JPH0824676B2 - X-ray CT system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to an X-ray CT apparatus for obtaining a CT image of a subject.

(Prior Art) X-ray CT technology includes an X-ray source (X-ray tube) that radiates fan-shaped X-rays having a flat fan-shaped spread angle, and a plurality of X-ray detection cells that detect the X-rays. The X-ray detectors arranged in parallel are opposed to each other with the subject interposed therebetween, and the X-ray tube and the X-ray detector are rotationally moved in the same direction around the subject at the same angular velocity to obtain the subject. After collecting X-ray projection data for various directions on the cross section and collecting sufficient data,
This data is analyzed by an electronic computer to calculate the X-ray absorptivity corresponding to each position on the cross section of the subject, and the gradation information according to the absorptivity is given to reconstruct the image information on the cross section of the subject. In this way, a clear tomographic image from soft tissue to hard tissue can be obtained.

By the way, in recent years, in an X-ray CT apparatus, more information is obtained by imaging multiple times with X-rays of different radiation qualities, and correction such as removal of metal pin artifacts and various information extraction can be performed by calculation processing such as data difference. Has been done.

(Problems to be Solved by the Invention) However, in the conventional X-ray CT apparatus, it is necessary to take an image under a certain condition, then change the radiation quality, and take an image again, which results in a long imaging time. If there is a point, or if the patient moves between the previous imaging and the subsequent imaging, there is a problem in that the result of calculation processing such as data difference is adversely affected and the image quality of the reconstructed image deteriorates. .

Also, a method of obtaining different X-ray scan data by one scan using a filter can be considered, but with this, the number of data by one type of X-ray energy becomes half of the number of conventional scan data, Spatial resolution deteriorates.

Therefore, it is an object of the present invention to provide an X-ray CT apparatus which shortens the projection time so as to collect data of different X-ray energies by one scan and does not deteriorate the spatial resolution. is there.

[Structure of Invention]

(Means for Solving Problems) The present invention has an X-ray tube and an n-channel detector group that are integrally rotated around a subject, and one detector of the detector group. Data acquisition for rotating n-channel X-ray fan data at each position by rotating the center of the X-ray tube and the center of rotation of the X-ray tube at a position displaced from the extension line connecting the X-ray tube by 1/4 of the detector array pitch. Section, an X-ray filter for making the X-ray quality different between the even channel and the odd channel of the detector group, and a data separation section for separating the data from the detector group into even and odd channels. A data processing unit that adds n to each of the n / 2 even-channel data groups and odd-channel data groups from this data separation unit as inter-channel data of each data group to create n data, respectively. And an image reconstruction for reconstructing a CT image based on two types of X-rays having different radiation qualities based on the data from the data processing unit of the X-ray CT apparatus. .

(Operation) In the present invention, the X-ray filter is arranged so that the even-numbered channel and the odd-numbered channel of the n-channel detector group have different X-ray qualities. As a result, X-ray data having different X-ray energies can be collected by one scan between the even channel group and the odd channel group,
The imaging time can be shortened, and the adverse effect on the image due to the movement of the subject can be reduced.

When such data collection is performed, the number of even channel group data and the number of odd channel group data obtained at each rotational position are n / 2, respectively, and the spatial resolution deteriorates.

Therefore, in the present invention, the CIA (Channel Interlace Arrenge
ment), that is, by supplementing zero as inter-channel data of each data group, each data group is treated as n pieces of data to reconstruct an image.

As described above, the inter-channel data is treated as zero and is processed as n-channel data, so that normal data can be obtained.
The principle of reconstructing a CT image is disclosed in Japanese Patent Application No. 61-74622 filed earlier by the present applicant, and the above principle will be described below by citing this.

The principle of CIA is that "opposite data" (details will be described later) corresponding to the middle of each of the n channels is selected from other fans by interpolation and once used for 2n.
A fan for a channel is created (hereinafter referred to as reflection processing), and reconstruction processing is performed using the fan data of this 2n channel, which is described in US Pat. No. 428489.
No. 6 discloses it.

5 and 6 are diagrams for explaining the above-mentioned "opposite data". In FIG. 5, reference numeral 1 is an X-ray tube, and D is a group of X-ray detectors that rotate integrally with the X-ray tube 1. Here, for convenience of explanation, 9-channel detectors D1 ...
It's called D9.

Here, it is assumed that the X-ray tube 1 and the detector D rotate in the direction of the arrow A at a rotation interval Δθ, the position of the X-ray tube 1 at the rotation angle θ is S, and the X-ray tube at the rotation angle θ ′. If the position 1 is S ', there is a common X-ray beam R in the fan beams F at the positions S and S'. If the X-ray beam R at the position S is (ψ, θ ₁ ), and the X-ray beam R at the position S ′ is (ψ ′, θ ₁ ′), then ψ ′ = − ψ (1) θ ₁ ′ = θ ₁ + 2π−φ (2) Here, the (ψ, θ ₁ ) X-ray beam R ₇ ′ corresponds to the data detected by the detector D-7.

By the way, as shown in the figure, the center line of one detector (detector D-5 in the figure) of the detector group D is the X-ray tube 1
Is rotated by P / 4 (P is the arrangement pitch of the detectors) from a line 1 connecting the rotation center O and
The X-ray beam R indicated by (ψ ′, θ ₁ ′) corresponds to the data at the center of the adjacent detectors (the midpoint between the detectors D-3 and D-4 in the figure).

Here, the data by the beam of (ψ, θ ₁ ) is P
(Ψ, θ ₁₎ and _{then, (ψ ', θ 1'} ) the data by the beam _{P (ψ ', θ 1'} ) of _{When, P (ψ, θ 1)} = P (ψ ', θ 1') … (3) P (ψ ′, θ ₁ ′) is data that cannot be obtained by the detector (because it is data at the intermediate position of the detector), but the corresponding data is P (ψ, θ 1).
₁ ).

Therefore, by setting P (ψ, θ ₁ ) to “opposite data”, data between channels can be obtained. Similarly, as shown in FIG. 6, “opposite data” corresponding to each channel is used. 2n channel data can be obtained.

According to the technique of the above-mentioned U.S. patent, the inter-channel data is obtained by reflection from "opposite data", whereas in the present invention, "opposite data" is treated as zero, and the "opposite data" is not waited for. In addition, it enables pipeline processing for each fan data.

That is, if the principle of the extended divergent method described in the US application (application number 732260) filed on September 5, 1985 by the same applicant as the present applicant is used, certain data P (ψ, θ) ) And its opposite data P (−ψ, θ + π
-2ψ) exists, even if both data are multiplied by arbitrary weights w (ψ, θ), w−ψ, θ + π−2ψ) and then reconstructed, the sum of the weights is “2”. It turns out that the result is the same as long as.

Here, as shown in FIG. 8, the X-ray beams emitted from the X-ray tube 1 at the rotational position S and detected by the detector group D are R _{1 to} R ₉
Then, it is shown in FIG. 5 that the X-ray beams R ₁ ′ to R ₉ ′ existing in the middle of the respective beams R _{1 to} R ₉ are obtained when the X-ray tube 1 reaches another rotation position. As shown. And
The beams as "opposing data" of these respective beams R _{1 to} R ₉ , R ₁ ′ to R ₉ ′ are shown by broken lines, and the X-ray beams R _{1 to} R ₉ , R ₁ ′ to R
_When the weighting of the data obtained by 9'and these "opposed data" is determined as shown in the figure, this satisfies the condition of the extended divergent method described above.

Therefore, focusing on the X-ray fan beam at the rotation position S, the data obtained by the X-ray beams R _{1 to} R ₉ are weighted with “2”, and the data between the channels are weighted with “0”. Even if it is configured, if it is considered in the entire data system, it means that the conditions of the extended divergent method are satisfied.

This will be described in more detail with reference to the sinogram of FIG. 9 showing the relationship between the spread angle of the X-ray path and the rotation angle of the X-ray source. In FIG. 9, when the rotation angle of the X-ray source is θ ₁ as shown in FIGS. 5 and 8, the X-ray detectors D _{1 to} D ₉ are X-ray paths indicated by black circles on the line l _1. R ₁ , R ₂ , ..., R
Detect ₉ data. At this rotation angle θ ₁ , the data of the X-rays R ₁ ′, R ₂ ′,…, R ₈ ′ that bisect the adjacent X-ray paths R ₁ , R ₂ , ..., R ₉ are detected. Not done. Here, as is clear from the geometrical positional relationship shown in FIG. 5, the path R ₇ ′ (white circle) deviated from the center of the fan beam by ψ ₁ at the rotation angle θ ₁ is the fan at the rotation angle θ ₁ + π−2ψ ₁ . It corresponds to the path R ₇ ′ (black circle) through the detector D ₃ that is −ψ ₁ shifted from the center of the beam. That is, line l _{1 in} FIG. 9 corresponds to l ₂ in FIG. 9, and if the X-ray source 1 collects data while rotating at an interval of Δψ, the data that cannot be obtained at line l ₁ at the rotation angle θ ₁ It is obtained by the path on the line l ₂ .

Therefore, if the line l ₁ (weighting in the ↓ direction) and the line l ₂ (weighting in the ↑ direction) according to the divergent method expanded as shown in FIG.
It can be reconstructed without embedding the data of the opposite path like Pat.4,284,896. Note that the weighting 0 is equivalently the same as the data value 0, so that the weighting 0 is 0 as shown in FIG. 7 (B) between the detected data as shown in FIG. 7 (A).
It is the same even if you put.

Further, if the data is sampled every 6Δφ, for example, the sinogram of FIG. 10 is obtained. Of the paths R ₁ ′, R ₂ ′, R ₃ , ..., R ₈ ′ on the line l ₁ ′ at the rotation angle θ _1, what is actually obtained is the path R ₁ ′, R ₄ ′ on the line l ₂ ′. , R ₇ ′, and the data of paths R ₂ ′, R ₃ ′, R ₅ ′, R ₆ and R ₈ ′ cannot be obtained. However 'there is data in the path r ₂ near the in the θ direction, r _3, moreover path R ₆ on the line l _1' pass R ₆ also to be present in the vicinity of θ direction. Also, for example, with 512 channels and a fan angle of 60 °, the data collection interval 6Δφ is 0.7.
Since it is about ゜, the path r ₃ or r ₂ is considered as R ₆ ′, and convolution and back projection do not have any artifacts.

In fact, the inventor used a computer
It was confirmed that no artifacts occurred in the simulated images in the case of 2 channels and the data collection interval, and the resolution was improved compared to the conventional one.

Now, let us consider an X-ray CT system having a 1024-ch detector, and assume that ψ is sampled at intervals of Δψ / 2 for all data P (ψ, θ) obtained by this system. Then, the above weighting is Then, this satisfies the condition of the extended divergent method. Therefore, the data that will be used for reconstruction will be Should be processed as Therefore, as a result, it is only necessary to collect data for 512 channels, for example, even channels of the 1024 channel detectors. For this reason, the detector was purposely set to 1024
It can be seen that the same image can be obtained without using ch, and even with half of them, 512 ch.

Also, as the function used for convolution And, Or Etc. can be used.

Therefore, the convolution function as shown by these equations (6), (7), (8) Then, the pixel value f at the point (r, φ) in FIG.
(R, φ) is Becomes

However, M = 360 ° / Δθ N = (fan angle / 2) / Δψ) Δθ represents the rotation angle interval of the X-ray source.

In equation (9), L _m (r, φ) is a point (r,
distance to φ) Indicates a convolution operation in the range of −NΔψ to + NΔψ, that is, the fan angle range, Indicates a back projection calculation.

In addition, Is the Jacobian for coordinate transformation.

Further, the weighting coefficient w takes 2 or 0. Here, kw (k is an arbitrary constant other than 0) may be used instead of w, and for example, k = 1/2 may be used. At this time, kw takes a value of 1 or 0. However, k ≠ 1
In this case, the grayscale value of the reconstructed image appears to be k times the original value, so it is necessary to multiply it by 1 / k after the image reconstruction. For example, when k = 1/2, the gray value of the created image may be doubled.

As described above, according to the CIA principle disclosed in Japanese Patent Application No. 61-74622, it is possible to reconstruct a CT image based on data for 2n channels even though the detector group is for n channels. According to the present invention, X-ray data having different radiation qualities are collected in the even-numbered channel and the odd-numbered channel of the n-channel detector group, and the data is separated into the even-channel data group and the odd-channel data group. , By executing CIA for each data group,
It is possible to obtain the same spatial resolution as an image obtained by two scans with different radiation qualities while collecting X-ray data having different radiation qualities by one scan with the detector group of the channel.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.
FIG. 1 is a block diagram of the embodiment apparatus.

In the figure, reference numeral 11 is a data collecting unit (DAS), which integrally rotates the X-ray tube 1 and the detector group D around the subject, for example, 360 ° to collect projection data from each direction. It is a thing. Where this data collector
As shown in FIG. 5, the relationship between the X-ray tube 1 and the detector group D in 11 is such that the center of one detector is P / 4 (rather than line 1 connecting the X-ray tube 1 and the rotation center O). P has a positional relationship in which it is displaced by the detector array pitch). Then, in this data acquisition unit 11, n is set for each injection position of the X-ray tube 1.
(= 2N) pieces of X-ray fan data P (nΔψ, θ) (where n = 1, 2, ..., 2N) are to be collected (therefore, the detector group D has n (= 2N) channels). Is).

Here, the data collection unit 11 is capable of collecting X-ray data having different X-ray quality in the even and odd channels of the 2N channels. For this purpose, as shown in FIG. 2, an X-ray filter 31 is arranged on the entrance surface of, for example, an even channel in the detector group D. Therefore, in the X-ray fan beam emitted from the X-ray tube 1, Ray1 does not pass through the X-ray filter 31, so the X-ray energy is A (kV), whereas Ray2 does not pass through the X-ray filter 31. Since it passes, the average X-ray energy becomes higher than A (kV), so that the X-ray quality can be made different between the even channel and the odd channel.

The log conversion unit 22 performs log conversion of the X-ray intensity collected by the DAS 11 in view of the exponential function.

The reference correction unit 12 is a correction unit that corrects the X-ray intensity.

The calibration unit 23 calibrates the data from the reference correction unit 12 based on the calibration data.

The data separation unit 24 separates the 2N channel data collected by the DAS 11 and processed by the respective units into N even channel data groups and N odd channel data groups and outputs them.

The data processing unit 13 pads the N even-numbered channel data groups and the odd-numbered channel data groups from the data separation unit 24 with zeros as inter-channel data of each data group, and outputs 2N channels of X-ray fan data. Is created and processed. In the present embodiment, in order to satisfy the above-mentioned conditions of the expanded divergent method, the data processing unit 13 uses the 2N data currently collected by the data collecting unit 11 as shown by the equation (5). Multiply "2" only for the data.

Reference numeral 14 is an image reconstruction unit, and a convolution unit 15,
It is composed of a back projection unit 16 and a memory 17 for back projection. The image reconstruction unit 14
Is the convolution as shown in equation (9) Back projection Is executed. Actually, these convolutions and back projections are processed by a high-speed arithmetic method equivalent to the equation (9).

Then, the reconstructed CT image is displayed on the display unit 18 or stored in the disc 19. Further, as is known as an analysis method of the dual energy method, an operation may be performed between images having different radiation qualities, and an image of effective atomic number or an image of effective electron density may be created thereafter.

The operation of the embodiment apparatus configured as described above will be described with reference to FIG. Data is collected by rotating the X-ray tube 1 and the detector group D in the DAS 11 360 degrees around the subject and irradiating X-rays at each rotation position. The data obtained as a result are shown in the upper part of FIG. In the figure, the horizontal direction corresponds to the channel number, the vertical direction corresponds to each projection position, and the white circles of the odd channels indicate the data (data that does not pass through the X-ray filter 31) collected with the energy of A (kV), and the even channels. The black circle is ≠ A (kV)
The data collected by the energy of (the data which passed the X-ray filter 31) are shown. These data are collected for each projection position, and then the log conversion unit
22 to the calibration unit 23 execute each processing, and the data is input to the data separation unit 24.

As shown in the middle part of FIG. 3, the data separation unit 24 separates and outputs an odd channel data group and an even channel data group.

Then, in the data processing unit 13, as shown in the lower part of FIG. 3, the odd-numbered channel data group and the even-numbered channel data group are padded with zeros as inter-channel data of each data group to generate 2N data each. Moreover, the weighting that satisfies the conditions of the divergent method extended here is performed. Therefore, in the subsequent image reconstructing unit 14, by treating the data based on the X-rays having different radiation qualities as the data of 2N channel and performing the image reconstruction, the data collected by the two scans with different radiation qualities are obtained. An image having a spatial resolution equivalent to that of the image can be obtained.

Moreover, in reconstructing such an image, pipeline processing can be performed for each X-ray fan data.

The present invention is not limited to the above embodiment,
Various modifications can be made within the scope of the present invention.

Although the X-ray filter 31 is arranged in front of the entrance surface of the detector group D in the above-described embodiment, the present invention is not limited to this, and such an X-ray filter 31 may be arranged between the X-ray tube 1 and the subject. Often,
The point is that the position of the detector group D does not matter as long as the X-ray quality can be changed between the odd-numbered channel and the even-numbered channel. Also,
When the apparatus of the present invention also performs a normal single energy scan in addition to the dual energy scan described above, the X-ray filter 31 is changed to a position where it intersects with the X-ray beam and a position where it does not intersect, as shown in FIG. When performing the single energy scan with the above configuration, the X-ray filter 31 may be arranged at a position not intersecting with the X-ray beam as shown by the broken line in the figure.

〔The invention's effect〕

As described above, according to the present invention, the data of different X-ray energies are collected by one scan to shorten the imaging time and reduce the adverse effect due to the movement of the subject, and further, the spatial resolution. Can be made equivalent to that of each image obtained by two scans with different beam qualities.

[Brief description of drawings]

FIG. 1 is a block diagram of an apparatus according to an embodiment of the present invention, FIG. 2 is a schematic explanatory view showing an example of an X-ray film, and FIG. 3 is a schematic explanatory view showing a flow of data processing in the same apparatus. The figure is X
FIG. 5 is a schematic explanatory view showing a modified example of the line filter, FIG. 5 is a schematic explanatory view of a data collection unit of the above-mentioned embodiment apparatus, FIG. 6 is a schematic explanatory view for explaining “opposite data”, and FIG. ),
(B) is a diagram showing the processing operation in the data processing unit, FIG. 8 is a diagram showing weighting based on the extended divergent method, and FIGS. 9 and 10 are X-ray path spread angles and X-rays. 2 is a sinogram showing the relationship with the rotation angle of. 1 ... X-ray tube, D ... Detector group, 11 ... Data collection unit,
13 ... Data processing unit, 14 ... Image reconstruction unit, 24 ... Data separation unit, 31 ... X-ray filter.

Claims (3)

[Claims] 1. An X-ray tube and an n-channel detector group that are integrally rotated around a subject, and one detector of the detector group has the center as the X-ray tube and its center. A data collecting unit that rotates in a positional relationship deviated from the extension line connecting the rotation center by 1/4 of the detector array pitch, and collects n-channel X-ray fan data at each position, and an even number of the detector groups. An X-ray filter for differentiating the X-ray quality between the channel and the odd channel, a data separation unit for separating the data from the detector group into even and odd channels, and n / n from each of the data separation units. A data processing unit that adds n to each of the two even-channel data groups and odd-channel data groups as inter-channel data of each data group to create n data, and the data from this data processing unit. Hazuki X-ray CT apparatus characterized by including a respective image reconstruction unit which reconstructs a CT image based on the operation to the radiation quality of two different X-rays as n-channel data. 2. The X-ray filter according to claim 1, wherein the X-ray filter is arranged on an incident surface of either an even channel or an odd channel of an n-channel detector group.
CT device. 3. The X-ray CT apparatus according to claim 2, wherein the X-ray filter is displaceably supported at a position intersecting with the X-ray beam and a position not intersecting with the X-ray beam.