Monitoring of tissue optical properties during thermal coagulation of ex vivo tissues

Abstract

Background and objective: Real-time monitoring of tissue status during thermal ablation of tumors is critical to ensure complete destruction of tumor mass, while avoiding tissue charring and excessive damage to normal tissues. Currently, magnetic resonance thermometry (MRT), along with magnetic resonance imaging (MRI), is the most commonly used technique for monitoring and assessing thermal ablation process in soft tissues. MRT/MRI is very expensive, bulky, and often subject to motion artifacts. On the other hand, light propagation within tissue is sensitive to changes in tissue microstructure and physiology which could be used to directly quantify the extent of tissue damage. Furthermore, optical monitoring can be a portable, and cost-effective alternative for monitoring a thermal ablation process. The main objective of this study, is to establish a correlation between changes in tissue optical properties and the status of tissue coagulation/damage during heating of ex vivo tissues.

Materials and methods: A portable diffuse reflectance spectroscopy system and a side-firing fiber-optic probe were developed to study the absorption (μa (λ)), and reduced scattering coefficients (μ's (λ)) of native and coagulated ex vivo porcine, and chicken breast tissues. In the first experiment, both porcine and chicken breast tissues were heated at discrete temperature points between 24 and 140°C for 2 minutes. Diffuse reflectance spectra (430-630 nm) of native and coagulated tissues were recorded prior to, and post heating. In a second experiment, porcine tissue samples were heated at 70°C and diffuse reflectance spectra were recorded continuously during heating. The μa (λ) and μ's (λ) of the tissues were extracted from the measured diffuse reflectance spectra using an inverse Monte-Carlo model of diffuse reflectance. Tissue heating was stopped when the wavelength-averaged scattering plateaued.

Results: The wavelength-averaged optical properties, <μ's (λ)> and <μa (λ)>, for native porcine tissues (n = 66) at room temperature, were 5.4 ± 0.3 cm(-1) and 0.780 ± 0.008 cm(-1) (SD), respectively. The <μ's (λ)> and <μa (λ)> for native chicken breast tissues (n = 66) at room temperature, were 2.69 ± 0.08 cm(-1) and 0.29 ± 0.01 cm(-1) (SD), respectively. In the first experiment, the <μ's (λ)> of coagulated porcine and chicken breast tissue rose to 56.4 ± 3.6 cm(-1) at 68.7 ± 1.7°C (SD), and 52.8 ± 1 cm(-1) at 57.1 ± 1.5°C (SD), respectively. Correspondingly, the <μa (λ)> of coagulated porcine (140.6°C), and chicken breast tissues (130°C) were 0.75 ± 0.05 cm(-1) and 0.263 ± 0.004 cm(-1) (SD). For both tissues, charring was observed at temperatures above 80°C. During continuous monitoring of porcine tissue (with connective tissues) heating, the <μ's (λ)> started to rise rapidly from 13.7 ± 1.5 minutes and plateaued at 19 ± 2.5 (SD) minutes. The <μ's (λ)> plateaued at 11.7 ± 3 (SD) minutes for porcine tissue devoid of connective tissue between probe and tissue surface. No charring was observed during continuous monitoring of thermal ablation process.

Conclusion: The changes in optical absorption and scattering properties can be continuously quantified, which could be used as a diagnostic biomarker for assessing tissue coagulation/damage during thermal ablation. Lasers Surg. Med. 48:686-694, 2016. © 2016 Wiley Periodicals, Inc.

Keywords: diffuse reflectance spectroscopy; thermal ablation; thermal tissue damage; tissue optical properties.