  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.
