Passive cavitation imaging with p ^th root coherence factor and sparse arrays: In vitro and in vivo assessment for histotripsy monitoring

BACKGROUND: root delay sum integrate (prDSI) and delay multiply and sum (DMAS) beamforming offer comparable or better performance than RCB without extensive computational overhead. However, enhancing imaging performance without compromising frame rates remains an active research area. PURPOSE: coherence factor-weighted delay sum and integrate (pCFwDSI) and evaluates its performance in vitro and in vivo. To reduce computational complexity, sparse array imaging was implemented with 32 and 16 transducer elements. We also employed graphical processing unit (GPU) computation to achieve real time frame rates. METHODS: Histotripsy bubble clouds were generated in vitro in a red blood cell-doped phantom and in vivo in a porcine femoral vein using focused sources with 1 MHz and 1.5 MHz center frequencies. The acoustic emissions were received passively with a curvilinear (C5-2v) and linear (L11-5v) array transducer, respectively. Beamforming was performed using RCB, prDSI, and pCFwDSI with 128, 32 and 16 elements. The performance achieved was compared using the following metrics: -6 dB axial width, signal-to-interference ratio (SIR), binary statistical analysis, and structural similarity index measure (SSIM). RESULTS: The pCFwDSI approach performed the best in terms of axial width and SIR, and prDSI showed intermediate performance between pCFwDSI and RCB. For binary statistical metrics, the performance achieved with p = 3 and 128 elements was similar for both approaches (within 2% of each other). In sparse array images implemented with 16 elements, grating lobes were observed in RCB and prDSI, but were suppressed in pCFwDSI. The SSIM of the sparse array (16 elements) with pCFwDSI (p = 3) was 96.34% relative to the images generated with the full aperture of 128 elements. Frame rates ranged from 18 to 91 Hz for pCFwDSI using RF data from 128 to 16 elements, respectively, which were similar to standard DSI. CONCLUSIONS: These results demonstrate pCFwDSI can improve axial resolution, reduce artifacts, and achieve clinically-relevant frame rates without increasing computational and data storage overhead. These findings could be a step towards enabling improved real time PCI monitoring of histotripsy.