MR-Cavitation Dynamics Encoded (MR-CaDE) imaging.

Purpose: To develop methods for dynamic cavitation monitoring of a non-invasive ultrasound mechanical ablation technology (histotripsy) in the brain and test its feasibility for treatment monitoring in ex-vivo brain in a human MRI scanner.

Methods: A Gradient Echo (GRE) pulse sequence was modified with a bipolar gradient to perform MR-Cavitation Dynamics Encoded (MR-CaDE) imaging. Cavitation generated by histotripsy sonication was monitored using MR-CaDE imaging in ex-vivo bovine brain tissues on a $3T$ human MRI scanner. Bipolar gradients with a b-value of $\text{b}=50s/{\text{mm}}^2$ and smaller were used while a trigger was sent from the MR scanner to the histotripsy driving electronics. MR acquisition was performed with TE/TR: $19\text{ms}/100\text{ms}$ with 1.5-cycle histotripsy sonications at 1 pulse/TR. Feasibility of treatment monitoring was also evaluated for histotripsy through an excised human skull.

Results: The MR-CaDE imaging pulse sequence was used to perform treatment monitoring of cavitation generated by histotripsy with a temporal resolution of $0.5\text{s}$ with a spiral readout. A decrease in the image magnitude and an increase in the phase was observed with an increasing number of histotripsy sonications. The magnitude image exhibited a peak loss of 50%, and the phase image exhibited a maximum increase of 0.64rad compared to the baseline signal level in the brain. The peak signal magnitude change aligned well with the array's geometrical focus, and the post-histotripsy lesion visualized on a DWI ( $\text{b}=1000\text{s/mm}{\hspace{0.17em}}^2$ ) scan with an alignment error of $0.71\text{mm}$ and $1.25\text{mm}$ in the transverse and longitudinal axes, respectively. The area of the histotripsy response using the spiral readout in the magnitude and phase images were $3.38\text{mm}\times 5.62\text{mm}$ and $10.92\text{mm}\times 20.28\text{mm}$ , respectively.

Conclusion: This work demonstrated the feasibility of the MR-CaDE pulse sequence, which can be used to monitor cavitation events in the brain generated by histotripsy.