.. ight- or left-handed symmetry group.
At the transition temperature the tetrahedral framework of beta-quartz twists, resulting in the symmetry of alpha-quartz; atoms move from special space group positions to more general positions. At temperatures above 867 C (1,593 F), beta-quartz changes into tridymite, but the transformation is very slow because bond breaking takes place to form a more open structure. At very high pressures alpha-quartz transforms into coesite and at still higher pressures, stishovite. Such phases have been observed in impact craters. Quartz is piezoelectric: a crystal develops positive and negative charges on alternate prism edges when it is subjected to pressure or tension. The charges are proportional to the change in pressure.
Because of its piezoelectric property, a quartz plate can be used as a pressure gauge, as in depth-sounding apparatus. Just as compression and tension produce opposite charges, the converse effect is that alternating opposite charges will cause alternating expansion and contraction. A section cut from a quartz crystal with definite orientation and dimensions have a natural frequency of this expansion and contraction (ie. vibration) that is very high measured in millions of vibrations per second. Properly cut plates of quartz are used for frequency control in radios, televisions, and other electronic communications equipment and for crystal-controlled clocks and watches.
3.3 WHAT IS ENDOSCOPIC PHOTOGRAPHY? With the use of modern light -weight single lens reflex cameras employing either automatic exposure control or through-the-lens metering, good half or whole frame 35mm colour photographs can be taken. Distal cameras (intragastric cameras), producing 5mm or 6mm colour pictures and electronic distal flash, are also available in some fibre-endoscopes. Endoscopic photography is the available equipment and the best method of obtaining the best possible colour photographs. It is possible to obtain high-quality colour transparencies of bowel lesions. These are generally employed for patient records, teaching and research.
They are not usually employed for diagnosis since visual inspection and biopsy will already have been performed. An exception is in so called gastro-camera diagnosis where miniature photographs are taken from within the stomach as an aid to the detection of early gastric cancer. Endoscopic cine-photography is useful for recording motility, endoscopic techniques, and unusual lesions. It can be also be used to make teaching films. Close circuit colour television endoscopy is already in routine use in some centres of Japan, the United States and Europe and will undoubtedly find a wider use, especially for teaching and training.
This equipment is naturally very costly but cheaper equipment can be anticipated. 4. ENDOSCOPIC PHOTOGRAPHY ELEMENTS 4.1 FIELD FLATTENER In lens design, it is desirable that the image coincide with the Gaussian image plane so that the whole field may be in focus simultaneously. In this case, the Petzval sum of the optical system must be zero or, at most, be a small residual to compensate for the secondary effects of higher-order astigmatism and oblique spherical aberration. When the third-order astigmatism coefficient is zero, it is well-known that the sagittal and tangential image surfaces coincide with the Petzval surface. The curved fields of such an astigmatic lens system can be flattened by using a bundle of fibers.
The shape and curvature of the entrance end of the bundle is determined by the image surface of the lens system that precedes it. The other end of the fiber bundle may be flat if the system is to be used for direct observation or photography, as shown in Fig. 4.1.However, when an image is field flattened in this manner, there is an interaction between the lens distortion coefficient and a distortion term introduced on field flattening. Distortion term shows the exit pupil of a lens system through which a principal ray passes at an inclination U and intersects the Petzval surface at the point P and the Gaussian image plane at the point Q. Since the principal ray does not intersect the Gaussian plane when a field flattener is used but is intercepted by a fiber at the Petzval surface, the effective image size is changed by an amount OQ = ?h.
And ?h = hG – h where hG is the Gausiian image height and h is the intersection height of the principal ray at the Gaussian image plane. There are several methods available for the production of a field flattener. In one of these methods, the fibers are ground and polished along the curve desired according to the Fresnel element, and then the entrance ends of the fibers are displaced to lie on the curved image surface. Obviously, this method suffers from technological limitations and is acceptable only when low-resolutison field flatteners are required. A second method consisting of lapping the field flattener in against a metallic master. In the third, most promising method, a Fresnel surface is produced at the curved surface of the fiber assembly with a master, employing an epoxy of the type used for making diffraction grating replicas.
4.2 CONICAL CONDENSER A conical fiber bundle is placed at the focal end of a lens system to increase the photographic speed of the system by utilizing the flux-condensing property of a cone. However, the condensing ratio of a glass-coated glass cone is determined by the ratio f- ratio and the field angle of the preceding image forming system, as well as the refractive indices of the fiber core and coating materials. If we make some simplifying assumptions of a meridional ray propagation in a cone with axial length many times greater than its diameter. For cones located off-axis at the image plane and with bend sides, there are obvious deviations. Figure 4.2 shows an image transmitted by a conical fiber bundle having a 2,5 : 1 ratio.
4.3 DISTORTION CORRECTOR It is possible to fabricate fiber bundles with the capability of correcting for pin-cushion and barrel distortion. It is also possible to evolve techniques for fabricating fiber bundles to compensate for the distortion term introduced in large-angle line scan systems and S-shaped distortion of the type introduced in electron-optical systems. Figure 4.3 shows images transmitted through two fiber plates, demonstrating the correction capability for pin-cushion and barrel distortion. Such fused fiber assemblies are fabricated by subjecting to well defined thermal and pressure gradients.
As another intersting example of the application of a combination of field flattener and distortion corrector, we shall cite the problem of a wide-angle spot scan systems in which a severe distortion term proportional to the field angle is introduced because of a change in spot size. In such a system, it is also desirable to use a curved image fieldto facilitate the mechanical synchronization of the two scanning functions of the data-acqusition and print-out systems. 4.4 FOCON RESOLUTION Of importance in the determination of the overall performance of a lens-fiber optics combination is the angular resolution (Rang) of an image-forming system of a aperture diameter, D, which, according to classical theory, is given by the formula: Rang = D/1.22? By inserting the value of the focal ratio (F), it is possible to determine the linear resolution (Rang), which is given by the following expression; Rlin = 1/1.22F? On the other hand, the linear displacement between two points which can be resolved by static fiber optics is between 2d + 3t and d + 2t, where d is the fiber diameter and t (? 0.5 ?) is the spacing between them. The resolution is then given by the reciprocal of this quantity. Waveguide effects and evanescent wave coupling between the fibers can be avoided if the fiber diameter is greater than or equal to ?? when the fiber numerical aperture is close to unity.
Such a fiber will propagate approximately 20 modes of wavelength, ?. Thus the optimum static resolution that can be obtained with fibers is approximately 1/ ?? + 2t. Consequently, for ? = 0.5 ?, a maximum static resolution of 220 to 350 lines / mm can be expected with high resolution fiber optics. Of course, dynamic scanning can be used to improve the resolution. Thus the highest linear resolution obtainable with a fiber bundle is considered to be equivalent to that of a diffraction-limited f/4 lens. Figure 4.4 shows a curve of the resolution of fiber conical condenser used in conjunction with diffraction-limited lenses of a given f-number.
Each curve corresponds to a conical condenser of ? = a2/a1 (no2 n2)1/2, where a1/a2 is the cone ratio, and no and n are the refractive indices of the fiber core and coating, respectively. 5. ENDOSCOPIC PHOTOGRAPHY TECHNIQUES 5.1 COLOUR PHOTOGRAPHY WITH FIBRE-OPTIC ENDOSCOPES This technique is the one of employed in great majority of endoscopic examinations. Photographs are taken through the endoscope by a camera placed on the eyepiece. This means that whatever the operator sees will be recorded photographically. The disadvantages of this method are that the fibre-matrix is also photographed.
In addition, any imperfections in the operators view, such as poor focus or bad picture composition, will be reflected in the photograph. To this extent the problems are similar to those of conventional photography, but otherwise there are few similarities. When employing a proximal camera for endoscopic photography the following points should be remembered. 1.
A single lens reflex (SLR) camera must be employed. 2. Through the lens exposure metering (TTL metering) must be employed, unless there is automatic exposure control of the light source output. 3. A medium focal length lens, eg 70-105 mm or telephoto lens, may be required with some endoscopes and must be focussed at infinity. 4.
The camera lens must be focussed at infinity. 5. Photography must be carried out at aperture if a camera lens is employed. 6. It may not with some endoscopes be necessary to use a camera lens. 7.
It is not usually possible to vary the ligthing. 8. High speed film is usually necessary and must be of the correct type. 5.2 CINE ENDOSCOPY Although cine endoscopy is employed routinely by some authorities to record lesions, motility , etc, it is usually reserved for occasional use in teaching because of the cost equipping with suitable cameras and films.
Suitable cine cameras include: Super-8 Kodak M-30 with power-operated zoom lens (from f/1.9) and Beaulieu R-16 B medical camera (16 mm). The Beaulieu R-16 B Euratom camera is undergoing evaluation at present. It houses an automatic light control system in place of the lens turret consisting of a graded neutral density filter wheel coupled to the exposure meter. This wheel is adjusted by a small servo motor so that the light reaching the film remains constant. This novel form of light control provides and alternative to the iris diaphragm which, as we have already seen, is not possible with endoscopy photography.
At the present, however, this camera is nut fully tested. Probably the best currently available system is the standard 16 mm Beaulieu R-16 B medical camera, employing a suitable adaptor supplied by the manufacturer for their endoscopes. 5.3 CLOSED CIRCUIT COLOUR TELEVISION ENDOSCOPY In a number of Japanese centers and in some centers in the USA and Europe, closed circuit colour television endoscopy is employed for demonstration and teaching. The results, as might be expected, are variable, but it is possible, by employing the best available equipment to produce excellent television images with good colour reproduction.
Television technology is highly developed, nevertheless it will be useful to discuss the items that make up an effective system for endoscopy and to point out the weak links. A succesful system for use in gastro-intestinal endoscopy would consist of: a colour television camera; a flexible optical coupling between the television camera and the endoscope; a light control system; colour television monitor(s); a fibre-optic endoscope, and a suitable light source. 5.4 GASTRO-CAMERA EXAMINATION Gastro-camera examination of the stomach is an investigation in which a flexible tube is passed into the stomach and multiple colour photographs taken employing a miniature camera and flash lamp mounted distally on the tube. This method was developed by the Japanese in 1950 in an attempt to diagnose gastric cancer, a disease that accounts for more deaths in Japan than any other form of cancer.
Diagnosis is based on a complete photographic survey of the stomach, followed by careful inspection of the transparencies. Suspicious areas are noted and the patient called back for full fibre-endoscopy and biopsy, or alternatively surgical biopsy. The term gastro-camera is understood to include blind gastro cameras which do not have visual control and visually controlled instruments with image blundles. With the blind gastro-cameras the tip of the instrument is positioned by observing the light from it through the abdominal wall. Clearly this must take place in darkened room. 6.
CONCLUSION Fibre-optic endoscopy has established itself as an important diagnostic tool in the investigation and management of disease of the gastric-intestinal tract. Considerable advances have been made in the design and construction of fibre-optic endoscopes and their support systems, over the past ten years. It is unlikely that development will take place at the same pace over the next decade. We are now entering a phase of consolidation during which objective evaluation of each area of endoscopy will take place as the techniques become more widely used.
Advances will be made in producing serviceable instruments and local servicing facilities are likely to be increased and streamlinid. Fibre bundle technology will probably not strive to produce smaller fibres since the limit has already been nearly reached. Design will probably concentrate on reliability, and cheaper meth-pds of production. Endoscope support systems, such as light sources, will probably improve with the development of more powerful, cooler and reliable lamps. The great advantage of flexibility provides the key to the use of optical communication within as well as outside medicine. As a result of this technology medical fibre-optics are likely to receive the benefit of cheaper more dispensible fibre-bundles.
These are, at present, the most expensive items in a Fibre endoscope. Bibliography 1) Kapany, N.S., Fiber Optics, Academic Press, New York, 1967 2) Buck, J.A., Fundamentals of Optical Fibers, Wiley-Interscience Publication, New York, 1995 3) Salmon, P.R., Fibre Optic Endoscopy, Pitman Medical Publishing, New York, 1974 4) http://www.britanicca.com 5) http://www.ibmpatent.com.
