Effects of respiratory liver motion on heating for gated and model-based motion-compensated high-intensity focused ultrasound ablation

To quantify the effects of respiratory motion on high-intensity focused ultrasound heating of liver tissue by comparing the simulated ablation using a conventional respiratory gating versus a MR-model-based motion compensation approach.

To measure liver motion, dynamic free-breathing abdominal MR scans were acquired for five volunteers. Deformable registration was used to calculate continuous motion models, and tissue heating at a moving single focus was computed in 3-D by solving the bioheat equation. Ablated volume ratios with respect to the static case, V(ab), were determined for a range of exposure times t(exp) and heating rates r.

To achieve V(ab) > 90% required t(exp) < 0.5s and r > 120 degrees C/s when gating, whereas t(exp) < 1s and r > 60 degrees C/s for motion-compensation.

Accurate compensation for respiratory motion is important for efficient tissue ablation. Model-based motion compensation allows substantially lower heating rates than gating, reducing the risk of skin burns and focal boiling.