Effect of ultrasound radiation force on the choroid.

PURPOSE: While visualization of the retina and choroid has made great progress, functional imaging techniques have been lacking. Our aim was to utilize acoustic radiation force impulse (ARFI) response to probe functional properties of these tissues. METHODS: A single element 18-MHz ultrasound transducer was focused upon the retina of the rabbit eye. The procedure was performed with the eye proptosed and with the eye seated normally in the orbit. The transducer was excited to emit ARFI over a 10-ms period with a 25% duty cycle. Phase resolved pulse/echo data were acquired before, during, and following ARFI. RESULTS: In the proptosed eye, ARFI exposure produced tissue displacements ranging from 0 to 10 µm, and an immediate increase in choroidal echo amplitude to over 6 dB, decaying to baseline after about 1 second. In the normally seated eye, ultrasound phase shifts consistent with flow were observed in the choroid, but enhanced backscatter following ARFI rarely occurred. ARFI-induced displacements of about 10 µm were observed at the choroidal margins. Larger displacements occurred within the choroid and in orbital tissues. CONCLUSIONS: We hypothesize that elevated intraocular pressure occurring during proptosis induced choroidal ischemia and that acoustic radiation force produced a transient local decompression and reperfusion. With the eye normally seated, choroidal flow was observed and little alteration in backscatter resulted from exposure. Clinical application of this technique may provide new insights into diseases characterized by altered choroidal hemodynamics, including maculopathies, diabetic retinopathy, and glaucoma.