Connecting tissue injury, temperature and mechanical properties

Abstract

Temperature related tissue injuries are often present in most hyperthermia treatments. Tissue temperature and its mechanical pro-perties are closely related due to the physiological changes caused by high temperature. An increase in tissue temperature will result in a decrease in moisture content in tissue, and permanent cell necrosis when temperature exceeds a threshold. It is therefore necessary to review the connection between temperature, mechanical properties and injury during hyperthermia treatment. A full understanding between the tissue tempera-ture and mechanical properties allow for possible injury assessment through methods for detecting mechanical properties change such as Magnetic Resonance Elastography (MRE) and vice versa. Many forms of hyperthermia treatment exist in clinical practices; Radio-frequency (RF) ablation, Microwave ablation, Laser ablation, Ultrasonic ablation and Cryoablation. This publication concentrates on thermal injury caused by RF ablation and its effects on liver tissue temperature and mechanical properties. RF assisted methods have been widely used in the treatment for hepatocellular cancers, breast tumors and cardiology treatments. RF ablation works on the principle of a high frequency electric current which generates ionic agitation and frictional heating in the target tissue. Catheters used for RF ablation are often needle electrodes for high current density and can be found in two forms; monopolar or bipolar electrodes. Monopolar electrodes work on the principle of a single polarity electrode with a large grounding pad attached to patients while bipolar electrodes are electrode pairs with dual polarity hence do not require a grounding pad. The electric field lines that are induced from the electrode tip by the applied voltage causes an electric force on the charged ions within the electrolytic medium of the liver tissue. This induced force produces a motion that causes ions in the tissue to rub against the surrounding fluid medium, causing friction and thus frictional heating. Increase in tissue temperature due to RF ablation results in moisture loss and protein denaturalization. Moffitt et al. (2002) and Walsh Jr et al (1989) both reported differences in mechanical property between native and thermally damaged liver. In our experi-ments, observations were also made for ablation time, to be positively related to a change in tissue stiffness. Ablation time is a good approxi-mation for tissue temperature and hence a relationship can be established between tissue temperature and mechanical properties. Mechanical properties of tissue can be measured by a compression test rig in-vitro or Magnetic Resonance Elastography (MRE) in-vivo. Temperature of tissue sample can be measured by thermocouple for point measurements and thermal cameras for plane measurements. Degree of tissue injury is not only a function of temperature but also a function of time exposed to a critical temperature. Liver tumor necrosis occurs at 45oC when held for long duration (hours) while necrosis occurs within minutes when temperature is above 60oC (Taton 2008, Baldwin 2001). Hence, tissue injury has to be correlated to temperature and exposed time. The degree of injury can be quantified by means of optical and fluorescence characteristics (Lin 2003) or Optical Coherence Tomography (OCT) (Wierwille 2010). Tissue variation is a big hurdle in the relationship between tissue injury, tissue temperature and mechanical properties. Microstructure variation in liver tissue causes non-systematic temperature distribution due to the many tissue variables; electric conductivity, thermal conductivity, blood perfusion rate, density, etc., governed by Joule heating and bioheat transfer equation. Hence affects homogeneity of RF ablation lesions and mechanical properties. A novel method is required to handle tissue variation for better simulation results and correlation between temperature and mechanical properties. A stochastic model might be a good approach to account for the tissue variation, while nondestructive medica. © 2012 by Nova Science Publishers, Inc. All rights reserved.