Cross-Modal CXR-CTPA Knowledge Distillation Using Latent Diffusion Priors Towards CXR Pulmonary Embolism Diagnosis

Pulmonary Embolism (PE) is a life-threatening condition. Computed tomography pulmonary angiography (CTPA) is the gold standard for PE diagnosis, offering high-resolution soft tissue visualization and three-dimensional imaging. However, its high cost, increased radiation exposure, and limited accessibility restrict its widespread use. In this work, we aim to introduce faster diagnosis opportunities by using 2D chest X-ray (CXR) data. CXR provides only limited two-dimensional visualization and is not typically used for PE diagnosis due to its inability to capture soft tissue contrast effectively. Here, we develop a novel methodology that distills knowledge from a trained CTPA-based teacher classifier model embedding to a CXR-based student embedding, by feature alignment - leveraging paired CTPA and CXR features as supervision, which can be readily acquired. This enables us to train without requiring annotated data. Our approach utilizes a latent diffusion model to generate CTPA-based PE classifier embeddings from CXR embeddings. In addition, we show that incorporating cross-entropy loss together with the corresponding loss of the teacher-student embeddings increases performance, bringing it close to clinical-level performance. We show state-of-the-art AUC in a PE categorization task using only the initial CXR input. This approach broadens the diagnostic capabilities of CXRs by enabling their use in PE classification, thereby extending their applicability beyond traditional imaging roles. The code for this project is available: https://github.com/meshims/Cross-Modal_CXR-CTPA_Knowledge_Distillation.