Computer-aided detection and visualization of pulmonary embolism using a novel, compact, and discriminative image representation.

Diagnosing pulmonary embolism (PE) and excluding disorders that may clinically and radiologically simulate PE poses a challenging task for both human and machine perception. In this paper, we propose a novel vessel-oriented image representation (VOIR) that can improve the machine perception of PE through a consistent, compact, and discriminative image representation, and can also improve radiologists' diagnostic capabilities for PE assessment by serving as the backbone of an effective PE visualization system. Specifically, our image representation can be used to train more effective convolutional neural networks for distinguishing PE from PE mimics, and also allows radiologists to inspect the vessel lumen from multiple perspectives, so that they can report filling defects (PE), if any, with confidence. Our image representation offers four advantages: (1) Efficiency and compactness-concisely summarizing the 3D contextual information around an embolus in only three image channels, (2) consistency-automatically aligning the embolus in the 3-channel images according to the orientation of the affected vessel, (3) expandability-naturally supporting data augmentation for training CNNs, and (4) multi-view visualization-maximally revealing filling defects. To evaluate the effectiveness of VOIR for PE diagnosis, we use 121 CTPA datasets with a total of 326 emboli. We first compare VOIR with two other compact alternatives using six CNN architectures of varying depths and under varying amounts of labeled training data. Our experiments demonstrate that VOIR enables faster training of a higher-performing model compared to the other compact representations, even in the absence of deep architectures and large labeled training sets. Our experiments comparing VOIR with the 3D image representation further demonstrate that the 2D CNN trained with VOIR achieves a significant performance gain over the 3D CNNs. Our robustness analyses also show that the suggested PE CAD is robust to the choice of CT scanner machines and the physical size of crops used for training. Finally, our PE CAD is ranked second at the PE challenge in the category of 0 mm localization error.