MR‐Cavitation Dynamics Encoded (MR‐CaDE) imaging

Abstract 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 a3 T $$ 3\mathrm{T} $$ human MRI scanner. Bipolar gradients with a b‐value ofb = 50 s / mm 2$$ \mathrm{b}=50\mathrm{s}/{\mathrm{mm}}^2 $$ 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 ms / 100 ms $$ 19\kern.2em \mathrm{ms}/100\kern.2em \mathrm{ms} $$ 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 of0.5 s $$ 0.5\kern.2em \mathrm{s} $$ 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 ( b = 1000 s/mm   2$$ \mathrm{b}=1000\kern.2em \mathrm{s}/{\mathrm{mm}}^2 $$ ) scan with an alignment error of0.71 mm $$ 0.71\kern.2em \mathrm{mm} $$ and1.25 mm $$ 1.25\kern.2em \mathrm{mm} $$ in the transverse and longitudinal axes, respectively. The area of the histotripsy response using the spiral readout in the magnitude and phase images were3 . 38 mm × 5 . 62 mm $$ 3.38\kern0.3em \mathrm{mm}\times 5.62\kern0.3em \mathrm{mm} $$ and10 . 92 mm × 20 . 28 mm $$ 10.92\kern0.3em \mathrm{mm}\times 20.28\kern0.3em \mathrm{mm} $$ , 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.