Aberration Correction for Transcranial Histotripsy
Ning Lu1, Sang Won Choi1, Timothy L. Hall1, Jonathan R. Sukovich1, John Snell2, Nathan McDannold3, Zhen Xu1
1Department of Biomedical Engineering, University of Michigan, Ann Arbor, USA
2Focused Ultrasound Foundation, Charlottesville, USA
3 Department of Radiology, Brigham and Women’s Hospital, Harvard Medical School, Boston, Massachusetts, USA.
Transcranial histotripsy has been shown to be effective for ablating a wide range of cerebral locations ex vivo and in vivo through an excised human skull. Acoustic aberrations introduced by the heterogeneities of skull thickness, composition, and speed of sound distort the shape of pressure field and decrease its amplitude at the target location, thereby limiting the treatment efficacy and efficiency. The goals of this study are: 1) To demonstrate the feasibility of aberration correction using the acoustic cavitation emission (ACE) shockwaves from inertially cavitating microbubbles generated by histotripsy for transcranial tissue ablation, and 2) To develop a novel 2-step aberration correction method for transcranial histotripsy treatment, which implements the CT-based analytical method as the first step, then followed by ACE-based aberration correction.
Step 1: CT-based analytical aberration correction
The head (or skull) CT scans are co-registered with the transducer array acoustically using pulse-echo data, and ray-tracing analysis is performed on the ray cast from the target location towards the element center for each of the array elements to estimate the phase delays along each ray’s path. These delays were applied as firing offsets to generate the initial cavitation at the target location.
Step 2: ACE-based aberration correction
A 700-kHz, 360-element histotripsy phased array capable of transmitting and receiving on all channels was used to acquire ACE shockwaves. Phase delays at each channel were calculated from the ACE shockwaves and applied to the transducer array to compensate for the phase aberration. These delays were then applied as firing offsets to deliver histotripsy treatment at the target location.
The performance of this 2-step approach is quantified by the percentage of pressure increase compared to no aberration correction, the relative pressure recovery compared to the aberration, the focal shifts, and the focal volume.
The 2-step approach has been shown to yield a focal pressure of 74% to 98% compared to hydrophone-based aberration correction. Beam profiles with aberration correction using ACE shockwaves showed smaller FWHMs and reduced side lobes in all three dimensions compared to no correction, suggesting successful refocusing of the pressure field.
We have demonstrated the feasibility of transcranial aberration correction using ACE shockwaves and the efficacy of using a 2-step approach. This presents a novel technique for aberration correction in transcranial applications without implanting a hydrophone or dependency on other imaging modalities such as CT or MRI.
Figure 1. 2D cross sections of the focal spot with no aberration correction (NAC), CT-based analytical method (CTAC), 2-step approach proposed in this study (2-step AC), directly implemented ACE-based aberration correction without CT-analytical method as a prior (Direct ACE), and hydrophone-based aberration correction (HPAC).
N. Lu, J. Sukovich, T. Hall, Z. Xu. “Aberration Correction for Transcranial Histotripsy”. Poster Presentation at 2021 IEEE International Ultrasonics Symposium (IUS) and paper in preparation.