Biomedical Engineering Reference
In-Depth Information
9
Interstitial Electromagnetic
Devices for Thermal Ablation
9.1 Introduction of Thermal Tissue Ablation ............................................................................159
9.2 Biophysics of Radiofrequency Ablation ...............................................................................159
Electrode Cooling • Electrical Tissue Conductivity at Radiofrequencies
9.3 Clinical Applications of Radiofrequency Ablation ............................................................162
Cancer Treatment (aka Radiofrequency Tumor Ablation) • Cardiac Arrhythmia
Treatment • Other Applications of RF Ablation
9.4 Biophysics of Microwave Ablation ........................................................................................164
Dielectric Properties of Biological Tissues • Electromagnetic Interactions with
Tissue • Microwaves Compared to Other Sources of Thermal Therapy
9.5 Microwave Ablation Systems .................................................................................................168
Power Generation and Distribution • Microwave Ablation Antenna
Design • Multiple-Antenna Arrays
9.6 Microwave Ablation Conclusions .........................................................................................171
References .............................................................................................................................................172
Dieter Haemmerich
Medical University of
South Carolina
Chris Brace
University of Wisconsin, Madison
9.1 Introduction of thermal
tissue ablation
as stimulation of excitable tissue and electrochemical reactions
(at DC) may occur. To avoid these effects, RF ablation-based
heating employs higher frequencies in the range of 450-500
kHz (this particular range is also used because electrosurgical
devices, which served as predicate devices, operate in the same
frequency band). Figure 9.1 illustrates the setup of an RF abla-
tion procedure, where an electrode is inserted into a target loca-
tion in tissue to be destroyed. Materials used for RF electrodes
include steel, platinum, and Ni-Ti alloys; parts of the RF elec-
trode are typically electrically insulated (e.g., to avoid heating of
the shaft region). Ground pads (dispersive electrodes) are placed
on the patient's skin (often thigh or back) to serve as a return
path for the RF current.
Inside the RF generator, cables, and RF electrode, electric
current is carried by electrons. Inside tissue, however, ions (Na
+
,
K
+
, Cl
−
) serve as carriers of the electric current. The resulting
oscillations of ions produce heat due to friction (i.e., electrically
resistive heating).
The specific absorption rate (SAR [W/kg]) describes the
amount of power deposited locally within tissue. SAR is typi-
cally high close to the interstitial applicator and drops rapidly
with distance from the applicator—i.e., direct heating by RF
current is limited to close proximity of the applicator, and ther-
mal conduction allows more distant tissue regions to obtain
sufficient temperatures (Schramm 2007). For interstitial RF
he term
thermal ablation
refers to localized destruction of tis-
sue via heating above ~50°C for typically a few minutes. Heating
via radiofrequency (RF) electric current (RF ablation) with the
goal of locally destroying tissue has been in use clinically for
treatment of cardiac arrhythmia since the 1980s by destroying
small tissue regions that are responsible for the arrhythmia.
Currently, thermal ablation is clinically used to treat various
other diseases including varicose veins and uterine bleeding
(Haemmerich 2006a, Cooper 2004, Markovic 2009) as well as
various forms of cancer. In this chapter we will discuss engi-
neering and biophysics of interstitial electromagnetic devices for
thermal ablation that employ heating either by radiofrequency
electric current or by microwaves.
9.2 Biophysics of radiofrequency
ablation
When electric current is applied to tissue, heating results due to
resistive losses. This heating occurs independent of frequency of
the current; at frequencies below ~10 kHz additional effects such
159
