Medical Applications of Lasers

The various interactions of laser light of different wavelengths with body tissue give rise to a myriad of medical applications.  One fundamental interaction is simply the heating of tisue through the absorption of light.  At elevated temperatures proteins will denature or coagulate just as an egg does when cooked.  Laser-induced denaturation is known as photocoagulation and is of primary importance in laser surgery.  

Infrared light is very strongly absorbed by water and is therefore particularly strongly absorbed by tissue since water is its major component.  Our body tissue is, on average, about 70% water.  Photocoagulation is used in surgery for the destruction of tumors, retinal surgery and in many internal surgeries using fiber optics to gain internal access without having to open up the body.  

A major advantage of laser surgery is the fact that small blood vessels are cauterized, or made to clot through the photocoagulation process, so that there is a large reduction in bleeding.  Furthermore, high photon powers in short pulses can deliver large doses of energy to actually vaporize tissue locally, in a process known as photovaporization.  Such high energy doses raise the local temperature above the boiling point of water for long enough to completely vaporize the tissue, resulting in clean cuts with no bleeding and very limited damage to neighboring tissue.  Usually this results in less pain and swelling (edema) and a more rapid recovery from surgery.  Laser surgery is particularly effective in areas of the body that are full of blood vessels and prone to much bleeding, such as the throat, intestines, or uterus.  By regulating the intensity of the laser and/or the number of pulses, the depth of the vaporization can be controlled.

Another major advantage of laser surgery is the ability to do microscopic internal (or external) surgery using fiber optics.  Visible or near infrared light can be steered using a fine fiber optics catheter to various internal organs through either blood vessels or the gastrointestinal (GI) tract.  These catheters are designed with many fibers, some of which allow imaging of the location of the fiber tip by collecting reflected light, while others are used to carry the surgical laser beam, and still others may be designed to suction off waste gases from the destruction of the tissue.  The device for viewing is called an endoscope and is fine enough to be "injected" into a blood vessels just like a hypodermic needle. 

The wavelength of laser light, and therefore the type of laser, used will depend on the tissue to be destroyed.   Strong absorption lines are used to insure specific destruction of that type of tissue; for example, blood rich tissue will absorb strongly at a wavelength of 575 nm due to a strong hemoglobin absorption line.  Similarly, in retinal surgery, the lens of the eye is transparent to visible light, so that visible light can be used to surgically seal leaky capillaries behind the retina or to re-attach a retina by spot-welding using coagulated blood.  Until this type of laser surgery was available, a detached retina was a leading cause of blindness.

Early laser surgery used a carbon dioxide laser because of its intense IR beam, but fiber optics do not work so well at the 10 mm wavelength and a more cumbersome mirrored articulated tool was used.  Note that the IR beam is invisible and so the surgeon could not see where the beam is without the use of a low-powered co-linear He-Ne laser.  The laser of choice for visible light has been the argon ion laser with its intense blue/green beam.  Excimer lasers are increasingly being used since their high energy uv pulses have enough energy to photodecompose tissue, essentially vaporizing it without any spread of heat to neighboring tissue. 

Some other applications of laser surgery include:

• laser lithotripsy -  A high powered laser to used to break up kidney stones or gallstones, mineral deposits that will not pass through ducts.  Stones can lead to painful incidents if they clog a duct.  One treatment uses lasers while a second, probably more common treatment uses focused ultrasonic (shock) waves to also break up stones.
• laser angioplasty -  Plaque is the buildup of fatty deposits on the arterial walls and it can lead to a blockage or heart problems from excess strain in pumping blood.  Laser light can be used, via insertion in a fiber optic catheter, to drill a hole in the plaque and clear a blockage.  Most often laser angioplasty is done in conjunction with balloon angioplasty which insures that the arterial wall stays open.   Also, often a stent, or wire mesh cylinder, is inserted to prevent the arterial wall from collapsing, although plaque sometimes tends to re-collect on the stent or near damaged scar tissue.  See this web site for more information: laser angioplasty
• photodynamic therapy - Certain types of chemicals, known as photosensitizing agents, are able to bind to cells and then kill them when they are exposed to light of a particular wavelength.  In treating cancer, these agents are injected into the blood stream and, after waiting an appropriate length of time for them to leave normal cells but still be present in malignant cells, light is directed on the tumors using a fiber-optic catheter.  The tumors must be on or near the surface of skin or the lining of internal organs so that the laser light can penetrate the full extent of the tumor.  The interaction with the light produces free radicals, very active chemical groups that oxidize tissue, killing it.  See: photodynamic therapy
• lasik eye surgery-  A laser beam is used to re-sculpt (flatten) the surface of the cornea in order to correct for myopia (nearsightedness) or make a series of cuts in the cornea to cause it to bulge more in order to correct for hyperopia (far-sightedness).  This procedure is becoming very common and has a high success rate.  Nearsighted people over about 40 years of age also have presbyopia, or blurry near vision with normal eyeglasses due to a stiffening of the lens with age.  Normally these people would wear bi-focals, allowing for good distance and reading vision.  In one variation of LASIK, one eye is left alone so that the person would have good near vision without glasses, while the other eye is treated to produce good distance vision.  Roughly 75% of people who have had this surgery are able to adjust to seeing independently with their eyes, while the others get LASIK on the other eye and will need to wear reading glasses for close vision.  See LASIK
• removing birthmarks, tatoos, port-wine stains, warts, etc.  
• imaging the lungs using MRI with  helium gas hyperpolarized with laser light.  Normal Magnetic Resonance Imaging does not produce high resolution pictures of the lungs.  But when filled with helium gas that has had its spin aligned through the interaction with a laser.  This newly developed method may also be useful for improving the imaging of brain and other types of tissue accessible to gas molecules.
• laser acupuncture - flooding nerve endings with laser light instead of using needles

Questions on Medical Applications of Lasers