AI-Driven Appendicular Skeletal Muscle Mass Index (ASMI) Prediction and Low Muscle Mass Detection from Routine Hip X-rays: A Novel Opportunistic Screening Tool

Purpose Sarcopenia diagnosis requires identifying low muscle mass (LMM), typically via dual-energy X-ray absorptiometry (DXA). However, DXA's limited accessibility restricts large-scale screening. This retrospective study aimed to develop and validate a deep learning model to predict DXA-derived ASMI from routine hip radiographs for opportunistic sarcopenia screening. Methods We included 1,267 patients with both hip radiography and DXA scans, split into development (n = 1,140) and external validation (n = 127) sets. A multimodal model integrating radiographic images (ResNet-34 backbone) and clinical variables (age, gender, height, weight, BMI) was trained to predict continuous ASMI and classify LMM per Asian Working Group for Sarcopenia (AWGS) 2019 criteria. Results On external validation, the model achieved strong performance with Pearson r = 0.806, R²=0.650, MAE = 0.414 kg/m², and AUC = 0.874 for LMM classification. Applying AWGS diagnostic thresholds yielded sensitivity of 70.5% and specificity of 83.3%, with consistent performance across gender and age subgroups. Gradient-weighted Class Activation Mapping confirmed focus on clinically relevant gluteal and proximal thigh muscles. Conclusions This deep learning approach enables automated LMM identification from routine hip radiographs, offering a cost-effective, accessible tool for opportunistic sarcopenia screening and early intervention in at-risk populations.