Phantom-based assessment of focused ultrasound thermal effects with conventional magnetic resonance imaging

This study presents key findings from Magnetic Resonance Imaging (MRI)-guided Focused Ultrasound (FUS) sonication experiments in a specialized gel phantom, aimed at demonstrating the effectiveness of using conventional T1-Weighted (T1-W) and T2-Weighted (T2-W) Turbo Spin Echo (TSE) sequences to assess FUS thermal effects and related system performance.

Three custom-manufactured, single-element spherically focused ultrasonic transducers were utilized in this study. The temporal regression of lesions induced by high-power FUS in the phantom model was investigated within a 3T MRI scanner for both employed sequences. Thermal effects within the phantom were characterized through visual assessment and quantitative measurements of lesion size and contrast-to-noise ratio (CNR) on TSE images, under different focal intensities, exposure durations, and focal depths, complemented by MR thermometry data.

The phantom exhibited hyperintense lesions with excellent contrast on T2-W TSE imaging, chosen for its superior contrast and faster lesion resolution. Lesion size increased with intensity and exposure time, though trends varied by transducer. Cigar-shaped or non-uniform elongated lesions developed, reflecting sharp or poor transducer focusing, with signal intensity decreasing as focal depth increased. A strong correlation between lesion CNR and accumulated thermal dose was observed, emphasizing that changes in imaging contrast can consistently reflect thermal effects.

The proposed phantom-based MRI approach shows promise as a tool for routine assessment of FUS ablation system performance by monitoring lesion dynamics using conventional T2-W TSE imaging, thereby streamlining the quality assurance workflow.