Ultrasonically induced necrosis through the rib cage based on adaptive focusing: ex vivo experiments

Ex vivo experiments have been conducted through excised lamb rib cages, with bone, cartilage, muscle and skin. The ribs were placed between a hydrophone and a programmable sparse array made of 200 high power transducers working at 1 MHz central frequency. First, adaptive focusing through ribs has been experimentally studied at low power. Without any correction, the pressure fields in the focal plane were affected by both inhomogeneous attenuation and phase distortion and three main effects were observed: a mean 2 mm shift of the main lobe, a mean 1.25 mm spreading in the half width of the main lobe and up to 20 dB increase in the secondary lobe level. Three adaptive focusing techniques were tested with continuous waves: phase shift, time reversal and time reversal coupled with amplitude compensation. We show that time reversal is more appropriate, as it minimizes the heating of the ribs by naturally sonicating between the ribs. It is also shown that a non invasive technique could be implemented by using the imaging capabilities of the therapeutic array.