Tracing Like a Clinician: Anatomy-Guided Spatial Priors for Cephalometric Landmark Detection

Sidhartha Mohapatra; Pallavi Mohanty

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arXiv · 2605.03358

Tracing Like a Clinician: Anatomy-Guided Spatial Priors for Cephalometric Landmark Detection

Sidhartha Mohapatra·Pallavi Mohanty

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Abstract

Clinicians trace cephalometric radiographs following a structured anatomical workflow, yet no prior system encodes this into computation. We present a five-phase anatomy-guided pipeline producing confidence-weighted spatial priors that shape HRNet-W32 training, achieving 1.04 mm mean radial error on 25 landmarks across 1,502 radiographs from 7+ imaging devices. A training x inference prior matrix isolates the mechanism: anatomical priors maintain a 1% validation-to-test gap versus 88% without priors (1.94 mm), despite identical validation convergence. The matrix establishes that all trained models are inference-independent, the expanded architecture alone provides no benefit, random priors yield partial but unstable improvement (1.72 mm), and only image-specific anatomically correct priors produce the 1.04 mm result -- functioning as a training-time regularizer requiring no automated prior generation at deployment. Five-fold cross-validation (p=0.0015), patient-level permutation testing (p<0.0001, n=151), quantified Grad-CAM analysis (88% vs. 74% in-zone activation, p<0.001), and clinical measurement validation (skeletal classification kappa=0.79-0.84, zero Class II<->III reversals, ICC>0.95) provide converging evidence. Cross-domain experiments on echocardiography, cervical spine, and hand radiography support the hypothesis that prior effectiveness scales with the spatial entropy of the landmark distribution.

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Sidhartha Mohapatra, Pallavi Mohanty. 2026-05-05. Tracing Like a Clinician: Anatomy-Guided Spatial Priors for Cephalometric Landmark Detection. https://arxiv.org/abs/2605.03358

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