Ultrasound Imaging of Cortical Bone

Abstract

Background It has been suggested that ultrasound (US) imaging can be used to assess cortical bone health, which is of particular interest owing to its major role in bone mechanical stability. Intra-cortical US imaging extends B-mode imaging into bone using a dedicated image reconstruction algorithm that corrects for refraction at the bone-soft tissue interfaces. It has shown promising results in a few healthy, predominantly young adults, providing anatomical images of the cortex (periosteal and endosteal surfaces) along with estimations of US wave speed. However, its reliability in older or osteoporotic bones remains uncertain. Objective In this study, we critically assessed the performance of intra-cortical US imaging ex vivo in bones with various microstructural patterns, including bones exhibiting signs of unbalanced intra-cortical remodeling. Methods We analyzed factors influencing US image quality, particularly endosteal surface reconstruction, as well as the accuracy of wave speed estimation and its relationship with porosity. We imaged 20 regions of interest from the femoral diaphysis of 5 elderly donors using a 2.5 MHz US transducer. The reconstructed US images were compared to site-matched high-resolution micro-computed tomography images. Results In samples with moderate porosity, the endosteal surface was accurately identified, and thickness estimates from US and high-resolution micro-computed tomography differed by less than 10%. In highly remodeled bones with increased porosity, pore size and an heterogeneous distribution of pores, the reconstructed endosteal surface appeared less bright and was located above the trabecularized cortex region. We observed a decrease in US wave speed with increasing cortical porosity, aligning well with literature data, suggesting that, based on wave speed value the method could discriminate between bones with low porosity (<5%) and those with moderate to high porosity (>10%). Conclusion This study paves the way for the application of US imaging in diagnosing cortical bone health, particularly for detecting increased cortical porosity and reduced cortical thickness.