Medical Lasers

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Medical Lasers

Definition

A laser is a device that transforms one type of energy, usually electrical, into optical energy. The light waves in the beam produced by a laser are nearly parallel (collimated), nearly monochromatic, and coherent. The light beam is produced by exciting atoms and causing them to radiate their energy in phase. The word laser is an acronym that stands for Lightwave Amplification by Stimulated Emission of Radiation.

Purpose

Lasers have proven useful in all medical specialties to vaporize or coagulate tissue. Surgeons use lasers to perform controlled linear vaporization in order to to cut tissue. Lasers can be used for surgery on all parts of the body, but are used most extensively in eye surgery and cosmetic skin procedures. An additional function of lasers is the sensing of physiological parameters.

Description

Lasers affect human tissue by transferring radiant energy to the target cells. The radiant energy turns into heat when the cells absorb it. As the target cells are heated, all their proteins are destroyed and their internal pressure rises rapidly. The cells then explode, giving off smoke-like steam called a laser plume. The major effects of most lasers on tissue are coagulation of blood and protein, and vaporization. Vaporization is the removal of tissue through its conversion from a solid to a gas.

Laser types

In general, there are two types of medical laser systems, contact and non-contact. Contact systems work by sending laser light through a fiber or sapphire crystal tip. The tip absorbs the radiant energy and becomes hot. Direct contact between the tissue and the heated tip causes conduction of the heat energy from the tip to the tissue, resulting in the vaporization of the target cells. In contrast, non-contact laser systems do not directly touch the tissue. Instead, the laser light transfers radiant energy to the tissue. Heat results when the cell absorbs the radiant energy and the molecules in the tissue begin to move. In both types of system, the laser light itself is not hot. Heat is created only after the laser's radiant energy is absorbed, either by the tip or by the tissue.

Laser components

All lasers, regardless of size, style, or application, have four main components: the active medium, the excitation mechanism, the feedback mechanism (high reflectance mirror), and the output coupler (partially transmissive mirror). Active media may be solid, liquid, gas, or electronic. Lasers are named for the medium that is used to produce the light. Some solid medium lasers commonly used in medical applications are erbium:yttrium aluminum garnet (Er:YAG); holium:-yttrium aluminum garnet (Ho:YAG); neodymium:-yttrium aluminum garnet (Nd:YAG); and alexandrite, ruby, and potassium titanyl phosphate (KTP). Carbon dioxide (CO₂), argon, copper vapor, and excimer lasers are examples of medical lasers with gas media. Dye lasers have liquid media and diode lasers have electronic media.

When energy is applied to the active medium of a laser, its electrons are raised to an unstable level of energy, from which they return spontaneously to a lower but relatively long-lived metastable (chemically unstable but not liable to spontaneous transformation) condition. These electrons will not return to their ground energy level. It is therefore possible to pump large amounts of energy into the active medium, to the point that most of its atoms are in a metastable state. The lasing action begins with an electron that returns to its ground state, producing a photon. If the photon has exactly the right wavelength, it will stimulate a metastable atom to emit another photon of the same wavelength. This process is called stimulated emission. If enough stimulated photons travel parallel to the long axis of the laser tube they will continue to stimulate the emissions of photons of the same wavelength. These photons combine coherently until they reach the mirrored ends of the laser tube. When the light beam strikes the reflecting mirror, it is reversed and continues to stimulate the emission of more photons. The beam increases in intensity until it reaches the partially reflecting mirror. A portion of the light is released while the rest is reflected back through the active medium to continue stimulating photon emission.

Medical lasers have three types of excitation mechanisms. In most gas lasers, high-voltage direct current electricity is used. With some CO₂ lasers, radiofrequency electricity excites the gas. This type of excitation is needed to produce an ultrapulsed output, which is the delivery of very fast, extremely powerful bursts of light. Media that do not conduct electricity, such as solid and liquid media, are excited with light produced by flashlamps or other lasers.

Specific medical applications

Certain lasers tend to be used for particular procedures to take advantage of the quality of the light and amount of absorption by different types of tissue. The CO₂ laser is quite versatile, able to perform both cutting and bulk vaporization. It is often used to perform gynecological procedures involving colposcopy as well as ear-nose-and throat (ENT) procedures using microlaryngoscopy, such as the treatment of snoring. The CO₂ laser is also useful for cosmetic skin resurfacing and in neurosurgery.

The Nd:YAG laser is a contact laser. It is used in abdominal, gynecological, or urological surgeries performed through laparoscopes, endoscopes, or hysteroscopes. The Er:YAG laser is used for bone cutting, hard tissue drilling in dentistry, and skin resurfacing. The Ho:YAG laser is useful for such orthopedic procedures as joint arthroscopies, as well as for urologic lithotripsy and ophthalmologic procedures.

Cosmetic laser hair removal is a very popular procedure that can be performed by diode, alexandrite, and ruby lasers. Ruby lasers can be used to remove tattoos. Argon and excimer lasers are used primarily to reshape the cornea in laser eye surgeries, although heart surgeons also use excimer lasers to perform angioplasties. Copper vapor or dye lasers are used to treat port-wine birthmarks. Tunable dye lasers and argon lasers are often used to repair such cosmetic vascular problems as varicose or spider veins.

Pulmonary and esophageal tumors are treated by a laser technique called photodynamic therapy (PDT). This technique has potential applications for treating many other types of tumors. In PDT, a photoreactive drug called dihematoporphyrin is administered systemically. The drug collects in tumor cells at a significantly higher concentration than in normal cells. Laser light from red dye lasers is then applied to the tumor site. The drug preferentially absorbs the light, causing the tumor cells to be vaporized and leaving the normal cells intact.

Operation

When a laser is used in surgery, there are three central control parameters—power in watts (set by the laser nurse); time of exposure (dependent on the speed of movement of the beam or tip); and spot size (an increase or decrease in the area contacted by the laser light, controlled by the surgeon in the field). In general, cutting is done with the smallest possible spot; that is, the beam is kept in tight focus. A change in power level changes the speed of incision. If the surgeon is vaporizing or debulking tissue, the key consideration is power density. Thus, the spot size can be increased but the power is increased proportionately. Rather than continuous use of a lower level of power that can cause thermal damage to surrounding tissue, the surgeon may use a higher level of power on a pulsed setting. Superpulsing and ultrapulsing are the two levels of pulsing available.

Setting tests

Laser settings can be tested on a wet tongue depressor blade before they are used to vaporize tissue. Ideally, a 0.1-second test shot will leave a scoop-shaped depression in the wood shaped like a golf ball cut in half, with no point. A point indicates that the power density is too high. Rather than vaporizing the tissue cleanly, the laser will carve ridges and furrows in the tissue that might cause bleeding. If the depression is too shallow, the laser will be in use too long and cause charring.

The spatial quality of a laser beam can also be tested on a piece of thermal paper.

Safety issues and precautions

The use of lasers raises important safety issues. Categorized as Class IV devices by the Bureau of Radiological Health (BRH), all medical laser systems are fire hazards. They are also chemical hazards because of the compressed gases required to operate them and the fumes produced from lasing of the active medium. In addition, the laser dyes or solvents may be toxic. Lasers can produce skin or eye burns, and can cause retinal damage from direct or reflected beams. Lastly, lasers are explosion hazards; lasing of the active medium may cause flying fragments that can injure nearby personnel. Accordingly, a significant number of safety precautions are recommended. The American National Standards Institute (ANSI) standard Z136.3 addresses the safe use of lasers in health care settings and is an excellent resource for laser safety concerns. The ANSI directive establishes both engineering and administrative/procedural controls for four classes of lasers.

The following are among the recommended precautions:

warning signs posted outside procedure room entrances
all windows protected from transmission of laser light (not required for CO₂ lasers because it does not transmit through glass)
protective eyewear rated for the wavelength being used for all personnel within the nominal hazard zone (which may be the entire procedure room)
protection of the patient's eyes
lasers operated only by those who have received formal training in laser theory, control techniques, and operation
the presence of a trained laser nurse or laser safety operator during the procedure
the proper use of a laser smoke evacuator, equipped with a 0.3-micron filter if viral contamination is a concern
judicious use of dulled (anodized) surgical instruments to reduce beam reflection
careful packing of tissue surrounding beam area to avoid accidental exposure
availability of water within the procedure room and a nearby fire extinguisher

Despite the numerous safety concerns associated with lasers, the light is not an ionizing radiation risk. Precautions such as those used with x-ray equipment are not necessary.

Maintenance

The maintenance of lasers requires specially trained laser technicians, often members of the hospital biomedical engineering department or an outsource company. Power calibrations are required every six months. Routine maintenance includes changing the laser's filters and deionizer water, replacing flash-lamps, checking alignments, power outputs and failsafe shields, and cleaning optics.

It is important to store lasers away from hightraffic areas to avoid miscalibration and damage to their internal mechanisms.

Health care team roles

Physicians who have received special training in the use of lasers are the only personnel who actually use the laser and control the foot pedal. Laser nurses aid in setting the controls of the device and are often responsible for filling out the laser log documenting the procedure. Laser technicians are responsible for the maintenance of the equipment and, if they are specially trained, laser repair.

The ANSI guidelines recommend the appointment of a laser safety officer (LSO) for the hospital. This person is responsible for ensuring that the safety procedures are followed for every laser procedure performed in the facility. Trained laser nurses or technicians may act under the LSO's authority. In many hospitals, the LSO is a senior laser nurse, a senior laser technician, or even a physician. This person has the authority to turn off the laser if he or she determines that its use would be hazardous to the patient or other personnel.

Training

Although there are no national accreditation standards for laser use, most hospitals have set up a laser committee that reviews applications from physicians who wish to perform laser procedures within the facility. In order to obtain operating privileges, many hospitals require training of at least eight hours with the particular type of laser that is to be used.

Some hospitals run their own training programs, while others rely on outside medical education companies. In either case, the training programs will cover the principles of laser use and safety; have a clinical practicum taught by specialists in the area of the physician's practice, and hands-on sessions with the laser. Many hospitals require practice with both inanimate and animal specimens.

Training for laser nurses is also run internally by each individual hospital. Course work includes basic information about laser function, operation, and safety as well as specific training with the lasers used in the different procedures. Because laser nurses are often the LSO's eyes and hands in the operating room, it is essential for them to understand and implement the procedures required for safe laser use.

KEY TERMS

Active medium— The solid, liquid, gas, or electronic substance used to produce the laser light. It contains atoms whose electrons can be excited to a metastable level of energy.

Excitation— The use of energy to move electrons present in the laser medium to a higher orbit around the atom nucleus.

Feedback— The use of mirrors in a laser tube to reflectively increase the intensity of the produced light.

Metastable— Chemically unstable but not liable to spontaneous transformation. Most of the atoms in the active medium of a laser must be raised to a metastable state before the lasing action can begin.

Output coupler— A mirror in the laser tube that part of the light beam can flow through because it is both reflective and transmissive.

Photodynamic therapy— The use of a photosensitive drug to selectively target tumor cells during laser treatment.

Pulsed laser— A laser that delivers energy in single or multiple pulses less than or equal to 0.25 second.