Biomedical Engineering Reference
In-Depth Information
80°C
Ablation zone
50°C
Tissue
Blood
Tumor
Electrode
5 mm
Liver
FIGURE 9.9
Tissue temperature around a cardiac RF ablation cath-
eter (2.3 mm diameter) at the end of a 45 s ablation. Outermost 50°C
boundary estimates boundary of ablation zone.
FIGURE 9.11
Illustration of a microwave ablation in liver. A zone of
complete cellular necrosis is created by heat generated from the appli-
cator antenna, which completely encompasses the tumor with a safety
margin.
heating (Figure 9.11). In this respect, microwave ablation is
the more acute and higher-temperature extension of micro-
wave hyperthermia, which has a longer history in the literature
and in clinical practice. Hyperthermia typically refers to the
temperature range of 41°C to 46°C, where most temperature-
induced physiological changes are reversible, while thermal
ablation at temperatures over 50°C is associated with irre-
versible changes such as denaturation of cellular proteins
and microvascular coagulation leading to rapid cell death
(Dewhirst 2003). Despite the difference in target temperature
range, the physics of heating tissues is relatively similar for
both therapies. A notable exception is when tissue tempera-
tures approach or exceed 100°C, when a series of temperature-
induced changes in tissue properties must be considered (Brace
2010b, Brace 2008).
Electromagnetic energy propagation can be described by
solving Maxwell's equations in a source-free lossless medium:
⋅=
E
0
(9.3)
×=−
∂
∂
B
t
E
(9.4)
⋅=
B
0
(9.5)
×=µε
∂
∂
E
t
B
(9.6)
Conduction to myocardium
Conduction to electrode
50°C after 1 s
Heart
Electrode
50°C after 60 s
Tissue
Blood
Catheter body
Convective cooling from
blood
FIGURE 9.10
Thermodynamics of cardiac RF ablation: RF electrode is in contact with myocardium (heart tissue), and RF current results in
tissue heating. Thermal conduction of heat into the tissue results in growth of the ablation zone (approximated by 50°C isotherm). Heat loss of
tissue in proximity of the electrode is due to thermal conduction through the electrode (black arrow); electrode and tissue surface experience
convective cooling from blood inside the chamber. Surface cooling produces the typical tear drop-shaped ablation zone. (Reproduced from
Tungjitkusolmun, S., Finite element modeling of radiofrequency cardiac and hepatic ablation. PhD thesis University of Wisconsin, Madison,
2000.)
