Slot-scan dual-energy bone densitometry using motorized X-ray systems.

Abstract

PURPOSE\nWe investigate the feasibility of slot-scan dual-energy (DE) bone densitometry on motorized radiographic equipment. This approach will enable fast quantitative measurements of areal bone mineral density (aBMD) for opportunistic evaluation of osteoporosis.\n\n\nMETHODS\nWe investigated DE slot-scan protocols to obtain aBMD measurements at the lumbar spine (L-spine) and hip using a motorized x-ray platform capable of synchronized translation of the x-ray source and flat-panel detector (FPD). The slot dimension was 5 × 20 cm2 . The DE slot views were processed as follows: 1) convolution kernel-based scatter correction, 2) unfiltered backprojection to tile the slots into long-length radiographs, and 3) projection-domain DE decomposition, consisting of an initial adipose-water decomposition in a bone-free region followed by water-CaHA decomposition with adjustment for adipose content. The accuracy and reproducibility of slot-scan aBMD measurements was investigated using a high-fidelity simulator of a robotic x-ray system (Siemens Multitom Rax) in a total of 48 body phantom realizations: 4 average bone density settings (cortical bone mass fraction: 10-40%), 4 body sizes (waist circumference, WC\xa0=\xa070-106\xa0cm), and 3 lateral shifts of the body within the slot field of view (FOV) (centered and ±1\xa0cm off-center). Experimental validations included: (i) x-ray test-bench feasibility study of adipose-water decomposition and (ii) initial demonstration of slot-scan DE bone densitometry on the robotic x-ray system using the European Spine Phantom (ESP) with added attenuation (PMMA slabs) ranging 2 to 6\xa0cm thick.\n\n\nRESULTS\nFor the L-spine, the mean aBMD error across all WC settings ranged from 0.08\xa0g/cm2 for phantoms with average cortical bone fraction wcortical \xa0=\xa010% to ∼0.01\xa0g/cm2 for phantoms with wcortical \xa0=\xa040%. The L-spine aBMD measurements were fairly robust to changes in body size and positioning, e.g., coefficient of variation (CV) for L1 with wcortical \xa0=\xa030% was ∼0.034 for various WC and ∼0.02 for an obese patient (WC\xa0=\xa0106\xa0cm) changing lateral shift. For the hip, the mean aBMD error across all phantom configurations was about 0.07\xa0g/cm2 for a centered patient. The reproducibility of hip aBMD was slightly worse than in the L-spine (e.g., in the femoral neck, the CV with respect to changing WC was ∼0.13 for phantom realizations with wcortical \xa0=\xa030%) due to more challenging scatter estimation in the presence of an air-tissue interface within the slot FOV. The aBMD of the hip was therefore sensitive to lateral positioning of the patient, especially for obese patients: e.g., the CV with respect to patient lateral shift for femoral neck with WC\xa0=\xa0106\xa0cm and wcortical \xa0=\xa030% was 0.14. Empirical evaluations confirmed substantial reduction in aBMD errors with the proposed adipose estimation procedure and demonstrated robust aBMD measurements on the robotic x-ray system, with aBMD errors of ∼0.1\xa0g/cm2 across all three simulated ESP vertebrae and all added PMMA attenuator settings.\n\n\nCONCLUSIONS\nWe demonstrated that accurate aBMD measurements can be obtained on a motorized FPD-based x-ray system using DE slot-scans with kernel-based scatter correction, backprojection-based slot view tiling, and DE decomposition with adipose correction. This article is protected by copyright. All rights reserved.