RU2754112C1 - Device for high-speed high-sensitivity registration of x-ray images with discrimination of secondary scattered radiation - Google Patents

Abstract

FIELD: X-ray technology.SUBSTANCE: invention relates to X-ray technology and can be used in medical X-ray equipment and tomographs with high spatial resolution, sensitivity and scanning speed. To achieve the effect, it is proposed to use a device for high-speed highly sensitive registration of X-ray images with discrimination of secondary scattered and transmitted radiation.EFFECT: increasing the sensitivity to the maximum possible, eliminating the parasitic background of scattered X-ray radiation, increasing the resolution and registration area, increasing the dynamic range, increasing the resource and durability of the device, reducing the dose load on the patient.3 cl, 3 dwg

Description

The invention relates to X-ray technology and can be used in medical X-ray equipment and tomographs with high spatial resolution, sensitivity and scanning speed.

Currently, there are three main methods of obtaining an X-ray image for analysis:

1. Conversion of an X-ray image into an optical one in two-dimensional scintillation screens with subsequent transfer of the latter through the transfer optics to a photosensitive surface (CCD matrix, image intensifier tube, photodiode array, etc.). In this case, the radiation source, the X-ray object and the screen, as a rule, are stationary relative to each other.

2. Scanning-type transducers, which are one-dimensional arrays of photodiodes or photomultipliers with an X-ray sensitive scintillator layer applied to the outer surface, moving along the scanning direction and constructing successively one after another a set of one-dimensional image slices, while X-ray diffraction can be carried out by a flat fan-shaped or plane-parallel beam. In this case, it is necessary to carry out the movement of objects relative to each other: either the object along the scanning direction, or the receiver and the source relative to a stationary object. The latter option is used for X-ray imaging in medical CT machines.

3. Recorders of direct conversion based on the internal photoelectric effect in a semiconductor matrix structure. In this case, when the X-ray quantum interacts with the material of the substance, electron-hole pairs are formed, which are then separated in the volume of the semiconductor by an electric field and then read by charge converters. The detectors can be used, as in the previous cases, both one-dimensional and two-dimensional.

The advantages of two-dimensional scintillation recorders include simplicity, convenience and low cost of operation: the X-ray projection of the object is immediately displayed on the imaging device. Their disadvantages include an excessively high excess dose of radiation required to obtain an X-ray diffraction pattern, low spatial resolution, and low shooting speed due to the inertia of the phosphor.

The advantages of scanning recorders include the possibility of obtaining three-dimensional tomographic images reconstructed from two-dimensional sections obtained in the course of X-ray diffraction with a narrow beam. Their disadvantages include complexity, cumbersomeness and high cost, lack of mobility and the need to use special resource-intensive algorithms to restore tomograms. In terms of total dose loads, scanning systems, despite the smaller beam cross section, are not inferior to scintillation recorders.

Since the quantum efficiency of the internal photoelectric effect can reach almost 100%, modern technologies for growing semiconductor materials make it possible to obtain high-quality structures with low readout noise and, due to the absence of an intermediate stage of X-ray quantum-photon conversion, recorders based on the internal photoelectric effect are highly sensitive to X-rays and high spatial resolution. Their disadvantages include the extremely high price, limited service life due to the accumulation of radiation-induced defects in the semiconductor structure, low data output rates, and small size of the sensitive area.

The method of obtaining X-ray images of high resolution, high sensitivity, with high data output rates with small mass-dimensional characteristics of the recorder, allowing to create a portable tomograph (Joseph Ladislas Wiza. MicroChannel plate detectors. Nuclear instruments and methods 162 (1979) 587-601), is the use of an alternately gated stack of microchannel plates, the output electronic image from which is displayed on a high-speed luminescent screen, transmitted to a high-speed matrix detector through a fiber-optic fox.

The property of microchannel plates (MCP) is known to convert X-ray radiation into an electron stream for use as highly sensitive X-ray counters with a speed of the order of a few nanoseconds.

However, these detectors are structurally constructed according to the scheme with a single metal anode and are not suitable for obtaining X-ray images.

Known (Frederic Ze, Otto L. Landen, Perry M. Bell, Robert E. Turner, Teresa Tutt, Sharon S. Alvarez, and Robert L. Costa. Investigation of quantum efficiencies in multilayered photocathodes for microchannel plate applications. Review of Scientific Instruments 70, 659 (1999).) A method for increasing the sensitivity of microchannel plates to soft X-ray radiation by applying an X-ray sensitive layer to the input surface of the microchannel plates.

The disadvantage of this solution is a decrease in resolution due to smearing of the electronic package in the lateral plane of incidence of the radiation flux.

It is known (RU, patent 2370789, publ. 20.10.2009) a device for registering γ-radiation, using microchannel plates for registering X-ray radiation, however, the device is one-dimensional, that is, it is not intended for obtaining two-dimensional images without moving the device along the object.

It is known (RU, patent 78955, publ. 10.12.2008) a device for forming and registering an X-ray image.

The device contains a projection optical system, a scintillation screen and an image intensifier tube with a photocathode. The use of a sequential conversion scheme "X-ray quantum-scintillation screen-visible light-optical system-photocathode-MCP-screen" leads to a low conversion efficiency of the device, which reduces its dynamic range of registration and requires an increase in the dose from the radiation source to increase it.

It is known (RU, patent 81810, publ. 03/27/2009) a device for the formation and registration of an X-ray image, in which a fokon is used to join the photosensitive matrix and the image intensifier screen, however, the formation of an X-ray image is carried out on a scintillation screen and then is projected onto photo cathode of the image intensifier tube, while the conversion efficiency of registration also decreases.

Known (RU patent 88164, publ. 27.10.2009) microchannel X-ray transducer, which uses a microchannel plate, the channels of which are filled with a phosphor. The disadvantage of such a converter is also a reduced conversion efficiency due to the use of a phosphor - an X-ray to visible radiation converter.

There is a known device (RU, patent 83623, publ. 06/10/2009) for the formation and registration of an X-ray image, in which the method of recording X-ray radiation without brightness amplifiers based on an image intensifier and without a projection optical system is used for registering X-rays, and parquetting of smaller matrices is applied. into a larger matrix by increasing their number. The disadvantage of the device is the gradual degradation of the sensitivity of the matrices due to the accumulation of radiation defects in them and, as a consequence, a decrease in the dynamic range and sensitivity.

The known device is adopted as the closest analogue.

The technical problem solved by the use of the developed device is to improve the means for recording X-ray radiation.

The technical result achieved by the implementation of the developed device consists in increasing the sensitivity to the maximum possible, eliminating the parasitic background of scattered X-ray radiation, increasing the resolution and registration area, increasing the dynamic range, increasing the resource and durability of the device, and, as a consequence, reducing the dose load per patient.

To achieve the specified technical result, it is proposed to use the developed device for high-speed high-sensitivity registration of X-ray images with discrimination of secondary scattered and transmitted radiation. It contains a housing in which an optical matrix sensor is located for obtaining images based on a device with a charge-coupled device or based on a CMOS structure, controller boards made with the ability to read a signal from the sensor and synchronously with reading an image from the matrix on an external command to start high-voltage generators of power pulses microchannel plates, the focal point, the smaller end of which is located on the specified sensor, and on the larger end there is a fluorescent screen made of phosphor with a fast decay of the persistence, a pulse generator, a low-voltage power supply that serves as a independent from each other identical outputs that generate high-voltage pulses with a delay relative to each other, and the number of outputs is equal to the number of microchannel plates used, the inner evacuated body, in which a window is made of material with low absorption p X-ray radiation for the passage of X-ray radiation, under the specified window there is a chevron assembly of microchannel plates, while inside the evacuated body there is a specified fluorescent screen, and each microchannel plate of the chevron assembly is made with an independent connection to the output of the pulse generator, and the generator is made with the possibility of production in series time-shifted pulses with respect to each other, while the delay and duration of the pulses are selected based on the transit time of the electron avalanche of secondary electrons in the channels of the microchannel plate.

The window in the inner evacuated body can be made of beryllium.

The optical matrix sensor can be made on the basis of a charge-coupled device or based on a CMOS structure.

To obtain high sensitivity to the input X-ray flux, up to the limiting one, when each X-ray quantum will be recorded on the output image, the invention proposes to use a multi-chevron assembly of microchannel plates (MCP), which are connected in series with each other, and the orientation of the channels of the microchannel plates is made as follows: that the outlet end of the previous channel is located above the inlet end of the subsequent channel (Fig. 1). Each MCP is powered separately and impulsely with a delay between each MCP equal to the time of flight along the channel of the cloud of electrons re-emitted from its surface (Fig. 2).

The principle of operation of the device is illustrated in FIG. 1. The X-ray quantum of the input image 1 interacts with the channel wall of the MCP 2, or is absorbed in the volume of the wall 3, causing an internal photoelectric effect. On the end face 5 of the first MCP, a layer of X-ray sensitive photoemission material, for example, CsI, CsBr, CuI and others, can be applied. When an X-ray quantum is absorbed, an electron is formed that goes out into the channel of the MCP 4. The electrodes of the ends of the MCP 6 and 7, brought out to the outside, are fed with a potential difference accelerating the knocked out electron (solid arrows). When moving in the channel, the electron knocks out the next portion of electrons from the channel wall at each subsequent collision. Thus, the effect of avalanche multiplication of charges in the channels arises. An electronic avalanche from each channel, passing through each channel formed by each MCP, at the exit from the last end 10 of the last MCP falls on a fast-illuminating X-ray fluorescent screen 11, applied to the large-diameter surface of the focal point 12, after which the resulting optical image of the X-ray object enters the articulated contact with the small the end of the focal point is a CCD or CMOS matrix 13, which transfers the resulting image to a computer. A phosphor with a fast decay time, for example, R-11, allows high-speed shooting with a reading speed of 1000 frames per second or more. The focal point is made of a material with a high lead content and, in addition to the function of transferring an image from a large area to a smaller area of the matrix, it also plays the role of protecting the matrix pixels from the effects of direct transmitted and scattered X-ray radiation. To make each MCP thinner, it is necessary to increase the content of heavy elements with a high atomic mass in its composition. The most suitable for this purpose is lead oxide, the mass fraction of which in the MCP must be at least 50%.

During the primary interaction of a quantum with the MCP material, secondary X-ray radiation arises, which causes unwanted emission of electrons from their walls in adjacent MCP channels (dash-dotted arrows). This effect is insignificant at a small MCP thickness, since the scattered radiation is rapidly absorbed by the MCP material between adjacent channels, and the scattering direction is mainly frontal in a small solid angle in the direction of radiation propagation. Thus, the largest contribution of scattered radiation to the noise of the MCP is made by the secondary X-ray quanta that fall into the underlying MCP. To significantly reduce the effect of secondary quanta and scattered radiation, each gap of the ends of neighboring MCPs is powered by a pulse with a delay between them. Since the speed of motion of the electron cloud in the channel is significantly less than the speed of light with which the photon propagates, the absence of potential in the MCP channel, when the electron avalanche has not yet reached it, and the quantum has already absorbed and emitted an electron, will create the absence of an accelerating field in the channel and as a consequence, the absence of the effect of avalanche multiplication, thus reducing the parasitic background illumination, increasing the sensitivity and dynamic range of registration. The pulse power supply circuit of the multi-chevron MCP assembly is illustrated in Fig. 2. The abscissa shows the relative time, and the ordinate shows the time of supply of the unlocking MCP electrical pulse for each interval corresponding to FIG. 1.

For overlapping large recording areas, the shown in FIG. 1, the recorder is mated to the same as shown in FIG. 3.

Claims (3)

1. A device for high-speed high-sensitivity registration of X-ray images with discrimination of secondary scattered and transmitted radiation, characterized in that it contains a housing in which an optical matrix sensor is located for obtaining images based on a charge-coupled device or based on a CMOS structure, a controller board, made with the possibility of reading the signal from the sensor and synchronously with reading the image from the matrix on an external command to start the high-voltage pulse generators of the microchannel plates, the focal point, the smaller end of which is located on the specified sensor, and on the larger end there is a fluorescent screen made of a phosphor with a fast decay of the afterglow, the generator pulses, a low-voltage power supply that serves as a power supply for the digital part of the device, and a high-voltage pulse power supply having more than one independent identical outputs, generating high-voltage pulses with a delay, etc. yr relative to the other, and the number of outputs is equal to the number of microchannel plates used, the inner evacuated housing, in which a window is made of a material with low absorption of X-ray radiation for the passage of X-rays, under the specified window is a chevron assembly of microchannel plates, while inside the evacuated housing is located the specified fluorescent screen, while each microchannel plate of the chevron assembly is made with an independent connection to the output of the pulse generator, and the generator is configured to produce pulses sequentially shifted in time relative to each other, while the delay and duration of the pulses are selected based on the transit time of the electron avalanche of secondary electrons in channels of the microchannel plate. 2. The device according to claim 1, characterized in that the window in the inner evacuated body is made of beryllium. 3. A device according to claim 1, characterized in that the optical matrix sensor is made on the basis of a charge coupled device or based on a CMOS structure.

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