Focus correction in MR thermography for increased targeting precision during focused ultrasound procedures.

Purpose: Accurate targeting during MR-guided focused ultrasound (FUS) procedures is essential for effective treatment to be achieved. However, spatial discrepancies frequently arise between the planned target and the observed thermal hotspot on proton resonance frequency (PRF)-based MR thermometry because of temperature-induced artifacts. This study aims to correct such displacements caused by chemical shift and k-space center offset.

Methods: Spatial misregistration was addressed using a two-step correction approach. The first step corrected pixel-wise displacements attributed to temperature-dependent resonance frequency shifts (chemical shift), based on local frequency offset maps. The second step compensated for TE errors induced by asymmetric phase gradients near the thermal focus, restoring accuracy in hotspot localization. Validation was performed in controlled phantom experiments, and the approach was retrospectively tested in vivo, in 121 sonications across seven essential tremor (ET) patients.

Results: Phantom experiments demonstrated that spatial shifts up to approximately 1.5 mm could be effectively corrected. Clinical analysis showed a strong correlation (R² = 0.852) between temperature rise and spatial displacement, with a mean shift of 0.5 Mm per 10°C. Combined correction significantly reduced temperature estimation bias, with the mean error decreasing from -0.11°C to -0.05°C, as evaluated by Bland-Altman analysis.

Conclusion: Temperature-related chemical shift and k-space offset substantially impact the spatial fidelity of PRF-based MR thermometry. The proposed correction framework improves thermal hotspot localization, enabling more accurate lesion targeting during FUS procedures.