  We present the first fully two-dimensional attenuation imaging technique
developed for pulse-echo ultrasound systems. Unlike state-of-the-art
techniques, which use line-by-line acquisitions, our method uses steered
emissions to constrain attenuation values at each location with multiple
crossing wave paths, essential to resolve the spatial variations of this tissue
property. At every location, we compute normalized cross-correlations between
the beamformed images that are obtained from emissions at different steering
angles. We demonstrate that their log-amplitudes provide the changes between
attenuation-induced amplitude losses undergone by the different incident waves.
This allows us to formulate a linear tomographic problem, which we efficiently
solve via a Tikhonov-regularized least-squares approach. The performance of our
tomography technique is first validated in numerical examples and then
experimentally demonstrated in custom-made tissue-mimicking phantoms with
inclusions of varying size, echogenicity, and attenuation. We show that this
technique is particularly good at resolving lateral variations in tissue
attenuation and remains accurate in media with varying echogenicity. Based on a
similar principle, this method can be easily combined with computed ultrasound
tomography in echo mode (CUTE) for speed-of-sound imaging, paving the way
towards a multi-modal ultrasound tomography framework characterizing multiple
acoustic tissue properties simultaneously.
