Abstract
Accurate automated segmentation of Lumbar Spine Structures (LSS) in Magnetic Resonance Imaging (MRI) is important for effective diagnosis and treatment planning. However, most current methods encounter a trade-off between segmentation precision and computational efficiency. In particular, several methods fall short in modelling the intricate multi-scale anatomical details and subtle boundary transitions typical of vertebral bodies and intervertebral discs, thereby limiting their clinical utility. This study introduces Multiscale Residual and Adaptive Spatial-Channel Feature Pyramid Network (MRAS + FPN), a novel Deep Learning (DL) designed to enhance Lumbar Spine Segmentation. The model integrates Multi-Scale Attention (MS) with Residual Learning (RL) to balance accuracy and efficiency. Two principal innovations are embedded in the design: (a) Multiscale Residual Attention Blocks (MRAB), which incorporate Directionally Dilated Convolution Modules (DDCM) coupled with spatial-channel attention to address anisotropic anatomical structures, and (b) Adaptive Spatial-Channel Attention Blocks (ASCAB), which refine Feature Selection (FS) across pyramid levels dynamically. Also, Cross-level Feature Gating (CFG) is employed to regulate information flow between the encoder and decoder. The model was trained and validated on the MRSpineSeg dataset, which comprises 215 T2-weighted lumbar MRI volumes annotated by experts, covering vertebrae T9-S1 and their corresponding intervertebral discs. A composite loss function was used, combining Dice, Binary Cross-Entropy, and a boundary-sensitive term. MRAS-FPN proved to have improved segmentation performance compared with established models, including 2D U-Net, 3D U-Net, and DeepLab v3+. It achieved a Dice Similarity Coefficient (DSC) of 87.8 ± 1.3% and a mean Intersection over Union (mIoU) of 78.2 ± 2.1%, outperforming the best baseline (3D U-Net: 83.4% DSC, 73.4% mIoU) by 4.4% and 4.8%. Evaluation of boundary accuracy showed marked improvements, with a 95th percentile Hausdorff Distance (HD95) of 3.18 ± 0.8 mm and an Average Symmetric Surface Distance (ASSD) of 1.32 ± 0.5 mm-representing gains of 19.9% and 25.0% compared to 3D U-Net. Structure-wise analysis indicated stable segmentation quality across all vertebral levels (T9-S1: 84.4% mean DSC) and intervertebral discs (85.6% mean DSC), with notable gains in regions of high anatomical complexity. Ablation experiments further confirmed the utility of each component, with MRAB generating a 3.9% improvement in DSC and ASCAB contributing an additional 2.1% over the base FPN. MRAS-FPN achieves a well-calibrated balance between segmentation fidelity and resource efficiency, generating SOTA results while requiring manageable computational resources (22.1 hours of training time and 15.8 GB of memory). Its MSA successfully captures intricate anatomical features and boundary definitions critical for clinical use. The model's strong performance across a standard range of spinal structures and its resilience to overfitting underscore its applicability in real-world scenarios, including surgical planning, diagnostic support, and automated testing of the Lumbar Spine (LS).