Ultrasound backscatter spectral analysis provides image feedback for histotripsy tissue fractionation

The feasibility of using spectral analysis as an image feedback modality for histotripsy tissue fractionation was investigated in this study. Spectral analysis analyzes the frequency domain of the backscattered ultrasound RF signal, and its corresponding spectral parameters have been shown to have relationships with scatterer diameter and concentration. Ex vivo canine liver was dissected and embedded in 1.5% (w:v) agarose hydrogel, and then treated with histotripsy pulses driven at 2.0 MHz frequency with pulse repetition frequency of 100 Hz (PRF), pulse duration of 10 cycles, P⁺/P^- = 31.8/17.4 MPa, and the number of pulses varied from 100 to 2000 pulses. After treatment, the specimens were scanned by a commercial ultrasound scanner, and the backscattered RF signals were collected. The power spectrum of the RF signals were analyzed by spectral analysis procedure with two approximation methods, one proposed by Lizzi et al. in 1996 [1], and the other by Oelze et al. in 2002 [2]. The results from both methods showed that the quantified scatterer diameter and acoustic concentration (=scatterer concentration × (relative acoustic impedance)²) decreased as the number of histotripsy treatment pulses increased. Furthermore, the quantified acoustic concentration had a strong correlation with the remaining nuclei density appeared in histological section of the treated specimen. These results suggest that the spectral analysis procedure could provide image feedback for histotripsy tissue fractionation.