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

This study aims to determine the relationship between Hounsfield Unit shift (
$\Delta {\mathrm{ HU}}$
), tissue temperature change (
$\Delta T$
), and tissue’s relative cross-sectional area change (
$\Delta A$
) of ex vivo bovine liver tissues. The fresh bovine livers (
$n = 3$
) 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
$\Delta A$
was calculated. Finally, a multivariate linear regression analysis was performed to determine the relationship between
$\Delta {\mathrm{ HU}}$
,
$\Delta T$
, and
$\Delta A$
. The results were compared with that of the conventional
$\Delta {\mathrm{ HU}}$
-
$\Delta T$
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
$\Delta {\mathrm{ HU}}$
-
$\Delta T$
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
$\Delta {\mathrm{ HU}}$
in the function of
$\Delta T$
and
$\Delta A$
shows better regression with the experimental data (
$R^{2} \in $
[0.838, 0.977]) than the conventional
$\Delta {\mathrm{ HU}}$
-
$\Delta T$
linear model. This finding indicates that tissue contraction is an important parameter to be considered in the development of CT thermometry. The
$\Delta {\mathrm{ HU}}$
-
$\Delta T$
-
$\Delta A$
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.