JPS5910216B2 - Electronic scanning computerized tomography device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electronic scanning computed tomography apparatus that makes it possible to realize a CT apparatus that does not require moving parts.

CT device (Computed Tomography Sc
In order to reconstruct the tomographic image, data regarding the absorption of transmitted radiation from all directions for each point on the tomographic plane is essentially required. Mechanical moving parts, especially rotating parts, were essential to the equipment.

Therefore, a large-sized mechanism for linear and/or rotational movement is required, and a fixed CT apparatus cannot be put to practical use. The present invention has been made in view of the above, and is an electronic scanning computed tomography system that enables the realization of a portable CT device that is small, lightweight, and does not require moving parts, and can be mounted on medical examination vehicles. The purpose is to provide a device.

The details will be explained below.

FIG. 1 is a perspective view showing a first embodiment of the present invention, and FIG. 2 is a longitudinal sectional view thereof. In this embodiment, an electron beam generation source 2 having an electron emission source 2a) a Wehnelt electrode 2b) an anode 2c provided in a vacuum container 1, a deflection system 4 for deflecting the generated electron beam 3, and a deflected electron beam A target 6 having a strip-shaped surface along the deflection direction that is impacted by a beam to generate a radiation beam 5, a slit-shaped radiation projection window T provided on the wall of the vacuum vessel facing the target The collimator 8 (Fig. 2) is equipped with a collimator 8 (Fig. 2) which is shaped like a thin plate and shaped so that a radiation beam with a spread centering on each point on the target can be taken out. As the fan beam is scanned from 5a to 5n, the fan beam is scanned along the beam surface from 5a to 5n. By making the surface of the target 6 a curved surface or an arc centered on the subject, it is possible to expand the obtained beam direction angle over a wide range. Further, cooling of the target in this apparatus can be accomplished relatively easily by providing a cooling pipe 10 attached directly to the target 6 as shown in FIGS. 1 and 2. In such a device, a subject 11 is placed in the fan beam, the transmitted radiation is detected by a detector 9 made up of a plurality of detection elements, and a tomographic image can be reconstructed by using the detected data. It is possible.

Although the directional angles of the data obtained with this device are limited, as will be explained in detail later, by utilizing the properties of the projection data on the Fourier plane, it is possible to perform projections from limited angular directions. It is possible to create data from other angular directions based on the data, and it is possible to reconstruct high-quality images by obtaining data from virtually all directions from data from a limited number of directional angles. There is no problem in practical use.

The rationality of the above method will be explained in detail below. The basic processing in a CT device is to reconstruct the distribution of absorption coefficients of the tomographic part, which is the original image, from radiation projection data determined by the distribution of absorption coefficients of the tomographic part of the subject. be. Various methods have been proposed for this reconstruction method, but in most cases, the most practically effective methods are the convolution method (or convolution method), the filter-back projection method, or the Fourier transform method. It is. These methods are completely different in terms of expression or actual data processing, but the basic mathematical formulas are the same and the mathematical meanings are the same. This will be briefly explained below. FIG. 3 is a diagram for explaining a method of reconstructing an original image from projection data.
First, in a coordinate system (X, y) fixed with respect to the subject 11, the distribution of the absorption coefficient of the subject 11 is expressed as f(X, y). f(X, y) is the desired reconstructed image. next,
For the coordinate system (X, y), the coordinate system (
Consider X, Y). A set of radiation beams 12 parallel to the Y-axis direction of this coordinate system (X, Y) is irradiated to obtain projection data g(X, θ) of the subject 11. In FIG. 3, projection data °g(X, θ) is indicated by reference numeral 13. At this time, one-dimensional Fourier transform G(ω
, θ) are defined as follows. G(ω, θ)=F. 2g(X, θ)e-1Ct)Xdx... (1) Same as 1C. The two-dimensional pulley transformation F(ξ, η) of the reconstructed image f(X, y) is defined as follows.

F (ξ, η) = LZf-: f (X, y) e-1 (ξX+
'1y) Dxdy... (2) Coordinate system (ξ, η)
If we replace with polar coordinate expression (ω, θ), we get 11 WV
There is a relationship between & Bano v.

Here, using equation (3) based on the theorem that the one-dimensional Fourier transform of projection data in a specific angular direction of an image gives the same direction component of the two-dimensional Fourier transform of that image, (1), (2 ), F(ωCOsθ, ωS
inθ)=G(ω,θ) It is shown that the relationship (4) holds true.

Equation (4) is the basis of the image reconstruction method. That is, the angle on the projection data directly corresponds to the angle in the polar coordinate representation of the Fourier transform of the reconstructed image. Therefore, the Fourier transform G(ω, θ) is obtained from the projection data g(X, θ), and the coordinate transformation is performed using the relationship of equation (3) to obtain F(
Find ξ, η).

A reconstructed image f(X, y) is obtained by inverse Fourier transforming F(ξ, η) according to the following equation. This is a reconstruction method called the Fourier transform method. On the other hand, the convolution integral method and the filter-one back projection method perform inverse Fourier transformation in polar coordinate representation without using equation (5). In either method, since the information of the entire large area centered on the origin of the Fourier transform is targeted for one transfer, information regarding O≦θ≦27 is required for the angle θ.

In other words, a reconstructed image cannot be obtained unless G(ω, θ) satisfying O≦θ≦2π is obtained. Therefore, in conventional CT apparatuses, a radiation source that emits a set of parallel radiation beams or a radiation source that emits a radiation beam that spreads at a predetermined angle called a Fan beam is used.
It has a mechanism that rotates and scans the outer circumference of the subject. Furthermore, one or more radiation detectors are provided which are rotated and scanned relative to this mechanism, and the amount of radiation beam transmitted by the subject is detected. By emitting a radiation beam over a complete rotation for each angular displacement of, for example, 1 degree, the radiation source can obtain data on the amount of radiation beam transmitted by a large number of objects from the detector. Next, from these transmission amount data, projection data g in a specific direction by a set of almost parallel beams is obtained.
(X, θ) (0≦θ≦2π) is extracted. The distribution of the absorption coefficient of the object is usually obtained from the obtained projection data using one of the above-mentioned reconstruction methods, but the image reconstruction described here specifically takes advantage of the analytical nature of the Fourier transform. It is characterized by This will be briefly explained below. First, equation (2) can be expressed as follows. As is clear from equation (6), the function F (ξ, η) consists of a real part ReF (ξ, η) and an imaginary part 1mF (ξ, η). Here it is.

Therefore, it is.

Similarly, it is.

On the other hand, according to equation (3), 1W-call↓-WV-gu-\V-?
! Therefore, expressions (8) and (9) can be expressed using ω and θ as follows.

Similarly, the function G(ω, θ) is also the real part ReG(ω, θ
) and the imaginary part 1mG (ω, θ), and (
4) From the relationship of formula, ReG (ω, θ + π) = ReF (ω
COs(θ+π), ωSin(θ+π))=ReF(ω
COsθ, ωSinθ) ReG(ω, θ)...
・・・・・・(self) Similarly, it is.

Therefore, for the function G(ω, θ), angular dependence on the same radius ω can be considered. That is, as is clear from equation (alternative), the real part ReG(ω, θ) is a periodic function of π with respect to the angle θ on the same radius ω. Also (self)
As is clear from the equation, the imaginary part 1mG(ω, θ) is a periodic function whose sign is reversed at π with respect to the angle θ on the same radius ω. FIGS. 4A and 4B illustrate this situation.
Figure 4a shows the θ dependence of the real part ReG(ω, θ),
FIG. 4b shows the θ dependence of the imaginary part 1 mG(ω, θ). Due to the periodicity and analyticity of the function G(ω, θ), its real part ReG(ω, θ) and imaginary part ImG(
It is guaranteed that each of ω, θ) can be expanded into a series as follows.

Here, the coefficient An(
ω), Bn(ω), Cn(ω) and Dn(ω) are the number of terms n
(12), 03)
It is also guaranteed that the formula holds strictly regardless of the value of θ.

Therefore, in actual processing, approximation can be achieved with sufficient accuracy for practical use by varying n until it reaches an integer value N that is appropriately selected according to the required accuracy.

In other words, it becomes.

These practically necessary coefficients An(ω), Bn(ω), C
n(ω) and Dn(ω) (where n=0, 1, 2,...
..., N), the function G(ω
, θ) Therefore, all the function data necessary for reconstruction on the function F(ξ, η) is given. Determination of these coefficients can be easily estimated from information obtained within a relatively small angle θ.

For example, in the range O≦θ≦ψ, projection data g(X, θ) has 2 (N+1) or more angles θ1, θ, .
If given for θ, (N+1), ......, these projection data g(X, θ1), g(X, θ,
),..., g(X, θ, (N+1))...
Fourier transform G(X, θ1), G(X, θ
,),...,G(X, θ2(N+1)),...
can be determined according to equation (1). Therefore, simultaneous linear equations regarding the above-mentioned coefficients according to the equation (self) are obtained, and by solving this, the value of each coefficient can be determined. In this way, the function G(ω, θ
), then the Fourier transform F(
ξ, η) can be obtained, and a reconstructed image f(X, y) can also be obtained using each of the above-mentioned reconstruction methods. The present invention is as described above, and by using the present invention, it is possible to obtain a CT apparatus that does not require moving parts, and the radiation source and the detector are installed facing each other, for example, on the ceiling and the floor. Since it can be formed into a simple shape with no space in between, it can be made into a CT that can be installed in ordinary medical examination cars.

In addition, since the imaging time can be very short (for example, less than a second), it has great effects, such as being able to obtain detailed still images at each moment even in objects with rapid movement such as the heart. .・Brief explanation of the drawings Figure 1 is a perspective view of the first embodiment of the present invention, Figure 2 is a vertical cross-sectional view of Figure 1, and Figures 3 and 4 are substantially drawn from data in limited angular directions. FIG. 3 is an explanatory diagram of a method for obtaining data in all directions.

1; Vacuum container, 2; Electron beam source, 3; Electron beam, 4; Deflection system, 5; Radiation beam, 6; Target, 7
; radiation projection window, 8; collimator, 9; detector, 11;
Object, 12; Radiation beam, 13; Projection data.

Claims (1)

[Claims] 1. A means for generating an electron beam, a fixed radiation generation source comprising a band-shaped target surface that generates a thin plate-shaped fan beam with the electron beam, and scanning the target surface with the electron beam, A fixed radiation detector arranged to obtain radiation absorption data or transmission data from a plurality of directions with respect to the object to be inspected; Means for obtaining projection data within a specified angular range, Fourier transform of these projection data, and further approximation of the real part and imaginary part of the Fourier-transformed projection data on polar coordinates in the Fourier series in the angular direction. An electronic scanning computed tomography apparatus characterized by comprising: means for reconstructing an image by obtaining projection data outside the angular range from the equation obtained from the equation.