Page 284 - Computational Modeling in Biomedical Engineering and Medical Physics

Hyperthermia and ablation 273

Fig. 8.13. It is important to underline that a supplementary restriction is introduced in
the study, and it refers to keeping the temperature within the therapeutic range. The
maximum temperature is easy to be monitored because it occurs near the radiating

slot of each antenna; it is set to 44 C, and the temperature control by the adjustment
of the feeding power of the antennae is thus applied if necessary.
For the next step of the analysis, the presence of a large blood vessel in the inter-
ventional zone is considered, as the computational domain in Fig. 8.15 is showing.
The numerical model is complemented with phenomena associated with the blood
flow through a large artery, that is, heat transfer by forced convection introduced in
Eq. (8.11b) and flow dynamics described in Eq. (8.13).
Some significant functional points are marked in Fig. 8.15: the Inlet is important
because an adequate boundary condition should be specified for the fluid flow at the
entrance of the bloodstream into the computational domain; Station Q represents the
exit point for the bloodstream; Station P marks the position of a temperature sensor
for the monitoring of the heating in the interventional region.
Three timescales govern the dynamics highlighted by the set of equations adopted
here: the problem of the electromagnetic wave evolves at high speed, or with the fast-
est time scale, followed by the flow problem at a slower time scale and finally the heat
transfer problem, which has the slowest evolution, that is, slowest time scale. The solv-
ing algorithm applied here for moderate hyperthermia does not take into consideration
the variation of physical properties with the temperature; in such circumstances, the
EMF problem, Eq. (8.10), is solved first and its solution is used for the estimate of the
resistive heat Q emf .
Heat transfer and flow, Eqs. (8.11b) and (13), should be solved in one step, due to
the convective term coupling. Aiming to get a proper balance between the actual
time step size, and the total time required to get the steady-state of the heating

Figure 8.15 Computational domain with an arterial tree segment. Temperature is recorded at
Station P.