Rheology and heat transport properties of a hydroxyethyl cellulose-based MRI tissue phantom

Abstract

A saline solution of hydroxyethyl cellulose has been recommended for use as a tissue phantom in testing the behavior of medical devices in MRI scanners. It has been stated in the standards governing these tests that the viscosity of the fluid used should be large enough that bulk transport or convection currents are not supported. In this study we evaluated a hydroxyethyl cellulose phantom based on an ASTM standard to determine the degree to which it supports convective, as compared to conductive, heat transport. We study the rheological properties of this fluid, and find that it behaves as a typical viscoelastic polymer solution. As a result, it flows in response to local heating, such as would occur due to eddy-current heating of a metallic device in an MR scanner. We use laboratory experiments and numerical simulations to determine the convective and conductive contributions to the heat transport in a simple model of this system. Our results indicate that convective heat transport is of the same order of magnitude as conductive transport under conditions typical of MRI device tests. This indicates that heating tests conducted with this fluid are not completely conservative in terms of estimating local temperature changes for medical devices in vivo. It also indicates that convective processes should be included along with conduction in computer simulations of device heating in order to allow accurate comparison with experimental measurements.