Ultrasound Imaging of Bone Cortex Using Block-Based Reconstruction

Without considering the significant acoustic impedance contrast between bone and soft tissue, traditional imaging methods with unique sound velocity assumption are challenging to generate accurate ultrasound image of bone cortex, such as time-domain synthetic aperture (TDSA), and phase shift migration (PSM). Furthermore, TDSA restores the image point-by-point, and PSM reconstructs the target layer-by-layer, leading to the relatively high computation cost. To overcome these limitations, this work proposed a block-based fast ultrasound imaging method using velocity model estimation (BR-FUI-VE). After the estimation of sound velocity model via travel-time inversion, the imaging process involved block-by-block reconstruction, merely extrapolating the received wavefield to the top of the target block. This facilitated the reconstruction of entire tissue block (e.g., soft tissue or bone tissue) in a single step through 2D fast Fourier transform (2D-FFT). The effectiveness of the method was demonstrated by one simulated specimen, two cortical phantoms, and two ex-vivo goat tibias. The estimated sound velocity model showed mean relative errors below 14%, and the mean errors of the cortical thickness were less than 0.32 mm. The results of ex-vivo experiments were in good agreement with the reference models measured by micro computed tomography (μCT). Moreover, BR-FUI exhibited significantly reduced time complexity compared to TDSA and PSM, allowing a 5-layer image (i.e., goat tibia) to be reconstructed in just 0.3 seconds. Unlike the conventional synthetic aperture (SA), the proposed BR-FUI-VE method was proven to be an effective modality for accurate and efficient cortical bone imaging.

I. Introduction

Osteoporosis is an age-related, chronic, metabolic disease [1] clinically featured by the degeneration of bone microarchitecture, thinning of bone thickness, and reduction of bone mass [2], which may finally develop to bone morphology change or even evolves to fracture when the bone fragility remarkably increases [3]. Although the onset of osteoporosis is not easily perceived, and no acute symptom can be observed, the life quality of patient would be significantly decreased due to the miss of best time for treatment and then the progress into severe osteoporosis [4]. The clinically gold standard of osteoporosis diagnosis is the measurement of bone mineral density (BMD) using dual-energy X-ray absorptiometry (DXA) [5], whereas BMD as inorganic composition only accounts for approximately 60%∼70% of bone strength [6]. Nevertheless, several other factors has been reported to be important determinants of bone strength, such as morphology, microstructure, and other contents in bone, which cannot be measured by DXA [6]. As two mature tools for clinical assessment of bone geometry and microstructure, the applications of computed tomography (CT) and magnetic resonance imaging (MRI) are limited [7], considering the relatively high costs, long time of inspection, ionizing radiation (CT), and poor mobility.