Measuring tissue elastic properties using acoustic radiation force on laser-generated bubbles

We apply acoustic radiation force to bubbles generated by laser-induced optical breakdown (LIOB) to study the mechanical response of the surrounding medium. Optical breakdown occurs when sufficiently high threshold fluence is attained at the focus of femtosecond pulsed lasers, inducing plasma formation and bubble generation. LIOB bubbles are of particular interest because they can be generated at very precise locations and optical parameters can be varied to control size. In this investigation, femtosecond laser pulses are focused in the volume of gelatin phantoms of different concentrations to form bubbles. A two-element confocal ultrasonic transducer generates acoustic radiation force on individual bubbles while monitoring their displacement within an elastic medium. Single tone burst pushes of varying duration have been applied by the outer element at 1.5 MHz. The inner element receives pulse-echo recordings at 7.44 MHz before, during, and after the excitation bursts and cross-correlation processing is performed offline to monitor bubble position. Maximal bubble displacements of 293 mum, 144 mum, and 88 mum have been measured in response to a single 6.7 ms ultrasound burst in 5%, 7.5%, and 10% gelatin phantoms, respectively. The time constants for bubble relaxation curves following the push bursts show a decreasing trend with increasing gel stiffness. These results demonstrate that bubble response to acoustic radiation force is directly related to gelatin concentration and, therefore, the viscoelasticity of the surrounding medium.