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

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. 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 <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><mrow><mn>3</mn> <mi>T</mi></mrow> <annotation>$$ 3\mathrm{T} $$</annotation></semantics> </math> human MRI scanner. Bipolar gradients with a b-value of <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><mrow><mtext>b</mtext> <mo>=</mo> <mn>50</mn> <mi>s</mi> <mo>/</mo> <msup><mrow><mtext>mm</mtext></mrow> <mrow><mn>2</mn></mrow> </msup> </mrow> <annotation>$$ \mathrm{b}=50\mathrm{s}/{\mathrm{mm}}^2 $$</annotation></semantics> </math> 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: <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><mrow><mn>19</mn> <mspace></mspace> <mtext>ms</mtext> <mo>/</mo> <mn>100</mn> <mspace></mspace> <mtext>ms</mtext></mrow> <annotation>$$ 19\kern.2em \mathrm{ms}/100\kern.2em \mathrm{ms} $$</annotation></semantics> </math> with 1.5-cycle histotripsy sonications at 1 pulse/TR. Feasibility of treatment monitoring was also evaluated for histotripsy through an excised human skull. The MR-CaDE imaging pulse sequence was used to perform treatment monitoring of cavitation generated by histotripsy with a temporal resolution of <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><mrow><mn>0.5</mn> <mspace></mspace> <mtext>s</mtext></mrow> <annotation>$$ 0.5\kern.2em \mathrm{s} $$</annotation></semantics> </math> 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 ( <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><mrow><mtext>b</mtext> <mo>=</mo> <mn>1000</mn> <mspace></mspace> <mtext>s/mm</mtext> <msup><mrow><mo> </mo></mrow> <mrow><mn>2</mn></mrow> </msup> </mrow> <annotation>$$ \mathrm{b}=1000\kern.2em \mathrm{s}/{\mathrm{mm}}^2 $$</annotation></semantics> </math> ) scan with an alignment error of <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><mrow><mn>0.71</mn> <mspace></mspace> <mtext>mm</mtext></mrow> <annotation>$$ 0.71\kern.2em \mathrm{mm} $$</annotation></semantics> </math> and <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><mrow><mn>1.25</mn> <mspace></mspace> <mtext>mm</mtext></mrow> <annotation>$$ 1.25\kern.2em \mathrm{mm} $$</annotation></semantics> </math> in the transverse and longitudinal axes, respectively. The area of the histotripsy response using the spiral readout in the magnitude and phase images were <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><mrow><mn>3</mn> <mo>.</mo> <mn>38</mn> <mspace></mspace> <mtext>mm</mtext> <mo>×</mo> <mn>5</mn> <mo>.</mo> <mn>62</mn> <mspace></mspace> <mtext>mm</mtext></mrow> <annotation>$$ 3.38\kern0.3em \mathrm{mm}\times 5.62\kern0.3em \mathrm{mm} $$</annotation></semantics> </math> and <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><mrow><mn>10</mn> <mo>.</mo> <mn>92</mn> <mspace></mspace> <mtext>mm</mtext> <mo>×</mo> <mn>20</mn> <mo>.</mo> <mn>28</mn> <mspace></mspace> <mtext>mm</mtext></mrow> <annotation>$$ 10.92\kern0.3em \mathrm{mm}\times 20.28\kern0.3em \mathrm{mm} $$</annotation></semantics> </math> , respectively. 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.