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

Background and objective

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.

Methods

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.

Results

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.

Conclusion

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.