| Treating
Cardiac Disease With Catheter-Based Tissue Heating |
Definition
In microwave ablation, electromagnetic energy would be delivered via a catheter
to a precise location in a coronary artery for selective heating of a targeted
atherosclerotic lesion. Advantageous temperature profiles would be obtained by
controlling the power delivered, pulse duration, and frequency. The major components
of an apparatus for microwave ablation apparatus would include a microwave source,
a catheter/transmission line, and an antenna at the distal end of the catheter
.The antenna would focus the radiated beam so that most of the microwave energy
would be deposited within the targeted atherosclerotic lesion. Because of the
rapid decay of the electromagnetic wave, little energy would pass into, or beyond,
the adventitia. By suitable choice of the power delivered, pulse duration, frequency,
and antenna design (which affects the width of the radiated beam), the temperature
profile could be customized to the size, shape, and type of lesion being treated. For
decades, scientists have been using electromagnetic and sonic energy to serve
medicine. But, aside from electro surgery, their efforts have focused on diagnostic
imaging of internal body structures-particularly in the case of x-ray, MRI, and
ultrasound systems. Lately, however, researchers have begun to see acoustic and
electromagnetic waves in a whole new light, turning their attention to therapeutic-rather
than diagnostic-applications. Current research is exploiting the ability of radio-frequency
(RF) and microwaves to generate heat, essentially by exciting molecules. This
heat is used predominantly to ablate cells. Of the two technologies, RF was the
first to be used in a marketable device. And now microwave devices are entering
the commercialization stage. These technologies have distinct strengths weaknesses
that will define their use and determine their market niches. The depth to which
microwaves can penetrate tissues is primarily a function of the dielectric properties
of the tissues and of the frequency of the micro waves.
The
tissue of the human body is enormously varied and complex, with innumerable types
of structures, components, and cells. These tissues vary not only with in an individual,
but also among people of different gender, age, physical condition, health and
even as a function of external in puts, such as food eaten, air breathed, ambient
temperature, or even state of minds. From the point of view of RF and Microwaves
in the frequency range 10 MHz ~ 10GHz, however biological tissue can be viewed
macroscopically in terms of its bulk shape and electromagnetic characteristic:
dielectric constant and electrical conductivity . These are dependent on frequency
and very dependent on the particular tissue type. All
biological tissue is somewhat electrically conductive, absorbing microwave power
and converting it to heat as it penetrates the tissue. Delivering heat at depth
is not only valuable for cooking dinner, but it can be quite useful for many therapeutic
medical applications as well. These includes: diathermy for mild orthopedic heating,
hyperthermia cell killing for cancer therapy, microwave ablation and microwave
assisted balloon angioplasty. These last two are the subject of this article.
It should also be mention that based on the long history of hi power microwave
exposure in human, it is reasonable certain that, barring overheating effects,
microwave radiation is medically safe. There have been no credible reported carcinogenic
, muragenic or poisonous effects of microwave exposure.
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