Bone-marrow edema estimation with a microwave imaging open-ended coaxial probe array in ex vivo bovine long bones: comparison with CT

OBJECTIVE: We evaluated a microwave imaging prototype to estimate bone-marrow edema in ex vivo bovine samples, using computed tomography (CT) and injected volumes as references. MATERIALS AND METHODS: Seven samples comprising distal/proximal halves of two femurs and two humeri were imaged before/after injection of iodine-enhanced gel into drilled holes (Ø 2 mm, depth 8 mm; 25.13 μL/hole). A 4 × 4 open-ended coaxial probe (OECP) array was used over 16 positions with four 90° rotations, simulating a 16 × 16 array of 256 antennas. Raw S11 scattering parameters across a 2.25-3.00 GHz sweep were calibrated and inverted using the Stuchly model with target permittivity at 2.5 GHz. Each OECP antenna was characterized to define its effective penetration depth and equivalent lateral footprint. Upscaled permittivity maps were then generated via nonlinear interpolation, while fluid estimation was computed leveraging a refractive index mixing model. RESULTS: Microwave estimated volumes were repeatable across the four rotations (mean coefficient of variation (CV) 2.8%; mean intraclass correlation coefficient (ICC) 0.999). Microwave estimates tracked the injected volumes with significant correlation (r = 0.81, p = 0.027) and moderate-to-good concordance (Lin's concordance correlation coefficient [CCC] 0.70), with a small bias (+12.4%; 95% limits of agreement (LoA) -17.4% to +42.2%; mean absolute percent error 12.4%). Agreement with CT-derived volumes was weaker (r = 0.58, p = 0.169; CCC 0.23) with a larger bias (+40.9%; 95% LoA -10.3% to +92.2%). CONCLUSION: Microwave imaging allowed for detecting and quantifying small, μL scale, fluid inclusions within ex vivo long-bone halves, using OECP-based arrays. RELEVANCE STATEMENT: Non-ionizing microwave imaging may represent a feasible complementary approach for imaging and quantitative assessment of fluid-related distributions in musculoskeletal tissues. KEY POINTS: A 16-element OECP microwave imaging array allows spatially resolved estimation of fluid-simulated inclusions in ex vivo bovine long bones. Experimental characterization of OECP antenna sensitivity enables resolution upscaling of dielectric permittivity maps. Fluid volumes derived from microwave permittivity maps show significant correlation with injected reference volumes (r = 0.81).