Simultaneous MRI and Ultrasound Guided Preclinical Histotripsy (MR-USgHt) System
Abstract
Purpose
To develop the first histotripsy system for non-invasive ultrasonic mechanical ablation with simultaneous, co-registered MRI and ultrasound feedback.
Methods
A custom 89-element transmit-receive capable, MRI-compatible, histotripsy array was designed and fabricated for preclinical murine tumor models. generated ablation zones in both red blood cell (RBC) phantoms and within an in-vivo murine brains. For each treatment pre and post T2* weighted MR sequences were acquired (7T Agilent scanner). In addition, acoustic emission signals generated by cavitation events were captured by histotripsy array elements. These signals were subsequently beamformed using a delay and sum algorithm in a process known as Passive Acoustic Mapping (PAM) to create a distribution map of cavitation activity (histotripsy treatment zone) during treatment. MR scans were co-registered to the array using fiducial markers. PAM distributions were then superimposed onto post treatment MR scans and compared to the prescribed ablation zone to determine a localization error. The localization error was defined as the difference between the centroid of the PAM distribution and the centroid of the ablation zone characterized by T2* contrast on the post treatment MRI.
Results
Ablation zones generated in RBC phantoms (n = 4) yielded a mean localization error of 0.7 ± 0.1 mm. Meanwhile in-vivo murine brain ablations (n = 2) yielded a mean localization error of 1.36 mm. Instances of unintended prefocal cavitation on the skull surface were also detected using PAM.
Conclusion
This study demonstrates the feasibility of the first histotripsy system with simultaneous, co-registered MRI and ultrasound feedback. We also demonstrated the first in vivo non-invasive brain ablation using this system in the vivo murine brain. The MRI and ultrasound based feedback techniques utilized in this study can be scaled up to human applications.