Biomedical Engineering Reference
In-Depth Information
0-1
1-2
2-3
3-4
4-5
5-6
6
5
4
3
20
2
16
1
12
0
8
900
800
700
600
500
400
4
300
200
100
Frequency (MHz)
FIGURE 4.6 (See color insert.)
Dependence of effective penetration depth
d
ef
into muscle-like tissue associated with square aperture sources
upon frequency
f
and aperture size
a
. The surface
d
ef
(
f
,
a
) is colored according to 1 cm increments in
d
ef
.
4.7 Invasive Heating techniques
in a relatively low current density in that region and therefore
relatively low SAR. Although rigid needle electrodes were used
originally, flexible electrodes became more common (Kapp et al
.
1988). To control the resulting elevated temperature distribution
within the targeted volume, longitudinally segmented electrodes
(Prionas et al
.
1988), hollow electrodes cooled by water flow-
ing through the lumen (Zhu and Gandhi 1988), and means of
multiplexing RF power to pairs of electrodes (Leybovich et al.
2000) have been introduced. An associated method operating
at 27 MHz, a frequency sufficiently high for the use of capaci-
tive coupling to the tissues rather than direct contact, has also
been described (van der Koijk et al
.
1997, Crezee et al
.
1999, van
Vulpen et al
.
2002).
4.7.1 rF Interstitial Hyperthermia
RF techniques for interstitial hyperthermia rely on Joule (
I
2
R
)
heating due to current f low between pairs of electrodes. A
frequency in the range 500 kHz to 1 MHz is usually chosen
for most RF interstitial hyperthermia techniques (referred to
as localized current field [LCF] methods), satisfying the need
to avoid direct stimulation of nerve and muscle fibers and for
conduction currents to dominate displacement currents (i.e.,
σ>> ω ε′ ε
o
).
Figure 4.7 shows a simple model of a bipolar technique in which
a pair of RF electrodes, each 1 mm in diameter, is implanted to
a depth of 5 cm and with center to center spacing of 10 mm in a
muscle-like phantom. The SAR falls off rapidly with distance from
the electrodes, although with a ratio of center to center spacing/
electrode diameter of 10, the regions around each electrode in
which SAR is greater than -3 dB of the peak SAR are contiguous.
For larger separations (e.g., a ratio of center to center spacing/
electrode diameter of 15), this is no longer the case, and heating
of the intervening tissue is essentially dependent upon thermal
conduction and tissue blood flow. Since the volume of tissue that
can be heated by a single pair of electrodes is relatively small,
an array of parallel electrodes is often implanted into the tissue.
Experience has shown that the diameter of the electrodes should
be 1-1.6 mm and that the interelectrode spacing should not be
greater than about 15 mm. It is important that the electrodes are
parallel since converging electrodes will lead to excessive heat-
ing in those regions where the interelectrode spacing is reduced.
If one of the needle electrodes is replaced by a larger electrode
at the skin surface, then a monopolar technique can be used. In
this case the SAR distribution around the needle remains local-
ized while the larger surface area of the return electrode results
4.7.2 rF ablation (rFa)
Techniques that lead to rapid heating and consequent tissue
destruction (thermal ablation) have been developed for treating
a range of conditions. Such minimally invasive, image-guided
therapy can provide effective local treatment and is also used
adjunctively with conventional surgery, systemic chemotherapy,
and radiation.
Cardiac catheter RFA involves the delivery of energy from a
catheter inserted into the heart through a vein to endocardial tis-
sue responsible for causing changes in the normal heart rhythm.
Both monopolar and bipolar techniques have been used to cause
desiccation of the targeted tissue. Control of the temperature at
the catheter tip is important in determining lesion size and also
in achieving reproducibility by preventing tissue boiling, forma-
tion of blood coagulum at the electrode surface, and subsequent
impedance changes at the catheter/tissue interface (Hindricks
et al
.
1989, Jackman et al
.
1988). Lesions produced by a typical RF
catheter with, say, a 4 mm diameter tip, are small (Huang 1991,
