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.