30.9

*Medical Applications of the Laser

One of the medical areas in which the laser has had a substantial impact is in ophthalmology, which deals with the structure, function, and diseases of the eye. Section 26.10 discusses the human eye and the use of contact lenses and eyeglasses to correct nearsightedness and farsightedness. In these conditions, the eye cannot refract light properly and produces blurred images on the retina.

The physics of PRK eye surgery.

A laser-based procedure known as PRK (photorefractive keratectomy) offers an alternative treatment for nearsightedness and farsightedness that does not rely on lenses. It involves the use of a laser to remove small amounts of tissue from the cornea of the eye (see Figure 26-34) and thereby change its curvature. As Section 26.10 points out, light enters the eye through the cornea, and it is at the air/cornea boundary that most of the refraction of the light occurs. Therefore, changing the curvature of that boundary can correct deficiencies in the way the eye refracts light, thus causing the image to be focused onto the retina where it belongs. Ideally, the cornea is dome-shaped. If the dome is too steep, however, the rays of light are focused in front of the retina and nearsightedness results. As Figure 30-28a shows, the laser light removes tissue from the center of the cornea, thereby flattening it and increasing the eye’s effective focal length. On the other hand, if the shape of the cornea is too flat, light rays would come to a focus behind the retina if they could, and farsightedness occurs. As part b of the drawing illustrates, the center of the cornea is now masked and the laser is used to remove peripheral tissue. This steepens the shape of the cornea, thereby shortening the eye’s effective focal length and allowing rays to be focused on the retina.

Figure 30-28 (a) To correct for myopia (nearsightedness) using the PRK procedure, the laser vaporizes tissue (dashed line) on the center of the cornea, thereby flattening it. (b) To correct for hyperopia (farsightedness), the laser vaporizes tissue on the peripheral region of the cornea, thereby steepening its contour.

The physics of LASIK eye surgery.

The LASIK (laser-assisted in situ keratomileusis) procedure uses a motor-powered blade known as a microkeratome to partially detach a hair-width flap (about 0.2 mm thick) in the front of the cornea (see Figure 30-29). The flap is pulled back and the laser beam then remodels the corneal tissue underneath by vaporizing cells. Afterward, the flap is folded back into place, with no stitches being required. The laser light in the PRK and LASIK techniques is pulsed and comes from an ultraviolet excimer laser that produces a wavelength of 193 nm. The cornea absorbs this wavelength extremely well, so that weak pulses can be used, leading to highly precise and controllable removal of corneal tissue. Typically, 0.1 to 0.5 m of tissue is removed by each pulse without damaging adjacent layers.

Figure 30-29 To correct for myopia (nearsightedness) using the LASIK technique, the laser vaporizes tissue (dashed line) on the cornea, thereby flattening it.

The physics of removing port-wine stains.

Another medical application of the laser is in the treatment of congenital capillary malformations known as port-wine stains, which affect 0.3% of children at birth. These birthmarks are usually found on the head and neck, as Figure 30-30a illustrates. Preferred treatment for port-wine stains now utilizes a pulsed dye laser. Figure 30-30b shows an example of an excellent result after irradiation with laser light of wavelength 585 nm, in the form of pulses lasting 0.45 ms and occurring every 3 s. The laser beam was focused to a spot 5 mm in diameter. The light is absorbed by oxyhemoglobin in the malformed capillaries, which are destroyed in the process without damage to adjacent normal tissue. Eventually the destroyed capillaries are replaced by normal blood vessels, which causes the port-wine stain to fade.

Figure 30-30 A patient with a port-wine stain (a) before and (b) after treatment using a pulsed dye laser (). [From E. Tan and C. Vinciullo, Pulsed Dye Laser Treatment of Port Wine Stains, Med J. Australia, 164 (6):333, 1996. © 1996. Reproduced with permission.]

The physics of photodynamic therapy for cancer.

In the treatment of cancer, the laser is being used along with light-activated drugs in photodynamic therapy. The procedure involves administering the drug intravenously, so that the tumor can absorb it from the bloodstream, the advantage being that the drug is then located right near the cancer cells. When the drug is activated by laser light, a chemical reaction ensues that disintegrates the cancer cells and the small blood vessels that feed them. In Figure 30-31 a patient is being treated for cancer of the esophagus. An endo-scope that uses optical fibers is inserted down the patient’s throat to guide the red laser light to the tumor site and activate the drug. Photodynamic therapy works best with small tumors in their early stages.

Figure 30-31 Photodynamic therapy to treat cancer of the esophagus is being administered to this patient. Red laser light is routed to the tumor site with an endoscope that incorporates optical fibers. (Fritz Hoffmann/The Image Works)

Test your understanding of the material in Sections 30.6, 30.7, 30.8 and 30.9 :

· | The Pauli Exclusion Principle and the Periodic Table of the Elements
· | X-Rays
· | The Laser

Copyright © 2007 John Wiley & Sons, Inc. All rights reserved.