# Forward model for quantitative pulse-echo speed-of-sound imaging

## TL;DR

This paper introduces an improved forward model for quantitative pulse-echo speed-of-sound imaging that enhances the robustness and accuracy of tissue SoS reconstruction in ultrasound tomography, demonstrated through phantom and volunteer studies.

## Contribution

The paper presents a novel forward model that accounts for specific phase shifts and position errors, significantly improving SoS imaging accuracy over previous models.

## Key findings

- Enhanced SoS reconstruction accuracy in phantom studies
- Improved robustness of quantitative results with the new model
- Preliminary volunteer results support clinical potential

## Abstract

Computed ultrasound tomography in echo mode (CUTE) allows determining the spatial distribution of speed-of-sound (SoS) inside tissue using handheld pulse-echo ultrasound (US). This technique is based on measuring the changing phase of beamformed echoes obtained under varying transmit (Tx) and/or receive (Rx) steering angles. The SoS is reconstructed by inverting a forward model describing how the spatial distribution of SoS is related to the spatial distribution of the echo phase shift. CUTE holds promise as a novel diagnostic modality that complements conventional US in a single, real-time handheld system. Here we demonstrate that, in order to obtain robust quantitative results, the forward model must contain two features that were not taken into account so far: a) the phase shift must be detected between pairs of Tx and Rx angles that are centred around a set of common mid-angles, and b) it must account for an additional phase shift induced by the error of the reconstructed position of echoes. In a phantom study mimicking liver imaging, this new model leads to a substantially improved quantitative SoS reconstruction compared to the model that has been used so far. The importance of the new model as a prerequisite for an accurate diagnosis is corroborated in preliminary volunteer results.