Patent Number: 
Section: description

Referring now to the drawings, and more particularly to FIG. 1, an anti-scatter X-ray raster according to the first embodiment has a plurality of parallel tubular channels 1 for an X-rays transporting, enclosed by two parallel planes 2, 3, being perpendicular to the longitudinal axes of the channels 1. The channels form a honeycomb structure (hereinafter referred to as a cellular structure). Their walls are in the form of a cylindrical surface or a lateral surface of a prism. FIG. 6 and FIG. 7 depict a form of the said cellular structure as viewed from of inputs or outputs of the channels. FIG. 6 and FIG. 7 illustrate as well embodiments, when the channels in the cross-section are round or oval, or they are in the form of a regular hexagon. In the latter embodiment there are no spaces between sidewalls of the neighboring channels. However the spaces, when they are represented, can operate as channels, and their availability has not an adverse effect on the transparency of a raster. An anti-scatter raster is produced according to the known technology of producing monolithic X-rays lenses (see, for instance: V. M. Andreevsky, M. V. Gubarev, P. I. Zhidkin, M. A. Kumakhov, A. V. Noshkin, I. Yu. Ponomarev, Kh. Z. Ustok. X-ray waveguide system with a variable cross-section of the sections. The IV-th All Union Conference on Interaction of radiation with Solids (May 15-19, 1990, Elbrus settlement, Kabardino-Balkarian ASSR, USSR). Book of abstracts. Moscow, 1990, pp. 177-178; U.S. Pat. No. 5,570,408 to Gibson). This technology consists of assembling the tubular stocks of starting diameter, heating up to the temperature of their material softening and drawing with compression in order to obtain a required form of the cross-section of the product. The walls of neighboring channels, which cross size is significantly smaller than a starting cross size of the stocks as a result of stretching and can reach a submicron level, become spliced (i.e., fused). The mentioned similarity of technologies of producing an anti-scatter raster and a monolithic X-ray lens does not mean that their technical principle is closely related. In an X-ray lens transporting of a radiation is based on the usage of an effect of multiple total external reflection from the interior side of the walls of the channels. Therefore they are designed to provide a capability of such a reflection. In the suggested anti-scatter raster, vice versa, a situation, when an effective component of a radiation passes immediately from the input to the output of a channel and a reflection of this radiation from the walls of the channels has interference affect on the raster coefficients, is ideal. A total absorption of a radiation by the walls of the channels without reflection from them is desirable for a secondary radiation. Following the condition 2d/h greater than xcex8c, (at this condition no more than one reflection is possible) provides the lack of multiple reflection of a radiation at its transporting along the channel. In the given inequality a critical angle of total external reflection is the following: xcex8c=hxcfx89p/E, where h is a Planck constant, xcfx89p is a plasma frequency for the material of the walls of the channels, E is energy of photon of radiation. Namely, for glass xcex8c [radian]=30/E[eV]. At E=17 keV xcex8c is on the order of 1.8xc3x9710xe2x88x923 radian. When an anti-scatter raster is used, it is placed between the object under study and the detecting device. The anti-scatter raster of the first embodiment is not focused and does not provide the identical conditions for passing an effective component of an X-rays for all channels. These conditions are the best for the channels of the central zone of the raster. Photons of a primary radiation, deflected from the direction to the geometrical center of a raster aperture at the angle exceeding the quantity inverse to the aspect relationship, cannot pass through the raster. Therefore the raster should he distanced well away from the source of a primary radiation, where a peripheral zone of the raster is still transparent for a primary radiation, passed through the object under study. Taking into account the above, if an anti-scatter raster is used according to the first embodiment there is no point in obtaining high values of an aspect relationship. Nevertheless if an anti-scatter raster is made as a cellular structure (and not a slot one), it provides good selection of a secondary radiation. The above is illustrated by FIG. 2 and FIG. 3. FIG. 2 depicts possible trajectories of photons of a secondary scattered radiation from one of the point of the object under study, when they can pass through a slot channel 1 to the detecting device. FIG. 3 depicts a raster, where a slot channel is substituted with channels-cells 1, having the same total size in a FIG. 1. The photons of a secondary radiation, reached a detecting device, have trajectories with smaller angle then shown on the FIG. 2, therefor a possibility of photons reaching the detecting device is reduced. An amount of suppressing of a secondary radiation has same proportional value to the relation of a length of a slot channel to its width, if the channels have a cross-section size equal to a width of a slot channel. It is possible to decrease an aspect relationship without the deterioration of a selection of a secondary radiation in comparison with the known slot anti-scatter grids in the limits of value, defined by this achievable gain. Owing to this fact a distance between a raster and a source of primary X-rays can be reduced, and if this distance remains the same a transparency of peripheral channels for a primary radiation in comparison with the known anti-scatter grids of the same sizes can be increased. The described anti-scatter raster according to the first embodiment is the simplest to produce. Producing a raster according to the second embodiment (FIG. 4) requires additional operations of shaping to make its channels 1 narrow simultaneously with a raster narrowing as from inputs to outputs of the channels. These operations can be carried out by using forming devices with spherical planes, a xe2x80x9cplanexe2x80x9d raster according to the first variant is fastened between, and the raster is simultaneously heated up to the temperature of the material softening. The input and output planes 4 and 5 are placed on the forming device. The walls of the channels have a form of side surfaces of truncated cones or truncated pyramids with a common top, coinciding with the center of the concentric spherical planes 4 and 5. A focused anti-scatter raster according to the second embodiment provides good selection of a secondary radiation in combination with a steady transparency along the whole aperture for a primary radiation. Therefore a choice of an aspect relationship in it is not limited by the factors, being taken into account when a raster according to the first embodiment is designed, and it can be realized with full usage of possibilities. Owing to this fact a degree of suppressing a secondary radiation can be very high with corresponding increasing of an image contrast. As the advantages, provided by high aspect relationship, can be realized fully in the raster according to the second embodiment, it is possible to assemble it from polycapillaries, and the sizes of a cross-section of a single channel of a capillary are already very small. In this case before assembling the polycapillaries can be shaped lengthwise to a required narrowing form, thus it is possible to form the raster in pieces rather than as a unit. The input and output apertures of an anti-scatter raster according to the second embodiment are non-planar, what makes it inconvenient in use (namely in storing and delivering). This inconvenience is removed in the design of a raster according to the third embodiment (FIG. 5), the channels 1 have the same form as in the raster according to the second variant, but the planes 6 and 7, their inputs and outputs are planar. Such surfaces can be obtained by cutting a raster according to the second variant from two sides (this raster has a xe2x80x9creservexe2x80x9d of a longitudinal size of a channel). Different longitudinal sizes of the channels, increasing to the periphery, are characteristic for the raster according to the third variant. It can cause unequal losses of radiation intensity in the channels, being at different distances from the central zone of the raster, particularly if the channels are not hollow. However the differences at the sizes of the raster and the focal distance (i.e. a distance from the input of the channels of the central zone of the raster to the source of a primary radiation), being characteristic for practical applications, are small. A form of a cross-section of the channels according to the second and the third embodiments can be the same, as according to the first embodiment (FIG. 6 and FIG. 7). In both embodiments, as well as in the first one, a condition 2d/h greater than xcex8c is followed, and owing to this fact when a radiation is transported along the channels its multiple reflection is lacking. An anti-scatter raster, intended for the use in a scanning system of X-ray diagnostics or flaw detection, can be made as a narrow brick (parallelepiped shaped) (FIG. 8), and the quantity of channels 1 along its larger side is significantly more than in a perpendicular direction (i.e. in direction of a raster moving when scanning is realized). Realizing a high aspect relationship may be not to the purpose if an anti-scatter raster is made as a narrow parallelepiped according to the first embodiment, and taking into account a straight path of its moving and the above said reasons. Therefore such a raster can be made with rather large cross size of the channels, placed in one row along the parallelepiped""s length. If an anti-scatter raster is made as a narrow parallelepiped according to the second and third embodiments, i.e. when they are focused, to realize advantages of this method it is reasonable to move the parallelepiped in an arc of a circle with a center in a focus (i.e. in a point where a source of a primary X-rays is placed) in order to keep an orientation of the longitudinal axes of the channels to the source. In this case it is reasonable to realize a high aspect relationship, and a parallelepiped can be made of several rows of channels as cross sizes of the channels are small. The indexes of a raster in a form of a narrow parallelepiped according to the first embodiment can be improved when a brick of channels is moved in an arc of a circle, as the conditions of a primary radiation passing through a raster will be equal in all points of a trajectory, it is moved along. The channels for radiation transporting, according to all three embodiments, can be made of glass; lead glass is preferable. It is possible to make the channels of lead or other heavy metals as well. An advantage of these materials is that in this case the walls of the channels can be very thin. For instance, if a raster is used for mammography, when an X-ray tube with molybdenum anode is used as a source (quanta photon energy is E=1.74 keV), the walls may be as thick as 10-20 xcexcm. It increases the transparency of a raster and makes possible to keep a raster immovable as a survey is realized, as the shadows of such thin walls are not practically visible on the film. Sometimes, in dependence of the energies to be used, it can be reasonable to make the walls of the channels of light metals (for instance aluminum) or dielectrics. It can be realized when a radiation fall on the wall made of heavy metal causes hard secondary radiation, which can reaches an X-ray film or a detector. When light metals and dielectrics are used an emerging secondary radiation is soft and it is absorbed in a air layer of some centimeters thickness. It is desirable to make the channels hollow in most cases, for instance a raster can be made of glass mono or polycapillaries. In this case a transparency of a raster (a relationship of an open area of the cross-section of a raster to a total area) can reach 80%. Such a high transparency makes possible to decrease an irradiation dose of a patient. A technology of such raster producing is complicated. Therefore sometimes in order to simplify producing it is possible to make a raster with the channels filled with an organic material (for instance, polyvinyl chloride) or light metal, which slightly absorbs a primary radiation, carrying the information about the object under study. The filled channels are less affected by uncontrollable deformations during forming of a raster. Experimental researches of a raster, produced according to the suggested inventions, show that it is real to produce a device with cross sizes of 20xc3x9720 cm2 and more. When the raster is made of lead glass, attained transmission factor for an effective component of a radiation is on the order of 0.85. If an aspect relationship H/d is on the order of 60 to 100, attenuation of a scattered radiation on the output of a raster reaches 100 to 1000 times. An image contrast increasing in 4 to 7 times (for instance, in mammographic researches) corresponds to such indexes. Thus the usage of a raster makes possible to decrease an irradiation dose in 3 to 5 times. While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.