Passive mapping for imaging thermal lesions in tissue.

Abstract

Passive acoustic mapping, a method of mapping sources of high‐frequency ultrasound emissions during focused ultrasound treatment, has been tested in ex vivo ox liver. The method uses an array of ultrasound sensors placed coaxially with a focused therapeutic ultrasound transducer. The ultrasound emissions received during treatment are first filtered and then beamformed to produce reconstructions of either broadband or harmonic source intensity within the tissue during the application of focused ultrasound. At pressure amplitudes in excess of the inertial cavitation threshold, passive mapping of broadband emissions is found to provide a good indication of the location and size of the lesion, while mapping of harmonic emissions adequately describes the location and onset of boiling. The results for different treatment parameters across several liver samples will be presented. Alternative methods of postprocessing the passively received data for improved spatiotemperoral resolution and signal‐to‐noise ratio will also be introduced and compared. These methods are being developed to characterize cavitation activity in order to give estimates of cavitation energy, tissue heating, and temperature during therapeutic ultrasound exposures, to ultimately enable prediction of cell death during treatment.