Imaging feedback of histotripsy treatments using ultrasound shear wave elastography

Histotripsy is a cavitation-based ultrasound therapy that mechanically fractionates soft solid tissues into fluid-like homogenates. This paper investigates the feasibility of imaging the tissue elasticity change during the histotripsy process as a tool to provide feedback for the treatments. The treatments were performed on agar tissue phantoms and ex vivo kidneys using 3-cycle ultrasound pulses delivered by a 750-kHz therapeutic array at peak negative/positive pressure of 17/108 MPa and a repetition rate of 50 Hz. Lesions with different degrees of damage were created with increasing numbers of therapy pulses from 0 to 2000 pulses per treatment location. The elasticity of the lesions was measured with ultrasound shear wave elastography, in which a quasi-planar shear wave was induced by acoustic radiation force generated by the therapeutic array, and tracked with ultrasound imaging at 3000 frames per second. Based on the shear wave velocity calculated from the sequentially captured frames, the Young´s modulus was reconstructed. Results showed that the lesions were more easily identified on the shear wave velocity images than on B-mode images. As the number of therapy pulses increased from 0 to 2000 pulses/location, the Young´s modulus decreased exponentially from 22.1 ± 2.7 to 2.1 ± 1.1 kPa in the tissue phantoms (R² = 0.99, N = 9 each), and from 33.0 ± 7.1 to 4.0 ± 2.5 kPa in the ex vivo kidneys (R² = 0.99, N = 8 each). Correspondingly, the tissues transformed from completely intact to completely fractionated as examined via histology. A good correlation existed between the lesions´ Young´s modulus and the degree of tissue fractionation as examined with the percentage of remaining structurally intact cell nuclei (R² = 0.91, N = 8 each). These results indicate that lesions produced by histotripsy can be detected with high sensitivity using shear wave elastography. Because the decrease in the tissue- elasticity corresponded well with the morphological and histological change, this study provides a basis for predicting the local treatment outcomes from tissue elasticity change.