Adjoint‑based pulse‑echo speed‑of‑sound tomography (Conference Presentation)

Existing pulse-echo speed-of-sound (SoS) imaging techniques rely on ray theory to linearly relate phase aberrations to tissue SoS variations. By neglecting refraction and diffraction, however, their accuracy and resolution remain fundamentally limited. To overcome this, we present a tomography technique that combines matrix imaging and the adjoint-state method. Under the matrix imaging formalism, we back‑propagate the measured wavefields using the Fourier split‑step method, enabling accurate phase-aberration estimations in heterogeneous media. We then define a misfit functional based on these estimations and use the adjoint method to compute its exact gradient with respect to the SoS. This gradient requires the computation of an adjoint wavefield generated by the upward propagation of spatially distributed phase aberrations. Numerical tests demonstrate that our approach drastically improves resolution compared to state-of-the-art methods. Beyond the current implementation, our framework can naturally incorporate misfit functionals that exploit other parts of the data, paving the way towards pulse-echo full waveform tomography.