JPH02291842A - X-ray ct apparatus - Google Patents

Abstract

PURPOSE:To obtain a reconstituted image having high spatial resolving power and to collect data within a short time by providing an operation means for operating calculation algorithm obtaining an X-ray detection signal free from crosstalk from an X-ray detection signal having crosstalk. CONSTITUTION:An X-ray shield body 6 shielding scattering X-rays 5 among X-rays obtained by irradiating an object 3 to be irradiated 3 with X-rays from an X-ray source 1 and the X-ray detector 7 provided in front of the X-ray shield body in the X-ray irradiation direction thereof and composed of a plurality of channels detecting X-rays transmitted through the object 3 to be irradiated are provided. An operation means 9 supposes linearity with respect to the crosstalk due to the scattering of X-rays generated between the respective adjacent channels of the X-ray detector 7 and expresses the detection outputs from the respective channels as plural simple simultaneous equations to solve the same to calculate the intensity of X-rays when it is supposed that there is no crosstalk and subjects said intensity to logarithmic conversion to multiply the same by a proper filter function and performs convolution integration and inversely projects the integrated result to calculate X-ray absorption and coefficient distribution to operate the algorithm subjected to image reconstitution.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to an industrial or medical X-ray CT device,
In particular, the present invention relates to an X@CT device that eliminates the influence of X-ray crosstalk between X-ray detectors that occurs when detecting X-rays from a limb irradiator.

[Prior Art] Fig. 8 is a cross-sectional view showing the schematic configuration of a conventional X-ray C'F device, in which (1) is an X-ray source, and (2) is a
An X-ray collimator (3) is placed in front of the X-ray collimator (2) and forms the X-rays from the X-ray source (1) into a parallel beam. The irradiated body (4) is the limb irradiated body (3).
), (5) is the scattered X-rays scattered by the irradiator (3), (6) is placed in front of the irradiator (3), and the scattered X-rays (5), etc. An X-ray shield body (7) is placed in front of this X-ray shield body (6) to block unnecessary X-rays from a large number of X-ray sub-detectors or channels as shown in the figure. This is an X-ray detector that detects transmitted X-rays (4). In addition, Defeat XI! FA (5
) is also caused by crosstalk between the X-ray sub-detectors as shown in the figure.

Figure 9 shows the geometric relationship between the X-rays irradiated to the irradiated object, the transmitted X-rays, the crosstalk between the X-ray detector (1) and its respective channels, etc. in the X-ray CT device of Figure 8. FIG. 10 shows the X-ray detector in more detail.
FIG. 11 is a schematic flowchart showing an image reconstruction algorithm when creating a tomographic image in such an X-ray CT apparatus.

In Figure 9, the X-ray source ((1) in Figure 8
)) is irradiated with X-rays, and the incident plane and the X-axis form an angle O. Transmitted X-rays (40), (4..
), (4-1) are the corresponding channels (7.), (7.1) of the X-ray detector (7). (7-1) is detected. The transmission data (hereinafter referred to as P (x, θ)) directly obtained by the channel (7) is transmitted through the amplifier (
8) is sent to an algorithm calculation means (not shown) that calculates an image reconstruction algorithm. be managed. (α) and (β) in the figure are scattered X-rays (
In 5) of Fig. 1, if the irradiated X-ray reaches a certain energy (for example, if the X-ray energy is Eγ, then Eγ > l MaV), it becomes impossible to ignore, and for example, at Eγ” 6 MeV, about 20% It comes to act as scattered X-rays due to crosstalk.

This is the problem that this invention seeks to solve, and will become clearer below. Transparent data P' (X, 0) when there is crosstalk is P' (X, 0)
It is expressed as =q(X, θ)+α+β.

In Figure 10, each channel of the X-ray detector (7)
...(71-,). (7υ, (7,.,)... is detected. There is an X-ray blocking body between each channel... (
6■1+). (at), (a■.,)... are provided.

In the conventional case, as shown in FIG. 11, image reconstruction data was obtained on the assumption that there was no crosstalk, that is, using transmission data P(X,0) according to a predetermined algorithm. When the radiation is high-energy, each channel receives scattered radiation (5
). That is, in FIG. 11°, (1) Transmission data (transmission ray intensity) P(X.θ) is also detected by a large number of channels for each projection angle 0.

■ This transmission data P (X, θ) and the irradiated object 3) ■
The fan beam data obtained by one exposure to the irradiator (3) is converted into parallel beam data using the internal beam method (in the case of a rotating XICT device).

■ The above transparent data P(X,0) after this conversion
, role data g ( X, θ) is obtained. That is, k(
X, e )=1/2 f g(X', θ)h(X-X
')dX'■ Perform a back projection operation on this deformed projection data k(X, θ),
Obtain the spatial distribution r(x.y) of the X-ray absorption coefficient. That is, r(x,y)=1/2πf k(x cosθ+y
sin θ, θ) dθ■ This spatial distribution r(x, y) is displayed on an image display device (not shown).

In this way, in conventional X-ray CT equipment, the Xll absorption coefficient distribution f(
x. y) is being calculated.

By the way, in conventional X-ray CT devices, γ is adjusted to the X-ray detector pitch, which is the interval between adjacent channels of the
This has not been sufficiently studied, resulting in a decrease in image resolution. That is, Nyquis
According to the sampling theorem of t), △7= (2 ・fm
ax)-'. rIlax is the highest spatial frequency of the image. As can be understood from this relationship, as γ increases with respect to the X-ray detector pitch, fmax decreases, and therefore the highest spatial frequency of the image decreases, resulting in a decrease in spatial resolution.

FIG. 12 is a schematic diagram illustrating a conventional scanning method for improving such a decrease in spatial resolution.

In the figure, by moving the winter channel (71) by half a pitch at an arbitrary irradiation angle, it is possible to obtain data equivalently halving the X-ray detector bit Δγ.

That is, it is possible to improve resolution degradation when Δγ becomes large.

However, in this case, it is necessary to move the X-ray detector, which significantly increases the scanning time.

In addition, there are the following conventional examples of dealing with crosstalk in X-ray detection, but the present invention, which will be explained later, uses a calculation algorithm to remove crosstalk between each channel of X-ray detection. None have. In JP-A-57-144478, the Urasa circuit for separating the crosstalk loss was configured as a hardware, and in JP-A-59-65487, the structure of the radiation detection element was designed to reduce the crosstalk loss. In JP-A-62-74334, a crosstalk loss absorbing material is arranged between adjacent elements, and in JP-A-59-183385, a crosstalk loss prevention material (reflecting material) is arranged between adjacent X-ray detection elements. JP-A-57-21.1538, JP-A-57
JP-A No. 195444 discloses that X-ray passing data is determined by a calculation algorithm, and JP-A No. 62-292142 discloses methods for preventing the occurrence of back sputtering.

Also, JP-A-60-108039. 60-181638
．． 61-290573 6260539. 62-7
0785. 62-191972. 62-26134
2, an attempt is made to remove the scattered X-ray components.

[Problems to be Solved by the Invention] In the conventional X-ray CT apparatus, the sub-detection channel of the X-ray detector (7) and the X-ray shield are arranged side by side at alternating pressure as shown in Figure 10. , When arranged in this way, crosstalk due to scattered X-rays occurs between adjacent channels, which cannot be ignored especially when high-energy rays are used.
Therefore, there is a problem that the X-ray intensity P(X, θ) for calculating the spatial distribution f(x, y) of the X-ray absorption coefficient does not represent the true transmitted X-ray output. Furthermore, if the X-ray shield is made thicker in order to eliminate this crosstalk, the pitch between the channels will increase, resulting in problems such as lowering the resolution of the reconstructed image and lengthening the data acquisition time.

This invention was developed to solve these problems, and uses an algorithm to remove scattered X-rays (crosstalk) between channels of an X-ray detector, making accurate image reconstruction possible. An object of the present invention is to obtain an X-ray CT apparatus that can improve resolution and shorten data acquisition time.

[Means for Solving the Problems] The X #jA CT device according to the present invention eliminates crosstalk due to X-ray scattering occurring between adjacent channels of an X-ray detector that detects X-rays transmitted through an irradiator. Assuming linearity for The apparatus is equipped with calculation means for calculating an algorithm for applying a filter function, performing convolution integration, and back projecting the results to obtain X-ray absorption and coefficient distribution and for image reconstruction.

[Function] In this invention, even if the crosstalk between channels is large, the true value can be derived by the crosstalk removal algorithm, and therefore a more accurate reconstructed image can be obtained. Furthermore, since the X-ray deflector can be made thinner, the pitch of each channel can be reduced, and the spatial resolution of the reconstructed image can be improved and the data acquisition time can be shortened.

[Example] FIG. 1 is a sectional view showing an example of the X-ray CT apparatus according to the present invention, in which (1) to (7) are the same as the conventional example shown in FIG. This is a calculation means for calculating the image reconstruction algorithm.

FIG. 2 is a configuration diagram emphasizing each channel and corresponding amplifier of the X-ray detector (7) in the embodiment of FIG.
A,) to (A,) are each channel of the X-ray detector (7),
(8,) to (8.) are amplifiers provided corresponding to each channel, and detector outputs Xll and Xfl+, respectively.
X311 X411 XSII X611
? Co-supply 1 ll Xllll.

FIG. 3 is a cross-sectional view showing the X-ray detector in more detail, showing the cross-tock due to X-ray scattering between adjacent channels. Each channel of (7), (6I) to (61.1
) is an X-ray shield provided between these channels (7.) to (71.1) to block crosstalk caused by X-ray scattering, and (q1) to (Q+-+) are
3), the true transmitted X-rays incident on the 6 channels (71) to (.7..,), (P1) to (P,.,") are the outputs of each channel corresponding to these, In other words, it is the actual transmitted X-ray intensity affected by crosstalk, and the broken line in the figure represents the scattered X-rays (5) between adjacent channels. The transmitted X-rays (4) from the body (3) are transmitted to each channel of the X-ray detector (7), for example (7l) to
(7,) for the detector output, for example, X. )~
(x-1).

These outputs are computed by the computing means (9) shown in FIG. 1 in accordance with the image reconstruction algorithm that characterizes the present invention, and the image is reconstructed.

FIG. 4 is a flowchart showing the above algorithm, which will be explained below. Steps 1--2 are the same as in the conventional example shown in FIG.
(X, θ) is created in step (2a). Next, a method for creating crosstalk-free data q (X, 0) in step (1a) will be explained.

As shown in Fig. 3, the intensity (q,) ~ (
q. ．． , ) to (qn) are incident, the outputs of each channel (7.) to (7l.1) to (7n) are assumed to be (P+) to (P+-+) to (Pn). The output (Pi) of the channel (71) is the intensity (qi) of the X-rays directly incident on it and the other channels (71)...+ (L
-t)+(7te+)+ ...+ Ratio of crosstalk due to scattered X-rays (5) from (7n) (a=1),
(a.)...(ai-+)+(at.+)+ ” ”
' + (ai). If we consider the output from all channels and express this in the form of a matrix, we get the following.

However, A is. From formula ■, true transmitted X-ray intensity (q+), (qt)
,...(qn) is as follows. However, Δ゜l is the inverse matrix of 8. As can be seen from formula ■, the true X-ray transmission intensity (q1), ... (qn
) can be calculated immediately if the inverse matrix A-11 and therefore matrix A1 or element a of matrix 8 is found experimentally, and by using this calculated value and using the above algorithm, Image reconstruction can be performed without the influence of crosstalk.

Note that in the above example, nxn experimental values are required to find the element aiJ of matrix 8, but since the characteristics (including the shape) of the X-ray detectors are the same, the arrangement of the X-ray detectors If the spacing is the same, the crosstalk between X-ray detectors will be the same between adjacent X-ray detectors, and will be the same between X-ray detectors that skip one. In this case, a I””a +2=ata=asa= ” ” ”
=a tn-znas+=as*=a+3=”
" =an+n-++a2 a+t=at+ a35 "'"ain-tina11"'a4
t a ,3== * e .

a n(n-y+ , and the matrix A is expressed as . Therefore, in this case, it is sufficient to experimentally determine (n1) matrix elements. Also,
If the crosstalk is small and we therefore only need to consider it between indirect detectors, then a,=0, so
What is matrix A? Output ("II) +...r (x■
), the following relational expression holds true.

Therefore, in this case, only matrix element a needs to be experimentally determined.

Next, a method for experimentally determining the above matrix element aiJ will be explained with reference to FIG. 2, assuming that the number of channels is eight.

■ First, squeeze the X-ray collimator (2) and set the channel (
7. ) so that the X-rays are incident only on the

■ Next, make this incident X-ray intensity a unit dose,
Each channel (7.). (7t),...(
7a) Detector output from (X.,). (X2.),
(X, lI) (X.1). (X-1). (X-,)
．． (X?.), (X-.) are obtained.

■ At this time, actually check the true XI; 1 output (X,,) ■ From the above formula, a I+”” l r 8 21= X
t+/Xa3+=Xs+/XI1+ ” '
``2ae+=Xa+/X, and the matrix elements are found.

Similarly, the X-ray collimator means (2) is connected to the channel (
Set so that X-rays are incident only on ”
"1+a3t"X3t/Xt! ＋ ” ”
” + all!=xsJxee.

? Perform the same door order for other channels and create matrix A
= (a+.+)(i, j=8) will be determined experimentally.

Here, as can be seen immediately, a l>a, 2>a, 3> 1 6・〉aa■〈a■
〉a. 〉...> a tsa ,,< a . ,＜
Since the following relationship holds true: a, 3>●・+>aS, when crosstalk rapidly attenuates, generally az+-
It is sufficient to find three matrix elements: ++ +ai++a+++*++.

Also, when the arrangement of each channel is regular and is recommended with equal pitch, a Hiroshi 1" a 3
t" a 41"' - fist = a 87
Mi aH3+t=ats°a,4=* ” =a?l
lmia, and only one matrix element (a only) needs to be found. (a) in this case can be obtained as follows.

As shown in FIG. 5, the X-ray collimator means (2) is set so that the X-rays enter only one channel (for example, (7)), and as shown in the figure, the
The detection outputs (x) and (y) from the channels may be obtained and the ratio y/x may be set as a. That is, a = y /
x. In this case, the true X-ray transmission intensity can be determined from the relationship:

As shown above, by removing crosstalk, it becomes possible to use the true X-ray transmission intensity q i (x, 0) that should originally be used, and therefore accurate image reconstruction with small blur can be performed. Become.

The density distribution as a limb irradiator as shown in Figure 6 is now 4.
X-ray detector (7
), if image simulation is performed by sampling every 1°, the result will be as shown in FIGS. 7(a) to (d). As can be seen, the blurring is larger in cases (b) to (d) than in case (a). By performing the above-mentioned crosstalk correction, (b) to (d) become as shown in (a).

[Effects of the Invention] As explained above, the present invention provides a calculation means for calculating a calculation algorithm for obtaining an X-ray detection signal without crosstalk from an X-ray detection signal with crosstalk, thereby achieving reproduction with high spatial resolution. This has the effect that constituent images can be obtained and data can be collected in a short time.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing an embodiment of the X-ray CT apparatus according to the present invention, and FIG. 2 is a configuration diagram showing in detail each channel of the X-ray detector and the corresponding amplifier in the embodiment of FIG. 1. , 3rd
The figure is a sectional view showing the X-ray detector in the above-mentioned embodiment in more detail, Figure 4 is a flowchart of an algorithm for calculating the true transmitted X-ray intensity and reconstructing an image according to the present invention, and Figure 5 is a flowchart of the algorithm for image reconstruction. FIGS. 6 and 7 are explanatory diagrams for experimentally determining the matrix elements necessary for calculating the true X-ray transmission intensity, and FIGS.
The figure shows the shape of the irradiator, Figure 7 shows the obtained results, Figure 8 is a cross-sectional view of a conventional X-ray CT device, and Figure 9 shows the X-ray CT device of Figure 8. Figure 10 is a cross-sectional view showing the situation near the X-ray detector, Figure 11 is a schematic diagram showing the image reconstruction algorithm used in the XlijC T device of Figure 8. The flowchart, FIG. 12, is a schematic diagram illustrating a conventional scanning method for improving the decrease in spatial resolution. In the figure, (1) is the X-ray source, (3) is the irradiated object, and (6
) is an X-ray beam body, (7) is an X-ray detector, and (9) is a detailed calculation means. Note that the same reference numerals in each figure indicate the same or corresponding parts.

Claims (1)

[Claims] An X-ray shield body that shields scattered X-rays from the X-rays obtained by irradiating an irradiated object with X-rays from an X-ray source, and
An X-ray detector is provided in front of the radiation shielding body in the X-ray irradiation direction and consists of a plurality of channels for detecting the X-rays that have passed through the irradiated object, and a A multidimensional linear simultaneous equation is established assuming linearity for crosstalk caused by lines, and by solving this, the X-ray intensity when there is no crosstalk is obtained, and this is logarithmically transformed to find an appropriate filter function. an X-ray CT apparatus, comprising: calculation means for calculating an algorithm for calculating an X-ray absorption coefficient distribution by performing convolution integration and back-projection;