Beyond Morphology: Quantitative Bilateral Self-Referenced Frequency-Domain Diffuse Optical Spectroscopy Reveals Functional Abnormalities in Patellar Tendinopathy.
Abstract
OBJECTIVE: Patellar tendinopathy (PT) involves functional and metabolic abnormalities that may precede structural degeneration, whereas MRI and ultrasonography primarily depict morphology. To bridge this gap, we aim to develop a quantitative optical method for detecting PT-related functional and hemodynamic abnormalities. METHODS: We developed an embedded multi-wavelength frequency-domain diffuse optical spectroscopy system integrating sinusoidal modulation, synchronized acquisition, real-time Goertzel demodulation, optical-property inversion, chromophore fitting, and standardized grid scanning over the patellar tendon region. To reduce inter-subject variability, a bilateral self-referenced strategy used the contralateral tendon as an endogenous reference. A composite optical biomarker, Tendon Hemodynamic Index (THI), was defined by integrating differential tissue oxygenation and blood-volume-related information. RESULTS: Phantom and in vivo measurements demonstrated robust quantitative performance. In a clinical cohort, THI was significantly elevated in PT patients and correlated with symptom severity and functional impairment across the full cohort, with Spearman correlations of 0.69 with the Numeric Rating Scale and -0.74 with the Victorian Institute of Sport Assessment-Patella score. In this exploratory cohort, THI distinguished PT from healthy controls with 91.8% accuracy, 95.8% sensitivity, 88.0% specificity, and an area under the receiver operating characteristic curve of 0.928. In imaging negative but symptom-positive cases, THI achieved 88.2% classification accuracy. In three exploratory follow-up cases, THI decreased toward the bilateral baseline as symptoms improved over 8 weeks. CONCLUSION: The proposed method enables self-referenced quantification of PT-related functional abnormalities. SIGNIFICANCE: By revealing tendon hemodynamic abnormalities beyond structural imaging, this work supports early PT detection, objective severity assessment, and quantitative monitoring of disease recovery.
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