Image-Based Analysis of Heat Transfer in Biological Systems
A. Benard and J.J. McGrath
Department of Mechanical Engineering
2555 Engineering Building

Abstract

Two important thermal therapies are currently being evaluated for the treatment of numerous cancers: cryosurgery and hyperthermia. In cryosurgery, freezing is used to destroy cancer cells of both external and internal tumors. A cryoprobe is typically used to treat internal cancers such as prostate or liver cancers. In hyperthermia, heat is applied to a small area and used as an adjuvant to other therapy such as radiation or chemotherapy to destroy or damage cancer cells.

The outcome of these therapies is strongly dependent upon the heating or cooling rates imposed, as well as the temperatures reached. This implies that a very careful staging prior to surgery is needed to ensure that all the diseased tissue is destroyed during the procedure while preserving the nearby delicate organs or tissues. In recent years, it was proposed to monitor the progress of the freezing front with magnetic resonance imaging (MRI) or ultrasound and this led to a renewed interest in these techniques. But even with this monitoring, it is difficult for a surgeon to evaluate precisely the required duration of the treatment and the amount of tissue destroyed after the procedure. We are therefore developing a computer model that will perform 3D heat transfer computations in complex biological systems. The computer model will be based of images and will be able to simulate heating as well as freezing processes. The results of these computations will help ensure that the optimal protocol is established prior to the surgery.

In order to address the needs of a specific patient, the computer model must use data that reflect the particular situation at hand, namely the layout of the tumor(s) and the delicate organs or tissue that must be protected. Also, in order to predict the survival of the cells affected by the thermal treatment, the damage mechanisms associated with heating or freezing must be well understood. Modern imaging techniques, such as magnetic resonance imaging (MRI), can provide detailed information about the layout of the tumors and delicate organs. MRI can also be used to measure temperature and blood perfusion. This allows MRI to be used for two purposes in this work: 1) it will provide the domain (images) required performing the computations; 2) it will be used to monitor temperature (freezing front position in cryosurgery) and estimate blood perfusion.

This work is now possible due to a convergence of recent mathematical and computational developments with imaging techniques. The computer simulation will use a novel image-based numerical technique to rapidly perform complex computations in domains of arbitrary shape. Also, current models used to describe freezing/heating processes in biological systems are relatively simple and further model development will be pursued to improve their accuracy. Finally, MRI will be used to perform non-invasive temperature measurements in test devices to further the development of the computer code, to evaluate the physical models used and developed, and assess the entire methodology proposed.