Multivariate Regression Between Hounsfield Unit Shift, Tissue Temperature, and Tissue Contraction: A Feasibility Study of Computed Tomography Thermometry

Abstract

This study aims to determine the relationship between Hounsfield Unit shift (<inline-formula> <tex-math notation= LaTeX >$\\Delta {\\mathrm{ HU}}$ </tex-math></inline-formula>), tissue temperature change (<inline-formula> <tex-math notation= LaTeX >$\\Delta T$ </tex-math></inline-formula>), and tissue’s relative cross-sectional area change (<inline-formula> <tex-math notation= LaTeX >$\\Delta A$ </tex-math></inline-formula>) of ex vivo bovine liver tissues. The fresh bovine livers (<inline-formula> <tex-math notation= LaTeX >$n = 3$ </tex-math></inline-formula>) were heated from 21 °C to 100 °C within 24 min using a custom-made ceramic hot plate and left for cooling to room temperature (~70 min). HU and tissue temperature measurement were obtained at 3-min intervals during the experiment. After that, the cross-sectional tissue area changes, and <inline-formula> <tex-math notation= LaTeX >$\\Delta A$ </tex-math></inline-formula> was calculated. Finally, a multivariate linear regression analysis was performed to determine the relationship between <inline-formula> <tex-math notation= LaTeX >$\\Delta {\\mathrm{ HU}}$ </tex-math></inline-formula>, <inline-formula> <tex-math notation= LaTeX >$\\Delta T$ </tex-math></inline-formula>, and <inline-formula> <tex-math notation= LaTeX >$\\Delta A$ </tex-math></inline-formula>. The results were compared with that of the conventional <inline-formula> <tex-math notation= LaTeX >$\\Delta {\\mathrm{ HU}}$ </tex-math></inline-formula>-<inline-formula> <tex-math notation= LaTeX >$\\Delta T$ </tex-math></inline-formula> linear model. Tissues that underwent a larger thermal dose experienced a higher degree of tissue contraction, an irreversible process that contributed to the non-linear behavior in the <inline-formula> <tex-math notation= LaTeX >$\\Delta {\\mathrm{ HU}}$ </tex-math></inline-formula>-<inline-formula> <tex-math notation= LaTeX >$\\Delta T$ </tex-math></inline-formula> characteristic curve. This finding was in agreement with the histological test, in which the tissues nearer to the heat source had higher cell counts than those farther away. A phenomenological equation for <inline-formula> <tex-math notation= LaTeX >$\\Delta {\\mathrm{ HU}}$ </tex-math></inline-formula> in the function of <inline-formula> <tex-math notation= LaTeX >$\\Delta T$ </tex-math></inline-formula> and <inline-formula> <tex-math notation= LaTeX >$\\Delta A$ </tex-math></inline-formula> shows better regression with the experimental data (<inline-formula> <tex-math notation= LaTeX >$R^{2} \\in $ </tex-math></inline-formula> [0.838, 0.977]) than the conventional <inline-formula> <tex-math notation= LaTeX >$\\Delta {\\mathrm{ HU}}$ </tex-math></inline-formula>-<inline-formula> <tex-math notation= LaTeX >$\\Delta T$ </tex-math></inline-formula> linear model. This finding indicates that tissue contraction is an important parameter to be considered in the development of CT thermometry. The <inline-formula> <tex-math notation= LaTeX >$\\Delta {\\mathrm{ HU}}$ </tex-math></inline-formula>-<inline-formula> <tex-math notation= LaTeX >$\\Delta T$ </tex-math></inline-formula>-<inline-formula> <tex-math notation= LaTeX >$\\Delta A$ </tex-math></inline-formula> characteristic equation presented in this work offers accurate estimation of tissue temperature based on the HU change and magnitude of tissue contraction during a CT-guided thermal ablation procedure.