Photoacoustic guidance of high intensity focused ultrasound with selective optical contrasts and time-reversal

The authors present a method of focusing high intensity ultrasound by time-reversing the photoacoustic response of an optically selective target in a nonselective background. The target’s photoacoustic response was isolated from the background by subtracting the photoacoustic waveforms obtained at different optical wavelengths and convolved with a continuous signal. It was found that the focus produced was comparable in quality to that obtained by delay-law beam-forming. The method holds the promise of allowing precise targeting of high intensity focused ultrasound on nonechogenic targets, in moving environments, independently of the presence of aberrating layers.