Laser and its Medical Applications
Dr. Dwijesh Kumar Panda
Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. It is a device that produces intense focused coherent (monochromatic) light. There are three types of lasers: gas, liquid and solid state lasers. All these have many medical uses. In this article, I shall outline these uses after discussing basic principles and operations of lasers.
When an atom is supplied energy from outside, it receives it in discrete amount because its energy states are discrete (quantized). The atomic electron “jumps” from its lowest (ground) state to upper (excited) states. This is absorption. Once in the excited state, the atom emits a photon after a short time which is called spontaneous emission. If there are other photon present, they stimulate the atom to emit more photons which is called stimulated emission.
In certain cases like ruby which is a crystal with CV2O3 impurity in AC2O3, the chromium atom has a long- lived or metastable state in between its ground and first excited state, i.e., the atom can stay in this state without emitting any photon for a time longer than its first excited state. As a result all the electron that are excited to its first excited state come down to the intermediate state as a result of which they stay there for a long time and this metastable state gets crowded. We can say that electrons are “pumped” from the ground state to the intermediate metastable state. This is called “optical pumping”. Eventually these electrons come down to the ground state by emitting large number of photons: each electron emitting one photon and the photons stimulate others to come down. All of these photons have some wavelength. As a result, the light is intense and coherent. All this is illustrated in Fig 1.
Fig. 1. Stimulated Emission
LASER IN OPERATION
Figure 2 is a schematic diagram of Ruby laser in operation showing a ruby rod (gain medium) inside a coiled lamp which is the source of power called the “optical pump”. At one end of the rod there is high reflecting mirror and at the other end there is a partially transparent mirror, which, apart from reflecting light, allows optical power of the laser to come out. The entire system is called a laser oscillator.
Fig.2. LASER OSCILLATOR
MEDICAL APPLICATION (LASER OSCILLATOR)
Lasers are used predominantly as a source of energy to effect tissue destruction but also provide a coherent energy source for phototherapy or confocal microscopy, which allows for real-time imaging of tissues.
Lasers destroy tissue through one of these mechanisms, including production of heat (photothermal), destruction of chemical bonds (photoablative), or reaction with a photosensitizer (photochemical). Selective photothermolysis describes the use of specific wavelengths of laser energy to target specific chromophores and thus minimize damage to adjacent tissues. The interaction between the laser and tissue depends upon the wavelength, power, duration of exposure, and properties of the target tissue.
Lasers are used in a variety of clinical applications commonly related to external structures such as the skin and cornea. However, minimally invasive technology allows access to deeper structures including the gastrointestinal mucosa, biliary tree, bronchial structures and urinary tract via endoscopy, and hollow and solid organ surfaces via laparoscopy and thoracoscopy.
The eye and skin are the organs that are more susceptible to damage by laser radiation. The biological effects induced by nonionizing radiation are similar for coherent and incoherent sources: however, laser radiation produces greater radiant exposures. In the workplace, eye protection is required by the United States Occupational Safety and health Administration (OSHA)
CLINICAL UTILITY OF LASERS — Lasers are used in a variety of clinical applications commonly related to external structures that are easily reached, such as the skin and cornea. Minimally invasive technology allows access to deeper structures including the gastrointestinal mucosa, biliary tree, bronchial structures, and urinary tract via endoscopy and hollow or solid organ surfaces via laparoscopy and thoracoscopy. Most commonly, tissue ablation occurs as a consequence of thermal effects.
· Dermatology – Progress in laser and light therapy technology has led to the development of safer and more efficient methods of achieving the desired effects on skin. Skin cooling limits inadvertent damage to tissues adjacent to the targeted sites.
· Gastroenterology – Pulse dye lasers and Q-switched solid-state lasers are used to fragment gallstones identified during choledochoscopy and ablate mucosal tumors. Confocal laser endomicroscopy has aided in the differentiation between dysplastic and malignant tissue.
· In bronchoscopy, lasers are used to either photocoagulate or vaporize tissues obstructing the airway. They can also be used to make concise radial incisions to enhance airway dilation in central airway strictures. Lasers have also been used to remove blebs and bullae during thoracoscopy.
· Cardiology and cardiac surgery – Laser energy is used to create transmural channels in ischemic myocardium to restore myocardial perfusion in a procedure called Trans myocardial laser revascularization. The procedure can be performed using a carbon dioxide or holmium: YAG laser.
· Ophthalmology – The excimer laser is used to remove a precise amount of corneal stroma during laser-assisted in-situ keratomileusis (LASIK), which is one of the most common applications of laser.
· General surgery – In general surgery, laser energy can be used to provide hemostasis or to ablate tumors identified in solid organs such as the liver.
· Gynecology – The Nd: YAG or carbon dioxide laser is used for tissue ablation in a variety of gynecologic applications.
· Urology – Lasers can be used to fragment ureteral stones but are becoming more popular for the treatment of benign prostatic hyperplasia. Benign prostatic hyperplasia is a condition that causes urinary symptoms in men, such as the frequent need to urinate, inability to empty the bladder, trouble starting urination and frequent urinary tract infections. These are caused by an enlarged prostate blocking normal urine flow.
· Green Light laser therapy treats benign prostatic hyperplasia by removing the excess prostate tissue that is blocking urine flow. After the patient is put to sleep, a fiber is placed into the urethra and passed to the prostate where it is quickly heated, vaporizing the extra prostate tissue. This procedure usually results in a very quick resolution of all urinary symptoms in most patients. Green Light laser therapy provides many benefits over the more traditional therapy for benign prostatic hyperplasia. There is a much quicker recovery time with laser therapy; most procedures are done on an outpatient basis and the patient goes home a few hours after surgery. There is a much lower risk for bleeding. Results are almost immediate, versus waiting several months for results with medications. The risk of complications, such as erectile dysfunction or bladder injury, are also lower. Greenlight laser therapy also has the additional benefit of not needing a catheter after surgery or only needing it for less than 24 hours
· Vascular surgery – Lasers are commonly used to manage the visible signs of chronic venous disease. Several types of lasers might be used depending upon the size and depth of the vein to be treated.
· Dentistry – Although incoherent light may be used in conjunction with photoactive dental bleaching systems, coherent light produced from a diode laser has also been found to be effective.