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

Abstract

Purpose To evaluate magnetization-prepared 3D T2-weighted MR imaging measurements of acute tissue changes produced during ablative MR-HIFU exposures. 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 approximately 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 over-estimated the lesion core by 22–33%, and under-estimated 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.

Evaluation of Focal Laser Ablation of Prostate Cancer Using High Spectral and Spatial Resolution Imaging: A Pilot Study

“…Furthermore, there was good agreement in Bland–Altman plots. Staruch et al reported similar correlations for lesion size in pig muscle measured between T 2 W and contrast‐enhanced T 1 W images, following MRI‐guided high‐intensity focused ultrasound therapy …”

“…In our study, the laser beam activation time at each location varied from 60 sec to 120 sec. This could produce variations in the tissue damage . These variations were not considered in the present study.…”

“…Spatially varying effects of FLA within each lesion may produce heterogeneous image contrast. For example, the inner core of the lesion may have coagulative necrosis, and the peripheral rim may have structural vacuolation and edema due to differences in perfusion . The contrast‐enhanced T 1 W images typically showed higher image contrast between the ablated lesion region and the periphery muscle (obturator internus) tissue compared with and WPH images.…”

“…Intraoperative MRI monitoring of the treatment region after each ablation is crucial for FLA. Currently, one of the challenges for FLA is accurately evaluating the ablation zone during the procedure without injecting intravenous contrast agent . Although the ablation zone becomes evident on contrast enhanced T 1 ‐weighted (T 1 W) MRI, intravenous contrast agent can only be administered once and only at the end of the procedure.…”

“…Currently, one of the challenges for FLA is accurately evaluating the ablation zone during the procedure without injecting intravenous contrast agent. 15 Although the ablation zone becomes evident on contrast enhanced T 1 -weighted (T 1 W) MRI, 16 intravenous contrast agent can only be administered once and only at the end of the procedure. Accurate characterization of the ablation zone with a noncontrast enhanced MRI scan during FLA would enable physicians to better plan the location of subsequent ablations and increase the efficacy of the procedure.…”