JPH10290798A - Projection data measurement method and device and x-ray ct device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide projection data measurement method and device for effectively reducing noise and an X-ray CT device for effectively reducing the noise of projection data. SOLUTION: The projection data of a testee body by radiation are respectively measured in a state where the position of opposing channels is shifted for a distance not more a channel width in the direction of the arrangement of the channel in the two directions for which the direction of the radiation becomes mutually opposite by a radiation detector 60 of multi-channel, the projection data measured in the two directions are interleaved and moving average is performed in the direction of the arrangement of the channels for that.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for measuring projection data, and an X-ray CT apparatus.
The present invention relates to an improvement of a measuring method and an apparatus for performing measurement using a detector of (el) and an X-ray CT apparatus equipped with such a measuring apparatus.

[0002]

2. Description of the Related Art In an X-ray CT (computed tomography) apparatus, an X-ray irradiation / detection system is rotated around an object to measure projection data in a number of view directions around the object. Then, an image is generated (image reconstruction) based on the projection data. To measure the projection data, a multi-channel X-ray detector in which a large number of X-ray detectors are arranged in an array is used.

[0003] In order to reduce noise included in the profile of the projection data in the channel direction, the profile is smoothed. The profile is smoothed by moving-averaging the measurement data of a plurality of channels in the direction in which the channels are arranged.

That is, the projection data of each channel is
It is determined by averaging the measurement data of the channel and the measurement data of a predetermined number of other channels adjacent to both sides of the channel. Such an average value calculation is sequentially performed for each channel, and each time the channel is updated, the channels participating in the average value calculation are switched one by one, so that a so-called moving average is performed. Since there is generally no correlation between noises between different channels, it is possible to smooth the profile, that is, reduce noise by moving average in the channel direction.

[0005]

The number of channels participating in the moving average is fixed. The larger this number is selected, the lower the noise variance .sigma. Is, and the higher the noise reduction effect. However, on the other hand, the influence of the data of the other channels, that is, the projection data of the other part of the subject becomes stronger, which causes an undesirable side effect that the projection data deviates from the original correct value.

[0006] As a result, the profile of the projection data is distorted, so that a correct reconstructed image cannot be obtained. For this reason, the number of channels participating in the moving average may have to be compromised where the noise reduction effect is insufficient.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a method and an apparatus for measuring projection data for effectively reducing noise, and an X method for effectively reducing noise in projection data. The object is to realize a line CT apparatus.

[0008]

[Means for Solving the Problems]

(1) A first aspect of the present invention for solving the above-mentioned problem is that the projection data of the object by the radiation is detected by using a multi-channel radiation detector in two directions in which the directions of the radiation are opposite to each other. The positions of the channels facing each other are measured in a state where the positions of the channels facing each other are shifted relatively less than the channel width in the direction in which the channels are arranged, and the projection data measured in the two directions are interleaved and interleaved. The projection data obtained is moving averaged in the direction in which the channels of the radiation detector are arranged.

In the first aspect of the present invention, the positions of the channels facing each other of the radiation detector are measured in a state where they are relatively shifted by a half of the channel width in the direction in which the channels are arranged. Is preferred in terms of uniformity.

In the first invention, it is preferable that the radiation is an X-ray because practical means for generating and controlling the radiation are the most substantial. (2) A second aspect of the present invention for solving the above-mentioned problem is that the projection data of the subject by the radiation is detected by using a multi-channel radiation detector in two directions where the directions of the radiation are opposite to each other. Measuring means for measuring the positions of the channels facing each other in a direction in which the channels are relatively displaced from each other in a direction less than the channel width, and interleaving means for interleaving the projection data measured in the two directions. Moving average means for moving and averaging the projection data interleaved by the interleave means in the direction in which the channels of the radiation detector are arranged.

In the second invention, the measuring means measures the radiation detectors in a state where the positions of the opposed channels of the radiation detector are relatively shifted by a half of the channel width in the direction in which the channels are arranged. Is preferable in that the intervals of the radiation paths in the interleaving are made uniform.

In the second aspect of the present invention, it is preferable that the radiation is an X-ray because practical means for generating and controlling the radiation are the most substantial. (3) According to a third aspect of the present invention to solve the above-mentioned problems, projection data by X-rays are obtained by using a multi-channel X-ray detector over the entire circumference of the subject, and the directions of the X-rays are opposite to each other.
Measurement means for measuring the positions of the mutually opposing channels of the X-ray detector in the direction in which the channels are arranged relative to each other in a direction less than the channel width for each of the directions, and Interleaving means for interleaving the projection data between two directions in which the directions of the X-rays are opposite to each other, and a direction in which the projection data interleaved by the interleaving means are arranged in the channels of the X-ray detector. And moving image averaging means for generating an image based on the projection data averaged by the moving averaging means.

[0013] In the third invention, the measurement means measures the X-ray detector in a state where the positions of the opposed channels of the X-ray detector are relatively shifted by half a channel width in the direction in which the channels are arranged. This is preferable in that the intervals between X-ray paths in interleaving are made uniform.

(Operation) In the two directions in which the directions of radiation (X-rays) are opposite to each other, the channels facing each other are displaced by a distance smaller than the channel width, and the projection data is measured and interleaved. And obtaining projection data at twice the channel density of the radiation (X-ray) detector, doubling the number of channels participating in the moving average without increasing side effects.

[0015]

Embodiments of the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiment.

FIG. 1 is a block diagram of an X-ray CT apparatus.
This device is an example of an embodiment of the present invention. Note that the configuration of the present apparatus shows an example of an embodiment relating to the apparatus of the present invention. Further, an example of an embodiment relating to the method of the present invention is shown by the operation of the present apparatus.

(Configuration) The configuration of the present apparatus will be described. As shown in FIG. 1, the X-ray CT apparatus 100 includes an operation console (c
An onsole 1, an imaging table 10, and a scanning gantry 20 are provided.

The operation console 1 includes an input device 2 for inputting instructions and information of an operator, a central processing unit 3 for performing scan control and image reconstruction, and a control signal and the like for transmitting a control signal and the like to a photographing table 10 and a scanning gantry. A control interface 4 for outputting to the scanning gantry 20; a data collection buffer 5 for collecting data provided from the scanning gantry 20;
The system includes a CRT (cathod-ray tube) 6 for displaying images and the like, and a storage device 7 for storing various data, reconstructed images, programs, and the like. Central processing unit 3
Is composed of, for example, a computer.

The imaging table 10 carries a subject (not shown) into and out of the X-ray irradiation space of the scanning gantry 20. The movement of the photographing table 10 is controlled by the control interface 4.

The scanning gantry 20 includes an X-ray tube 30 and a collimator 50 for forming an X-ray beam.
And a detector array 60 and an X-ray controller 2 for adjusting the timing and dose of X-ray irradiation
1 and the X-ray passing aperture of the collimator 50 (aperture (ape
rture)), a data collection unit 23 that collects data detected by the detector array 60, and a rotation that rotates the X-ray tube 30, the detector array 60, and the like around the body axis of the subject. And a controller 24.

The X-ray controller 21, collimator controller 22, data collection unit 23, and rotation controller 24 are controlled by control signals provided from the control interface 4.

FIG. 2 shows a schematic configuration of the detector array 60. Detector array 60 may have many (eg, about 1000)
X-ray detectors 60 (i) are arranged in an arc to form a multi-channel X-ray detector. Here, i is a channel number, and i = 1 to I. The detector array 60
1 is an example of an embodiment of multi-channel radiation detection according to the present invention. Also, in the present invention, the multi-channel X
It is an example of an embodiment of a line detector.

The X-ray detector 60 (i) is constituted by a solid state detector such as a scintillation X-ray detector or a semiconductor X-ray detector. Of course, for example, Xe
It is also possible to use an ionization chamber type using an ionized gas such as (xenon) gas.

FIG. 3 shows the X-ray tube 30 and the collimator 50.
FIG. 4 is a schematic diagram showing a mutual relationship between the detector array and the detector array; 3A is a front view, and FIG. 3B is a side view. X
The X-rays emitted from the X-ray tube 30 are converted into a flat fan-shaped X-ray beam Xr by a collimator 50, and the detector array 60
Is irradiated.

Here, a virtual straight line L passing through the focal point of the X-ray tube 30 and the rotation center C of the scanning gantry 20 is defined as an angle reference axis. The extension of the angle reference axis L reaches approximately the center of the detector array 60. The details of the relationship between the focal point of the X-ray tube 30, the angle reference axis L, and the detector array 60 will be described later.

The focus of the X-ray tube 30 and the individual X-ray detectors 60
The angle formed by the virtual straight line connecting (i) with the angle reference axis L is called the channel angle γ. Detector array 60
In the center X-ray detector 60 (I / 2), the channel angle is γ = 0. In the X-ray detector 60 (1) at the left end of the detector array 60 in the drawing, the channel angle is γ = −γm. The X-ray detector 60 at the right end of the detector array 60 in the drawing.
In (I), the channel angle is γ = + γm. Since the channel number i and the channel angle γ have a one-to-one correspondence, the X-ray detector 60 (i) will be hereinafter referred to as the X-ray detector 60.
(Γ).

The subject is carried in with the body axis crossing the fan surface of the X-ray beam Xr. This state is shown in FIG. As shown in the figure, a projection image of the subject OB sliced by the X-ray beam Xr is projected on the detector array 60, and the projection data is measured. The projection data is an example of an embodiment of the projection data in the present invention.

The thickness of the X-ray beam Xr at the isocenter of the object OB gives the slice thickness th. The slice thickness th is adjusted by the aperture of the collimator 50. The X-ray tube 30, the collimator 50, and the detector array 60 rotate (scan) around the subject OB while maintaining this relationship. The center of rotation is the rotation center C of the scanning gantry 20. X-ray tube 30, collimator 50
The detector array 60 constitutes an X-ray irradiation / detection system.

A plurality (for example, about 1
The projection data of the subject is collected at a view angle of 000). The collection of the projection data is performed by a system including the detector array 60, the data collection unit 23, and the data collection buffer 5. X-ray tube 30, collimator 50, detector array 60,
The data collection unit 23 and the data collection buffer 5 are an example of an embodiment of a measuring unit according to the present invention.

The view angle will be described with reference to FIG.
The angle θ formed by the angle reference axis L and the vertical axis at one angular position where the X-ray irradiation / rotation system is rotated is called a view angle.
Data collected at a view angle θ by an X-ray detector with a channel angle γ is represented by D (γ, θ). For example, if γ = 0, it is D (0, θ), and if γ = −γm, it is D (−γm,
θ), and D (+ γm, θ) if γ = + γm.

Since a fan-shaped X-ray beam is used, the view data has so-called fan beam data in which the direction of the X-ray is different for each channel. With regard to fan beam data, by sorting it over multiple views, the X-ray directions are parallel,
It is known to use so-called parallel beam data.

That is, as shown in FIG. 6, for example, data D (−γm, θ) obtained at the view angle θ and data D (−γj, θ ′) obtained at the view angle θ ′ are X
The directions of the lines are parallel. Regarding data of another channel, data of another channel in another view has a similar relationship.

Therefore, by selecting data having such a relationship, data of a plurality of views by a fan beam can be reorganized into data of a plurality of views by a parallel beam.

Such conversion from fan beam data to parallel beam data is called fan / parallel conversion. In this apparatus, the central processing unit 3 performs fan-to-parallel conversion on the projection data collected in the data collection buffer 5. The parallel beam data obtained by the fan / parallel conversion is stored in the storage device 7.
Is stored.

For each view, there is data of the opposite view. This will be described with reference to FIG. 7. In FIG. 7, when attention is paid to data D (−γm, θ) shown in (a), as shown in (b), the view angle θ ″ = θ
Data D (+ γm, θ ″) at + π−2 (−γm) is data of the facing view.

The data D (−γm, θ) and D (+
γm, θ ″) is projection data obtained by X-rays passing through the same part of the imaging space in opposite directions. Similarly, the data of the opposite view exists for the data of all other channels. Generally, data D
The data of the opposite view of (γ, θ) is D (−γ, θ + π)
-2γ). The central processing unit 3 obtains facing view data for each projection data after the fan / parallel conversion.

FIG. 8 shows a focal point (X-ray focal point) F of the X-ray tube 30.
FIG. 4 conceptually shows the relationship between the angle reference axis L and the detector array 60. As shown in FIG.
When a virtual straight line connecting the angle reference axis L, that is, the X-ray focal point F and the rotation center C of the scanning gantry 20 is extended in the direction of the detector array 60, the straight line becomes the center channel 60 (0). The relative positional relationship is formed such that the position is shifted (offset) in the channel arrangement direction by a distance corresponding to １ ／ of the channel width d from the center. Such an arrangement of the detector array 60 is known as a so-called quarter offsett arrangement.

In the detector array 60 in the quarter offset arrangement, the channels facing each other at an angle different by 180 °, that is, the opposite views, have a channel facing each other at half the channel width d as shown in FIG. It is relatively shifted in the channel arrangement direction by a corresponding distance.

The path of the X-ray that gives the channel data is
Since it is represented by a line connecting the X-ray focal point F and the center of the channel, the path of the X-ray that provides the data of the opposed view is relatively shifted by d / 2 in the channel arrangement direction.

For this reason, between the opposing views after the fan-to-parallel conversion, the paths of the X-rays that provide the data of the respective channels have a relation of interleaving or interleaving as shown in FIG.

The quarter offset of the detection array 60 is such that the channels facing each other in the facing view are d /.
This is preferable because the distance between the X-ray paths to be interleaved becomes uniform because two shifts occur.

In order to interleave the X-ray path, it is not necessarily limited to the quarter offset.
It is sufficient that the offset amount is less than d / 2. Thereby, the relative shift between the channels facing each other in the facing view becomes less than d, and, for example, an interleaving X-ray path is formed as shown in FIG.

FIG. 7A shows that the offset amount is 0 and d / 4.
And the shift between the opposing channels is smaller than d / 2. In (b), the offset amount is d /
4 and d / 2, and the deviation between the opposing channels is d /
This is the case where it is larger than 2 and smaller than d.

In this apparatus, the central processing unit 3 interleaves the data of the opposite views after the fan / parallel conversion in accordance with such properties of the projection data. The central processing unit 3 is an example of an embodiment of an interleaving unit in the present invention. The interleaved data is stored in the storage device 7.

By interleaving, the number of data in each view is twice the number of channels. Since these data are projection data by X-rays having different paths, they have unique information. In other words, the view data after interleaving is substantially equivalent to the detector array 60.
The projection data of the subject is obtained at a density twice as high as the channel density.

The central processing unit 3 performs a moving average on the interleaved view data.
The central processing unit 3 is an example of an embodiment of a moving average unit in the present invention. The moving average is calculated by the following equation.

[0047]

(Equation 1)

Here, y: moving average value x: measured value i: absolute number of discrete value j: relative number of discrete value w: weighting factor

W is a coefficient, which is given by the following equation.

[0050]

(Equation 2)

According to equation (1), the i-th measurement data is
Weighted averaging is performed with m pieces of measurement data on each side. That is, 2m + around the i-th measurement data
A weighted average of one measurement data is obtained, which is i
This is the second new measurement data. As i updates,
The data of 2m + 1 added to the average is sequentially replaced.
That is, a moving average is performed.

Since the data density has been doubled by the interleaving, the number of data to be averaged is 2m + 1
Is, for example, twice as high as when no interleaving is performed.
By doing so, the noise reduction effect is, for example, 2
Up to double.

Even if the number of data to participate in the averaging is doubled, the corresponding distance in the physical space is the same as in the case where no interleaving is performed. Therefore, the influence of other data having different X-ray paths is not different from the case of not interleaving. That is, side effects such as distorting the profile of the projection data do not increase.

When the weight coefficient w is constant regardless of j, for example, as shown in FIG. 11A, uniform weight averaging is performed. This is preferable because the calculation becomes simple.
Alternatively, as shown in (b), the weight coefficient may be increased as the distance from the center increases. This is preferable in that the influence of data having a far-distant number is reduced. In addition, as the weight coefficients, those having appropriate function characteristics can be used as needed.

An image is generated (image reconstructed) by the central processing unit 3 based on the projection data on which the moving average has been completed.
The central processing unit 3 is an example of an embodiment of an image generating unit according to the present invention. The image reconstruction is performed by, for example, performing a filtered back-projection process on the projection data of, for example, 1000 views.

The reconstructed image, that is, the tomographic image of the object OB is stored in the storage device 7. The tomographic image stored in the storage device 7 is read out according to a command given by an operator through the input device 2 and displayed on the CRT 6 as a visible image.

(Operation) The operation of the present apparatus will be described. FIG.
2 shows a flowchart of the operation of the present apparatus. X-ray CT apparatus 1 according to a command given by the operator through input device 2
00 starts scanning.

Thus, in step ST1,
Collection of fan beam data is performed. That is, the X-ray irradiation / detection system rotates around the subject, and X-ray transmission data in a plurality of views is collected by the data collection unit 23 by the fan-shaped X-ray beam. The collected data is transmitted to the data collection buffer 5.

Next, in step ST2, fan-to-parallel conversion is performed on the data collected in the data collection buffer 5. This is performed by the central processing unit 3. The operation of each of the following steps is also performed by the central processing unit 3.

In step ST3, data pre-processing is performed. This performs, for example, sensitivity correction of the detector array 60 and other corrections. Next, interleaving is performed in step ST4. The interleaving process is performed between those having different view angles by 180 °. As a result, data of a plurality of views whose data density is doubled is formed.

Next, in step ST5, a moving average is performed on the data of each view. The moving average is performed according to the above equation (1). The number of pieces of data to participate in the moving average is set to be twice that when no interleaving is performed, in accordance with the doubled data density. As a result, view data with reduced noise can be obtained without increasing side effects.

Therefore, it is possible to obtain projection data with little noise even when the attenuation of X-rays is large, for example, when a part having a very long X-ray transmission length such as a shoulder of a subject is imaged. Can be.

Next, image reconstruction is performed in step ST6. Image reconstruction is performed on the moving averaged view data by, for example, a filtered back projection method. Since the noise of the view data is effectively reduced, even for a part having a very long X-ray transmission length such as a shoulder of a subject, for example, a streak artifact due to an error in a measurement value does not occur. Thus, a reconstructed image with good image quality can be obtained.

Next, in step ST 7, the reconstructed image is displayed on the CRT 6 and stored in the storage device 7. The example in which X-rays are used as radiation has been described above, but the radiation is not limited to X-rays, and for example, γ-rays or the like may be used. However, at present, it is preferable to use X-rays because practical means for generation and control are the most substantial.

[0065]

As described above in detail, according to the present invention, in two directions in which the directions of radiation (X-rays) are opposite to each other, channels facing each other are shifted by a distance smaller than the channel width. Since each projection data is measured and interleaved to obtain projection data having a density twice as high as the channel density of the radiation (X-ray) detector, the number of channels to participate in the moving average does not increase the side effect. Can be doubled. Thereby, a projection data measurement method or apparatus that effectively reduces noise,
Or X-ray C that effectively reduces noise in projection data
A T device can be realized.

[Brief description of the drawings]

FIG. 1 is a block diagram of a device according to an example of an embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of a detector array in an apparatus according to an embodiment of the present invention.

FIG. 3 is a schematic configuration diagram of an X-ray irradiation / detection system in an apparatus according to an embodiment of the present invention.

FIG. 4 is a schematic configuration diagram of an X-ray irradiation / detection system in an apparatus according to an embodiment of the present invention.

FIG. 5 is a diagram illustrating a relationship between a view angle and view data when a fan-shaped X-ray beam is used.

FIG. 6 is a diagram illustrating a relationship between view angles and view data when a fan-shaped X-ray beam is used.

FIG. 7 is a diagram illustrating the relationship between view angles and view data when a fan-shaped X-ray beam is used.

FIG. 8 is a conceptual diagram of a quarter offset of a detector array.

FIG. 9 is a conceptual diagram of X-ray path interleaving.

FIG. 10 is a conceptual diagram of X-ray path interleaving.

FIG. 11 is a graph illustrating an example of a moving average weight coefficient.

FIG. 12 is a flowchart showing an operation of the apparatus according to the embodiment of the present invention;

[Explanation of symbols]

 REFERENCE SIGNS LIST 100 X-ray CT apparatus 1 Operation console 2 Input device 3 Central processing unit 4 Control interface 5 Data acquisition buffer 6 CRT 7 Storage device 10 Imaging table 20 Scanning gantry 21 X-ray controller 22 Collimator controller 23 Data collection unit 24 Rotation controller 30 X-ray Tube 50 collimator 60 detector array 60 (i) X-ray detector OB subject

Claims (3)

[Claims] 1. Using a multi-channel radiation detector, projection data of an object by radiation is read in two directions where the directions of radiation are opposite to each other. In the direction of the arrangement, each measurement is performed in a state where the distance is less than the channel width, and the projection data measured in the two directions are interleaved with each other. A moving average in a direction in which the projection data is arranged. 2. Using a multi-channel radiation detector, projection data of an object by radiation is used to determine the position of the channel of the radiation detector facing each other in two directions where the directions of the radiation are opposite to each other. Measuring means for measuring each in a state in which the distance is less than the channel width in the direction of arrangement, interleaving means for interleaving projection data measured in the two directions, and projection data interleaved by the interleaving means Moving average means for moving and averaging in a direction in which the channels of the radiation detector are arranged. 3. The X-ray detector according to claim 1, wherein the X-ray projection data is projected over the entire circumference of the subject using a multi-channel X-ray detector in two directions in which the X-ray directions are opposite to each other. Measuring means for measuring the positions of the channels facing each other in a state in which they are relatively displaced from each other by a distance less than the channel width in the direction in which the channels are arranged; and projecting data measured by the measuring means. Interleaving means for interleaving between those measured in two opposite directions; moving average means for moving and averaging projection data interleaved by the interleaving means in the direction of the channel arrangement of the X-ray detector; Image generating means for generating an image based on the projection data averaged by the moving average means. An X-ray CT apparatus characterized by the above-mentioned.