Advancements in Microwave Bone Imaging: Tomographic Reconstruction of an Anthropomorphic Calcaneus Phantom Using a Specialized Imaging Prototype for Bone Health Applications

The evaluation and characterization of microwave imaging (MWI) prototype along with the optimization of imaging algorithm is a precursor step before the clinical study to measure in vivo dielectric properties. This study presents the microwave tomographic (MWT) image reconstruction of anthropomorphic calcaneus phantoms by using a specialized microwave bone imaging prototype. To this end, the 2D tomographic images of 3D experimental multi-layered (including skin, fat, cortical and trabecular bone) calcaneus-shaped phantoms were reconstructed. Liquid tissue-mimicking mixtures (TMM) for skin, fat, normal bone, osteoporotic bone, and osteoarthritic bone were prepared. The TMMs were dielectrically characterized over the frequency range of 1 - 4 GHz. The TMMs were poured into a calcaneus-shaped phantom that was placed in the imaging prototype. The imaging prototype was equipped with nine microstrip antennas, connected to a 2-port VNA through a 24-port switching matrix. The tomographic reconstructions were achieved around 3 GHz by using a Distorted Born Iterative Method (DBIM) with an Iterative Method with Adaptive Thresholding for Compressed Sensing (IMATCS). Qualitative and quantitative findings indicate that the employed method is suitable for reconstructing and distinguishing the properties of the mimicked human bone tissues. Indeed, the osteoporotic and osteoarthritic bone phantoms can be differentiated with an average percentage difference of 25%. The target region (i.e., trabecular tissue) is correctly located and its diseased condition can be determined from its retrieved properties for all the phantoms, showing a Structural Similarity Index of at least 82%. This work proves the potential clinical utility of MWI in bone health assessment.