Multilevel Lumbar CT Attenuation Beyond L1: Comparison with QCT-Derived Volumetric Bone Mineral Density and Prevalent Fragility Fracture Status in a Diagnostic Referral Cohort

Background/Objectives: CT-based trabecular attenuation in Hounsfield units (HU) is used as a surrogate marker of bone quality, but most approaches rely on a single vertebral level, usually L1, where local abnormalities may limit reliability and availability. This study evaluated whether multilevel lumbar, reference-adjusted HU assessment beyond isolated L1 improves the identification of QCT-defined osteoporosis and the discrimination of prevalent fragility fracture status. Methods: Between 2021 and March 2024, 800 patients referred for evaluation of bone mineral density underwent QCT of the lumbar spine with a calibration phantom. Cancellous attenuation was measured in HU at L1–L3 using manually positioned ellipsoid regions of interest, with predefined adjacent levels (T12, L4) substituted when a target vertebra was unsuitable. Single-level HU, the mean multilevel HU value and the lowest valid HU value were compared with QCT-defined osteoporosis and prevalent fragility fracture status (vertebral or sacral) using ROC analyses and pairwise DeLong testing. Results: Mean multilevel HU correlated closely with mean vBMD (Spearman ρ = 0.988; p < 0.001); as both derive from the same acquisition, the same vertebral bodies and the same phantom, this reflects a strong association between two related measurements rather than validation against an independent standard. Osteoporosis was present in 483 patients (60.4%). Mean multilevel HU was associated with osteoporosis with an AUC of 0.996 (95% CI, 0.993–0.998) at an internally derived cut-off of ≤99.6 HU. In the 699 patients with a valid L1 measurement, in whom a paired comparison was possible, mean multilevel HU exceeded isolated L1-HU (0.992; p = 0.0028). For fracture status, the AUC was 0.975 (0.966–0.983) at ≤80.5 HU, without advantage over L1-HU (p = 0.816). A valid value was obtainable in all 800 patients versus 699 (87.4%) with L1 alone. All patients with a fracture had a vBMD below the osteoporosis threshold, so this endpoint is structurally dependent on the densitometric one. Conclusions: Multilevel lumbar HU assessment provides a practical marker of trabecular bone quality beyond isolated L1. Its principal advantage is measurement availability rather than a clinically meaningful gain in discrimination. The reported thresholds are internally derived, cohort- and protocol-specific, and require external validation.