JPH02201254A - Scanning for radiation ct apparatus - Google Patents

Abstract

PURPOSE:To prevent a streak artifact by adding a fine rotating action to one translating action and rotating action to sample a data at a dead angle between detectors. CONSTITUTION:A case herein considered is a detector in which detectors each with an effective width of P/2 are arrange in a range of a pitch P. With both of an X-ray source 1 and a detector 2 left at a specified position, a translating is performed to obtain a data and thereafter, a fine rotating is performed by a portion corresponding to P/2. Then, a translating is performed again to obtain a data. Then, rotating is performed by a portion equivalent to subtraction of an angle corresponding to P/2 from a prescribed rotating value. This operation is repeated to sample all data. This enables the sampling of a data by method corresponding to a gap liable to be of oversight for detectors unable to be measured traditionally thereby preventing a streak artifact.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a radiation CT apparatus that irradiates radiation such as X-rays and R-rays at a fan beam angle, and particularly relates to a radiation CT apparatus that irradiates radiation such as X-rays and R-rays at a fan beam angle. The present invention relates to a scanning method in which a tomographic image of a subject is reconstructed by sequentially repeating a translation operation for relatively parallel translation and a rotation operation for rotationally moving around the subject.

[Conventional technology]

Conventional techniques for radiation CT apparatuses are shown in FIGS. 6 to 9.

As shown in FIG. 6, this prior art includes an X-ray source 1 as a radiation source, a detector 2, a translation mechanism 3, and a rotation mechanism 4. The X-ray source 1 is installed on a movable frame 6 provided on a rotating body 5, and is configured to irradiate the subject 100 at a fan beam angle (approximately 5 to 10 degrees) α as shown in FIG. There is. The movable frame 6 is mounted on a guide rail 7 fixed to the rotating body 5 and is configured to move along the guide rail 7. The detector 2 is arranged so that the X-ray source 1 is sandwiched between the object 100 in the movable frame 6.
are installed in a position facing each other, and each detection element 21
24 are arranged along the width direction of the X-ray fan beam angle α0 and have the same dimensions as the detection elements, as shown in FIG. Furthermore, the detection element 21":24 is definitely XvA
The X-ray dose at each corresponding angle within the fan beam angle α is measured. The translation mechanism 3 consists of a rack 3a provided on the movable frame 6, a binion 3b for moving the rack, and a motor for driving the binion 3b, and the movable frame 6 is moved together with the rack 3a by rotation of the binion 3b. Accordingly, the X-ray source 1 and the detector 2 are relatively moved in parallel, thus performing a translation operation. The rotation mechanism 4 consists of a rotating motor 4a, a sprocket 4b attached to its rotating shaft, and a chain 4c wound between the sprocket 4b and the rotating body 5. The rotating body 5 is rotated by the drive of the sprocket 4b. is sequentially rotated at an angle corresponding to the X-ray fan beam angle α, thereby rotating the X-ray source 1 and the detector 2 around the subject 100, thus performing a rotation operation.

Then, as shown in FIG. 7, the translation mechanism 3 moves the X-ray source 1 and the detector 2 relatively in one direction so as to cover the entire area of the subject 100, and performs the first translation. At this time, each detection element of the detector 2 detects that the X-rays at the respective angles within the X-ray fan beam angle α are detected by the object 100 at each point during the movement of the X-ray source 1.
Measure the data that passes through. That is, X-ray transmission data in the direction corresponding to each detection element within the fan beam angle α is measured for each point in the entire area of the subject 100.

Next, the rotation mechanism 4 performs a first rotation operation by relatively rotating the X-ray source 1 and the detector 2 by an angle corresponding to the fan beam angle α, and at that time, each detection of the detector 2 The element measures the X-ray transmission data of the object 100 at each point during the rotation of the X-ray source 1 in the same manner as described above.
X-ray transmission data is measured by moving the radiation source 1 and the detector 2 in the opposite direction to the first translation operation and performing a second translation operation, thus performing multiple translation and rotation operations sequentially. By repeatedly measuring transmission data in 360-degree directions for each point on the subject 100, a tomographic image of the subject 100 can be reconstructed.

[Problem to be solved by the invention]

By the way, in general, in a radiation CT apparatus, it is desired that the density of each measurement angle measured by the detector is high, and for this reason, it is preferable to provide a large number of each detection element of the detector and to make it as small as possible. .

However, in the conventional technology shown above, each detection element 21
The density of angles to be measured is limited by the number of ~24, and as the size of the detection elements 2I-24 becomes smaller, the amount of radiation incident on each of these detection elements 21-24 decreases, making measurement difficult. The reality is that.

Furthermore, each of the detection elements 21 to 24 needs to be partitioned off with a septa as a shielding member made of tungsten or the like in order to prevent radiation from entering at angles beyond a desired angle.

With the above-mentioned conventional technology, in the case of a medical CT apparatus that targets the human body, the X-ray tube voltage is used at about 120 KV, so there is no problem, but in particular, materials with a higher radiation absorption coefficient than the human body, such as iron, are used. Industrial CT for copper, copper, etc.
In the case of a device, an X-ray tube with a high voltage of several hundred KV or an isotope that generates high-energy R-rays may be used as a radiation source, so each detection element of the detector 2 is Since the thickness of the dividing septa must be increased, each detection element 2L to 2. The reality is that the distance between them is widening. The relationship between the fan beam angle α and the detector 2 is as shown in FIG. (twice the width of the element).

Then, if the distance between each detection element becomes wide (as shown in FIG. 7), there will be an angle that is not measured within the fan beam angle α, and the measurement angle will become coarser. As shown in the figure.

Figure 9 shows data sampling points during translation operation, with the horizontal axis X representing the coordinates of the measurement position during movement in the direction of translation operation from the center of rotation, and the vertical axis 0 representing the coordinates of the measurement angle. . In this case, there are four detection elements 21 to 24 corresponding to FIG. 7, and they are arranged at an angle 2P (twice the width of the detection elements) corresponding to twice the gap dimension. shall be.

From Figure 7, when the number of measurement positions during translation operation is increased, the data density in the horizontal axis X direction (translation operation direction) increases, but the data density in the vertical axis 0 direction (angular direction) does not increase. I understand that.

Therefore, there are angles that are not measured within the fan beam angle α, and as a result, the measurement angle interval becomes coarse. When an image is reconstructed, radial noise, that is, noise called streak artifact occurs.

SUMMARY OF THE INVENTION In view of the problems of the prior art, an object of the present invention is to provide a scanning method for a radiation CT apparatus that can reliably prevent the occurrence of strike and soak artifacts even if the distance between each detection element of a detector is wide. Another object of the present invention is to provide a rotation control method for a radiation CT apparatus that can reliably prevent streak artifacts from occurring without requiring a long scanning time.

[Means to solve the problem]

The above purpose is to sequentially repeat a translation operation in which the fan beam radiation source and the detector are moved in parallel, and a rotation operation in which the radiation source and the detector are rotated at a specific angle. In a scanning method for a TR-type radiation CT apparatus that measures transmission data of (a) a translation operation with the radiation source and detector both remaining in their predetermined positions, (b)
Either the radiation source or the detector repeatedly executes k-1 micro-rotate operations and the translation operation that should be moved after each micro-rotate operation (fan beam). When the translation operation is performed at the position of angle α = degrees, the radiation source and detector are rotated to the position of α angle, and (d) The following processes (a) to (c) are performed for each α angle. Sequential N
It is characterized by being executed repeatedly.

(e) Below, process (a) to (d) sequentially for each α angle N
It is characterized by being executed repeatedly.

[Effect]

In the method of the present invention, as described above, (a) the radiation source and the detector are translated while keeping both of them in their predetermined positions, and then (b) k-1 times of minute rotation operations and Either the radiation source or the detector sequentially executes the translation operation to be moved after each minute tJz' rotation operation up to the minute range (
When the translation operation is performed at the fan beam position,
Move the radiation source and detector in a large rotation to the α angle position, and (d) perform the following steps (a) to (c) sequentially for each α angle N
Since the execution is repeated a number of times, for example, if the value is k-2 and the detector is rotated once, the vertical rotation in Fig. 6 can be achieved by performing a translation operation before and after performing one minute rotation operation in (b). As a result of being able to increase the density in the direction, even if the distance between the detection elements increases, the measurement angle can be reliably made denser, and noise such as streak artifacts can be prevented.

〔Example〕

Embodiments of the method of the present invention will be described below with reference to FIGS. 1 to 5. 1 and 2 show a first embodiment of the method of the present invention, in which the same parts as in the conventional example are given the same reference numerals, respectively.

The scanning method in the radiation CT apparatus of the embodiment is as follows:
and detector 2, both of which detect element 2. of detector 2. ~2
After performing the translation operation while leaving 4 in the specified position,
An X-ray source 1 and a detector 2 are connected to a detection element 2. ~2
Pitch between 4. Perform a minute rotation operation at a position shifted by 2 angles, then perform a translation operation on both the X-ray source 1 and the detector 2 while the detector 2 remains in the same position, and at the point when the translation operation is executed. , a large rotation operation is performed, and the above processing is sequentially repeated N times for each α angle, thereby obtaining twice the data density.

Specifically, after both the X-ray source 1 and the detector 2 perform the first translation operation, the X-ray source 1 and the detector 2 are moved to the gap between each detection element 21 to 2 degrees. By rotating by the corresponding angle P, the first minute rotation operation is performed. That is, during the first minute rotation operation, as shown in FIG.
Both the radiation source 1 and the detector 2 are rotated by an angle corresponding to a half pitch between each of the detection elements 21 to 24 (see FIG. 7).

In that case, each detection element 2. ~24 is of course,
It is designed to fall within the range of the fan beam angle α.

Further, after the second translation operation, the second rotation operation is performed by rotating both the X-ray source 1 and the detector 2 by an angle obtained by subtracting the angle P from the fan beam angle α. There is.

That is, during the second rotation operation, the rotation is made at an angle of α-P.

Then, during the third and subsequent rotation operations, both the X-ray source 1 and the detector 2 are rotated by sequentially repeating the rotation angle during the first rotation operation and the rotation angle during the second rotation operation. ing.

Therefore, when the XmI source 1 and the detector 2 are rotated an even number of times, they are positioned at an angular position divisible by the fan beam angle α.

360° Therefore, the fan beam angle α=2N is such that the X-ray source l rotates 360° around the subject (N is an integer).

In this way, both the X-ray source 1 and the detector 2 rotate at an angle 2 corresponding to the pitch dimension between the detection elements 21 to 21 during the first minute rotation operation, and during the second rotation operation. Since the above-mentioned angles are repeated sequentially during the α-operation, compared to the conventional example in which the fan beam angle α is simply used for the roach operation, the first, third, and fifth Although the measurement data during odd-numbered translation operations such as the 2nd, 4th, etc. are the same as before, the measurement data during even-numbered translation operations such as the 2nd, 4th, etc. It is possible to measure data that is shifted by an angle P with respect to data measured during the translation operation.

Therefore, in the even-numbered translation operations, data is measured with a shift of the angle P, so that the density of the X-ray measurement angles can be increased accordingly.

As a result, even if the distance between the detection elements 2I to 21 of the detector increases, the measurement angle will not become coarse, so when reconstructing an image, it is possible to prevent noise such as streak artifacts from occurring. . Also, according to the example,
Since only the rotation angle of the rotation operation is changed, the device can be used by only slightly changing the sea case.

In the illustrated embodiment, in order to double the density in the angular direction between the detection elements, the detector sequentially performs one minute rotation operation and the subsequent translation operation. In order to obtain double the density, the X-ray source 1. The detector 2 is rotated to the position of angle a, and the above process is repeated sequentially to obtain 3 as shown in FIG.
Obtaining twice as dense data can be achieved.

Therefore, in order to obtain higher density, if the value of k is increased, the number of minute rotation operations and the number of subsequent translation operations will increase, but this can be achieved more reliably.

Assume that the fan beam angle α of the X-ray source l is set as shown in the following equation.

Then, the X-ray source 1 and the detector 2 are placed around the subject at a 360-degree angle.
When repeating the translation operation from the ° direction and the rotation operation for each fan beam angle α, as shown in Fig. 4, the measurement line a at the time of the Nth rotation operation,
The measurement lines at the time when the first translation operation is started are parallel to each other. Therefore, when the X-ray source l moves from S to S' by the N+1 translation operation, data in the same direction as the first measurement line will be measured. In addition to this, after the N+1 time,
If you repeat the translate and rotate operations, 1
Data in the same direction as the data measured by the Nth to Nth translation and rotation operations will be measured.

Furthermore, when X-rays pass through a subject with an uneven distribution of radiation absorption coefficients, the spread of the X-ray beam from the X-ray source 1 to the detector 2 may differ, and the effective energy of the X-rays may vary in the direction of transmission. Because the difference may differ depending on the
If it is reversed, a difference may occur in the measurement data.

However, normally, the width of the detection elements 21 to 21 is only a few -, the spread error of the X-ray beam is minute, and the change in the effective energy of the X-ray is also extremely small, so it can be ignored.

Therefore, even if all subsequent movements up to the 2N-1st rotation are omitted from the Nth microrotation operation, within that range, the data measured in advance from the 1st to Nth rotations can be used as a substitute for the subject. Since data has been measured in a 360° direction around , and there is virtually no problem with the data, processing up to the 2N-1st time is not required. Therefore,
α=18°.

The image can be reconstructed by setting the X-ray source l and the detector 2 as N times.

FIG. 5 shows a second embodiment of the method of the invention.

In this embodiment, the data density is increased by changing the processing contents of the 1st to Nth times and the processing contents of N to 2-1 times out of the X-ray source fan beam angle α-N times.

Specifically, for example, k-2 and 1-N-
X-ray source until the first rotation operation.

Both detectors rotate with a fan beam angle α.

Next, during the N-th rotation operation, the X-ray source 1 and the detector 2 are rotated at an angle P+α obtained by adding the fan beam angle α to the angle P corresponding to the gap size between the detection elements 21 to 21. .

Thereafter, during the N+1st to 2N-1st rotation operations, the fan beam angle α is again rotated.

In addition, when rotating 2N times, the remaining angle,
That is, the X-ray source 1 and detector 2 are rotated by an angle of α-P.

In this way, the X-ray source 1 and the detector 2 rotate at the angle P+α during the N-th rotation operation, and thereafter rotate at the angle P+α
During the first rotation operation, when rotating at the fan beam angle α, the data is measured by an angle P compared to the first to N rotation operations due to the Nth to 2N-1st rotation operation. Can be done.

Therefore, according to this embodiment, compared to the conventional rotating operation at the fan beam angle α, the measurement is shifted by the angle P from the Nth time to the 2N-1st time, so the angular density can be increased by that much. This has the same effect as the first embodiment. In addition, since it is only necessary to rotate by the angle P + α during the Nth rotation operation, the number of rotation operations and the number of translation operations can be significantly reduced compared to the first embodiment, so that the scanning time is longer. Not at all.

In addition, in this embodiment, an example was shown in which the X-ray source 1 and the detector 2 were rotated at the angle α+P during the N-th rotation operation, but the same effect can be obtained by using the angle α−P. .

In addition, in the illustrated embodiment, the density in the angular direction between the detection elements is set to 2.
In order to double the number, the detection elements 21 to 24 perform one minute rotation operation and the subsequent translation operation from the Nth time.
An example has been shown in which the rotation is performed while the angle is rotated to α±P/, and then the translate operation and the rotate operation are sequentially repeated up to the 2N-1st time. However, the method of the present invention is limited to the illustrated embodiment. It's not something you can do.

That is, when obtaining a density three times (=k), up to the 1-N-1th time, one minute rotation operation and the subsequent translation operation are performed on the detection element 2. ~ When executed at a variable position, the X-ray source 1. The detector 2 is rotated to the position of the α angle, and these operations are repeated in sequence.

rotate the X-ray source 1. High density can be obtained by repeating the operation of the detector 2 sequentially up to 2N-1 times with the detection elements 21 to 24 remaining at the predetermined positions and at angles of α.

Therefore, in order to obtain higher density, if the value of k is increased, the number of minute rotations and the number of subsequent translation operations will increase, but this can be achieved more reliably [Effect of the invention] As described above, the method of the present invention can increase the density of measurement angles even if the distance between the detection elements of the detector increases, and as a result, has the effect of preventing noise such as streak artifacts from occurring. be.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of the rotation angle during the rotation operation showing the first embodiment of the method of the present invention, FIG. 2 is an explanatory diagram showing the data sampling points measured by the rotation operation, and FIG. An explanatory diagram of data sampling points showing a modification of the first embodiment, FIG. 4 is a fan beam angle of 180
5 is an explanatory diagram of the rotation angle showing the second embodiment of the method of the present invention, and FIG. 6 is an explanatory diagram of the rotation angle of the conventional X&IC.
A schematic diagram of the main parts showing an example of the configuration of the T device, FIG. 7 is a diagram showing the relationship between the X-ray fan beam angle and the detector, FIG. 8 is an explanatory diagram showing the conventional scanning method, and FIG. 9 is the conventional scanning method. FIG. l... Radiation source, 2... Detector, 21-24...
Detection element, 3... Translate mechanism, 4... Rotate mechanism, α... Fan beam angle, P... Pitch between detection elements. Figure 1 Agent Patent attorney Tadashi Akimoto 1 other person Figure IX Engakure 2: Examination 21-24: Autumn + Kunia Ambie lA angle Figure 1 0 Roko-to Figure 1 : X-ray source 2; Test pig 100; Sample sample

Claims (1)

[Claims] 1. By sequentially repeating a translation operation in which the fan beam radiation source and the detector are moved in parallel, and a rotation operation in which the radiation and the detector are rotated at a specific angle, In a scanning method in a TR-type radiation CT apparatus that measures radiation transmission data around the area, (a) after performing a translation operation with the radiation source and detector both remaining in their predetermined positions, (b) Either the radiation source or the detector performs k-1 micro-rotate operations and the translation motion that should be moved after each micro-rotate operation. The position is rotated sequentially by k angles, and the execution is repeated sequentially up to a range of α-(P/k)(k-1) angles (fan beam angle = 180°/N, P is each detection of the detector). pitch between elements, k is an integer of 2 or more), (c) When the translation operation is performed at the position of α-(P/k) (k-1) angle, move the radiation source and detector to the position of α angle. Large rotation, (d) below, (b)
A scanning method in a radiation CT apparatus, characterized in that the processes of (c) to (c) are sequentially repeated N times for each α angle. 2. A translation operation in which the fan beam radiation source and the detector are relatively translated in parallel, and a rotation operation in which the radiation and the detector are rotated at a specific angle are sequentially repeated to improve the transmission of radiation around the subject. In the scanning method of the TR type radiation CT device that measures data, (a) the radiation source fan beam angle α is set as α=360°/2N (N is an integer);
The repetitive processing of the translation operation and rotation operation of both the radiation source and the detector is set separately for 1st to N-1 times and N to 2-1th times, and 1 to N-1. Up to the first time, (b) both the radiation source and the detector are translated while remaining in the predetermined position; (c) thereafter, the radiation source and the detector are moved in a micro-rotation motion km times, and after each micro-rotation motion, they are moved; The translation operation to be performed is set at a position that is sequentially shifted by the P/k angle for each minute rotation operation, and is sequentially repeated up to the range of α-(P/k)(km) angle. (P is the angle corresponding to the pitch between each detection element of the detector, k is an integer greater than or equal to 2, and m is an integer smaller than k), (d) α-(P/k) (k-m) When the translation operation is executed at the angle position, the radiation source and detector are rotated in a large rotation to the α angle position, and (e) the following processes (b) to (d) are performed at α until the N-1st time.
It is executed sequentially for each angle, and in the N to 2-1 times, (f) both the radiation source and the detector are further translated while leaving one at the predetermined position, and (g) after that, m-1 times of minute rotation operation and the translation operation to be moved after each minute rotation operation, one of the above is calculated by an angle of α-(P/k)(k-m) for each minute rotation operation. Further P/k from the position
(H) After that, the radiation source and detector are further rotated to α-
It is characterized by performing a translation operation after performing a rotation operation at an angle of (P/k) (k-m), and repeating the processes of (i), (g), and (h) sequentially up to the 2N-1st time. A scanning method in a radiation CT apparatus.