Photoacoustic imaging of ultrasound‐induced displacements in ocular tissues.

Abstract

Acoustic radiation force produces compression that can be used for assessment of tissue stiffness, generally by detection of displacements in the phase‐resolved pulse/echo ultrasound waveform. In this report, we describe use of photoacoustics for tracking displacements produced by acoustic radiation force in the iris and retina. The probe consisted of a 20 MHz transducer with a central aperture through which 532 nm laser pulses were introduced. Laser pulses were emitted simultaneously with excitation of the transducer by a 20 MHz monocycle. For a 12 ms period, we interleaved tonebursts (generating force) between successive laser pulses/monocycles. Iris displacements averaged 26.5 and 76.7 μm photoacoustically versus 24.9 and 71.3 μm by pulse/echo at 60 and 100 W cm‐2 force, respectively. For the retina, the displacements were 14.6 and 25.3 μm photoacoustically versus 12.4 and 25.8 by pulse/echo. Pulse/echo displacements were more difficult to discern due to their broadness and anatomic non‐specificity. The photoacoustic signal is advantageous because of its broadband character and because the photoacoustic signal is only generated by specific molecules, which in the retina would correspond to melanin in the pigment epithelium. As little is known about retinal elasticity, this technique offers an avenue toward investigation of this tissue property.