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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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Abstract
In microwave ablation, electromagnetic energy would be delivered through a catheter to a precise location in a coronary artery for the selective heating of a directed atherosclerotic lesion. Advantageous temperature profiles would be obtained by controlling the delivered power, pulse duration and frequency.

The main components of a 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 directed atherosclerotic lesion. Due to the rapid decay of the electromagnetic wave, little energy would pass towards, or beyond, adventitia. By appropriate choice of power supplied, pulse duration, frequency and antenna design (affecting radiated beam width), the temperature profile could be adapted to the size, shape and type of injury Is being treated.

For decades, scientists have been using electromagnetic and sonic energy to serve medicine. But, apart from electro surgery, his efforts have focused on the diagnostic imaging of internal structures of the body, especially in the case of X-rays, MRI and ultrasound systems. Lately, however, researchers have begun to see the acoustic and electromagnetic waves in a completely new light, turning their attention to therapeutic applications rather than diagnostic applications. Current research is exploiting the ability of radiofrequency (RF) and microwaves to generate heat, essentially by exciting molecules. This heat is used predominantly to flap cells. Of the two technologies, RF was the first to be used in a marketable device. And now microwave devices are entering the marketing stage. These technologies have different strengths weaknesses that will define their use and determine their market niches. The depth at which microwaves can penetrate tissues is primarily a function of the dielectric properties of tissues and the frequency of microwaves.

The tissue of the human body is enormously varied and complex, with innumerable types of structures, components and cells. These tissues vary not only in an individual, but also among people of different sex, age, physical condition, health and even depending on external factors, such as food eaten, air breathed, room temperature, or even mood . From the RF and Microwave point of view in the frequency range 10 MHz ~ 10GHz, however, biological tissue can be seen macroscopically in terms of its mass shape and electromagnetic characteristics: dielectric constant and electrical conductivity. These are frequency dependent and highly dependent on the particular tissue type.

All biological tissue is electrically conductive, absorbing microwave energy and converting it into heat as it enters the tissue. Delivering heat in depth is not only valuable for cooking dinner, but can also be very useful for many therapeutic medical applications. These include: 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 mentioned that based on the long history of exposure to high power microwaves in humans, it is reasonable that, except for the effects of overheating, microwave radiation is medically safe. No credible, carcinogenic, muragenic, or poisonous effects have been reported from microwave exposure.
A phenomenal force
Electromagnetic radiation begins with a phenomenon that occurs when the electric current flows through a conductor, such as a copper wire. The movement of the electrons through the wire produces an energy field that surrounds the wire and floats just off its surface. This floating zone or cloud of energy is formed by two different energy fields, one electric and the other magnetic. The electric and magnetic waves that combine to form an electromagnetic wave travel at right angles to each other and to the direction of movement.

If the current flowing through the wire is oscillated at a very fast speed, the floating electromagnetic field will be released and released into space. Then, at the speed of light, the energy will radiate outward in a pulsating pattern, like the waves in the pond. It is theorized that these waves are formed by small packets of radiant energy called photons. The photon currents, each carrying energy and momentum, travel in waves like an undulating chain of cars on a speed roller coaster.

Microwave radiation versus radioactive radiation
There is a very important difference between these radiations. As shown in the right frequency spectrum, microwaves used in microwave ovens, similar to microwaves used in radar equipment, and telephone, television and radio communications, are in the non-ionizing range of electromagnetic radiation. Non-ionizing radiation is very different from ionizing radiation. Ionizing radiation is unusually high in frequency (millions of trillion cycles per second). Therefore, it is extremely powerful and penetrating. Even at low levels, ionizing radiation can damage living tissue cells. In fact, these dangerous rays have enough energy and intensity to actually change (ionize) the molecular structure of matter. At sufficient doses, ionizing radiation can even cause genetic mutations. As shown in the frequency spectrum, the range of ionizing frequencies includes x-rays, gamma rays and cosmic rays. Ionizing radiation is the type of radiation we associate with radioactive substances such as uranium, radium and the fall of atomic and thermonuclear explosions.

Non-ionizing radiation is very different. Due to lower frequencies and reduced energy, it does not have the same damaging and cumulative properties as ionizing radiation. Microwave radiation (at 2450 MHz) is not ionizing, and at sufficient intensity it will simply cause the molecules in matter to vibrate, thus causing friction, which produces the heat that cooks food.
GLOBAL ANGIOPLASTY


Balloon angioplasty (or percutaneous transluminal coronary angioplasty) has become one of the most commonly performed major cardiac operations in the United States. Compared with other surgical procedures, balloon angioplasty is relatively simple. A special catheter with a narrow collapsed inflatable balloon is inserted into a vein through an incision in the neck or leg and is fed through the blood vessels until it reaches the diseased arteries of the heart. The fluid is then pumped to the balloon by inflating several times its nominal diameter. The enlarged tip quickly compresses the plaque layer that is clogging the artery, leaving a much wider opening for blood flow. The balloon is then deflated and withdrawn with the catheter. The procedure avoids cardiac bypass surgery, or another more traumatic operation, and has been very successful in both expanding and improving the quality of life.

Unfortunately, in 3-5% of cases where balloon angioplasty is used, sudden suddenness occurs, and gradual restenosis of the artery occurs in 17-34% of cases. Fiber optic guided laser light has been used to thermally irradiate and fuse plaque fragments after coronary angioplasty. Beneficial welding effects have been obtained for the fabric temperature between 95-135 ° C. Although these previous studies have used laser radiation to supply power to the plate, it is concluded that the welding is mainly a thermal process that depends on the maintenance of a high temperature level. If sufficient heat can be supplied to the plate, it will be thermally fixed in place and compressed against the wall of the artery. However, when using laser energy it is difficult to determine the proper laser intensity and duration of exposure. Physicians should be extremely careful to avoid burning the healthy tissue of the artery and piercing the wall of blood vessels. Insufficient exposure results in poor welding while too much damage to the sensitive coronary artery.

GLOBE ANGIOPLASTY MICROWAVE AID
An alternative physical process that can rapidly deposit energy in conductive media is microwave irradiation. Since the atherosclerotic plaque, which accumulates on the inner walls of blood vessels, is composed of lipids and calcium particles, it can be considered LWC tissue. The healthy wall of blood vessels that extends through the plaque layer is mostly muscular like HWC tissue. The challenge of microwave assisted balloon angioplasty (MABA) is to sufficiently heat the plaque layer without overheating the surrounding vessel wall. In addition, since plate occlusions occur asymmetrically, it is essential to demonstrate that the electric field strength and deposited potency are also concentrated in this LWC tissue layer even when it is predominantly on one side of the artery.

MABA devices were first reported by Rosen and subsequently studied. A patent was granted in 1991 which described a variety of antennas built into and around a catheter balloon. These applications made use of varied narrow antennas that could be easily guided through the blood vessels in the coronary arteries, and were based primarily on field attenuation in the plaque layer to avoid damaging the healthy arterial wall. Since coaxial cables are commonly used as microwave transmission lines, they are naturally suitable for connecting a microwave antenna to a power source in catheter-based applications.

The earliest applications of MABA were dipoles and small radio helices, which tends to radiate with electric fields aligned parallel to their axes and, therefore, to the wall of the artery. With this orientation, there is a tendency to deposit more energy in the healthy tissue than in the external surface of the plaque, it is important to avoid overheating of the arterial wall, if possible. Understanding the role played by wave polarization has led to an alternative design of MABA application that minimizes the heating of healthy tissue.