Lec.3( Light in Medicine

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• Spectrum of light from the sun: Fig.1
Figure 1. Relative intensity of solar energy of different wavelength at the earth s
surface.
• Properties of light
1-The speed of light changes when it goes from one material into another.
The ratio of the speed of light in a vacuum to its speed in a given material is
called the index of refraction. If a light beam meets a new material at an
angle other than perpendicular, it bends, or is refracted.
2- Light behaves both as a wave and as a particle. As a wave, it produces
interference and diffraction, which are of minor importance in medicine. As
a particle, it can be absorbed by a single molecule. When a light photon is
absorbed, its energy is used in various ways. It can cause a chemical change
in the molecule that in turn cause an electrical change. This is basically what
happens when a light photon is absorbed in one of the sensitive cells of the
retina (the light-sensitive part of the eye).
3- When light is absorbed, its energy generally appears as heat. This property is
the basis for the use in medicine of IR light to heat tissues. Also, the heat
produced by laser beams is used to "weld" a detached retina to the back of the
eyeball and to coagulate small blood vessels in the retina.
4- Sometimes when a light photon is absorbed, a lower energy light photon is
emitted. This property is known as fluorescence; as you may guess, it is the basis
of the fluorescent light bulb. Certain materials fluoresce in the presence of UV
light, sometimes called "black light" and give off visible light.
The amount of fluorescence and the color of the emitted light depend on the
wavelength of the UV light and on the chemical composition of the material that
Sub. : Medical physics Light in Medicine مدرس المادة
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is fluorescing. One fluorescence is used in medicine is in the detection of
porphyria, a condition in which the teeth fluoresce red when irradiated with UV
light. Another important application is in fluorescent microscopes.
5- Light is reflected to some extent from all surfaces. There are two types of
reflection. Diffuse reflection occurs when rough surfaces scatter the light in many
directions. Specular reflection is a more useful type of reflection; it is obtained
from very smooth shiny surfaces such as mirrors where the light is reflected at an
angle that is equal to the angle at which it strikes the surface.
1. Measurement of Light and its Unit
?Ultraviolet (UV) light: ??= 100 ~ 400 nm
?UV-C: 100 ~ 290 nm.
?UV-B: 290 ~ 320 nm.
?UV-A: 320 ~ 400 nm.
??Visible light: ??= 400 ~ 700 nm
??Infrared (IR) light: ??= 700 ~ 10,000 nm
??Photometric unit for visible light (Table:1)
?Illuminance: quantity of light striking a surface
?Luminance: intensity of a light source
??Radiometric unit for all lights (Table:1).
?Irradiance: quantity of light striking a surface.
?Radiance: intensity of a light source.
?Light has wavelengths much shorter than TV and radio waves but
much longer than x-rays and gamma rays. Visible light has energies
ranging from2 ~ 4 eV. For comparison, the kinetic energy of a molecule
in air at room temperature is about 0.025 eV and the energy of a typical
x-ray photon used in medicine is about 50,000 eV, 50 KeV(Fig.2).
Table 1. Light Quantities and units.
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Fig.2. The relationship of light wavelengths to the entire spectrum of electromagnetic
radiation
2. Applications of Visible Light in Medicine
??Visual information about a patient: color, structure
- Mirror, ophthalmoscope, otoscope.
- Endoscope.
- Cystoscope for bladder.
- Proctoscope for rectum.?
-Bronchoscope for air passages to lungs.
-Flexible endoscope for stomach fiberoptic technique.???????
-Biopsy channel.
-Cold-light endoscope: very little IR radiation to minimize heating effect
??Transillumination
- Detection of hydrocephalus (water-head) in infants.?
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-Detection of pneumothorax (collapsed lung) in infants.
-Sinuses, gums, breasts, testes, etc.
??Therapeutic uses.
-?Jaundice (excessive secretion of bilirubin by the liver) in infants ?
Phototherapy using visible light (usually blue light ~ 450 nm).
3. Applications of UV and IR Light in Medicine
??Energy of photon: UV > visible > IR
1-?UV light
- < 290 nm: germicidal (kill germs) ??sterilize medical instruments.
- ?Conversion of molecular products into vitamin D.
- ?May improve certain skin conditions.
- Affect melanin to cause tanning or sunburn.
- Solar UV light: major cause of skin cancer (light absorption by
DNA) ?cancer?
- Absorption of UV light in the eye.
2-?IR light
- About half of the energy from the sun is in the IR region (Fig: 1). The
warmth we feel from the sun is mainly due to the IR component. The rays
are not usually hazardous even though they are focused by the cornea and
lens of the eye onto the retina. However, looking at the sun through a filter
(plastic sunglasses) that removes most of the visible light and allows most
of the IR wavelengths through can cause a burn on the retina. Some people
have damaged their eyes in this way by looking at the sun during a solar
eclipse. Dark glasses absorb varying amounts of the IR and UV rays from
the sun.
- Heat lamps that produce a large percentage of IR light with wavelengths
of 1000 to 2000 nm are often used for physical therapy purposes. Infrared
light penetrates further into the tissues than visible light and thus is better
able to heat deep tissues.
- Two types of IR photography are used in medicine: reflective IR
photography and emissive IR photography. The latter, which uses the long
IR heat waves emitted by the body that give an indication of the body
temperature, is usually called thermography.
- Reflective IR photography, which uses wavelengths of 700 to 9oo nm to
show the patterns of veins just below the skin. Some of these veins are
visible to the eye, but many more can be seen on a near-IR photograph of
the skin.
- There is considerable variation in the venous patterns of normal
individuals. Even in the same individual the venous patterns in the two
breasts may be quite different, but these changes can be masked by the
normal variations. A layer of fat beneath the skin can reduce the
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appearance of the venous pattern. IR photography can be used to follow
changes in the venous pattern.
- Near IR penetrates about 3mm below the skin regardless of the color of the
skin.
- Infrared can also be used to photograph the pupil of the eye without
stimulating the reflex that changes its size.
4. Lasers in Medicine
??Laser (Light Amplification by Stimulated Emission of Radiation).
? A laser is a unique light source that emits a narrow beam of light of a
single wavelength (monochromatic light) in which each wave is in phase
with the others near it (coherent light).
??Laser materials: gases, liquids, solids.?
??A laser beam can be focused to a spot only a few microns in
diameter. When all of the energy of the laser is concentrated in such a
small area, the power density becomes very large.
? In medicine lasers are used primarily to deliver energy to tissue, the
laser wavelength used should be strongly absorbed by tissue (Fig. 3).
Fig.3.The absorbance and reflectance of a white woman s skin.
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? This curve varies for different individuals, but the short wavelengths
(400-600 nm) are always absorbed better than the long wavelengths
(~700 nm).
??Effect of laser
1- Laser energy directed at human tissue causes a rapid rise in temperature
and can destroy the tissue. The amount of damage to living tissue
depends on how long the tissue is at the increased temperature.
2- 1064 nm laser ??retina damage without heating.
3- 441.6 nm laser ??photochemical damage.
??The laser is routinely used in clinical medicine only in ophthalmology.
Its effectiveness in treating certain types of cancer and its usefulness as a
"bloodless knife" for surgery are under active investigation. Lasers are used
in medical research for special three-dimensional imaging called
holography.
? In ophthalmology lasers are primarily used for photocoagulation of the
retina, that is, heating a blood vessel to the point where the blood
coagulates and blocks the vessel (Fig. 4).
Fig.4. A laser beam is focused by the cornea and lens to a small spot on the retina
where it photocoagulates a small blood vessel.
The amount of laser energy needed for photocoagulation d e p e n d s
on the spot s1ze used. In general, the proper dose is determined visually
by the ophthalmologist at the time of the treatment. The minimum
amount of laser energy that will do observable damage to the retina is
called the minimal reactive dose M R D ). For example, the MRD for a 50
?m spot in the eye is about 2.4 mJ de1ivered in 0.2 5 sec. Typical
exposures needed for photocoagulation are 10 to 50 times the MRD
(i.e., 24 to 120 mJ for a 50?m spot in 0.25 sec).
Photocoagulation is useful for repairing retinal tears or holes that
develop prior to retinal detachment. When the retina is completely
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detached, the laser is of no help. A complication of diabetes that
affects the retina, called diabetic retinopathy, c a n also be treated with
photocoagulation.
? Protective glasses must be worn in medical laser areas to protect
the eyes of the patient and the workers. The area should have adequate
warning signs and a system that prevents outsiders from entering
while lasers are in use.
5. Applications of Microscopes in Medicine
??1670 by Leeuwenhoek.
??Pathology lab.
???1000 magnification ??study of cells (cytology) and study of tissue
(histology).
??Standard light microscope: (Fig.5).
Fig.5. The standard light microscope. E indicates the replaceable eyepieces and O
indicates the various objective lenses that can be used by rotating the turret
- The highest magnification t h a t c a n be obtained is limited by the
wavelength of visible light. Since the wavelengths of visible light
range from 400 to 700 nm (0.4 to 0.7 ?m), the smallest object that can
be resolved is about 1 ?m in diameter. Since most cells are 5 to 50 ?m in
diameter, this type of microscope is adequate for resolving all but
subcellular objects.
- In order to distinguish different cells it is usually necessary to stain
them with a chemical that strongly absorbs certain visible
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wavelengths. Different chemicals are used to stain the various cell
components and aid in the identification of cell structures.
- Other techniques in addition to staining are useful in microscopy. One
technique takes advantage of the different indexes of refraction of
different cell parts. Since light travels at different speeds in the various
part s of a cell, the phase relationships of the light waves change in
passing through a specimen.
- The phase-contrast microscope takes advantage of this phenomenon to
allow cell structures to be seen without the use of stain.
- It is sometimes advantageous to use UV light or x-rays in microscopy. Sine
our eyes cannot see wavelengths shorter than those of visible light,
it is necessary to convert the images produced by UV light or x-ray
beams into images that use visible light. There are two common ways
to produce visible images from UV light and x -rays. Neither
increases the resolution but both offer other advantages.
?? Fluorescent microscopy:
Ultraviolet light is used in fluorescent microscopy. T h e tissue sample
is stained with a dye that fluoresces when it is irradiated with UV light
in a fluorescent microscope.
???Historiography:
Low-energy x-rays are used in a microscopy technique called
historiography. X-rays are beamed through a tissue sample that is
held in close contact with fine-grain film, and the resultant x-ray
image is examined under a conventional microscope. Since lowenergy
x-rays are strongly absorbed by heavy elements, such as
calcium, historiography is often used to study bone samples that
have been cut in thin slices (~0.1mm).
???Electron microscope:
- Very short wavelength electron beam?? better resolution.
- ? 250,000 magnification.
- In transmission electron microscopy (TEM) exceedingly thin
specimens must be used so that the electrons can pass through them. It
is also usually necessary to evaporate a very thin layer of a heavy metal
on each sample to act as a stain.
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- In scanning electron microscopy (SEM) a finely focused beam of
electrons scans the surface of the specimen and a detector measures the
number of scattered electrons from each point on the surface.