JP4871056B2 - Cradle position measurement method, cradle sag compensation method, and X-ray CT apparatus - Google Patents

Description

  The present invention relates to a cradle position measurement method, a cradle sag compensation method, and an X-ray CT (computed tomography) apparatus, and more particularly, an object mounted on a cradle is scanned with an X-ray irradiation / detection apparatus ( The present invention relates to a method for measuring a cradle position in a scan plane, a method for compensating sag of a cradle, and an X-ray CT apparatus for performing cradle position measurement and cradle sag compensation.

  An X-ray CT apparatus collects transmitted X-ray signals of multiple views of an object by an X-ray irradiation / detection device that rotates in a gantry, and reconstructs a tomographic image based on the transmitted X-ray signals. Is configured to do. The rotation of the X-ray irradiation / detection device is called a scan.

  The subject is mounted on the cradle and carried into the scan plane. The cradle is cantilevered from the table that supports it. Since the cradle that is fed out is bent (sag) due to a load, the amount is compensated by measuring the amount.

Sensors provided at various locations on the cradle are used to measure the position of the table including the sag, and sag compensation is achieved by tilting the table supporting the cradle in the direction opposite to the tilt of the cradle due to the sag. (For example, refer to Patent Document 1).
JP 2004-180846 A (page 4-8, FIG. 1-8)

  As described above, a special sensor is required for measuring the table position. In addition, a special table mechanism is required to tilt the table in the direction opposite to the tilt of the cradle for sag compensation.

  Therefore, an object of the present invention is to realize a method for measuring a cradle position by using a normal function of an X-ray CT apparatus. Another object of the present invention is to realize a method for performing cradle position measurement and cradle sag compensation by utilizing the normal functions of an X-ray CT apparatus. Another object of the present invention is to realize an X-ray CT apparatus that performs cradle position measurement and sag compensation without using a special sensor or table mechanism.

  The present invention in one aspect for solving the above-mentioned problem is to measure the cradle position in the scan plane for an X-ray CT apparatus that scans a subject mounted on the cradle with an X-ray irradiation / detection device. A cradle position measuring method, characterized in that a cradle position in a scan plane is obtained based on a channel position where X-rays passing in a tangential direction through the immediate outer side of the back surface of the cradle and the geometry of the X-ray irradiation / detection device are obtained. It is.

It is preferable to obtain a sag from a normal position with respect to the cradle position in order to clarify a necessary amount of sag compensation.
In another aspect of the present invention for solving the above-described problems, an X-ray CT apparatus that scans an object mounted on a cradle with an X-ray irradiation / detection apparatus compensates for sag of the cradle in the scan plane. The cradle position in the scan plane is obtained based on the channel position where the X-ray passing in the tangential direction on the outer side of the cradle and the X-ray irradiation / detection device geometry is detected, and the cradle position is determined from the normal position. The cradle sag compensation method is characterized in that the sag is obtained and the cradle is lifted by the sag.

Lifting the cradle when the sag exceeds a predetermined threshold is preferable in terms of improving the stability of sag compensation.
Another aspect of the present invention for solving the above-described problems is an X-ray CT apparatus that scans a subject mounted on a cradle with an X-ray irradiation / detection apparatus. An X-ray CT apparatus comprising: a cradle position measuring means for obtaining a cradle position in a scan plane based on a channel position where X-rays passing in a tangential direction are detected and the geometry of the X-ray irradiation / detection apparatus. is there.

It is preferable to provide a sag calculation means for obtaining a sag from a normal position with respect to the cradle position in order to clarify the necessary amount of sag compensation.
It is preferable that a height adjusting means for lifting the cradle by the sag is provided from the viewpoint of compensating for a change in the table position due to the sag.

  The height adjusting means preferably lifts the cradle when the sag exceeds a predetermined threshold from the viewpoint of improving the stability of sag compensation.

  According to the present invention from one aspect, in measuring the cradle position in the scan plane for an X-ray CT apparatus that scans a subject mounted on the cradle with an X-ray irradiation / detection apparatus, The cradle position in the scan plane is obtained based on the channel position where X-rays passing through the outside in the tangential direction are detected and the geometry of the X-ray irradiation / detection device. A method of performing measurement can be realized.

  According to another aspect of the present invention, in the X-ray CT apparatus that scans the subject mounted on the cradle with the X-ray irradiation / detection apparatus, the cradle sag in the scan plane is compensated for immediately after the back of the cradle. The cradle position in the scan plane is obtained based on the channel position where the X-ray passing through the outside of the tangential direction is detected and the geometry of the X-ray irradiation / detection device, the sag from the normal position is obtained for the cradle position, Since the cradle is lifted by the sag, it is possible to realize a method of performing cradle position measurement and sag compensation using the normal function of the X-ray CT apparatus.

  According to another aspect of the present invention, an X-ray CT apparatus that scans a subject mounted on a cradle with an X-ray irradiation / detection apparatus, the X-ray CT apparatus passing in the tangential direction on the outermost side of the back surface of the cradle. X-ray CT that measures the cradle position without using a special sensor, because it has cradle position measurement means for determining the cradle position in the scan plane based on the channel position where the line is detected and the geometry of the X-ray irradiation / detection device An apparatus can be realized.

  Also, an X-ray CT apparatus that performs sag compensation without using a special table mechanism by including sag calculation means for obtaining a sag from a normal position with respect to the cradle position and a height adjusting means for lifting the cradle by the sag. Can be realized.

  The best mode for carrying out the invention will be described below with reference to the drawings. Note that the present invention is not limited to the best mode for carrying out the invention. FIG. 1 shows a schematic configuration of an X-ray CT apparatus. This apparatus is an example of the best mode for carrying out the present invention. An example of the best mode for carrying out the invention related to the X-ray CT apparatus is shown by the configuration of the apparatus. An example of the best mode for carrying out the invention related to the cradle position measuring method and an example of the best mode for carrying out the invention related to the cradle sag compensation method are shown by the operation of this apparatus.

  The apparatus includes a gantry 100, a table 200, and an operator console 300. The gantry 100 scans the subject 10 carried by the table 200 with the X-ray irradiation / detection device 110 to collect a plurality of views of transmitted X-ray signals (scan data: scan data) and inputs them to the operator console 300. . The operator console 300 performs image reconstruction based on the scan data input from the gantry 100 and displays the reconstructed image on the display 302.

  The operator console 300 also controls the operation of the gantry 100 and the table 200. Under the control of the operator console 300, the gantry 100 scans under a predetermined scanning condition, and the table 200 positions the subject 10 so that a predetermined part is scanned. Positioning is performed by adjusting the height of the top plate 202 and the horizontal movement distance of the cradle 204 on the top plate by a built-in position adjustment mechanism.

  By scanning with the cradle 204 stopped, an axial scan can be performed. By continuously performing a plurality of scans while continuously moving the cradle 204, a helical scan can be performed. A cluster scan can be performed by scanning the cradle 204 for each stop position while moving the cradle 204 intermittently.

  The height of the top plate 202 is adjusted by swinging the support column 206 around the attachment portion to the base 208. The top plate 202 is displaced in the vertical direction and the horizontal direction by the swing of the column 206. The cradle 204 moves in the horizontal direction on the top plate 202 to cancel the horizontal displacement of the top plate 202. Depending on the scanning conditions, scanning is performed with the gantry 100 tilted. The gantry 100 is tilted by a built-in tilt mechanism.

  As shown in FIG. 2, the table 200 may be of a type in which the top plate 202 moves up and down vertically with respect to the base 208. The top plate 202 is moved up and down by a built-in lifting mechanism. In this table 200, the horizontal movement of the top plate 202 accompanying the raising and lowering does not occur.

  FIG. 3 schematically shows the configuration of the X-ray irradiation / detection device 110. The X-ray irradiation / detection device 110 detects an X-ray 134 emitted from the focal point 132 of the X-ray tube 130 with an X-ray detector 150.

  The X-ray 134 is formed by a collimator (not shown) and is a symmetrical cone beam (fan beam) or fan beam X-ray. The X-ray detector 150 has an X-ray incident surface 152 that expands two-dimensionally corresponding to the spread of X-rays. The X-ray incident surface 152 is curved so as to constitute a part of a cylinder. The central axis of the cylinder passes through the focal point 132.

  The X-ray irradiation / detection device 110 rotates around a central axis passing through an imaging center, that is, an isocenter O. The central axis is parallel to the central axis of the partial cylinder formed by the X-ray detector 150.

  The direction of the center axis of rotation is the z direction, the direction connecting the isocenter O and the focal point 132 is the y direction, and the direction perpendicular to the z direction and the y direction is the x direction. These x, y, and z axes are three axes in a rotating coordinate system with the z axis as the central axis.

  FIG. 4 schematically shows a plan view of the X-ray incident surface 152 of the X-ray detector 150. The X-ray incident surface 152 is configured such that detection cells (cells) 154 are two-dimensionally arranged in the x direction and the z direction. That is, the X-ray incident surface 152 is a two-dimensional array of detection cells 154. In the case of using fan beam X-rays, the X-ray incident surface 152 may be a one-dimensional array of detection cells 154.

  Each detection cell 154 constitutes a detection channel of the X-ray detector 150. Thereby, the X-ray detector 150 becomes a multi-channel X-ray detector. The detection cell 154 includes, for example, a combination of a scintillator and a photodiode.

  The cradle 204 is fed out from the top plate 202 in a cantilever manner. For this reason, the extended part is bent by the load, and displacement from the normal position, that is, sag occurs. This state is indicated by a broken line in FIG. In FIG. 5, the sag is exaggerated. The amount of sag increases as the cradle feed amount increases and the load increases.

  In this apparatus, the actual height of the cradle 204 having such a sag is measured by using the function of the X-ray CT apparatus, and compensation according to the amount of sag is performed. Hereinafter, a method for measuring the height of the cradle 204 and a method for compensating for the sag of the cradle 204 will be described.

  FIG. 6 shows a geometric relationship between the cradle 204 on which the subject 10 is mounted and the X-ray irradiation / detection apparatus 110. In FIG. 6, the X-ray irradiation / detection device 110 is represented by a fan-shaped X-ray beam 134 irradiated from a focal point 132 and an X-ray detector 150 that receives the fan-shaped X-ray beam 134.

  The fan angle (fan) of the X-ray beam 134 is θ. The fan angle θ is a known number defined by the specification of the system, that is, the geometry of the X-ray irradiation / detection device. The number of channels of the X-ray detector 150 corresponding to the X-ray beam 134 having the fan angle θ is N. Channel numbers are 1, 2, 3,..., N from the right end to the left end. The number of channels N is also a known number defined by the system specifications, that is, the geometry of the X-ray irradiation / detection apparatus.

  The X-ray irradiation / detection apparatus 110 scans the subject 10 and the cradle 204 while rotating, for example, clockwise around the isocenter O. The subject 10 and the cradle 204 are in the scan plane of the X-ray irradiation / detection device 110.

  During the scan, the focal point 132 moves on the circumference of the radius R with the isocenter O as the center. The radius R is a known number defined by the specification of the system, that is, the geometry of the X-ray irradiation / detection device. Hereinafter, the rotational position of the X-ray irradiation / detection device 110 is represented by the position of the focal point 132 on the circumference.

  The focal point 132 sequentially passes through M trigger points set on the circumference. Transmission X-ray signal acquisition is performed at each of the M trigger points. The trigger point number M is a known number defined in the system specifications.

  Among the M trigger points, as shown in FIG. 7, there is a trigger point at which the X-ray 136 emitting the focal point 132 passes in the tangential direction outside the nearest side of the back surface of the cradle 204. If the angle between the straight line connecting the focal point 132 and the isocenter O and the X-ray 136 is α, the distance h from the isocenter O to the X-ray 136 is given by the following equation.

距離ｈはアイソセンタＯからクレードル２０４の裏面までの距離を表す。

The distance h represents the distance from the isocenter O to the back surface of the cradle 204.

  The channel number n of the X-ray detector 150 on which the X-ray 136 is incident is given by the following equation.

したがって、チャンネル番号ｎが分かれば（２）式から角度αを求めることができ、角度αを用いて（１）式から距離ｈを求めることができる。

Therefore, if the channel number n is known, the angle α can be obtained from the equation (2), and the distance h can be obtained from the equation (1) using the angle α.

  Among the M trigger points, as shown in FIG. 8, there is also a trigger point in which the X-ray 136 ′ emitting the focal point 132 passes in the tangential direction over the immediate outer side of the back surface of the cradle 204. The X-ray 136 'passes through the same path as the X-ray 136 shown in FIG. 7 in the reverse direction. Such a trigger point is also called a mirror point.

  The relationship between the angle α between the straight line connecting the focal point 132 and the isocenter O and the X-ray 136 ′ and the distance h from the isocenter O to the X-ray 136 is given by equation (1). The channel number n ′ of the X-ray detector 150 on which the X-ray 136 ′ is incident is given by the following equation.

したがって、チャンネル番号ｎ’が分かれば（３）式から角度αを求めることができ、角度αを用いて（１）式から距離ｈを求めることができる。

Therefore, if the channel number n ′ is known, the angle α can be obtained from the equation (3), and the distance h can be obtained from the equation (1) using the angle α.

  Since the distance h is the distance in the scan plane, when the gantry 100 is tilted and the scan plane is tilted as shown in FIG. 9A, the true distance H is obtained as shown in FIGS. The apparent distance Htilt also increases.

  When the tilt angle of the gantry 100 is ψ, the distance Htilt is expressed by the following equation.

そこで、ガントリ１００がチルトしている場合は、（３）式を用いて本当の距離Ｈが求められる。

Therefore, when the gantry 100 is tilted, the true distance H is obtained using equation (3).

  Since the height of the isocenter O from the floor surface is defined by the system specification, that is, the geometry of the X-ray irradiation / detection device, the height of the cradle 204 can be known if the distance h is known. Since the normal height of the cradle 204 is defined by the system specifications, the sag can be understood from the difference from the normal height. By determining the sag, the necessary amount of sag compensation can be clarified.

  A method for specifying the channel number n will be described. FIG. 10 shows the X-ray passing state when the X-ray focal point 132 sequentially passes through three adjacent trigger points. The trigger point interval is exaggerated.

  In FIG. 10, the trigger point * 1T is a trigger point at which the X-ray passes in the tangential direction on the outer side closest to the back surface of the cradle 204. The trigger points * 1T-1 and * 1T + 1 are the trigger points one before and one after, respectively. At both of these trigger points, X-rays partially pass through the cradle 204.

  The detection signals of channel n at the trigger points * 1T-1, * 1T, * 1T + 1 are YT-1, YT, and YT + 1, respectively. The detection signals YT-1, YT, and YT + 1 are obtained at different timings in this order. If these timings are T-1, T, and T + 1, respectively, the signal intensity at each timing is as shown in FIG. 11, for example. The signal intensity corresponds to the attenuation rate of X-rays.

  As shown in FIG. 11, the signal YT has the minimum signal strength. This is because X-rays have passed through a route that is entirely air. Signal strength of signals YT-1 and YT + 1 is greater than that of signal YT. This is because both are caused by X-rays partially passing through the cradle 204. There is a sudden change in signal level between the signal YT and the signals YT−1 and YT + 1.

  As described above, the X-ray detection signal passing in the tangential direction through the immediate outer side of the rear surface of the cradle 204 exhibits a characteristic signal intensity change pattern (pattern) from the front and rear trigger points. Hereinafter, this change pattern is also referred to as a cradle bottom pattern.

  The channel number n is specified using this cradle bottom pattern. That is, the channel in which the detection signal indicates the cradle bottom pattern is specified from the channels of the X-ray detector 150. The channel number n 'at the mirror point can be specified in the same manner.

  The approximate position of the trigger point when X-rays pass tangentially through the immediate outer side of the back of the cradle 204 can be predicted from the system specifications, and the channel number n ( Alternatively, an approximate value of n ′) can also be predicted from the system specifications.

  Therefore, it is efficient to specify the channel number n (or n ′) by searching for the vicinity of the predicted trigger point and channel number. The distance h is preferably obtained by averaging the calculated value based on the channel number n and the calculated value based on the channel number n ′ from the viewpoint of improving accuracy.

  In order to increase the certainty of the channel number n, it is preferable to use detection signals of a plurality of channels near the channel number n. That is, as shown in FIG. 12, when attention is paid to channel numbers n-1, n-2, n-3, and n-4 in the vicinity of channel number n, there is an angular difference between X-rays incident on these channels. Since they are small, the detection signals of those channel numbers also show a change pattern similar to the detection signal of channel number n.

  Therefore, if the detection signals of a plurality of channels adjacent to each other in the direction in which the channel number decreases indicate a pattern similar to the cradle / bottom pattern, it is possible to increase the certainty of the channel number n. it can.

  The certainty of the channel number n 'can be increased accordingly. However, the condition is that the detection signals of a plurality of channels adjacent in the direction in which the channel number increases indicate a pattern similar to the cradle / bottom pattern.

  In order to increase the certainty of the channel number n, it is preferable to use detection signals of a plurality of channels at the trigger point near the trigger point * 1T. That is, as shown in FIG. 13, when focusing on the trigger point * 1T + 1 next to the trigger point * 1T, the X-ray angle difference from the trigger point * 1T is small. As shown in FIG. 14, a value similar to the detection signal group of the trigger point * 1T is shown.

  Therefore, if the detection signal group of the trigger point adjacent to the later one shows the same value, the certainty of the channel number n can be increased.

  The certainty of the channel number n 'can be increased accordingly. However, it is a condition that the detection signals of the trigger points adjacent to the earlier one show the same value.

  FIG. 15 shows a flowchart of the operation of this apparatus. In step 101, a trigger point and a channel number are predicted. That is, the trigger point at which X-rays pass in the tangential direction through the immediate outer side of the back surface of the cradle 204 and the channel number on which such X-rays are incident are predicted based on the system specifications.

  In step 103, scanning is performed. The scan is performed by an axial scan, a helical scan, a cluster scan, or the like. Thereby, scan data is collected.

  In step 105, the channel number is specified and the height of the cradle 204 is calculated. The channel number is specified by searching the cradle bottom pattern in the scan data using the trigger point and the channel number predicted in step 101. The identification of the channel number and the calculation of the cradle height are performed every scan. This provides a height measurement of the cradle 204 in real time.

  The operator console 300 performs channel number specification and cradle height calculation. The operator console 300 is an example of a cradle position measuring unit in the present invention.

In step 107, the cradle height measurement value is supplied to the image reconstruction unit. The cradle height measurement value is used by the image reconstruction unit for position correction of the reconstructed image.
In step 109, the sag of the cradle 204 is calculated. The sag is obtained as a difference between the normal height of the cradle 204 and the height measurement value of the cradle 204. The sag calculation is performed by the operator console 300. The operator console 300 is an example of a sag calculation means in the present invention.

  In step 111, it is determined whether or not the sag is larger than a threshold value. The threshold corresponds to the allowable amount of sag. When it is larger than the threshold value, sag compensation is performed at step 113. The sag compensation is performed by increasing the height of the top plate 202 supporting the cradle 204 by the sag. When the sag is within the threshold value, sag compensation is not performed.

  In the method of swinging the support column 206 as shown in FIG. 1, the horizontal position changes when the height of the top plate 202 is changed. Therefore, it is necessary to displace the cradle 204 in the horizontal direction by a distance that cancels the horizontal position. is there. On the other hand, such an operation is not necessary in the system in which the top plate 202 is moved up and down vertically with respect to the base 208 as shown in FIG.

  Sag compensation is performed by the table 200 under the control of the operator console 300. The operator console 300 and the table 200 are examples of height adjusting means in the present invention.

  In step 115, it is determined whether or not the scan is finished. If not, the process returns to step 103. While scanning is not finished, the operations of steps 103-115 are repeated. Thus, during scanning, the cradle height is measured every scan, and sag compensation is performed every time the sag exceeds the threshold value.

  The state of sag compensation is shown in FIG. FIG. 16 shows a state in which a sag (thin line) that gradually increases as the cradle 204 is extended is compensated based on a threshold value. As shown in FIG. 16, the sag of the cradle 204 is compensated each time the threshold is reached and always remains within the allowable range. Thus, since the cradle is lifted when the sag exceeds the threshold value, the stability of the sag compensation can be improved.

  When sag compensation is not necessary, as shown in the flowchart of FIG. 17, the sag calculation and compensation may be omitted by simply supplying the cradle height measurement value to the image reconstruction unit.

It is a figure which shows the structure of the X-ray CT apparatus of an example of the best form for implementing this invention. It is a figure which shows the structure of the X-ray CT apparatus of an example of the best form for implementing this invention. It is a figure which shows the structure of a X-ray irradiation / detection apparatus. It is a figure which shows the structure of the X-ray entrance plane of an X-ray detector. It is a figure which shows the sag of a cradle. It is a figure which shows the geometric relationship of the to-be-examined object mounted in the cradle, and X-ray irradiation / detection apparatus. It is a figure which shows the geometric relationship of the to-be-examined object mounted in the cradle, and X-ray irradiation / detection apparatus. It is a figure which shows the geometric relationship of the to-be-examined object mounted in the cradle, and X-ray irradiation / detection apparatus. It is a figure which shows a real distance and an apparent distance when a gantry is tilted. It is a figure which shows the passage state of X-ray when an X-ray focus passes three trigger points sequentially. It is a figure which shows the signal strength of a detection signal. It is a figure which shows the X-ray passage state of a some channel. It is a figure which shows the X-ray passage state of a some channel. It is a figure which shows the signal strength of a detection signal. It is a flowchart which shows operation | movement of the X-ray CT apparatus of an example of the best form for implementing this invention. It is a figure which shows a sag compensation. It is a flowchart which shows operation | movement of the X-ray CT apparatus of an example of the best form for implementing this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10: Subject 100: Gantry 110: X-ray irradiation / detection apparatus 130: X-ray tube 132: Focus 134: X-ray 150: X-ray detector 152: X-ray incident surface 154: Detection cell 200: Table 202: Top plate 204: Cradle 206: Prop 208: Base 300: Operator console 302: Display

Claims (8)

When measuring the cradle position in the scan plane for an X-ray CT apparatus that scans the object mounted on the cradle with an X-ray irradiation / detection device,
Finding the cradle position in the scan plane based on the channel position where the X-ray passing near the back surface of the cradle made of a flat surface and passing in the direction along the back surface is detected and the geometry of the X-ray irradiation / detection device.
A cradle position measuring method characterized by the above. Obtain a sag from a normal position for the cradle position,
The cradle position measuring method according to claim 1. In order to compensate for sag of the cradle in the scan plane for the X-ray CT apparatus that scans the object mounted on the cradle with the X-ray irradiation / detection device,
Find the position of the cradle in the scan plane based on the channel position where the X-ray passing near the back surface of the cradle consisting of a flat surface and passing in the direction along the back surface is detected and the geometry of the X-ray irradiation / detection device,
Obtain a sag from the normal position for the cradle position,
Lift the cradle by the sag,
A cradle sag compensation method characterized by the above. Lifting the cradle when the sag exceeds a predetermined threshold;
The cradle sag compensation method according to claim 3. An X-ray CT apparatus that scans an object mounted on a cradle with an X-ray irradiation / detection apparatus,
A cradle for obtaining a cradle position in a scan plane based on a channel position where an X-ray passing in the direction along the back surface in the vicinity of the back surface of the cradle having a flat surface is detected and the geometry of the X-ray irradiation / detection device Position measuring means,
An X-ray CT apparatus comprising: Sag calculating means for obtaining a sag from a normal position with respect to the cradle position;
The X-ray CT apparatus according to claim 5, comprising: A height adjusting means for lifting the cradle by the sag,
The X-ray CT apparatus according to claim 6, comprising: The height adjusting means lifts the cradle when the sag exceeds a predetermined threshold;
The X-ray CT apparatus according to claim 7.

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