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 microwaves.
Balloon angioplasty ( or Percutaneous Transluminal Coronary Angioplasty) has become one of the most commonly performed major cardiac operations in the United States. Compared to other surgical procedure, balloon angioplasty is relatively simple. A Special Catheter with a collapsed narrow inflatable balloon is inserted into a vein through an incision in the neck or leg and fed through blood vessels until it reaches the diseased arteries of the heart. Fluid is then pumped into the balloon inflating it to several times its nominal diameter. The enlarged tip quickly compresses the layer of plaque which is clogging the artery, leaving a much wider opening for blood flow. The balloon is then deflated and it is withdrawn with the catheter. The procedure avoids cardiac bypass surgery, or other more traumatic operation, and has been very successful at both extending and improving the quality of life.
Unfortunately, abrupt reclosure occurs in 3-5 % of the cases in which the balloon angioplasty is used and gradual restenosis of the artery occurs in 17-34% of the cases. Fiber optic guided laser light has been used to irradiate and thermally fuse fragmented plaque pieces following coronary angioplasty . Beneficial welding effects have been obtained for tissue temperature between 95 – 135 C. Although these previous studies have used laser radiation to deliver power to plaque, it is concluded that welding is primarily a thermal process dependent in maintaining an elevated temperature level. If sufficient heat can be delivered to the plaque it will become thermally fixed in place and compressed against the artery wall. However when using laser energy it is difficult to determine the proper laser intensity and length of exposure. Physicians must be extremely careful to avoid burning the healthy artery tissue and perforating the blood vessel wall. Insufficient exposure results in poor welding while too much injures the sensitive coronary artery.
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.
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