Spatio-temporal quantitative thermography of pre-focal interactions between high intensity focused ultrasound and the rib cage

Abstract

Objective: The aim of this paper is to quantitatively investigate the thermal effects generated by the pre-focal interactions of a HIFU beam with a rib cage, in the context of minimally invasive transcostal therapy of liver malignancies. Materials and methods: HIFU sonications were produced by a phased-array MR-compatible transducer on Turkey muscle placed on a sheep thoracic cage specimen. The thoracic wall was positioned in the pre-focal zone 3.5 to 6.5 cm below the focus. Thermal monitoring was simultaneously performed using fluoroptic sensors inserted into the medullar cavity of the ribs and high resolution MR-thermometry (voxel: 1 × 1 × 5 mm³, four multi-planar slices). Results: MR-thermometry data indicated nearly isotropic distribution of the thermal energy at the ribs’ surface. The temperature elevation at the focus was comparable with the pericostal temperature elevation around unprotected ribs, while being systematically inferior, by more than a factor of four on average, to the intra-medullar values. The spatial profiles of the pericostal and intra-medullar thermal build-up measurements could be smoothly connected using a Gaussian function. The dynamics of the post-sonication thermal relaxation as determined by fluoroptic measurements was demonstrated to be theoretically coherent with the experimental observations. Conclusion: The experimental findings motivate further efforts for the transfer towards clinical routine of effective rib-sparing strategies for hepatic HIFU.

• MRgHIFU
• thermal effects
• pre-focal interactions
• rib cage

Acknowledgements

The authors thank the Centre for Biomedical Imaging (CIBM), Switzerland, for providing access to the MR imaging infrastructure and Stéphanie Gardier for useful advice and assistance with the in vitro experiments.

Declaration of interest

This work was partly funded by the Swiss National Foundation of Science (Grant no. CR32I3_125499 and CR33I3_143980) and La fondation pour la lutte contre le cancer et pour des recherches médicobiologiques, Geneva, Switzerland. Real-time data transfer software from the MR host computer was provided by Siemens MR Division, Erlangen, Germany. The authors alone are responsible for the content and writing of the paper.