Page 265 - Computational Modeling in Biomedical Engineering and Medical Physics

254 Computational Modeling in Biomedical Engineering and Medical Physics

hotspot that kills the cancer cells situated in the electrode range (Radiology). The heat
source is then the electrothermal (Joule) effect. Numerical modeling may complement
other medical assertive elements in the preinterventional phase with the aim of pre-
cisely positioning the antenna and adjust its power level. An as simple and as accurate
possible prediction of the RFA protocol is a desideratum, and to this aim difficulties
related to the realistic representation of the tumor volume have to be surmounted.
Microwave ablation (MWA) is also a minimally invasive intervention used for the
same indications as RFA to heat and kill the tumor. MWA provides low risk and a
short hospital stay as an outpatient procedure, with overnight supervision in the hospi-
tal if general anesthesia is recommended. The electrodes for RFA and interstitial
MWA, specialized needle-like probes (Mulier et al., 2005), are inserted using an
image-guided technique: magnetic resonance imagery (MRI), ultrasound (US), or
computed tomography (CT). Multiple tumors may be treated simultaneously, and the
procedure can be repeated if remittance occurs. RFA electrodes for soft tissue (kidney
and liver) are expanding rapidly (Goldberg et al., 2003). Recently the International
Working Group on Image-Guided Tumor Ablation (IWGIGTA) proposed their clas-
sification. Many new electrodes, commercial and experimental, have been introduced
since then, including “multiple electrode systems”.
The heat source characterization and its numerical modeling are one significant
part of the problem. The other part is the heat transfer in vascularized tissues. This
“physics” is usually addressed by the continuous media bioheat equation (Pennes)
(Chapter 1: Physical, Mathematical, and Numerical Modeling), which accounts for the
blood vessels heat transfer. However, this homogenization approach may be prone to
underestimation of the required power levels for a successful procedure when larger
than the capillaries vessels are present too.
This chapter presents several models of localized (interstitial) thermal therapy
(hyperthermia and ablation) with a focus on modeling the power sources (power level
and duration) and their sizing when applied to tissues where heat is conveyed through
hemodynamic flow in larger size vessels and the tissue, approximated to be a saturated
porous medium, called here the general heat transfer (GHT) model. The results are
compared to those obtained by using the bioheat (BHT) model. The exposure to
EMF is limited in time and power level to provide for the success of the procedure
and to avoid the thermally produced damage of the neighboring healthy tissue.
Numerical simulations may add to the experimental work in solving this matter (Sands
and Layton, 2000), as affordable, noninvasive and accurate methods used to solve
thought experiments and studies focused on patient-specific data, electrode design
(Koda et al., 2011), procedure duration, and power level. Numerical experiments are
normally performed on validated and reasonably realistic models of utmost utility for
exploring a wide spectrum of correlations between various interventional parameters
and for the assessment of corresponding physical consequences. For instance the power