Assessment of acute thermal damage volumes in muscle using magnetization-prepared 3D T₂-weighted imaging following MRI-guided high-intensity focused ultrasound therapy

Purpose: To evaluate magnetization-prepared 3D T₂-weighted magnetic resonance imaging (MRI) measurements of acute tissue changes produced during ablative MR high-intensity focused ultrasound (MR-HIFU) exposures. Materials and Methods: A clinical MR-HIFU system (3T) was used to generate thermal lesions (n = 24) in the skeletal muscles of three pigs. T₁-weighted, 2D T₂-weighted, and magnetization-prepared 3D T₂-weighted sequences were acquired before and after therapy to evaluate tissue changes following ablation. Tissues were harvested shortly after imaging, fixed in formalin, and gross-sectioned. Select lesions were processed into whole-mount sections. Lesion dimensions for each imaging sequence (length, width) and for gross sections (diameter of lesion core and rim) were assessed by three physicists. Contrast-to-background ratio between lesions and surrounding muscle was compared. Results: Lesion dimensions on T₁ and 2D T₂-weighted imaging sequences were well correlated (R² ∼0.7). The contrast-to-background ratio between lesion and surrounding muscle was 7.4 ± 2.4 for the magnetization-prepared sequence versus 1.7 ± 0.5 for a conventional 2D T₂-weighted acquisition, and 7.0 ± 2.9 for a contrast-enhanced T₁-weighted sequence. Compared with diameter measured on gross pathology, all imaging sequences overestimated the lesion core by 22–33%, and underestimated the lesion rim by 6–13%. Conclusion: After MR-HIFU exposures, measurements of the acute thermal damage patterns in muscle using a magnetization-prepared 3D T₂-weighted imaging sequence correlate with 2D T₂-weighted and contrast-enhanced T₁-weighted imaging, and all agree well with histology. The magnetization-prepared sequence offers positive tissue contrast and does not require IV contrast agents, and may provide a noninvasive imaging evaluation of the region of acute thermal injury at multiple times during HIFU procedures. Level of Evidence: 1. Technical Efficacy: Stage 2. J. MAGN. RESON. IMAGING 2017;46:354–364.