Endoscopy
Principle
Endoscopy
refers to looking inside the human body using an endoscope. Endoscopy
is a minimally invasive diagnostic technique used to assess the interior
surfaces of an organ by inserting a tube into the body. The instrument may have
a rigid (borescope) or flexible tube (fiberscope) and not only provide an image
for visual inspection, photography and video-scopy, but also enable taking
biopsies and retrieval of foreign objects. Endoscopy is the vehicle for minimally
invasive surgery.
An endoscope comprises an eyepiece, the ocular, producing
a parallel exit bundle or virtual image and light source to illuminate the
object on one end. At the other end is an objective lens producing a real image
(see Light: the ideal lens).
Both are linked by a tube mounting an optical
fiber system (see Fiber
optics). So, the fundamental principle of operation
is transmitting optical information through a bundle of optical fibers such
that an image can be observed. However, a classical boroscope may comprise instead of the fiber system a whole series
of lenses as transmission system. Basically, an endoscope is a kind of
microscope. The light source may provide wide band visible light, whether or
not spectrally scanned, and for specific applications narrow band, (near) IR light.
For Fluorescence
(natural or artificial), e.g. applied in examination the esophagus, UV light
can be used. Often, the light is provided by a Laser. An additional channel allows entry of air, fluid, as well as remote control of medical instruments such as biopsy forceps or cytology brushes.
A flexible endoscope.
Application
Endoscopy is applied in nearly every medical discipline,
also for outdoor patients. Endoscopes can be
divided into two distinct categories according to their medical application.
These are the regular, macroscopic endoscopes such as
the gastroscope, colonscope, and bronchoscope to inspect epithelial surfaces. The
second category comprises the miniaturized types. They includes ultrathin endoscopes for use as ophthalmic
endoscopes, angioscopes, robotic surgery and needlescopes. The latter, with a diameter less than one mm, have
been developed to examine very small parts of internal organs. The images of ultrathin needlescopes
contain 2,000 to 6,000 pixels with a smallest resolution of about 0.2mm. They
can be inserted into for instance mammary glands to detect breast cancer at
early stages.
A borescope is used in arthroscopy (and also in engineering).
Non-medical uses are in architectural design (pre-visualization
of scale models) and internal inspection of complex technical systems (borescope)
and examination of improvised explosive devices by bomb robots.
The type of fibers used
is dependent on the type of the illuminating light and the image specifications.
Often, the fibers to deliver the light, the light guide, (mostly with coherent
light, so light with the same frequency and
intensity) and those to transmit the image information,
the image guide, are of different types of fibers (see Fiber optics).
Recent developments are
fiber-optic fluorescence imaging systems. Until recently, fiber-based
fluorescence imaging was mainly limited to epifluorescence and scanning
confocal modalities (confocal micro-endoscopy) (see Light microscopy:
confocal). New classes of photonic crystal fibers (see Fiber optics) facilitate ultra-short
pulse delivery for fiber-optic two-photon fluorescence imaging. This can be combined with two-photon fluorescence and second harmonic generation microscopy,
miniaturized in a nonlinear optical endoscope based on a double-clad photonic
crystal fiber to improve the detection efficiency and a MEMS (MicroElectroMechanical
System) mirror to steer the light at the fiber tip (see Fiber optics, Light microscopy: two-photon fluorescence,
and the chapters about light microscopy).
Another new application is combining laser holographic
interferometry with an endoscope (see Holography and Huygens’principle).
Another combination is laser Doppler imaging (see Doppler principle) of blood flow
with endoscopy.
With the application of robotic systems, telesurgery was
introduced as the surgeon could operate from a site physically removed from the
patient.
Wireless
capsule endoscopy is another emerging technology.
This technique uses an ingestible
capsule comprising a miniature camera with a MEMS mirror for scanning and a
transmitter. In this way some part of the gastrointestinal tract can be visualized.
Nowadays, this application in the esophagus is more or less standard, but other
parts of the tract are still experimentally due to peristaltic movements. MEMS
technology may provide a solution for this.