Tissue-mimicking thermochromic phantom for characterization of HIFU devices and applications

Abstract

Purpose: Tissue-mimicking phantoms (TMPs) are synthetic materials designed to replicate properties of biological tissues. There is a need to quantify temperature changes following ultrasound or magnetic resonance imaging-guided high intensity focused ultrasound (MR-HIFU). This work describes development, characterization and evaluation of tissue-mimicking thermochromic phantom (TMTCP) for direct visualization and quantification of HIFU heating. The objectives were to (1) develop an MR-imageable, HIFU-compatible TMTCP that reports absolute temperatures, (2) characterize TMTCP physical properties and (3) examine TMTCP color change after HIFU.

Methods and materials: A TMTCP was prepared to contain thermochromic ink, silicon dioxide and bovine serum albumin (BSA) and its properties were quantified. A clinical MRI-guided and a preclinical US-guided HIFU system were used to perform sonications in TMTCP. MRI thermometry was performed during HIFU, followed by T2-weighted MRI post-HIFU. Locations of color and signal intensity change were compared to the sonication plan and to MRI temperature maps.

Results: TMTCP properties were comparable to those in human soft tissues. Upon heating, the TMTCP exhibited an incremental but permanent color change for temperatures between 45 and 70 °C. For HIFU sonications the TMTCP revealed spatially sharp regions of color change at the target locations, correlating with MRI thermometry and hypointense regions on T2-weighted MRI. TMTCP-based assessment of various HIFU applications was also demonstrated.

Conclusions: We developed a novel MR-imageable and HIFU-compatible TMTCP to characterize HIFU heating without MRI or thermocouples. The HIFU-optimized TMTCP reports absolute temperatures and ablation zone geometry with high spatial resolution. Consequently, the TMTCP can be used to evaluate HIFU heating and may provide an in vitro tool for peak temperature assessment, and reduce preclinical in vivo requirements for clinical translation.

Keywords:

• Tissue-mimicking phantom
• thermochromic
• hyperthermia
• HIFU
• MRI

Acknowledgments

The authors thank Drs. Subha Maruvada and Yunbo Liu for characterization of TMTCP acoustic and thermal properties.

Disclosure statement

Dr. Ari Partanen is a paid consultant for Profound Medical Inc. Dr. Bradford Wood is NIH Principal Investigator on related Cooperative Research and Development Agreements HIFU-related with Philips and with Profound Medical Inc. (under negotiation). The other authors have no additional relevant conflicts of interest to disclose. The authors alone are responsible for the content and writing of the article. The mention of commercial products, their source or their use in connection with material reported herein is not to be construed as either an actual or implied endorsement of any products by the US Government nor the National Institutes of Health.

Additional information

Funding

This research was supported in part by the Center for Interventional Oncology and Intramural Research Program of the National Institutes of Health (NIH) and by NIH and National Cancer Institute grants ZID# BC011242 & CL040015 and NIH Clinical Center.