MEDICAL
INSTRUMENTS
Audiogram Kidney- Dialysis E.K.G. Stethoscope
Pioneer of Endoscopy;
Maximillian Nitze (150th birthday ) cancellation
He discovered in 1877
first endoscopic instrument
Endoscopy is a method of viewing the inside
of the body using an endoscope, especially useful in diagnosis and treatment of
disorders of the gastrointestinal tract. An instrument employing fiber-optic
technology, an endoscope contains up to 20,000 coherent quartz fibers. In order
to provide an image of the area of the body being investigated, light is shone
down the endoscope and is reflected back up the bundle of fibers and viewed
through an eyepiece by the endoscopist. Because the alignment of each fiber is
maintained throughout the length of the endoscope, there is no distortion in
the final image. The tip of the endoscope can be maneuvered through 180° and
has mechanisms for cleaning the lens or the tissue under examination.
Microscope is any of several types
of instruments used to obtain a magnified image of minute objects or minute
details of objects.
The most widely used microscopes are
optical microscopes, which use visible light to create a magnified image of an
object. The simplest form of optical microscope is the double-convex lens with
a short focal length. These lenses can magnify an object by up to 15 times. In
general, however, a compound microscope is used, which has multiple lenses to
provide more magnification than a single convex lens could alone. Some optical
microscopes can magnify an object by 2,000 times or more.
The compound microscope consists
essentially of two lens systems, the objective and the ocular, mounted at
opposite ends of a closed tube. The objective lens is composed of several lens
elements that form an enlarged real image of the object being examined. The
microscope lenses are set up so that the real image formed by the objective
lies at the focal point of the ocular; the observer looking through the ocular
sees an enlarged virtual image of the real image. The total magnification of
the microscope is determined by the focal lengths of the two lens systems.
The accessory equipment of an
optical microscope includes a firm stand with a flat stage for holding the
material to be examined, and some means for moving the microscope tube towards
and away from the stage so that the specimen can be brought into focus.
Ordinarily, specimens for microscopic examination are transparent and are
viewed by using light that passes through the specimen. The specimens are
usually mounted on thin, rectangular glass slides. The stage has a small hole
through which light passes. Underneath the stage there is a mirror that
reflects light through the specimen, or a special electric light source that
directs light through the specimen.
In photomicrography, which is the
process of taking photographs through a microscope, a camera is mounted
directly above the microscope's eyepiece. Normally the camera does not contain
a lens, because the microscope itself acts as the lens system. The term
microphotography, sometimes used instead of photomicrography, is usually
applied to the technique of duplicating and reducing a picture or a document to
a miniature size for storage.
Microscopes used for research
commonly have a number of refinements to enable a complete study of the
specimens. Because the image of a specimen is highly magnified and inverted,
manipulating the specimen by hand is very difficult. As a result, the stages of
high-powered research microscopes are mounted so that they can be moved by
means of micrometer screws; in some microscopes, the stage can also be rotated.
In addition, all research microscopes are equipped with three or more
objectives, mounted on a revolving head, so that the magnifying power of the
microscope can be varied.
Special-Purpose Optical Microscopes
A number of types of microscopes
have been developed for specialized uses. One such type is the stereoscopic
microscope, which is actually two low-powered microscopes arranged so that they
converge on the specimen. These instruments provide a three-dimensional image
that has its right side up.
The ultraviolet microscope uses the
ultraviolet region of the spectrum rather than the visual region, either to
gain resolution because of the shorter wavelength or to emphasize detail by
selective absorption at different wavelengths within the ultraviolet band. Because
glass does not transmit the shorter ultraviolet wavelengths, the optics used in
this type of microscope are usually quartz, fluorite, or aluminized-mirror
systems. Further, because ultraviolet radiation is invisible, the image is made
visible through phosphorescence, photography, or electronic scanning. The
ultraviolet microscope is used in medical research.
The petrographic microscope is used
to identify and quantitatively estimate the mineral components of igneous rock
and metamorphic rock. It is equipped with a Nicol prism or other polarizing
device to polarize the light that passes through the specimen being examined.
Another Nicol prism or analyzer determines the polarization of the light after
it has passed through the specimen. The microscope also has a rotating stage
that, by suitable adjustment, indicates the change in polarization caused by
the specimen.
The dark-field microscope employs
illumination in the form of a hollow, extremely intense cone of light, which is
concentrated on the specimen. The field of view of the objective lies in the
hollow, dark portion of the cone and thus picks up only scattered light from
the object. As a consequence, the clear portions of the specimen appear as a
dark background, and the minute objects under study glow brightly against this
dark field. This form of illumination is useful for transparent, unstained
biological material and for minute objects that cannot be seen in normal
illumination under the microscope.
The phase microscope illuminates the
specimen with a hollow cone of light, as in the dark-field microscope. In the
phase microscope, however, the cone of light is narrower and enters the field
of view of the objective. Within the objective is a ring-shaped device that
both reduces the intensity of the light and introduces a phase shift of a
quarter of a wavelength. This form of illumination causes minute variations of
refractive index in a transparent specimen to become visible. This type of
microscope is particularly effective for studying living tissue; hence, it is
used widely in biology and medicine.
Very advanced optical microscopes
include the near-field microscope, through which even details slightly smaller
than the wavelengths of light can be seen. A light beam shining through a tiny
hole is played across the specimen at a distance of only about half the
diameter of the hole, until an entire image is obtained.
Electron Microscope
The magnifying power of an optical
microscope is limited by the wavelength of visible light. An electron microscope
uses electrons to “illuminate” an object; since electrons have a much smaller
wavelength than light, they can resolve much smaller structures than light can.
The smallest wavelength of visible light is about 4,000 angstroms (1 angstrom
is 0.0000000001 meters); the wavelength of electrons used in electron
microscopes is usually about 0.5 angstrom.
All electron microscopes comprise
several basic elements. They have an electron gun emitting electrons that
strike the specimen and create a magnified image. Magnetic “lenses” that create
magnetic fields are used to direct and focus the electrons, because the
conventional lenses used in optical microscopes to focus visible light do not
work with electrons. A vacuum system is an important part of any electron microscope.
Electrons are easily scattered by air molecules, so the interior of an electron
microscope must be at a very high vacuum. Finally, electron microscopes also
have a system that records or displays the image produced by the electrons.
There are two basic types of
electron microscopes: the transmission electron microscope (TEM), and the
scanning electron microscope (SEM). In a TEM, the electron beam is directed
onto the object to be magnified. Some of the electrons are absorbed or bounce
off the specimen; others pass through and form a magnified image of the
specimen. The sample must be cut very thin to be used in a TEM; usually the
sample is no more than a few thousand angstroms thick. A photographic plate or
fluorescent screen is placed beyond the sample to record the magnified image.
Transmission electron microscopes are capable of magnifying an object up to 1
million times.
A scanning electron microscope
creates a magnified image of the surface of an object. When using an SEM, the
object to be magnified does not need to be thinly sliced; the sample can be
placed in the microscope with little, if any, preparation. An SEM scans the
surface of the sample bit by bit, in contrast to the TEM, which looks at a
relatively large part of the object all at once. In an SEM, a tightly focused
electron beam moves over the entire sample, much the way an electron beam scans
an image onto the screen of a television. Electrons in the tightly focused beam
might scatter directly off the sample, or cause secondary electrons to be
emitted from the surface of the sample; these scattered or secondary electrons
are collected and counted by an electronic device located to the side of the
sample. Each scanned point on the sample corresponds to a pixel on a television
monitor; the more electrons the counting device detects, the brighter the pixel
on the monitor is. As the electron beam scans over the entire sample, a
complete image of the sample is displayed on the monitor. Scanning electron
microscopes can magnify objects 100,000 times or more. SEMs are particularly
useful because, unlike TEMs and powerful optical microscopes, SEMs produce
detailed pictures of the surface of objects, providing a realistic
three-dimensional image.
Various other electron microscopes
have been developed. A scanning transmission electron microscope (STEM)
combines elements of an SEM and a TEM, and can resolve single atoms in a
sample. An electron probe microanalyser, which is an electron microscope fitted
with an X-ray spectrum analyzer, can examine the high-energy X-rays that are
emitted by the sample when it is bombarded with electrons. Because the identity
of different atoms or molecules can be determined by examining their X-ray
emissions, electron probe analyzers not only provide a magnified image of the
sample as a conventional electron microscope does, but also information about
the sample's chemical composition.
Scanning Probe Microscope
A scanning probe microscope uses a
probe that scans the surface of a sample to provide a three-dimensional image
of the network of atoms or molecules on the surface of the sample. A probe is
an extremely sharp metal point that can be as narrow as a single atom at the
tip. An important type of scanning probe microscope is the scanning tunneling
microscope (STM). Invented in 1981, the STM uses a quantum physics phenomenon
called tunneling to provide detailed images of substances that can conduct
electricity. The probe is brought to within a few angstroms of the surface of
the material being viewed, and a small voltage is applied between the surface
and the probe. Because the probe is so close to the surface, electrons leak, or
tunnel across the gap between the probe and surface, generating a current. The
size of the tunneling current depends on the distance between the surface and
the probe; if the probe moves closer to the surface, the tunneling current
increases, and if the probe moves away from the surface, the tunneling current
decreases. As the scanning mechanism of the STM moves the probe along the
surface of the substance, the mechanism constantly adjusts the height of the
probe to keep the tunneling current constant. By tracking these minute
adjustments, a sketch of the contours of the surface is produced. After many
scans back and forth along the surface, a computer is used to create a
three-dimensional representation of the surface.
Another type of scanning probe
microscope is the atomic force microscope (AFM), which does not use a tunneling
current, so the sample does not need to be able to conduct electricity. As the
probe in an AFM moves along the surface of a sample, the electrons in the metal
probe are repelled by the electron clouds of the atoms in the sample. As the
probe moves along the sample, the AFM adjusts the height of the probe to keep
the force on the probe constant. A sensing mechanism records the up-and-down
movements of the probe, and feeds the data into a computer; from this data, the
computer constructs a three-dimensional image of the surface of the sample.
Carl Zeiss
1816 – 1888
German
manufacturer of optical instruments, born in 
1846 opened a shop in which he
produced and repaired optical equipment for the
