SKIALOGIYA (Greek skia a shadow + logos the doctrine) — the section of a radiology studying patterns of formation of the x-ray image and developing rules of definition of a structure and function of the studied part of a body or body is normal also at pathology on shadows and enlightenments.
The visible x-ray image owing to physical laws not always reflects a true form, size, situation and structure of the studied object. For the correct understanding rentgenol. pictures it is necessary to know patterns of a x-ray teneobrazovaniye, i.e. to own bases of a skialogiya.
The Skialogichesky analysis of the x-ray image begins with assessment of its quality determined by a number of parameters, among to-rykh the greatest practical value have contrast and sharpness of the image; estimates of skialogiche-sky properties of shadows (their provision, quantity, size, form, intensity, structure, character of contours, smeshchayemost).
There are four laws C.: law of absorption, summation of shadows, projective law and law of a tangentsialnost.
Law of absorption. According to this law intensity of a shadow of an object on the receiver of radiation is proportional to attenuation range objektokhm x-ray emission. Absorbing capacity of an object depends on its atomic structure, density and thickness, and also on the nature of x-ray emission.
At roentgenoscopy and a X-ray analysis of a body of the person the Wednesdays making four levels of optical density matter: gassy environments, soft tissues, dense fabrics and exogenous substances. Gassy environments (air in lungs, gases in intestines, the gas produced by microorganisms, the gas entered artificially for creation of contrast) cause effect of an enlightenment on the receiver of radiation. Soft tissues (excepting fatty) are almost equally transparent for x-ray emission and, therefore, give a uniform shadow. Treat soft tissues as normal myag-kotkanny formations of a body of the person (a muscle, parenchymatous bodies, a brain) and physiological liquids (blood, a lymph, cerebrospinal liquid, urine, bile), and patol. fabrics and liquids (tumors, fabric with inflammatory infiltration, exudate, transudate). The x-ray image of myagkotkanny objects, difficult and diverse on character, is created by hl. obr. thanks to their various thickness, orientation in space and to relationship with surrounding structures. Fatty tissue has absorbing capacity, a little smaller in comparison with other soft tissues, that creates premises for the differentiated its image against the background of other fabrics (an intermuscular fatty tissue, lipomas). Dense fabrics (enamel, dentine, bone, sites of heterogeneous calcification and ossification) have high absorbing capacity and create a shadow of high density on the receiver of radiation. Exogenous substances of very high density — the metal foreign bodys, positive X-ray contrast means containing elements with high atomic weight (iodine, barium, tantalum, ferroalloys) cause a shadow of very high «metal» density on the receiver of radiation.
Law of summation of shadows. Being flat display of a difficult three-dimensional object, the x-ray image contains information relating to all details of an object located on the course of a bunch of x-ray emission thanks to addition, or superposition of shadows (projective stratification of several educations). Owing to superposition on the roentgenogram there are shadows higher, than in the neighboring sites, density. Superposition can be full or partial (sectoral). In the latter case on border of two educations (e.g., in bones of a shin or a forearm) sometimes there is a ka*rtina of a break of contours called by tangential effect. Superposition along with anatomic factors causes on the roentgenogram more dense shadow of a joint hollow in comparison with contours of heads of the jointed bones. At superposition of dense objects with air spaces there is an effect of subtraction, or subtraction that can lead to disappearance or weakening of a shadow. The similar picture arises, e.g., at superposition of tumoral and inflammatory infiltration with emphysematous sites of a lung.
Projective law defines creation of the x-ray image. As the bunch of x-ray emission has the dispersing character, its section in the plane of the receiver of radiation is always more, than at the level of the studied object, i.e. any x-ray image always is increased. Extent of this increase is defined by a ratio of distances between a radiator, an object of a research and the receiver of radiation. It the is more, than further from an object the receiver of radiation is located and than closer to it there is a X-ray emitter. Receiving pictures with direct blowup is based on this principle. During considerable removal of a radiator from an object and the receiver beams in a x-ray bunch become almost parallel, at the same time the sizes of an object and its image practically match (see. Telerentgenografiya ).
The fact that objects, various in a form, can give the image, identical in a form, and, on the contrary, the same object depending on its arrangement in space is an important consequence of the projective law it can be displayed variously on the roentgenogram. So, the cylinder, a sphere and a cone can look on the roentgenogram in the form of a circle, and the hollow cylinder depending on its arrangement is displayed in the form of a shadow, ring-shaped, rectangular with underlined contours. Therefore on roentgenograms of lungs the image of bronchial tubes variously at axial and their longitudinal section.
From positions of the projective law it is possible to explain distortion of a form and the sizes of the object having considerable extent, napr, a vessel, a tubular bone. In case of an arrangement of a long axis of an object perpendicular to the central beam (a perpendicular aksialnost) only small projective increase in an object is observed. At reduction of a corner between the central beam and a long axis of an object (a slanting aksialnost) there is a projective shortening of an object. At coincidence of the central beam to a long axis of an object (a longitudinal aksialnost) the sizes of the image of an object are minimum and at the same time intensity of a shadow increases. Essential distortion of size and a form of an object is observed at its arrangement away from the central beam or in the absence of perpendicularity between this beam and the receiver of radiation — so-called slanting projections.
The important skialogichesky phenomenon following from the projective law is parallax or parallactical shift (Greek parallaxis evasion). In a radiology understand the shift of the x-ray image on the receiver of radiation arising during the movement of a radiator or object of a research, or at rotation of the last as the phenomenon of parallax. Parallactical shift of elements of the x-ray image matches the direction of the movement of the studied object and is opposite to movement of a radiator. At rotation of an object of a research of a shadow from its elements located kpered from an axis of rotation shadows of the elements located kzad from an axis of rotation — in an opposite direction are displaced on the receiver of radiation in the direction of turn, and. The size of parallactical shift depends on an arrangement of an element in the studied object. During the movement in space of a radiator or object the parallactical shift of subjects is more, than is spaced far apart an element of an object from the receiver of radiation. At rotation of the studied object degree of parallactical shift is proportional to distance of an element of an object from a rotation center. The phenomenon of parallax is widely used in a radiology during the definition of localization of foreign bodys (see. Rentgenogrammetriya ).
Law of a tangentsialnost defines a konturnost of an object and differentiation of its separate details thanks to what the outside contour of an object is defined only when the x-ray beam passes on a tangent (tangentsialno) to its surface, and details, various on density, are differentiated only when the surface of their section (boundary surface) has the orthograde (perpendicular) direction.
See also Radiology .
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I. P. Korolyuk.