Virtual initialised ray tomography: Towards contact-free realistic ultrasonic bone imaging

Ultrasound tomography struggles with high-contrast and/or extended-range problems, such as site-specific in vivo bone imaging, which is crucial for assessing fracture risk. Two main obstacles arise. (a) An external region prohibits access to the region of interest (ROI), and (b) high contrast exists between the two regions. These challenges hinder existing algorithms-including bent-ray tomography (BRT). Virtual initialised ray tomography (VIRT) is proposed, which addresses these challenges through (a) placement of virtual transducers on the ROI, facilitating (b) rapid initialisation before BRT inversion. In silico validation against BRT with and without prior information shows superior resolution and robustness while improving computational speed. Validation against BRT directly on the ROI shows that VIRT is approaching the resolution limit. Additionally, VIRT performs well under 20 dB signal-to-noise ratio. The ability to solve high-contrast extended-range tomography problems without prior knowledge about the interior of the ROI has many implications. VIRT has the potential to unlock site-specific in vivo bone imaging for assessing fracture risk. It may also replace BRT to improve resolution, robustness, and computational speed-especially where the ROI does not cover the entire imaging array.