Laser and its
Medical Applications
Dr.
Dwijesh Kumar Panda
INTRODUCTION
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.
FUNDAMENTAL PRINCIPLES
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.
I acknowledge
discussion with Prof. Trilochan Pradhan, on fundamental principles and
operational aspects of laser.
References:
Declaration: I declare that this article has not been
reproduced. Submitted to the Secretary, Odisha Bigyan Academy for publication
in “Science Horizon”
Dr.
Dwijesh Kumar Panda, M.D, Ph.D. (Medicine)
Senior Scientist awardee, Odisha Bigyan
Academy.
Contact:
(0674) 2543122.