Magnetic resonance imaging-guided large-scale lesion patterning with high-intensity focused ultrasound in homogeneous phantoms.

Abstract

Background: In typical high-intensity focused ultrasound (HIFU) therapy, energy is delivered sequentially to multiple focal points to collectively shape the overall treatment zone. However, the behavior of large-scale grid sonication patterns in homogeneous media under magnetic resonance imaging (MRI) guidance has not been fully characterized.

Purpose: To characterize the spatial fidelity and MRI appearance of large-scale HIFU sonication grids in an agar-silica tissue-mimicking phantom and to describe their observed behavior relative to biological tissue.

Methods: Rectangular and irregular large-scale (∼100-point) grids were executed in agar-silica phantoms using an MRI-guided HIFU robotic system with a single-element transducer under 3T MRI guidance. T2-weighted (T2-w) Turbo Spin Echo (TSE) images were acquired after each sonication row for lesion monitoring and post-sonication quantification. Each grid configuration was evaluated in a separate phantom preparation. Descriptive metrics were used to characterize lesion morphology and spatial fidelity, quantified using lesion diameter and Euclidean localization error when individual lesions were distinguishable, and region-level descriptors (area and centroid position) for merged lesion regions. One of the grid protocols was also applied to freshly excised porcine skeletal muscle, with lesion formation monitored using the same MRI-based approach.

Results: In the phantom, lesions appeared as hyperintense regions at the intended focal depth and closely reproduced the prescribed grid geometry. For a standardized 10 × 10 grid (4 mm spacing) in the ablative regime, individual lesions exhibited a mean diameter of 3.0 ± 0.7 mm and a mean Euclidean localization error of ∼2.5 ± 0.7 mm relative to the planned coordinates. When inter-point spacing approached the lesion diameter, adjacent sonications produced merged regions that preserved the overall grid footprint while forming contiguous thermal coverage. In the irregular configuration, grid coverage was 96%, with a centroid offset of ∼1 mm. Under the tested conditions, ex vivo porcine muscle exhibited incomplete lesion formation and a shift of ∼15 mm from the prescribed depth, producing an irregular hypointense region displaced toward the tissue surface.

Conclusions: This study documented the successful generation and MRI visualization of large-scale HIFU grid patterns in an agar-silica phantom using T2-w imaging. Under the tested conditions, the observed hyperintense regions showed spatial correspondence with the prescribed grid geometry. Observations in freshly excised porcine skeletal muscle recorded variations in lesion formation in the presence of biological heterogeneity, while the agar-silica phantom provided a standardized platform for system-level testing as configured in this study.