Biomedical Engineering Reference
In-Depth Information
For 3-5 GHz, the tube radius (
a
) is generally in the range of 3-5 mm. A 3-mm-
size tube filled with high dielectric constant material can be utilized in many
medical applications, including MBA, and when truncated will radiate energy
into the tissue at 3-5 GHz.
Simulation of the power density distribution (pointing vector) of a trun-
cated circular waveguide is shown in Figure 6.3
c
.
It is worthwhile mentioning that in a flexible circular waveguide (flexible
catheter) the waveguide can introduce mode conversion/reconversion as a
consequence of bending or shape transformation (circular waveguide to ellip-
tical waveguide), which in turn increases the waveguide's loss. In our particu-
lar application, this is not critical.
Power Capacity of a Circular Waveguide
For the TE
1,1
mode, the relation-
ship between the power capacity (
P
) and the maximum allowable field
strength
E
m
ax
is
P
E
Ê
Á
l
l
ˆ
˜
=
199
.
10
¥
-
3
a
2
(6.10)
2
max
g
where
E
max
30,000 V/cm in an air-filled guide under standard sea-level con-
ditions of temperature, pressure, and humidity and between 3-5 times higher
in a deionized-water (e
2
80) filled guide. In standard applications, the
maximum power that the guide will safely carry is a fraction of the theoreti-
cal maximum.
6.3
ANTENNAS
6.3.1
Fundamentals
To effectively deliver microwave energy to heat myocardial tissue and create
histological changes, it is essential to control the size and location of the
radiated field, thus controlling the affected tissue volume. In our studies,
three types of microwave antennas at the tip of a coaxial cable were investi-
gated: (1) the whip antenna (Fig. 6.1
a
), (2) the gap antenna (Fig. 6.1
b
), and
(3) the helical antenna (Fig. 6.1
c
). Temperature distributions along the tissue
cylinder in the direction of the antenna length for whip and gap antennas
were measured to have a Gaussian shape with peaks adjacent to the junction
in the whip antenna, and a fall of temperature rise, and therefore tissue
damage, was observed near the tip of the antennas. These two types of anten-
nas were ideal for the treatment of most cases of atherosclerotic arteries and
were utilized through all of the MBA studies. The helical antenna—that has
some end-fire characteristics, if designed correctly, thus facilitating heating
next to the antenna tip—was not used for MBA since the very small coil size
(0.5 mm diameter) needed for coronary angioplasty failed to show any dis-
advantages in preliminary testing in vitro and is so much more difficult to
produce.
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