JPH0747061A - Computerized x-ray tomographic system - Google Patents

Abstract

PURPOSE:To shorten the restraining time by moving an object to be inspected in the body axis direction at a speed as advancing for a distance smaller than the specified slice thickness in the period the X-ray source makes one turn. CONSTITUTION:An X-ray tube 3 and an X-ray sensor 4 are opposingly installed while pinching the object to be inspected M. X-ray generation in the X-ray tube 3 is controlled by a high voltage generating device 7, and the tube 3 is revolved round an insert hole 6 by an X-ray tube drive/control device 5. A bed top plate 2 is continuously moved a bed drive/control device 8 in the body axis direction of the object M. A fan beam is generated by the X-ray tube 3, and the object M is moved by the bed drive/control device 8 continuously all the time while the fan beam emitted is in rotation continuously and data acquisition takes place. This constitution shortens the time required to feed the object at the time of data acquisition, decreases the restraining time for the object, shortens the data acquiring time for a plurality of slice surfaces, and reduces the arch fact based upon the clipping effect.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention belongs to the technical field of X-ray tomography apparatus (hereinafter referred to as X-ray CT apparatus).

[0002]

2. Description of the Related Art Conventionally, when an X-ray CT apparatus obtains an X-ray tomographic image (hereinafter referred to as a tomographic image) of a desired slice plane of a subject, for example, a patient, the position of the patient is fixed.
By collecting X-rays from the X-ray tube while rotating the X-ray tube around the patient in a vertical plane having the slice plane, all projection data from all directions on the slice plane are collected. Image reconstruction is performed based on all projection data, and a tomographic image of a desired slice plane is displayed on the display device.

Then, when it is desired to obtain a plurality of tomographic images for a plurality of different slice planes of the patient by the X-ray CT apparatus, the X-ray tube is 180 ° for the first slice plane.
Alternatively, after 360 ° rotation to collect all projection data for the first slice plane, the operation of the X-ray tube is stopped and the X-ray tube is positioned in a vertical plane having the second slice plane, Spend time to move the patient horizontally,
The rotation and X-ray exposure of the X-ray tube must then be carried out on the second slice plane.

[0004]

Therefore, the conventional X
The X-ray CT apparatus has a problem that when a plurality of tomographic images are obtained on different slice planes, the patient's restraint time is long and therefore the operation efficiency of the X-ray CT apparatus is deteriorated.
Further, in the conventional X-ray CT apparatus, when acquiring a plurality of tomographic images for different slice planes of a patient into which a contrast medium is injected, projection data is collected for the first slice plane and the projection data for the last slice plane. There is also a problem that a plurality of tomographic images under the same physiological condition cannot be obtained because the physiological condition of the patient changes depending on the time of collection.

The present invention has been made in view of the above circumstances, and shortens the time required for feeding an object at the time of data acquisition to reduce the time for restraining the object and the acquisition time for a plurality of slice planes. X-ray CT that can be realized
The purpose is to provide a device.

[0006]

In order to achieve the above object, the present invention irradiates X-rays which pass through a subject in a direction perpendicular to the plane of rotation while rotationally moving around the subject. An X-ray source, an X-ray detector that detects X-rays that have passed through the subject, and a rotational movement of the X-ray source so that the X-ray source travels a distance shorter than a predetermined slice thickness in one rotation time. A subject moving means for moving the subject in the body axis direction, a data collecting means for collecting the data obtained from the detector, and an image for reconstructing an image based on the data supplied from the data collecting means And a reconstructing means.

[0007]

It is possible to perform scanning so that the thickness of the one-slice tomographic plane can be moved in a short time.

[0008]

The present invention will be described in detail with reference to the following examples.

FIG. 1 is a system block diagram of an X-ray CT apparatus showing an embodiment of the present invention. Reference numeral 1 denotes a gantry, which is provided with an insertion hole 6 into which the subject M placed on the bed top plate 2 is inserted, and an X-ray tube 3 serving as an X-ray source with the inserted subject M interposed therebetween. The X-ray detector 4 is arranged so as to face it. Here, the X-ray tube 3 is configured such that the high-voltage generator 7 controls the generation of X-rays, and the X-ray tube drive controller 5 rotationally moves around the insertion hole 6. The line detector 4 is configured by arranging a plurality of single detectors in an array along the circumferential surface of a cylindrical holding member that is obliquely arranged. The X-ray of the object transmitted from the X-ray tube 3 is detected. Is always received by a part of the detector 4.
The couch top 2 can be continuously moved along the body axis direction of the subject M by the couch drive control device 8 (also referred to as subject moving means). Reference numeral 9 is a data collection device that collects data obtained by the X-ray detector 4, 10 is a correction calculation device for making the data in the data collection device appropriate reconstruction data, and 11 is a correction calculation device. An image reconstructing device that reconstructs an image based on the data sent from 10.
A display device 12 performs display based on the image data from the image reconstruction device 11. Reference numeral 13 is a system control device that controls the above-described devices.

Next, the operation will be described.

In the above apparatus, a fan beam-like X-ray (hereinafter also simply referred to as a fan beam) is emitted from the X-ray tube 3.
The X-ray detector 4 detects the fan beam as one unit, and the fan beam does not stop rotating at 360 °. It shall be capable of continuous rotation. For such continuous rotation, a known slip ring may be used, or USP No. 41581 may be used.
It can be realized by employing an electron beam scan as disclosed in No. 42. Then, the subject M is continuously moved by the middle bed drive control device 8 while the fan beam is continuously rotated to collect data. It is assumed that this movement amount is advanced by P mm per one rotation of the fan beam. With such a configuration, it is equivalent to, for example, a translational movement in the body axis direction while the fan beam rotates with respect to the stationary subject M, and the fan beam moves around the subject M in a spiral shape. Data will be collected (spiral scan). That is, the spiral scan is performed by the combination of the movement of the X-ray source and the subject. The data thus obtained can be defined as spiral data. Therefore, with the X-ray detector 4 fixed as in this embodiment, the X-ray tube 3 is
Not only the CT device that rotates only the fourth generation CT device, but also the CT device that relatively rotates the opposed X-ray tube and the X-ray detector (the third generation CT device) has been described above. It is possible to obtain spiral data such as When the positions of the X-ray source and the fan beam of the spiral scan are observed from the direction perpendicular to the body axis direction, a sine waveform XL with a period Pmm as shown in FIG. 2 is drawn.

Next, a method of reconstructing an image from the data obtained as described above will be described. First, in general, (1) a method of dividing the entire volume of the scan range into small elements and reconstructing them at once, (2) for example, one rotation of the X-ray tube from the slice points S ₁ to S ₂ in FIG. Considering the obtained data, the fan beam positions are X ₁ and X in the X direction of FIG.
_A known method of performing image reconstruction for each plane by approximating that fixed in the center of ₂ (USP No. 4149
247) is possible. Further, if the data obtained by the two rotations from the points S ₁ to S ₃ are bundled (superposed) by the following equation 1 and made into data for one rotation, the data is reconstructed as scan data representative of the slice position X _2. It can also be configured.

[0013]

[Equation 1]

Here, θ is a rotation angle of the X-ray tube 3 and the X-ray fan beam FB as shown in FIG. 3, and takes a value of 0 to 360 °. P ₁₂ (θ, ψ) is the X-ray tube 3 for the subject M
The projection data obtained while the relative position of X is from S ₁ to S ₂ in FIG. 2, and P ₂₃ (θ, ψ) is the projection data obtained from the same relative position of the X-ray tube from S ₂ to S _3. The data.

The above method can be deduced up to the case of obtaining an image for one slice by three rotations or more.

Further, the same effect as the above case can be obtained by independently reconstructing the projection data obtained at each rotation and averaging a plurality of obtained images.

As described above, it is useful to bundle continuous data for a plurality of rotations to form one image in order to reduce artifacts. That is, generally, in an X-ray CT apparatus, the thickness of the X-ray fan beam viewed from the side is
Since it is not a parallel X-ray, the thickness is approximately proportional to the X-ray tube. When an object is inspected with an X-ray beam in this manner, if the object has a large change in the slice thickness direction, it will include a slightly contradictory portion for each angle θ at which projection data is taken, and an artifact often called a clipping effect is generated. Will be born. A phenomenon similar to this occurs in the case of the present invention, which will be described with reference to FIG. In FIG. 4, A is the intensity profile in the slice thickness direction of the X-ray fan beam obtained when the X-ray tube is at the S ₁ position (that is, X = X ₁ ) in FIG. The slice thickness at this time is tmm. As the X-ray tube rotates, the slice plane moves in the body axis direction of the subject. For example, at θ = 180 °, the X-ray tube position is the initial position X.
It is not at ₁ , but at a position advanced by P / 2 from that point (X = X ₁
It will be located at + P / 2). If P = t, the intensity profile and position of the X-ray fan beam in the slice thickness direction at θ = 180 ° are as shown in B of FIG. Here, in the waveforms of A and B, the hatched portions mean that the objects that are not common are measured.
Since the image reconstruction calculation is made on the assumption that all projection data are the results of measuring the same subject, the hatched portions in the waveforms A and B are considered to cause some distortion in the image. . This is true not only for the relationship between θ = 0 ° and 180 ° but also for the entire range of θ. In particular, in the data collection method as in this embodiment, much development similar to the clipping effect is likely to occur.

The present invention solves such a problem by using the following principle. For example, when it is desired to obtain an effective slice thickness of tmm, the subject M is sent at a rate of t / 2mm per one rotation of the X-ray fan beam, and the X-ray fan beam FB is narrowed down to t / 2mm by a collimator or the like. ing. As a result, the intensity profile and position shown in FIG. 5 are obtained. In the figure, A and B are the intensity profile and position in the slice thickness direction of the X-ray fan beam obtained when the X-ray tube relative position is X = X ₁ and X = X ₂ , respectively, and C and D are also numbers. Two

[Equation 2]

It was obtained in. As a result, if the projection data are bundled as shown in the above equation 1, the profile and position as shown in FIG. 6 can be obtained. That is, θ = 0
Waveforms E and F are obtained in the slice thickness direction of the projection data obtained at ° and 180 °, respectively. The hatched portion is relatively small as compared with the case of FIG. 4 described above. That is, the distortion of the image is reduced.

Further, when the spiral scan as described above is performed, there are the following problems. If the collection of projection data is started at θ = 0 ° and the collection of projection data for one image is completed at a position θmax that is close to θ = 360 °, P (0, ψ) and P (θmax, ψ) Since the scan planes to be measured are different in and, the contents of the data will be quite different. It is well known that when there is a discontinuous difference between adjacent data in this way, an artifact is more likely to occur as compared with a continuous shift. In order to solve such a problem, the present invention includes a correction calculation device 10 that performs the following processing. The principle of this correction arithmetic device is that one part obtained at the beginning or one part obtained at the end of the data to be used for image reconstruction of one slice plane (slice plane) is obtained before or after that. It is corrected by the data obtained at the same rotation angle in the data of the tomographic plane.

The projection data obtained from θ = 0 to θX is substituted by the data P '(θ, ψ) which has been subjected to the arithmetic processing of the following equation (3) (θX is the necessary image reconstruction area). The width and the degree of reduction of artifacts are arbitrarily set).

[0022]

[Equation 3]

Here, W (θ) is θ = 0 as shown in FIG.
It is a function that connects 0 at θ and 1 at θ = θX, and connects them with a straight line, for example, without a sharp change.

In place of such correction, conversely, the data P '(which has been subjected to the arithmetic processing of the following equation 4 for correcting the data obtained at θ = θY to θmax with the data obtained by the previous rotation is used. θ, ψ) is substituted (θY has the same meaning as θX).

[0025]

[Equation 4]

Here, W (θ) is θ = θY = 1, θmax
Is set to 0, and there is no abrupt change between them, for example, a function that connects with a straight line.

By providing such a correction arithmetic unit 10, adjacent data can be evaluated as a continuous deviation, so that the occurrence of artifacts can be reduced.
Note that the above correction was for the case where an image is created by one rotation from S ₁ to S ₂ and slightly extended from that, but one image is created by rotating it twice or three times and slightly extending it. It goes without saying that it is possible to create.

In the above embodiment, when the relationship between the movement amount P of the bed per one rotation of the fan beam and the slice thickness t is selected as P <t, the deviation of the scan plane per one rotation of the spiral is small. Therefore, the occurrence of artifacts is reduced.

The present invention is not limited to the above embodiment, and various modifications can be made. For example, in the above embodiment, 0 to 3
Although the X-ray CT for forming one image from the projection data obtained over 60 ° has been described, the X-ray C for performing image reconstruction from scan data of less than 360 ° as shown in FIG.
It can also be applied to the T device. That is, the X-ray source reciprocates at a high speed on the orbit XL or moves one way, and the detector group 4'has a circumference of 2
They are arranged along a range of about / 3, and the subject M may be continuously sent during the repeated scanning. Also in this case, it is possible to obtain projection data for one image from the X-ray source 3'from a to b. With such a device, a locus in which U-shaped scans are continuous can be obtained as shown in FIG. The data obtained by this can be defined as modified spiral data. In this case, in FIG. 8, the movement of the X-ray source 3 ′ is such that the moving speed from b to a (at the time of returning) can be ignored compared to the moving speed from the position a to b (at the time of data collection). It must be done at a high speed, but this is fully possible by employing the known electron beam scan. According to the apparatus of this embodiment, since the moving time of the X-ray source 3'can be shortened, the slice interval Pmm can be minimized. Therefore, by expanding the equation (1), the projection data of many times can be superposed. Creating images for slices makes it easy to reduce artifacts.

[0030]

According to the present invention described in detail above, it is possible to shorten the time required for feeding the object at the time of data acquisition and reduce the time for restraining the object, and to reduce the acquisition time of a plurality of slice planes. It is possible to provide an X-ray CT apparatus that can be shortened and that can reduce artifacts due to the clipping effect.

[Brief description of drawings]

FIG. 1 is a system block diagram showing an embodiment of the present invention.

FIG. 2 is a schematic explanatory view showing a relative orbit of the X-ray source according to the present embodiment.

FIG. 3 is a schematic explanatory view showing a state of a fan beam according to this embodiment.

FIG. 4 is an explanatory diagram of a reason why distortion occurs in an image.

FIG. 5 is an explanatory view of the reason that distortion occurring in an image can be reduced by adopting the apparatus of the embodiment of the present invention.

FIG. 6 is an explanatory view of the reason that distortion occurring in an image can be reduced by adopting the apparatus of the embodiment of the present invention.

FIG. 7 is an explanatory diagram of a function used for correction calculation.

FIG. 8 is a schematic explanatory view showing another embodiment of the present invention.

FIG. 9 is an explanatory diagram of a relative orbit of the X-ray source according to the other embodiment.

[Explanation of symbols]

 1 gantry 2 bed top plate, 3,3 'X-ray source 4 X-ray detector 5 X-ray drive control device 6 detector drive device 7 high voltage generator 8 bed drive control device 9 data acquisition device 10 correction calculation device 11 image re-correction Component device 12 Display device 13 System control device

Claims (1)

[Claims] 1. An X-ray CT apparatus for obtaining a tomographic image of a predetermined slice thickness on a desired slice plane of a subject, while rotatively moving around the subject and transmitting the subject in a direction perpendicular to the plane of rotation. An X-ray source that emits X-rays, an X-ray detector that detects X-rays that have passed through the subject, and an X-ray source that travels a distance shorter than the predetermined slice thickness during one rotation of the X-ray source. A subject moving means for moving the subject in the body axis direction during the rotational movement of the X-ray source, a data collecting means for collecting data obtained from the detector, and data supplied from the data collecting means. And an image reconstructing means for reconstructing an image based on the X-ray tomography apparatus.