08-02-2013, 01:56 PM
TREATING CARDIAC DISEASE WITH CATHETER-BASED TISSUE HEATING
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ABSTRACT
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.
ABSTRACT
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.
HUMAN BODY
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.
MICROWAVE ASSISTED BALLOON ANGIOPLASTY
An alternative physical process which can quickly deposit power in conductive media is microwave irradiation. Since the artherosclerotic plaque , which collects on the inner walls of the blood vessels, is composed of lipids and calcium particles, it can be considered LWC tissue. The healthy blood vessel wall out side the plaque layer is mostly muscle like HWC tissue. The challenge of microwave assisted balloon angioplasty (MABA) is to sufficiently heat the plaque layer without over heating the surrounding vessel wall. In addition, since plaque occlusions occur asymmetrically, it is essential to show that the electric field intensity and the deposited power 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 incorporated within and surrounding a catheter balloon. These applications made use of vary narrow antennas that could easily be guided through blood vessels into coronary arteries, and they relied primarily on field attenuation in the plaque layer to avoid harming the healthy artery wall. Since coaxial cables are commonly used as microwave transmission lines, they are naturally suitable to connect a microwave antenna to a power source in a catheter based applications. The first reported MABA applications were dipoles and small radius helices, which tends to radiate with electric fields aligned parallel to their axes and thus, the artery wall. With this orientation, more power tends to be deposited in the healthy tissue than in the outer plaque surface, it is important to avoid overheating the artery wall, if possible. Understanding the role played by the wave polarization has led to an alternative MABA application design which minimizes the heating of healthy tissue.
Microwave Cardiac Ablation
Another application of catheter based microwave heating is the treatment of abnormal heart rhythm, or cardiacarrhythmia .this life threatening disease , which affects over 300,000 Americans yearly, is caused by anomalous electrical activity in certain areas of the heart. Although drugs can be used to control the excessively rapid heart beat, mechanically removing or destroying section of this tissue is more effective in curing arrhythmias. Selective catheter fed ablation, or excessive heating of tissue, destroys the region of the heart responsible for the anomalous electrical activity. Radio frequency (RF) ablation, operating at frequencies between 100KHz and 10 MHz has a high success rate in treating a wide range of atrial ventricular cardiac arrhythmias . However with absorbed power decreasing uniformly in all directions 1/r4, the heated region tends to be small. Also, there is no control of the shape of the heating patterns; for large lesions, the electrode must be positioned and moved many times to ablate the entire region. Larger lesions cannot be created by increasing the power to the RF electrode, as this leads to tissue charring which introduces impedance mismatches and prevent additional power transfer . To treat ventricular tachycardia, a particularly large volume of tissue usually must be ablated. RF ablation is generally limited to a depth of 0.5 cm, in sufficient for eliminating deep diseased tissues.