HOLOGRAPHY

From Big Medical Encyclopedia

HOLOGRAPHY (Greek holos everything, full + grapho to write, represent) — a method of obtaining the volume image of an object. It is based on the registration and the subsequent recovery of the front reflected from an object electromagnetic (optical G.) or sound (acoustic G.) waves.

Opening of a method G. is connected with a name of the English physicist Gabor (D. Gabor, 1948). However technically it was at that time extremely difficult to realize a method, and G. did not gain distribution. The first holograms were received at the beginning of the 60th an amer. physics Leith (E. Leith) both I. Upatnieks and the Soviet physicist Yu. M. Denisyuk in connection with emergence lasers (see). At the same time numerous opportunities of practical use of G. in radio electronics, physics, optics, various areas of the equipment, art and in medicine opened.

In medicine optical G. is especially perspective in microscopy and endoscopy. Use of a method will allow to reach high resolving power of microscopes thanks to for registration of an object of radiation with small wavelength (e.g., x-ray), and for recovery of the image — radiation with the bigger wavelength (visible light). Thanks to property of sound waves to get into optically opaque environments acoustic G. will allow to obtain information on internal structures of an object. In this regard it can be perspective at a nondestructive research of a brain, heart and other bodies and large blood vessels, during the definition physical. - chemical properties of blood and dynamics of various changes in its structure, at control of functional characteristics of blood circulation.

Fig. 1. The scheme of installation for obtaining the hologram: the optical quantum generator (1) through system of lenses (2 and 3) directs a basic beam of light (4) to a mirror (5) and the studied object (6); having been reflected from a mirror, the basic bunch gets on a photographic plate (8) where also the scattered light wave (7) which was reflected from an object gets, fixing the hologram on a photographic plate.
Fig. 2. The scheme of installation for recovery of the hologram: a basic beam of light (4) from the optical quantum generator (1) through system of lenses (2 and 3), having been reflected from a mirror (5), (would) light the hologram on a photographic plate; an eye of the observer (7) sees a virtual image of an object (M) behind the hologram; an eye of the observer from other point (8) sees the valid image of an object (D) before the hologram.

It is carried out in two stages: obtaining the hologram (record of all front of the electromagnetic or sound wave reflected from an object) and recovery of the image of an object on the hologram. The principles of obtaining the electromagnetic and acoustic hologram are identical. In both cases apply coherent (identical) sources, only instead of change of intensity of light owing to imposing of waves at optical G. in case of use of an acoustic method register change of acoustic pressure. At first photograph the diffraction picture (or a picture of change of acoustic pressure) given by an object together with a coherent background. Then the hologram is lit with a parallel monochromatic beam of light and owing to its diffraction on the hologram receive the image of an object. For color G. use three-dimensional holograms: an object is photographed in the light of three waves of different length on a thick plate. The relief color image turns out after consecrations of the three-dimensional hologram by this world. In installations for receiving and recovery of the hologram (fig. 1 and 2) as a light source use the optical quantum generator. An object is lit with light received from the optical quantum generator. The scattered light wave reflected from an object gets on a photographic plate, on to-ruyu also basic beam of light created by means of the same generator and a mirror falls and thus the image of an object (hologram) is fixed. For recovery of the image of an object the hologram is lit with monochromatic light and consider a virtual image on a gleam. The valid image hangs before the hologram, to see it with the naked eye quite difficult. Observing and photographing a virtual image of an object (e.g., an internal wall of a bladder or a stomach), it is possible to record depending on location of the device and its focusing this object from the different parties in several pictures. Studying of pictures and viewing of the hologram allow to gain complete idea of the explored site of a mucous membrane thanks to dimensions of the image.

At acoustic G. recovery of the wave front is carried out at direct interaction of light with a sound wave. As the probing radiation it is most effective ultrasound (see) since it quite meets the main requirements of medical diagnosis: it is capable to get through organic fabrics, provides contrast of borders of various organic fabrics, in the applied dose (1 MW/cm 2 ) significantly does not influence the current fiziol, processes and structure of the studied fabrics. Ultrasonic diagnostic units have rather high resolving power. Parameters of the most continuous environment (as exert impact on the nature of interaction of ultrasound with substance not so much at x-ray emission) how many the borders of environments having various acoustic properties (contrast and transparency).

The size of contrast (α) between two various environments and transparency of a layer (β) can be determined on formulas:

α = (W '-W) / W and β = W' / W 0 ,

where W and W' — shares of the energy which passed in Wednesdays, W 0 — incident radiant energy.

Variability of values of coefficients for ultrasound is especially big in comparison with x-ray emission. E.g., during the use of ultrasound (frequency of 1 MHz) the size of contrast of environments a bone — a muscle, blood — a muscle and blood — the bone at a thickness of layer of 1 cm (the contact environment water) makes respectively 71, 20 and 91%; the size of contrast of the same environments during the use of x-ray emission (wavelength of 0,1 nanometers, energy of a bunch 0,01 mev, the contact environment air) respectively 15; 0 and 15%. Value of these coefficients testifies to a possibility of receiving with the help ultrasonic G. of the layer-by-layer image of internal structure of the studied object.


Bibliography Babin L. V. and Kovalyov O. A. Problems of use of acoustic holography for a research of internals and the blood circulatory system, Nauch. works Leningr, in-that usovershenstvo. doctors, century 105, p.1, page 141, L., 1971, bibliogr.; Vyeno Zh.-Sh., Smigil-skiyp. and Ruaye A. Optical holography, Development and use, the lane with fr., M., 1973, bibliogr.; The island-with to and y Yu. I. Golografiya, L., 1970, bibliogr.; Franson M. Holography, the lane with fr., M., 1972, bibliogr.; Acoustical holography, in book: Proc, of the intern, symposium on acoustical holography, v. 1 — 2, California, 1969, N. Y. — L., 1969 — 1970; Brenden B. B. Acoustical holography as a tool for nondestructive testing, Materials evolution, v. 27, p. 140, 1969, bibliogr.

V. S. Andreyev, V. I. Belkevich.

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