I. Introduction
Thermoacoustics has the potential to join quantitative imaging techniques such as X-ray CT and shear wave elastography that provide images representing tissue density and stiffness, respectively. Thermoacoustic images represent an induced pressure jump in Pascals from which intrinsic tissue properties can be inferred. When very high-frequency (VHF) irradiation is used, thermoacoustic pulse amplitudes are most closely correlated to electrical conductivity. Thermoacoustic imaging is an inverse acoustic source problem, in which a broadband thermoacoustic pulse travels from internal source to external detector that passively records the thermoacoustic pulse in receive-only mode. Challenges to thermoacoustic imaging over large fields of view are the ability to generate detectable thermoacoustic pulses throughout large volumes and adequate receive chain bandwidth. The first challenge can be overcome by irradiating with VHF [1] or ultra high-frequency [2] irradiation. In the following, we demonstrate that a clinical ultrasound array augmented with the addition of one element sensitive to very low frequencies can overcome the challenge of receiver bandwidth. Combining data from a clinical array with 1–4-MHz sensitivity band with lower frequency single-element transducer measurements preserve resolution of the P4-1 images and improves quantitative accuracy.