Diffusion-weighted imaging in acute pulmonary embolism: a feasibility study

Background Magnetic resonance imaging (MRI) can be an alternative method to computed tomography angiography (CTA) for pulmonary embolism. Purpose To evaluate the feasibility of diffusion-weighted imaging (DWI) detecting acute pulmonary embolism (PE) in free-breathing humans. Material and Methods Twenty patients with PE verified by CTA and 20 controls were investigated with MRI (1.5 Aera, Siemens Healthcare). All sequences were performed in the transversal plane using free-breathing without gating. The protocol consisted of a two-dimensional steady-state free precession (SSFP) and a single-shot DWI echo-planar imaging sequence with a voxel resolution of 2 × 2 × 5 mm. Three b values were used: 50, 400, and 800 s/mm2. Images were analyzed in two orders: an open source analysis (OSA); and a blinded only DWI analysis (BDA) simulating clinical work. Results OSA of corresponding images showed 370 findings on CTA (i.e. one elongated emboli could be represented in multiple images). SSFP identified 237 of those (64%). DWI with b values of 50, 400, and 800 identified 327 (88%), 245 (66%), and 138 (37%), respectively. In BDA we found 160 true emboli (according to CTA) on b50, 78 on b400, and 54 on b800. Fifty-two of these findings at the subsegmental level could be correlated to PE on CTA but were not visible on SSFP. Conclusions DWI has a high sensitivity for detecting PE but suffers from poor specificity. It could potentially be used as an eye catcher, i.e. where to look for PE in other MRI sequences.