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Laser (Light Amplification by Stimulated Emission of Radiation) is an important technology in the field of medicine today.
Laser Modes
Different laser operation modes are available:
Continuous or CW mode
Continuous pumping of energy into the lasing medium keeps the population inversion steady, so that steady light emission is achieved, resulting in a continuous wave laser.
Chopped Mode
The CW laser may have a shutter-interrupted output to artificially produce short laser pulses (usually 100-500 ms), each with the same maximum power level as in the CW mode. The period of the laser beam when the laser is in use is called the Duty Cycle or DC.
Pulsed Mode
Intermittent pumping in a gas medium gives rise to pulsed lasers, often with resulting higher peak power than in CW mode.
Q-switched laser creates a giant laser pulse (a very high-powered flash) by using a high-speed shutter inside the resonant (reflecting) cavity to allow extremely fast discharge of the stored energy at high peak intensity.
Types of Lasers
The following are some common medical lasers:
Gas Lasers
The carbon dioxide laser is a gas laser that uses infrared radiation at 10600 nm which is absorbed by water in the tissues, vaporizing it at only 100 oC, and sealing blood vessels. It also closes lymph vessels to prevent tumor cells from spreading, and seals the cut ends of nerves to prevent the formation of a neuroma. It is used in ENT, gynecology, plastic and maxillofacial surgery, and urology. It cannot be carried by flexible fibers at present.
The argon laser is another blue-green gas laser with several wavelengths, mostly at 488 and 514 nm. It coagulates blood vessels efficiently, and is used in microsurgery on the ear, bleeding ulcers of the stomach and duodenum, and the removal of vascular lesions or polyps, and small tumors of the skin, both benign and malignant. It can be used with flexible optical fibers.
Fiber Lasers
The Nd-YAG (Neodymium-YAG) laser is a doped fiber laser that produces infrared light at 1060 nm and is used to seal both normal and abnormal blood vessels. It is absorbed in colorless as well as pigmented tissues of all sorts. It is used to treat stenotic or granulomatous lesions, benign tumors and to debulk cancers, in the trachea, bronchial tree, gastrointestinal and urologic tract.
Dye Lasers
The pulsed yellow dye laser is an example of a dye laser (580–595 nm), and is widely used to disrupt blood vessels as oxyhemoglobin absorbs yellow light best.
Solid-State Lasers
The KTP laser uses a potassium-titanyl phosphate crystal pumped by an Nd: YAG laser, producing green light at 532 nm. It coagulates blood vessels.
Excimer Lasers
The UV excimer lasers is produced from ionized noble gases and performs ablation by converting tissue from solid to vapor directly without much heating.
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Laser-Tissue Interactions
These may be wavelength-dependent or independent.
Wavelength-Dependent Mechanisms
Photothermal Interactions
Laser light energy is converted to heat, which destroys tissue by coagulation, vaporization or cutting. After heating is accomplished, the heat diffuses into neighboring tissues. For minimal unwanted thermal damage, the thermal relaxation (cooling) duration should not be exceeded by the duration of the laser pulse.
Photochemical Effects
These are caused by laser excitation of electronic bonds which rupture to cause molecular fragmentation (photodecomposition) but without significant tissue heating effects.
- Photodynamic Therapy (PDT)
This uses photosensitizers to accept the light energy and become activated, following which they energize other molecules nearby which are often toxic and cause oxidative damage and cell death. Sometimes PDT acts by activating molecules in the irradiated tissue to alter the metabolism.
This photochemical phenomenon goes by many names such as Low Laser Level Therapy (LLLT) and low intensity-low power therapy. It uses near-infra red light in a monochromatic beam to stimulate growth and regeneration of soft tissues as well as promote pain relief.
Also called ablative photodecomposition, this refers to the breaking of molecular bonds to produce precise and controlled tissue ablation without heating.
Wavelength-Independent Mechanisms
When the power density crosses a certain limit with very short pulses, the tissue undergoes optical breakdown due to multiphoton ionization of atoms and molecules in both pigmented and other tissue. This causes plasma to form and generates a shock wave, often leading to cavitation and jetting inside soft tissues. Controlled plasma formation can be used for clean and precise surgery without thermomechanical damage if limited to pico- or femtosecond pulses.
Applications
Laser Surgery
In surgery, lasers can do three things based on their peculiar attributes:
- Cut deeply and cauterize as it cuts, producing a precise wound with little hemorrhage
- Ablate tissue by vaporizing the tissue surface
- Allow internal surgery without an open wound using specialized optical fiber
Uses
- Lasers can be used to remove superficial cancers or very early neoplasms of the genital organs or sometimes non-small cell lung cancers
- Tumors can be shrunk using CO2 or Nd:YAG lasers
- They may also relieve bleeding or obstructive complications of tumors by sealing blood vessels or debulking large growths.
- When used for curative therapy, lasers are combined with chemo- or radiotherapy, or surgery
- Lasers are also used to close off nerve endings or lymph vessels to prevent neuromas and tumor cell metastasis
- As mentioned earlier, PDT uses the photochemical effects of lasers to selectively destroy cancer cells
- Laser ablation is used to treat skin lesions, using CO2 lasers.
- Laser treatment of vascular lesions is with green and yellow light, originally with the argon laser but now with the pulsed yellow dye laser
- Angioplasty and cardiac revascularization using laser drilling
- Cosmetic and dermatologic surgery
- Laser applications for brain tumor treatment include:
- Obtaining a biopsy for diagnosis
- Removal of tumor tissue wholly or partially
- Palliative tumor debulking to reduce intracranial pressure which is causing symptoms
- Removal of hard or very fragile tumors or growths which are deep within the brain or have infiltrated the skull
- Removal using PDT
- Creating a route for chemotherapy or internal radiation therapy implants, or for hypertherapy and brain mapping during surgical procedures
Advantages
- Laser beams can cut accurately and rapidly through tissues
- Less pain and bleeding
- Reduces scarring
- No direct contact with the tissue
- Better visualization
- The skin is less damaged and the skin edges are free of infection
- Can be combined with flexible fiberoptics, low-power microscopes, and micromanipulators to avoid large incisions
- Shorter recovery times
Disadvantages
Laser therapy must be done only by well-trained specialists with stringent safety regulations, the cost of the setup and the size of the required equipment. Some effects are short-lived and treatment must be repeated.
Sources and Further Reading
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