Selective photothermolysis

The aforementioned laser parameters—power density, fluence, and wavelength—are the fundamental principles in the operation of medical lasers in the concept known as SP. Anderson and Parrish described SP in 1983, when they outlined the essential factors necessary for discrete laser-induced tissue damage to occur. SP is a method for localizing tissue damage to specific chromophore targets at the cellular level; therefore, it can be used to minimize undesired thermal damage to the surrounding tissue caused by thermal diffusion.

The rate of thermal diffusion of a given tissue is known as the thermal relaxation time (T_R) and is defined as the time required for a given heated tissue to lose 50% of its heat through diffusion. It is measured in terms of the area affected and the thermal diffusivity (D) of the target tissue, as follows:

T_R = r²/4 D, where r is the radius of target tissue.

Therefore, significant thermal diffusion (and hence thermal damage) is minimized if the duration of the laser pulse is shorter than the T_R of the target tissue.

For example, water (the primary constituent by weight of living cells) has a high absorption coefficient of 230 cm^-1 at 10,600 nm, the wavelength emission of a CO₂ laser, and a T_R of 326 µs. With these properties, if a CO₂ laser contacts the skin for less than 326 µs, most of the radiation is absorbed by the water in the targeted skin, with almost no thermal diffusion. However, if the duration of the laser impingement on the tissue is longer than 326 µs, heat is transmitted to the surrounding nontargeted tissue and results in undesirable thermal injury.

Therefore, for proper SP to occur, the target tissue (through its chromophores) must possess greater optical absorption than the nontargeted surrounding tissue does, and the laser of choice must have a pulse duration shorter than the T_R of the target tissue. Because soft tissues in humans generally have a T_R of less than 1 ms, the laser pulse must be extremely short and high-powered to be medically beneficial and minimally destructive. Because many types of lasers exist, selection is crucial and must be tailored to the specific procedure.

Modes: Continuous Wave, Pulsed, and q Switching

The light generated with a laser, in general, can be delivered in 2 ways: as a constant flow of energy (continuous-wave [CW] laser or as multiple discrete pulses (pulsed laser). The 2 types of lasers are fundamentally different in design, light delivery, and operation.

A CW laser is generated by continuously pumping energy into the active medium to achieve an equilibrium between the number of atoms raised to the excited state and the number of photons emitted. At such an equilibrium, continuous laser output results. The duration of a CW laser pulse is approximately 0.25 s. With this duration and with relatively constant power delivery to tissues, significant thermal damage occurs. To minimize their destructive effects, CW lasers have been modified to emit beams in a pulsatile fashion by adding electronically controlled, mechanically gated, timed shutters to interrupt the output beam at preset intervals. This system is not a true pulsed laser per se because the laser beam is physically chopped off to produce the pulse effect.

Pulsed lasers, in contrast, deliver high-energy beams in very short pulses in the range of milliseconds without the use of a shutter. Emissions are produced when the pump is modulated to create discrete laser pulses, which usually are broad and randomly shaped.

Both types of lasers can be further modified to produce even shorter pulses, usually in the range of 10-250 ns, by using a method referred to as Q switching. With this technique, a large population inversion builds before emission is stimulated. This population inversion is accomplished by using a mechanical opaque shutter or by inserting a high-speed, electrically sensitive, polarizable optical shutter known as a Pockel cell between the 2 mirrors of the laser.

Because this shutter effectively blocks the photons' path between the 2 mirrors and prevents resonation, stimulated emission does not occur. When the population inversion reaches its maximal level, the opaque mechanical shutter opens. When Pockel cells are used, an electrical pulse is applied to the cell that changes it from opaque to transparent, allowing the photons to reflect back and forth and subsequently generate an intense beam.

Since their initial use to coagulate ophthalmic lesions in 1963, laser-equipped devices are now used in almost every specialty of medicine. They have revolutionized the science of medicine and have become valuable and indispensable medical tools. With the rapid pace of technologic advances, other novel applications are likely to be discovered.

Clinical and Diagnostic Applications

Fields of Practice

Laser technology is widely used in medical research, diagnostics, and treatment. The clinical efficacy of laser therapy is well known, and lasers are used in many fields and settings.

Dentistry

Lasers have a wide range of applications in various fields of dentistry, especially when conventional treatments are not effective. In endodontics, laser technology is being considered for dentin structure modification, cleaning and shaping of the root canal system, pulp diagnosis, and endodontic surgery. Lasers such as the Nd:YAG, CO₂, and semiconductor Diode lasers have been used in soft tissue treatment of the oral cavity. In periodontics, the Er:YAG laser has potential for clinical application in hard tissue treatment due to its ability to cut or contour bone with minimal damage and fast healing. In orthodontics, lasers have been used in laser surface scanning of craniofacial anomalies.

Dermatology

Advances in laser technology have provided dermatologists with more choices and have contributed to improved clinical results. Lasers are widely used in the treatment of dermatologic conditions, including acne vulgaris, pseudofolliculitis barbae, and vascular and pigmented lesions. They are also used for removal of unwanted hair (by the Nd:YAG laser), tattoos, and scars. Diagnostically, they are used in the laser-capture microdissection technique, which provides faster identification of infectious agents than standard diagnostic techniques based on the polymerase chain reaction (PCR).

Oncology

Lasers are used in photodynamic therapy of patients with both malignant tumors and non-tumoral illnesses. Photodynamic therapy involves the absorption of a photosensitizing agent (which interacts with visible light), retention of the agent in tumor tissue, and selective laser irradiation of tumor tissues, which were previously sensitized by dyes. This technique plays a large role in preserving healthy organ issue. Lasers, particularly the laser-capture microdissection technique, are an important part of proteomic technology for cancer diagnosis and the development of markers for early detection. In oncologic surgery, CO₂ lasers have been used for the surgical treatment of carcinomas of the oral cavity, pharynx, and larynx.