/41). Subsegmental clots had a lower attenuation than all other clots as a group. Kilovoltage peak (kVp) and proximal vessel attenuation were poor predictors of clot attenuation . CONCLUSIONS: Most acute PE demonstrated a CT attenuation ≤125 HU on standard CTPA, an upper threshold higher than previously reported, while artifacts demonstrated both lower and higher attenuation. An attenuation >125 HU on standard CTPA, favoring artifact rather than acute PE, may be a useful adjunct to morphologic differentiation between these entities.