JP2004301861A - Computer tomographic apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a computer tomographic apparatus capable of setting a reconstitution method to the same reconstitution method (one kind) while enabling offset scanning and usual scanning. <P>SOLUTION: The computer tomographic apparatus is equipped with an X-ray source 1, a turntable 5 on which a subject is placed and an X-ray detector 3. The tomographic image of the subject is obtained from a plurality of the transmission data of the subject detected at a plurality of the rotary positions of the turntable by the X-ray detector. This computer tomographic apparatus is equipped with a reconstitution part 24 which sets an expanded sinogram, which has the larger distance of the distances between the center of rotation of the turntable and both edges of an X-ray beam on both sides of the center of rotation of the turntable, on the sinogram formed by a plurality of the transmission data of the subject and completes the same by employing the transmission data of the reversely turned X-ray routes of the X-ray routes at the respective points with respect to at least an expanded region to reconstitute a tomographic image from the expanded sinorgam. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a computer tomography apparatus among nondestructive inspection apparatuses, and more particularly to an improvement in a high-resolution computer tomography apparatus for inspecting, for example, small electronic components at a high resolution.

  In recent years, high-resolution industrial computed tomography apparatuses (also referred to as CT scanners) have been manufactured for the purpose of inspecting, for example, small electronic components at high resolution.

  In this conventional high-resolution computed tomography apparatus, an X-ray beam generated from an X-ray tube and transmitted through a subject is detected by a two-dimensional detector to obtain a transmitted image of the subject. I have.

  When capturing a cross-sectional image, a number of transmission images are obtained while the subject is rotated once (hereinafter, referred to as scanning).

  Then, data processing is performed on the plurality of transmission images to obtain cross-sectional images (one or more) of the subject.

  For reconstruction of the cross-sectional image, a filtered back projection (FBP (Filtered Back Projection) method) is generally used.

  This high-resolution computed tomography apparatus has a feature that the X-ray geometry can be freely set and can cope with various objects.

  The rotating table for mounting and rotating the subject and the detector can be moved closer to or farther away from the X-ray tube (X-ray beam focal point), and the imaging distance FCD (Focus to Center Distance) and the detection distance FDD (Focus to FOC). Detector Distance) can be changed continuously, and the imaging magnification (magnification ratio) (= FDD / FCD) can be changed according to the size of the subject.

  FIG. 22 is a conceptual diagram illustrating an example of a scan area of a normal scan and an offset scan.

  As shown in FIG. 22, the scan area (cross-sectional image field) is a circle An centered on the rotation center Cn included in the imaging X-ray beam 102 on the rotation plane, and becomes smaller as the imaging magnification increases.

  The rotation center Cn is usually slightly deviated from the center due to a mechanical error. However, if the deviation is large (at the same photographing magnification), the scan area becomes narrow, which is not preferable.

  In order to process a large number of transmission images and obtain a cross-sectional image with good resolution, the position of the rotation center on the transmission image needs to be accurately known in units smaller than one pixel.

  In a conventional high-resolution computed tomography apparatus, before scanning a subject after geometric setting, it is mounted on a pin-shaped phantom and photographed, and the center of rotation is calibrated (scaled).

  The calibration of the rotation center is performed by obtaining a detection channel position corresponding to the rotation center and storing the same in the data processing unit.

  On the other hand, there is known a computed tomography apparatus in which a rotation center is shifted, an object is scanned out of one side, and a large object can be imaged (for example, JP-A-58-116342).

  This scan is called an offset scan because it is set (offset) by shifting the rotation center.

  As shown in FIG. 22, the scan area (cross-sectional image field) in the offset scan is a circle Aof that is in contact with one side of the imaging X-ray beam around the rotation center Cof on the rotation plane. growing.

  By the way, it is preferable to use the offset scan in a high-resolution computed tomography apparatus because the scan area is widened.

  However, when an offset scan is employed by adding a table offset mechanism to a high-resolution computed tomography apparatus, the X-ray geometry can be freely set, but the following problems arise. Occurs.

  That is, first, it may be difficult to set the table at the offset position.

  An appropriate X-ray geometry is to select an FCD so that the subject fits in the scan area and offset the rotation center to the end of the field of view including a margin. When the FCD (or FDD) is changed, the center of rotation may be shifted on the screen. Therefore, the subject and the pin-shaped phantom are replaced, or the FCD and the offset are alternately switched. The geometrical setting is performed while performing.

  If the center of rotation deviates from the field of view, the cross-sectional image will not be reconstructed correctly, and if it is too inside, the scan area will be narrow.

  These are very cumbersome tasks.

  Second, it is necessary to mount a pin-shaped phantom at the stage when the geometric setting is completed, and to calibrate the center of rotation.

  Further, after the calibration is completed, the subject must be replaced in the original position.

  At this time, if the transfer position shifts, the subject may not be able to fit within the field of view of the imaging.

  These are also very cumbersome tasks.

  In a normal scan, there is a problem that a scan area becomes narrow due to a shift of the rotation center Cn due to a mechanism error (at the same imaging magnification).

  In other words, in the case of the same scan area, the imaging magnification is not increased to the maximum, and it can be said that there is waste.

  On the other hand, when the normal scan and the offset scan can be switched, there is a problem that the reconstruction method is different.

  Conventionally, as a reconstruction method of the offset scan, a fan-parallel FBP method of rearranging parallel beams, correcting the filters, and projecting back has been used.

  In the case of normal scanning, a direct FBP method is used, in which a fan beam is filtered and backprojected with the fan beam unchanged.

  Therefore, it is necessary to mix two types of reconstruction methods in the reconstruction means.

  An object of the present invention is to provide a computed tomography apparatus capable of performing the same reconstruction method (one type) while allowing an offset scan and a normal scan.

  In the invention corresponding to claim 1, an X-ray source that emits an X-ray beam, a rotary table on which the subject is mounted, and a fan-shaped orthogonal to at least the rotation axis of the rotary table from the X-ray source that has passed through the subject. X-ray detector for detecting an X-ray beam of the subject, and obtaining a cross-sectional image of the subject from a plurality of transmission data of the subject detected by the X-ray detector at a plurality of rotation positions of the rotary table. An extension in which, on a sinogram created by a plurality of transmission data of a subject, a longer distance between the rotation center of the rotary table and both edges of the X-ray beam is provided on both sides of the rotation center of the rotary table. A sinogram is set, and an extended sinogram is completed by adopting transmission data of the X-ray path in the opposite direction to the X-ray path of each point with respect to at least the extended area. And a reconstruction means for reconstructing a sectional image.

Therefore, in the computer tomography apparatus according to the first aspect of the present invention, if the center of rotation (vertical line) is known on the sinogram, a point on the sinogram that forms an opposite X-ray path from the point on the sinogram is calculated. The extended sinogram can be completed by adopting the transmission data of the X-ray path in the opposite direction of the X-ray path of each point to the extended area of the extended sinogram.
By using this extended sinogram and reconstructing by the ordinary direct FBP method, the larger one of the distances between the center of rotation and both edges of the X-ray beam can be used as the maximum cross-sectional image radius. A cross-sectional image within the region radius can be reconstructed.
Thereby, in the offset scan in which the rotation center is set to the offset position, the reconstruction can be performed by the same reconstruction method (direct FBP method) as the normal scan, and in a computer tomography apparatus capable of performing the offset scan and the normal scan, The configuration of the reconfiguration means can be arranged by using one type of reconfiguration method.

  According to the present invention, it is possible to provide a computed tomography apparatus capable of performing the same reconstruction method (one type) while allowing offset scanning and normal scanning.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(First Embodiment)
FIG. 1 is a schematic diagram showing a configuration example of a computer tomography apparatus according to the present embodiment.

  In FIG. 1, as an X-ray tube 1, a micro-focus X-ray tube having a focus F of an X-ray beam 2 of several to several tens of μm is used, and an X-ray detector (hereinafter simply referred to as a detector) 3 is used. As an example, an X-ray II (image intensifier) and a television camera are used.

  The X-ray tube 1 and the detector 3 are arranged to face each other, and are supported by an x shift mechanism 8.

  The subject 4 is placed on a rotating table 5, rotated along an imaging surface 14 (of a cross-sectional image) in the X-ray beam 2 by a rotation / elevation mechanism 6, and moved up and down substantially at right angles to the imaging surface 14. Is done.

  Further, the subject 4 is moved (in the y direction in the drawing) across the X-ray beam 2 by the y-shift mechanism 7 together with the turntable 5, and the X-ray tube 8 and the detector 3 are moved by the x-shift mechanism 8. Then, the shooting distance FCD is changed.

  The detector 3 is moved by the x shift mechanism 8, and the detection distance FDD is changed.

  In FIG. 1, C indicates a rotation center, and D indicates a detection center.

  In addition, as other components, a data processing unit 19 that processes a transmitted image from the detector 3, a display unit 20 that displays a processing result and the like, and a mechanism that controls the mechanism unit by a command from the data processing unit 19. The control unit 18, the X-ray control unit 17 for controlling the tube voltage and tube current of the X-ray tube 1 and the high-voltage generator 16, and the part including the X-ray tube 1, the subject 4, and the detector 3 are housed. There is an X-ray shielding box (not shown) and the like.

  On the other hand, an encoder (not shown) is attached to the x shift mechanism 8 and the y shift mechanism 7, and the FCD value, the FDD value, and the y value are read and sent to the data processing unit 19 through the mechanism control unit 18, respectively.

  The data processing unit 19 and the display unit 20 are ordinary computers, and include a CPU, a memory, a disk, a keyboard, an interface, and the like, and previously store software for reconstructing a cross-sectional image from tomographic sequences and data. ing.

  The operator uses the data processing unit 19 and the display unit 20 to perform menu selection and condition setting, manual operation of the mechanical unit, start of tomography, reading of the apparatus status, display of a cross-sectional image, analysis of a cross-sectional image, and the like.

  The data processing unit 19 includes, as functional blocks of software, a scan control unit 21 for tomography, a shift calculation / control unit 22 that calculates and controls a shift amount of the turntable 5, and a rotation center on the image from the transmission image. The apparatus includes a rotation center finding unit 23 for finding a position and a reconstructing unit 24 for creating a cross-sectional image.

  Next, the operation of the computed tomography apparatus according to the present embodiment configured as described above will be described.

  Here, each scan mode will be described separately.

<Normal scan> (same operation as before)
First, a case where a cross-sectional image of the subject 4 is photographed in the normal scan mode will be described.

  The operator places the subject 4 on the turntable 5 and inputs a command to the data processing unit 19 to cause the X-ray tube 1 to emit the X-ray beam 2 and display a transmission image of the subject 4. It is rotated manually or electrically while displaying it as a real-time moving image on the unit 20, so that the transmission image (at the position where a cross-sectional image is to be obtained) is completely within the field of view of the image at all rotation positions. The photographing magnification is adjusted by changing the FCD (and the detection distance FDD).

  Normally, the mechanism is adjusted so that the rotation center C is near the center of the image (at the y-shift origin position), but there is a slight error, and this error varies depending on the shooting distance FCD (and the detection distance FDD). .

  The maximum field of view of the cross-sectional image in the normal scan is the scan area An shown in FIG.

  If the deviation of the rotation center C is too large, the scan area decreases, which is not preferable.

  However, in general, there is not such a shift as to hinder the adjustment of the photographing distance FCD.

  Next, the subject 4 is moved up and down to adjust the inspection position to the imaging surface 14.

  Next, when a normal scan command is input to the data processing unit 19, the scan control unit 21 of the data processing unit 19 captures the transmission image output from the detector 3 while rotating the turntable 5, and transmits the transmission image over 360 °. From the transmission data at the position of the imaging plane 14 of the image, first, a rotation center position on the transmission data is determined by the rotation center finding unit 23 as described later, and the reconstruction unit 24 uses the rotation center position as in the related art. Then, a cross-sectional image at the position of the photographing surface 14 is reconstructed by a filter correction back projection method or the like.

  The reconstruction unit 24 or the Feldkamp method reconstructs a large number of cross-sectional images using transmission data other than the position of the imaging surface 14.

  Here, the deviation of the rotation center C from the image center is an allowable deviation in terms of reduction of the scan area, but is an unacceptable deviation for reconstruction. The adjustment is made by seeking <End>.

<Offset scan 1>
Next, a case where a cross-sectional image of the subject 4 is captured in the offset scan 1 mode will be described.

  The operator places the subject 4 on the turntable 5 and inputs a command to the data processing unit 19 to cause the X-ray tube 1 to emit the X-ray beam 2 and display a transmission image of the subject 4. While displaying the moving image on the unit 20 as a real-time moving image, it is rotated by electric power, and the shooting distance FCD (at the position where a cross-sectional image is to be obtained) is exactly included in the field of view of the image at all rotational positions. And the detection distance FDD) to change the photographing magnification.

  Normally, the mechanism is adjusted so that the rotation center C is near the center of the image (at the y-shift origin position), but there is a slight error, and this error varies depending on the shooting distance FCD (and the detection distance FDD). .

  The maximum field of view of the cross-sectional image in the normal scan is the scan area An shown in FIG.

  If the deviation of the rotation center C is too large, the scan area decreases, which is not preferable.

  However, in general, there is not such a shift as to hinder the adjustment of the photographing distance FCD.

  Next, the subject 4 is moved up and down to adjust the inspection position to the imaging surface 14.

  Next, when a command to move to the “optimal offset position 1” is input to the data processing unit 19, the shift calculation / control unit 22 of the data processing unit 19 sets the offset scan area having the same size as the normal scan area at the current position. Is calculated, and a command is issued to the mechanism control unit 18 to move it.

  FIG. 2 is a geometric diagram showing the movement to the “optimal offset position 1”.

  This indicates the positional relationship between the X-ray focal point F, the rotation center C, the detection center D, and the like on the imaging surface 14.

  FIG. 3 is a flowchart showing the movement to the “optimal offset position 1”.

  The movement procedure will be described with reference to FIGS.

  When a movement command is issued, first, in step S1, a deviation (mechanism error) of the rotation center C from the center line 30 is measured.

  The X-ray beam 2 is turned on, and the transmission data is collected while rotating the turntable 5 once.

  Next, in step S2, a rotation center position dc1 on the transmission data is obtained from the transmission data by the rotation center obtaining unit 23.

  The calculation of the rotation center position by the rotation center calculation unit 23 will be described later in detail.

  Next, in step S3, an offset position is calculated.

  This calculation is simplified in the figure and will be described in detail below.

As a known quantity,
L _w : effective half width (mm) of the detector. Fixed value.
Lof: Offset position (mm).
To 70% to the L _w set in advance 90% of the value.
(Offset line 31 is defined by Lof and focal point F.)
FDD: Current value of FDD (mm).
FCD1: Current value of FCD (mm).
There is.

  Using this, the following calculation is sequentially performed to calculate the offset position (FCD2, y2).

θ _w = atan (L _w / FDD) (1)
θ _of = atan (Lof / FDD) (2)
θ _c1 = atan (dc1 / FDD) (3)
r = FCD1 · sin (θ _w − | θc1 |) / cos (θc1)
≒ FCD1 · sin (θ _w − | θc1 |) (4)
FCD2 = r · cos (θof) / sin (θ w + | θof |) ... (5)
y2 = FCD2 · tan (θof) (6)
Next, in step S4, only the shooting distance FCD is moved to FCD2.

  Next, in step S5, as a measurement of the deviation (mechanism error) of the rotation center C from the center line 30, the X-ray beam 2 is turned on, and the transmission data is collected while rotating the turntable 5 once.

  Next, in step S6, a rotation center position dc2 on the transmission data is obtained from the transmission data by the rotation center obtaining unit 23.

  Next, in step S7, the movement amount Δy of y is calculated.

Δy = y2-dc2 · FCD2 / FDD (7)
Next, in step S8, by moving y by Δy, the mechanism error can be corrected and the rotation center C can be set to the offset position (FCD2, y2) on the offset line 31.

  As described above, the calculation and control (command) are performed by the shift calculation / control unit 22 except for the rotation center position finding steps S2 and S6.

  Next, when an offset scan command is input to the data processing unit 19, the scan control unit 21 of the data processing unit 19 captures the transmission image output from the detector 3 while rotating the turntable 5, and transmits the transmission image over 360 °. First, the rotation center is obtained from the transmission data at the position of the image capturing surface 14 by the rotation center calculation unit 23 as described later, and the reconstruction unit 24 uses the rotation center to perform “reverse transformation reconstruction” described later. A cross-sectional image at the position of the imaging plane 14 is reconstructed.

  The reconstruction unit 24 applies the reconstruction method of “window function multiplication + three-dimensional back projection (BP)” of offset scanning described in, for example, “Japanese Patent Application Laid-Open No. 2001-330568”, and Many cross-sectional images other than the imaging surface 14 are reconstructed using transmission data other than 14.

  Here, a cross-sectional image of the offset scan area having the same size as the normal scan area before the movement can be obtained by the offset scan.

  That is, if the imaging magnification is adjusted in the normal scanning state and the subject 4 is set so as to just fit in the scanning area, it is possible to automatically move to the offset scanning position having the same scanning area. <End>.

<Offset scan 2>
Next, a case where a cross-sectional image of the subject 4 is photographed in the offset scan 2 mode will be described.

  The difference from the offset scan 1 described above is only the movement to the “optimal offset position 1”.

  That is, the only difference is that an assumed radius r 'without a mechanism error is used as a substitute for the normal scan area radius r at the actual current position when there is a mechanism error.

  When the mechanism error is small, r 'is substantially equal to r and slightly larger, so that offsetting results in a slightly larger scan area, which is on the safe side.

  As a result, the rotation center measurement at the current position can be omitted, and the movement becomes faster.

  FIG. 4 is a geometric diagram showing the movement to “optimal offset position 2”.

  FIG. 5 is a flowchart showing the movement to “optimal offset position 2”.

  The movement procedure will be described with reference to FIGS.

  When a movement command is issued, first, in step S11, an offset position is calculated.

  This calculation is simplified in the figure and will be described in detail below.

  The following formulas are sequentially calculated to calculate the offset position (FCD2, y2).

θ _w = atan (L _w / FDD) (1)
θ _of = atan (Lof / FDD) (2)
r ′ = FCD1 · sin (θ _w ) (8)
FCD2 = r '· cos (θof ) / sin (θ w + | θof |) ... (9)
y2 = FCD2 · tan (θof) (10)
Next, in step S12, only the shooting distance FCD is moved to FCD2.

  Next, in step S13, as a measurement of the deviation (mechanism error) of the rotation center C from the center line 30, the X-ray beam 2 is turned on, and the transmission data is collected while rotating the rotary table 5 once.

  Next, in step S14, a rotation center position dc2 on the transmission data is obtained from the transmission data by the rotation center obtaining unit 23.

  Next, in step S15, the movement amount Δy of y is calculated.

Δy = y2-dc2 · FCD2 / FDD (7)
Next, in step S16, by moving y by Δy, the mechanism error can be corrected and the rotation center C can be set to the offset position (FCD2, y2) on the offset line 31.

  As described above, except for the rotation center position finding step S14, the calculation and control (command) are respectively performed by the shift calculation / control unit 22. <End>.

<Optimal normal scan>
Next, a case where a cross-sectional image of the subject 4 is photographed in the optimal normal scan mode will be described.

  The operator places the subject 4 on the turntable 5 and inputs a command to the data processing unit 19 to cause the X-ray tube 1 to emit the X-ray beam 2 and display a transmission image of the subject 4. While displaying the moving image on the unit 20 as a real-time moving image, it is rotated by electric power, and the shooting distance FCD (at the position where a cross-sectional image is to be obtained) is exactly included in the field of view of the image at all rotational positions. And the detection distance FDD) to change the photographing magnification.

  At this time, the center of rotation slightly deviates from the center of the image, but usually does not occur so much as to hinder the adjustment of the shooting distance FCD.

  Next, the subject 4 is moved up and down to adjust the inspection position to the imaging surface 14.

  FIG. 6 is a geometric diagram showing the movement to the “optimum normal scan position”.

  This is a diagram of the photographing surface 14 viewed from above.

  When the adjustment of the photographing magnification is completed, the rotation center C is generally at a position C1 shifted from the center line 30.

  When the operator inputs a movement command to the “optimum normal scan position”, the shift calculation / control unit 22 of the data processing unit 19 has a normal scan area A2 having the same size as the normal scan area A1 at the current position. The amount of movement of the rotation center C located on the center line 30 to the optimum normal scan position C2 is calculated, and a command is issued to the mechanism control unit 18 to move the rotation.

  FIG. 7 is a flowchart showing the movement to the “optimal normal scan position”.

  The movement procedure will be described with reference to FIGS.

  When there is a movement command, first, in step S20, a deviation (mechanism error) of the rotation center C from the center line 30 is measured.

  The X-ray beam 2 is turned on, and the transmission data is collected while rotating the turntable 5 once.

  Next, in step S21, a rotation center position dc1 on the transmission data is obtained from the transmission data by the rotation center obtaining unit 23.

  The calculation of the rotation center position by the rotation center calculation unit 23 will be described later in detail.

  In step S22, the movement amount is calculated.

Δy = −dc1 · FCD1 / FDD (11)
ΔFCD = − | Δy | · FDD / L _w (12)
Next, in step S23, the movement of Δy and ΔFCD is performed.

  After the rotation of the rotary table 5, scanning is performed in the same manner as in the above-described "normal scan", and a cross-sectional image is reconstructed. <End>.

<Rotation center calculation 1>
Using a so-called sinogram in which the transmission data of the position of the imaging surface 14 of the transmission image of the subject 4 over 360 ° is arranged with the detection channel on the horizontal axis and the rotation angle on the vertical axis, the rotation center position on the sinogram is determined. Ask.

  The sinogram is described by the set angle θ of the detection channel (arrangement angle in the fan-shaped X-ray beam) and the rotation angle φ, and is obtained as transmission data at equal angular intervals of each angle.

  The basic principle of this rotation center calculation 1 is that “transmission data of X-ray paths in mutually opposite directions are almost the same”.

  In this rotation center calculation, a reverse transformation is used.

  FIG. 8 is a diagram showing an X-ray path for explaining the reverse conversion.

  This indicates an X-ray path viewed from the direction of the rotation axis 13 at coordinates fixed to the rotary table 5, and the focal point F of the X-ray beam 2 rotates.

  The reverse transformation 40 determines a reverse path (θr, φr) from one X-ray path (θ, φ).

  The reverse transform 40 is represented by the following equation.

θr−θ0 = − (θ−θ0)
That is,
θr = 2 · θ0−θ (13)
φr = φ + 180 ° -2 · (θ−θ0) (14)
Here, θ0 is (the set angle of) the rotation center.

  FIG. 9 is a diagram showing a sinogram P (θ, φ).

  In FIG. 9, when the virtual rotation center θ0 is set, the reverse path (θr, φr) can be calculated from (θ, φ).

  In order to obtain the rotation center, the sinogram value P is correlated with (θ, φ) in a predetermined area and each of the reverse paths (θr, φr).

  When θ0 is correctly set to the center of rotation, P (θ, φ) substantially matches P (θr, φr) by matching (opposite) the respective paths, and the correlation is improved.

  The center of rotation can be obtained by taking the correlation while changing θ0.

  The sinogram P (θ, φ) is usually obtained by performing transmission data up to logarithmic conversion, but may be data before logarithmic conversion or data at any stage of processing.

  This can be understood from the basic principle described above.

(Theta, phi) as the predetermined area of, typically, left and right narrow side (θ0~θ _w, all phi) of .theta.0 either the region .theta.0 center folded region (2 · θ0-θ _w a ~θ0 , All φ), but may be part of each.

  Since θ and φ of the sinogram P (θ, φ) are discrete values at regular intervals, they are specifically described as P (m, n).

  FIG. 10 is a diagram showing a sinogram P (m, n).

θ, φ and m, n are
θ = m · Δθ, (Δθ = θ _w / M) (15)
φ = n · Δφ, (Δφ = 360 ° / N) (16)
There is a relationship.

  The reverse transform 40 for obtaining the reverse path (mr, nr) from (m, n) can be obtained by using equations (13) to (16).

  FIG. 11 is a flowchart showing the algorithm of rotation center calculation 1.

  With reference to FIGS. 10 and 11, the rotation center calculation 1 will be specifically described.

  Step S30: Initialize θ0.

  Step S31: Reset the correlation value SOKAN.

  Step S32: Find a loop start value ms of m.

ms = INT (θ0 / Δθ) +1 (17)
Step S33: Enter a loop of m = ms to M.

  Step S34: Mr is obtained by reverse conversion.

  Here, mr is generally not a whole number but a real number.

θ = m · Δθ (15)
θr = 2 · θ0−θ (13)
mr = θr / Δθ (18)
Step S35: Enter a loop of n = 0 to N-1.

  Step S36: Find nr by reverse conversion.

  Here, nr is generally not a whole number but a real number.

φ = n · Δφ (16)
φr = φ + 180 ° -2 · (θ−θ0) (14)
nr = φr / Δφ (19)
Step S37: Since mr and nr are real numbers, the point (mr, nr) is different from the data point.

  Therefore, a value Pr (mr, nr) at this point is obtained from surrounding data by interpolation calculation.

  Step S38: Integrate the absolute value of the difference between the data values of the reverse paths into the correlation value.

SOKAN = SOKAN + ABS (Pr (mr, nr) -P (m, n))
… (20)
Here, instead of taking the absolute value here, it may be squared to find a value equivalent to the mean square error as the correlation value, but there is no significant difference in the final result (rotation center).

  Step S39: The loop of m and n is repeated.

  Step S40: Determine whether the SOKAN value is minimum.

  Step S41: If it is determined that the SOKAN value is not the minimum, θ0 is changed and the process returns to step S31.

  Step S42: If it is determined that the SOKAN value is the minimum, the process ends with θ0 or mc as the rotation center.

mc = θ0 / △ θ (21)
Here, the loop format for finding the minimum SOKAN value is described in a simplified manner for easy understanding.

  Actually, for example, the SOKAN value is obtained by changing θ0 in a certain step, an area of θ0 having a small SOKAN value is determined, the SOKAN value is calculated in fine steps, and the θ0 of the smallest SOKAN value is set as the rotation center. Is calculated.

  The flowchart shows only basic calculations, and in actuality, various processes for improving calculation accuracy may be added.

  For example, by applying a low-cut (frequency) filter to the sinogram P (m, n), the instability of the low-frequency component is reduced, and the accuracy can be improved.

  In addition, a weight depending on P (m, n) can be applied to ABS (Pr (mr, nr) -P (m, n)) by integrating the correlation values (Equation (20)).

  In this case, a small weight is used for a portion where absorption is too strong and the amount of transmitted X-ray beam is small (large P) or a portion where the subject 4 hardly stays and the amount of transmitted X-ray beam is too large (P small). The accuracy can be improved by using.

  In addition, various modifications are possible without regard to accuracy.

  For example, the SOKAN value may be obtained as an average value by dividing by the number of (m, n) points in the calculation.

  The “rotation center calculation 1” described here is directly correlated from the sinogram as compared with the “rotation center calculation 2” described later, so that the accuracy of the center calculation is good. Even in the case of an offset scan near the edge (although the area to be correlated becomes smaller), the center can be found well.

  In addition, even when the subject 4 is elongated and the absorption becomes extremely large in one direction, the center can be found well by lowering the weight of the area on the sinogram (P increases). There are advantages. <End>.

  Next, the interpolation method used in the above-described “rotation center calculation 1” will be described.

<Interpolation method>
Assuming that the data point interval is constant (= 1), the interpolation can be described by an interpolation function.

  FIG. 12 is a diagram illustrating an example of the interpolation function.

  FIG. 12 shows two types of interpolation functions gI and fI.

  gI is an interpolation function of “primary interpolation” that is often used, and is represented by the following equation.

gI (Δi) = 0 (when | Δi | ≧ 1)
= 1− | Δi | (when | Δi | <1) (22)
Here, Δi is i-ir, i is the position of the interpolation source (integer), and ir is the position of the interpolation destination (real number).

  The interpolation gI is interpolation from two points within ± 1 from the position ir of the interpolation destination.

  fI is a function that is rarely used, but is set as “COS interpolation” here.

  fI is represented by the following equation.

fI (Δi) = 0 (when | Δi | ≧ 1.5)
= (1 + COS (2π · Δi / 3)) / 3
(When | Δi | <1.5) (23)
Here, Δi is the same.

  This function has a raised cosine curve.

  The interpolation fI is interpolation from three points within ± 1.5 from the position ir of the interpolation destination.

  When Pr (mr, nr) is obtained by interpolation, two-dimensional interpolation is performed using any of the above functions vertically and horizontally.

First, when using the interpolation gI, the interpolation is performed by the following equation:
Pr (mr, nr) = {mΣn {gI (m-mr) .gI (n-nr) .P (m, n)}
… (24)
It is performed in.

  Here, Δm performs addition at m within ± 1 before and after mr, and Δn performs addition at n within ± 1 before and after nr.

  The interpolation is a four-point interpolation.

When using the interpolation fI, the interpolation is given by the following equation:
Pr (mr, nr) = {mΣn {fI (m-mr) .fI (n-nr) .P (m, n)}
… (25)
It is performed in.

  Here, Δm performs addition at m within ± 1.5 before and after mr, and Δn performs addition at n within ± 1.5 before and after nr.

  The interpolation is 9-point interpolation.

  Further, the interpolation function does not necessarily have to be the same function in the vertical and horizontal directions, and gI and fI may be mixed.

For example,
Pr (mr, nr) = {mΣn {fI (m-mr) .gI (n-nr) .P (m, n)}
… (26)
It may be.

  The interpolation is six-point interpolation.

  Normally, it is desirable to use the nine-point interpolation of the equation (25) in the “rotation center calculation 1”.

  This is to reduce the influence of noise included in the data.

  The details will be described below.

  Generally, data contains noise, and noise is reduced by averaging data at two points.

  In the case of using the primary interpolation, when the interpolation source data point and the interpolation destination data point match, the one point average is obtained. When the noise is not changed, and when the pitch is shifted by a half pitch, the two point average is obtained. The phenomenon of decreasing occurs.

  Depending on the degree of coincidence between the interpolation source point and the interpolation destination point (interpolation phase), the noise of the value obtained by interpolation changes.

[In the case of “rotation center calculation 1”, when θ0 is set to １ ／ and ／ of the data point, the interpolation is shifted by a half pitch, the noise of the interpolation value is reduced, and the correlation value is reduced. Therefore, there is a problem that the 1/4 and 3/4 positions are preferentially selected as the rotation centers. ]
On the other hand, if COS interpolation is used, the increase / decrease of noise is always uniform.

  This is proved as follows.

Assuming that the interpolation phase is x (-1/2 to 1/2) and the noise of the original data is σ, the converted noise σ ′ is a three-point average with weight,
σ ′ = σ · √ {(fI (x−1)) ² + (fI (x)) ² + (fI (x + 1)) ² } (27)
When the equation (23) is specifically substituted for fI and deformed, the right side becomes constant at σ / √2 regardless of x. <End of proof>.

  Here, even if the COS interpolation is not an accurate cosine curve, the same effect can be obtained by performing three-point interpolation using an approximate curve.

  [In the case of “rotation center calculation 1”, if COS interpolation is used, noise will not be uniform and the correlation value will not be reduced at the ４ and ／ positions, so a unit finer than the data pitch The center of rotation is required with high accuracy. ] <End>.

<Rotation center calculation 2>
This rotation center calculation 2 utilizes the fact that “transmission data obtained by adding 360 ° is bilaterally symmetric”.

  This is true at any stage of the data pre-processing.

  This is, for example, the rotation center calculation described in “Japanese Patent Application Laid-Open No. 2000-298105”. Here, a brief description will be given and new improvements will be described.

  FIGS. 13A and 13B are conceptual diagrams for explaining the algorithm of rotation center calculation 2.

  First, the sinogram P (m, n) is added (averaged) in the direction of the rotation angle φ, that is, n, to obtain average projection data (transmission data after logarithmic conversion is referred to as projection data) P (m).

  Next, a virtual rotation center m0 is set, the folded P '(m) is obtained, the absolute value of the difference from P (m) is added for m, a correlation value is obtained, and the correlation value is changed by changing m0. Is the rotation center mc.

  FIG. 14 is a flowchart showing the algorithm of the rotation center calculation 2.

  With reference to FIGS. 13 and 14, the rotation center calculation 2 will be specifically described.

  Step S50: Apply a low cut (frequency) filter to P (m).

Further, the above values predetermined value P _H replaced by P _H, the following values predetermined value P _L replaced by P _L (saturation processing). (This saturation process is equivalent to a weight hanger to 0 Waits correlation calculated by the following P value P _H or more and P _L.)
Step S51: Initialize m0.

  Step S52: Reset the correlation value SOKAN.

  Step S53: Find a loop start value ms of m.

ms = INT (m0) +1 (28)
Step S54: Enter a loop of m = ms to M.

  Step S55: Find a turning point mr.

  Here, mr is generally not a whole number but a real number.

mr = 2 · m0−m (29)
Step S56: The point mr is different from the data point (because it is a real number).

  Therefore, a value P '(m) at this point is obtained from surrounding data by interpolation calculation.

  COS interpolation is used for the interpolation.

The interpolation is performed by using a function fI as shown in Expression (23).
P ′ (m) = Σi (fI (i−mr) · P (i)) (30)
Perform in.

  Here, Σi is added at i within ± 1.5 before and after mr, and three-point interpolation is performed.

  Step S57: Integrate the absolute value of the difference between the data values of the reverse route into the correlation value.

SOKAN = SOKAN + ABS (P '(m) -P (m)) (31)
Here, instead of taking the absolute value here, it may be squared to find a value equivalent to the mean square error as the correlation value, but there is no significant difference in the final result (rotation center).

  Step S58: The loop of m is repeated.

  Step S59: It is determined whether the SOKAN value is minimum.

  Step S60: If it is determined that the SOKAN value is not the minimum, m0 is changed and the process returns to step S52.

  Step S61: If it is determined that the SOKAN value is the minimum, the processing is terminated with m0 as the rotation center.

mc = m0 (32)
Here, as in the case of the rotation center calculation 1 described above, the loop format for finding the minimum SOKAN value is described in a simplified manner for easy understanding.

  In addition, only the basic calculation is described as in the case of the rotation center calculation 1 described above.

  Various modifications are possible as in the case of the rotation center calculation 1 described above.

  New improvements from the rotation center calculation described in “JP-A-2000-298105” described above are as follows.

  First, in step S50, by applying a low cut filter to P (m), the instability of the low frequency component is reduced, and the accuracy can be improved.

  Second, COS interpolation is used in the interpolation calculation in step S56.

  When the primary interpolation is used, if the setting of m0 is 1/4 and 3/4 of the data point, the interpolation is shifted by a half pitch, the noise of the interpolation value is reduced, and the correlation value is reduced. There is a problem that the 1/4 and 3/4 positions are selected as the rotation centers.

  When COS interpolation is used, such a point is eliminated, the noise becomes homogenous, and the correlation value does not decrease at the 1/4 and 3/4 positions. Therefore, the accuracy is improved with a unit smaller than the data pitch. The center of rotation is required. <End>.

<Reverse transformation reconstruction>
Conventionally, as a reconstruction method of the offset scan, a fan-parallel transformation FBP method of rearranging parallel beams, performing filter correction, and performing back projection has been used.

  In the case of normal scanning, a direct FBP method is used in which a filter is corrected and back-projected with the fan beam unchanged.

  For this reason, in a computed tomography apparatus in which normal scan and offset scan are mixed, two types of reconstruction methods are mixed.

  By employing the “reverse transform reconstruction” described here for the offset scan, the reconstruction method can be directly used as one type of the FBP method.

  “Reverse transformation reconstruction” can be referred to as a sinogram reverse process + direct FBP method.

  FIG. 15 is a conceptual diagram for explaining the reverse processing of the sinogram.

  The area of m = -M to M is the original sinogram 45, the area of ms to M 'is the recalculation unit 46, and the area of -M to M' is the sinogram 47 after the reverse processing.

  A recalculation unit 46 is added to the original sinogram 45 (partially overwritten) to obtain a sinogram 47 after the reverse processing.

  In principle, by using the data of the reverse path (mr, nr) as the data of the uncollected X-ray path (m, n), a sinogram in the same range on the left and right around the rotation center mc is created. Processing.

  (Mr, nr) is calculated from (m, n) by the above-described reverse transformation 40.

  FIG. 16 is a flowchart showing the sinogram reverse process.

  Step S70: Using the rotation center mc (real number), a loop range between the set angle θ0 of the rotation center and m is obtained.

θ0 = mc · Δθ (33)
ms = INT (mc) +1 (34)
M ′ = INT (2 · mc + M) (35)
Step S71: Enter a loop from m = ms to M '.

  Step S72: mr is obtained by reverse conversion.

  Here, mr is generally not a whole number but a real number.

θ = m · Δθ (15)
θr = 2 · θ0−θ (13)
mr = θr / Δθ (18)
Step S73: Enter a loop of n = 0 to N-1.

  Step S74: nr is obtained by reverse conversion.

  Here, nr is generally not a whole number but a real number.

φ = n · Δθ (16)
φr = φ + 180 ° -2 · (θ−θ0) (14)
nr = φr / Δφ (19)
Step S75: Since mr and nr are real numbers, the point (mr, nr) is different from the data point.

  Therefore, a value Pr (mr, nr) at the point is obtained from surrounding data by interpolation calculation.

  Here, for the interpolation, the primary interpolation of the equation (24) is usually sufficient, but the COS interpolation of the equation (25) or the mixed interpolation of the equation (26) may be used.

  Step S76: Replace P (m, n).

P (m, n) = Pr (mr, nr) (36)
Step S77: The loop of m and n is repeated.

  Step S78: A sinogram is completed in the region of m = -M to M '.

  In the above-described calculation flow, the range of m = ms to M uses the original value P (m, n) or the average of P (m, n) and Pr (mr, nr). It is possible.

  By using the sinogram 47 after the reverse process and reconstructing by the normal direct FBP method, the larger one of the distances between the center of rotation and both edges of the X-ray beam can be set as the maximum sectional image radius. Thus, a cross-sectional image within the radius of the offset scan area can be reconstructed.

  By employing the “reverse transform reconstruction” as described above, even if the normal scan and the offset scan are mixed, the reconstruction method can be directly used as one type of the FBP method.

  This makes it possible to arrange the configuration (software) of the reconfiguration unit 24. <End>.

  As described above, the following effects can be obtained with the computed tomography apparatus according to the present embodiment.

  In the above-described “offset scan 1”, simply adjusting the imaging magnification in the normal scan state and setting the subject 4 to just fit in the scan area, the same scan area in which the subject 4 just fits is automatically set. Can be moved to “optimal offset position 1”.

  In the above-described “offset scan 2”, the imaging magnification is adjusted in the normal scan state, and the setting is made so that the subject 4 just fits in the scan area. It is possible to move to “optimal offset position 2” having a slightly larger scan area (earlier than offset scan 1).

  In the above-described offset scan, a transmission image can be obtained with a large imaging magnification, and thus a high-resolution cross-sectional image can be obtained.

  Thus, in the present embodiment, even in a computed tomography apparatus in which the X-ray geometry can be freely set, the geometry of the offset scan can be easily set, so that a high-resolution cross-sectional image can be easily obtained. Become.

  In the “optimal normal scan”, the photographing magnification is adjusted in the normal scan state, and the setting is made so that the subject 4 just fits in the scan area. Can be moved to the “optimal normal scan position” in which the shooting magnification is increased to the maximum.

  Further, in the “rotation center calculation 1”, the rotation center can be obtained with high accuracy from the transmission image itself of the subject 4 without using a special jig.

  Since the correlation is directly obtained from the sinogram as compared with the above-described “rotation center calculation 2”, the accuracy of the center calculation is good. In spite of the smaller size, the center search can be performed well.

  In addition, even when the subject 4 is elongated and the absorption becomes extremely large in one direction, the center can be found well by reducing the weight of the area on the sinogram (where P becomes large). .

  Further, in the “rotation center determination 2”, similarly, the rotation center can be accurately determined from the transmission image of the subject 4 without using a special jig.

  In this method, the center of rotation is determined by autocorrelation of the symmetry from the added transmission data (projection data) obtained by adding the sinogram in the rotation direction. In particular, the conventional method employs a low cut filter applied to the added transmission data. Since the three-point interpolation using the interpolation function of the cosine curve of the interpolation at the time of the correlation is adopted, it is possible to obtain the rotation center with high accuracy.

  In addition, since "reverse transform reconstruction" is adopted, even if the normal scan and the offset scan are mixed, the reconstruction method can be a direct FBP method.

  This makes it possible to arrange the configuration (software) of the reconfiguration unit 24.

(Modification)
(A) In the first embodiment, by simply switching the movement command, the rotary table 5 is automatically set to “optimal offset position 1”, “optimal offset position 2”, and “optimal normal scan position”. It is also possible to add a function of moving between the positions.

  (B) In the first embodiment, the rotary table 5 and the X-ray tube are automatically moved when moving to the “optimal offset position 1 (or 2)” or the “optimal normal scan position”. 1 or the subject 4 may interfere with the X-ray tube 1 or the like.

  Therefore, the shift calculation / control unit 22 is provided with an interference prevention function (software) to read the photographing distance FCD, the detection distance FDD, y, the table height h, the subject maximum radius ro (manual input), etc., and calculate. The mechanism can be controlled so as to stop when the interference condition is satisfied (immediately before) by the processing.

  (C) In the first embodiment, instead of incorporating the interference prevention function into the shift calculation / control unit 22, the shift calculation / control unit 22 moves toward the target position only while the operator keeps pressing the operation button “table move”. It is also possible to add a function that keeps moving.

  The operator presses this manual operation button while visually checking for interference (through the lead glass window of the X-ray shielding box).

  The shift calculation / control unit 22 stops the operation when the target position is reached, and displays the completion.

  This makes it possible to prevent interference that can flexibly cope with any case with a simple function.

  (D) In the first embodiment, step S9 and step S10 can be performed in “offset scan 1” instead of steps S4 to S8 in FIG.

In step S9, y2 ′ is obtained by adding a mechanism error correction to y2,
y2 ′ = y2-δy (37)
In step S10, the rotation center is directly moved to the offset position (FCD2, y2 ').

  Similarly, in “offset scan 2”, steps S17 and S18 can be performed instead of steps S12 to S16 in FIG.

  In step S17, y2 'is obtained by adding a mechanical error correction to y2 in the above equation (37), and in step S18, the rotation center is directly moved to the offset position (FCD2, y2').

  The mechanism error δy is obtained by the following “mechanism error δy calculation 1” or “mechanism error δy calculation 2”.

<Mechanism error δy calculation 1>
FIG. 17 is a diagram for explaining an example of the mechanism error δy calculation 1.

  This shows the positional relationship between the focal point F, the rotation center C, and the detection center D of the X-ray beam 2 on the imaging surface 14.

  The calculation of the mechanism error δy here is based on the premise that the points C and D move almost linearly when the shooting distance FCD and the detection distance FDD are changed.

  Coordinates XY are immovable coordinates, 50 is a movement locus of the rotation center C, and 51 is a movement locus of the detection center D.

  The direction connecting the focal point F of the X-ray beam 2 and the detection center D is the center line 30, that is, the x-axis, and the x-axis changes as the detection center D moves.

  In FIG. 17, the Y-axis direction is extended to emphasize the mechanism error δy.

When the coordinates of the rotation center C and the detection center D are (Yc, FCD) and (Yd, FDD), respectively, the movement trajectory 50 of the rotation center C and the movement trajectory 51 of the detection center D are expressed by equations, respectively.
Yc = Ac + Bc · FCD (38)
Yd = Ad + Bd · FDD (39)
It becomes.

  Ac, Bc, Ad, and Bd are constants.

The mechanism error δy, which is the deviation of the point C from the x-axis, is calculated using Yc and Yd.
δy = Yc−Yd · FCD / FDD (40)
It can be seen from FIG.

Substituting the above equations (38) and (39) into equation (40),
δy = Ac + (Bc−Bd) · FCD−Ad · FCD / FDD (41)
It becomes.

  By changing the name of the constant, the following equation is obtained as the mechanism error δy calculation equation.

δy = a + b · FCD + c · FCD / FDD (42)
If the constants a, b, and c are obtained in advance, the equation can be used to calculate the mechanism error δy from the photographing distance FCD and the detection distance FDD.

  Next, calibration for obtaining constants a, b, and c will be described.

  Since there are three unknowns, the constants a, b, and c can be obtained if the mechanism error δy is known for at least three combinations of the photographing distance FCD and the detection distance FDD.

  This three-point calibration is performed as follows.

  First, three points are measured with the y shift as the origin, that is, with the rotation center C aligned with the center line 30 (deviation from the mechanism error).

  (1) The rotation center position dc is measured at the FCD1 (C1) point and the FDD1 (D1 point), and the mechanism error δy is obtained by the following equation.

δy11 = dc11 · FCD1 / FDD1 (43)
(2) The rotation center position dc is measured at the FCD1 (C1) point and the FDD2 (D2 point), and the mechanism error δy is obtained by the following equation.

δy12 = dc12 · FCD1 / FDD2 (44)
(3) The rotation center position dc is measured at the FCD2 (C2) point and the FDD2 (D2 point), and the mechanism error δy is obtained by the following equation.

δy22 = dc22 · FCD2 / FDD2 (45)
Next, substituting each measured value into equation (42),
δy11 = a + b · FCD1 + c · FCD1 / FDD1 (46)
δy12 = a + b · FCD1 + c · FCD1 / FDD2 (47)
δy22 = a + b · FCD2 + c · FCD2 / FDD2 (48)
It becomes.

  Solving the simultaneous equations (46), (47) and (48), constants a, b and c are obtained.

By subtracting equation (47) from equation (46) and solving for constant c,
c = (δy11−δy12) / {FCD1 · (1 / FDD1-1 / FDD2)}
… (49)
It becomes.

By subtracting equation (48) from equation (47) above and solving for constant b,
b = (δy12−δy22) / (FCD1-FDD2) −c / FDD2
… (50)
It becomes.

From the above equation (47) × FCD2−the above equation (48) × FCD1, the constant a is solved.
a = (δy12 · FCD2-δy22 · FCD1) / (FCD2-FCD1)
… (51)
It becomes.

  As a result, the constants a, b, and c are obtained by using the above equations (43), (44), (45), (49), (50), and (51), and calibration is completed.

  As shown in FIG. 17, the points C1 and C2 are points close to F in the calibration.

  This is because the effect of the mechanism error δy becomes large when the photographing magnification is large, so that the calibration is performed with a weight in an arrangement where the photographing magnification is large.

  Thereby, even when the movement locus 50 of the point C has a slight deviation from the straight line, the influence can be reduced.

  The calibration can be performed at three or more points to improve the statistical accuracy. <End>.

<Mechanism error δy calculation 2>
In the calculation of the mechanism error δy, the mechanism error δy can be accurately calculated even when the movement trajectory of the point C and the point D deviates greatly from the straight line.

  FIG. 18 is a diagram illustrating an example of calibration of the mechanism error δy calculation 2.

  FIG. 18 shows a calibration point 53 based on a combination of the photographing distance FCD and the detection distance FDD.

  The photographing distance FCD is set to increase the number of calibration points at a position where the photographing magnification is large so as to increase the accuracy.

  The photographing distances FCD of the calibration points are arranged in ascending order, FCD0 (i) and FDD are also arranged in ascending order, and set as FDD0 (j).

Adjust the y shift to the origin, measure the rotation center position dc at each calibration point,
δy0 (i, j) = dcij · FCD0 (i) / FDD0 (j) (52)
Obtains δy0.

  FCD0 (i), FDD0 (j), and δy0 (i, j) are stored as error tables, and the calibration is completed.

  The above calibration can be performed fully automatically.

  FIG. 19 is a flowchart illustrating an example of the algorithm of the mechanism error δy calculation 2.

  This is to calculate the mechanism error δy in this arrangement from the photographing distance FCD and the detection distance FDD.

  Step S80: Find i satisfying FCD <FCDO (i) while changing i in the increasing direction.

  Step S81: A division ratio gi between i-1 and i is obtained.

gi = (FCD−FCD0 (i−1))
/ (FCD0 (i)-FCD0 (i-1))
… (53)
Step S82: Find j satisfying FDD <FDD0 (j) while changing j in the increasing direction.

  Step S83: The division ratio gi of j-1 and j is determined.

gj = (FDD-FDD0 (j-1)) / (FDD0 (j) -FDD0 (j-1))
… (54)
Step S84: δy is obtained by primary interpolation (four-point interpolation).

δy = {δy0 (i, j) · gi + δy0 (i−1, j) · (1−gi)} · gj
+ {Δy0 (i, j−1) · gi + δy0 (i−1, j−1) · (1−gi)} · (1−gj)
… (55)
Thereby, even when the movement locus of the point C and the point D deviates from the straight line, the mechanism error δy in this arrangement can be accurately calculated from the photographing distance FCD and the detection distance FDD.

  When the deviation from the straight line is large, the accuracy can be prevented from being lowered by increasing the number of calibration points. <End>.

  (E) In the above description, two contrasting methods have been described for calculating the mechanism error δy.

  One is correction using a trajectory as a straight line, and the other is one using interpolation.

  In addition to these, various modifications are possible. For example, the relationship of δy-FCD is considered as a straight line or a curve (polynomial), and the coefficient of the straight line or the curve is obtained for several points of the detection distance FDD. When calculating the mechanism error δy, there is a method in which a coefficient is interpolated with the value of the detection distance FDD and used.

  (F) In the “offset skin 1” and the “offset scan 2”, the offset line 31 is set as a single straight line, but may be a curved line or a broken line.

  This means that the value of Lof may be changed by the photographing distance FCD or the photographing magnification.

  However, in order to uniquely determine the offset position, the offset line needs to be determined such that the offset scan area radius r monotonically decreases when the shooting distance FCD is reduced.

(Second embodiment)
The computer tomography apparatus according to the present embodiment has the same hardware configuration as that of the first embodiment, and therefore, the same parts as those in FIG. Here, only the different parts will be described.

  That is, the computer tomography apparatus according to the present embodiment changes the shooting distance FCD and the detection distance FDD by the offset scan described above to the shift calculation / control unit 22, which is one of the functional blocks of the software in FIG. Then, when the adjustment is made so that the subject 4 just fits in the scan area, a function of controlling the turntable 5 so that the rotation center position (dc) does not shift is added.

  Next, the operation of the computed tomography apparatus according to the present embodiment configured as described above will be described.

  First, when capturing a cross-sectional image of the subject 4, the operator selects between the normal scan mode and the offset scan mode.

  The operator places the subject 4 on the turntable 5 and inputs a command to the data processing unit 19 to cause the X-ray tube 1 to emit the X-ray beam 2 and display a transmission image of the subject 4. The photographing magnification is adjusted by changing the photographing distance FCD (and the detection distance FDD) while displaying the real-time moving image on the unit 20.

  At this time, the shift calculation / control unit 22 of the data processing unit 19 controls the y shift so that the rotation center C is always on the center line 30 or the offset line 31 depending on the mode.

  This control will be described later in the section “Rotation center continuous setting”.

  In the case of the normal scan mode, the image pickup distance FCD (and the detection distance FDD) is rotated by electric power so that the transmission image (at the position where a cross-sectional image is to be obtained) is exactly within the field of view of the image at all rotation positions. ) Is set.

  In the case of the offset scan, the object is rotated by electric power, and the photographing distance FCD (at the position where a cross-sectional image is to be obtained) is set so that one side of the transmitted image (at the position where a cross-sectional image is to be obtained) just fits in the field of view of the image. And the detection distance FDD).

  Next, in the same manner as in the first embodiment, a normal scan or an offset scan is performed to obtain a cross-sectional image.

  Since the “continuous rotation center setting” is not sufficiently accurate for reconstruction, the rotation center is obtained from the transmission data of the position of the transmission surface 14 of the transmission image in the same manner as in the first embodiment. Later, reconstruction is performed using this.

<Rotation center continuous setting> (Operation according to the present embodiment)
FIG. 20 is a geometric diagram showing the state of “continuous rotation center setting”.

  This indicates the positional relationship between the focal point F, the rotation center C, the detection center D, and the like of the X-ray beam 2 on the imaging surface 14.

  "Continuous rotation center setting" means that when the operator changes the shooting distance FCD and the detection distance FDD manually or electrically, the shift center C is automatically set to the offset line 31 or the center line by the shift calculation / control unit 22. The y-shift is controlled so as to be above 30.

  FIG. 21 is a flowchart illustrating an example of the algorithm of “continuous rotation center setting” algorithm.

  Hereinafter, description will be made in the order of steps.

  Step S90: When the end of the continuous setting of the table position is input, the processing is ended.

  Step S91: The technique is divided according to the movement command of the photographing distance FCD or the detection distance FDD.

  Step S92: If there is a movement command, the shooting distance FCD and the detection distance FDD are moved.

  Step S93: Calculate the y-direction mechanism error δy at the photographing distance FCD and the detection distance FDD.

  This uses the aforementioned “mechanism error δy calculation 1”, “mechanism error δy calculation 2”, and the like.

  Step S94: The technique is divided according to the mode.

Step S95: In the case of the normal scan mode,
y = −δy (56)
Then, y is obtained.

Step S96: In the case of the offset scan mode,
y = Lof · FCD / FDD-δy (57)
Then, y is obtained.

  Step S97: Move to y and return to step S90.

  By moving y every fine movement of the photographing distance FCD and the detection distance FDD in the above flow, the mechanism error δy is corrected and the rotation center C is moved along the offset line 31 or the center line 30. Can be.

  Further, by switching the mode, the rotation center C can be switched between the offset line 31 and the center line 30. <End>.

  As described above, the following effects can be obtained with the computed tomography apparatus according to the present embodiment.

  In the present embodiment, when the photographing distance FCD and the detection distance FDD are changed by the offset scan so as to adjust the subject 4 to just fit in the scan area, the rotation center position (Lof) is calculated by the shift calculation / control unit 22. Since the rotary table 5 is controlled so as not to shift, the adjustment can be performed very easily.

  Similarly, in the case of the normal scan, even if the photographing distance FCD and the detection distance FDD are changed, the mechanism error δy is corrected, and the rotation table 5 is controlled so that the rotation center position does not deviate from the detection center D. As a result, the geometric setting can be performed without waste (the scan area can be maximized at the same shooting distance FCD).

  Further, the rotary table position can be easily switched between the center position and the offset position by the mode switching input.

  In the offset scan, a transmission image can be obtained at a large photographing magnification, so that a high-resolution cross-sectional image can be obtained.

  Thus, in the present embodiment, even in a computed tomography apparatus in which the X-ray geometry can be freely set, the geometry of the offset scan can be easily set, so that a high-resolution cross-sectional image can be easily obtained. Become.

  In addition, it is possible to obtain the same effects as in the case of the above-described first embodiment.

(Other embodiments)
It should be noted that the present invention is not limited to the above embodiments, and can be implemented in various modifications without departing from the spirit of the invention at the stage of implementation.
In addition, the above embodiments may be implemented in appropriate combinations as much as possible. In such a case, the combined effects can be obtained.
Furthermore, the above embodiments include inventions at various stages, and various inventions can be extracted by appropriately combining a plurality of disclosed constituent features.
For example, even if some components are deleted from all the components shown in the embodiments, at least one of the problems described in the section of the problem to be solved by the invention can be solved, and the effects of the invention can be solved. In the case where (at least one of) the effects described in the section is obtained, a configuration from which this component is deleted can be extracted as an invention.

FIG. 1 is a schematic diagram showing a first embodiment of a computed tomography apparatus according to the present invention. FIG. 4 is a geometric diagram showing movement to “optimal offset position 1” in the computer tomography apparatus of the first embodiment. 9 is a flowchart showing a movement to “optimal offset position 1” in the computer tomography apparatus of the first embodiment. FIG. 3 is a geometric diagram showing movement to “optimal offset position 2” in the computer tomography apparatus of the first embodiment. 5 is a flowchart showing a movement to an “optimal offset position 2” in the computer tomography apparatus of the first embodiment. FIG. 6 is a geometric diagram showing movement to the “optimal normal scan position” in the computer tomography apparatus of the first embodiment. 5 is a flowchart showing a movement to an “optimum normal scan position” in the computer tomography apparatus of the first embodiment. FIG. 3 is a diagram showing an X-ray path for explaining a reverse transformation in the computer tomography apparatus of the first embodiment. FIG. 3 is a diagram showing a sinogram P (θ, φ) in the computer tomography apparatus according to the first embodiment. FIG. 3 is a diagram showing a sinogram P (m, n) in the computer tomography apparatus according to the first embodiment. 4 is a flowchart showing an algorithm of rotation center calculation 1 in the computer tomography apparatus of the first embodiment. FIG. 3 is a diagram illustrating an example of an interpolation function in the computer tomography apparatus according to the first embodiment. FIG. 3 is a conceptual diagram for explaining an algorithm of rotation center calculation 2 in the computer tomography apparatus of the first embodiment. 9 is a flowchart showing an algorithm of rotation center calculation 2 in the computer tomography apparatus of the first embodiment. FIG. 3 is a conceptual diagram for explaining a sinogram reverse process in the computer tomography apparatus according to the first embodiment. 5 is a flowchart showing a sinogram reverse process in the computer tomography apparatus of the first embodiment. FIG. 11 is a diagram for explaining an example of a mechanical error δy calculation 1 in the computed tomography apparatus of the modification of the first embodiment. FIG. 9 is a diagram for explaining an example of calibration of a mechanical error δy calculation 2 in the computer tomography apparatus according to the modified example of the first embodiment. 9 is a flowchart illustrating an example of an algorithm of a mechanical error δy calculation 2 in the computer tomography apparatus according to the modification of the first embodiment. FIG. 14 is a geometric diagram showing a state of “continuous rotation center setting” in the computer tomography apparatus according to the second embodiment of the present invention. 9 is a flowchart illustrating an example of an algorithm of “continuous rotation center setting” in the computer tomography apparatus according to the second embodiment. FIG. 3 is a conceptual diagram illustrating an example of a scan area of a normal scan and an offset scan.

Explanation of reference numerals

DESCRIPTION OF SYMBOLS 1 ... X-ray tube, 2 ... X-ray beam, 3 ... Detector, 4 ... Subject, 5 ... Rotary table, 6 ... Rotating / elevating mechanism, 7 ... Y shift mechanism, 8 ... X shift mechanism, 9 ... Detector Support frame, 10: X-ray tube support frame, 13: rotating shaft, 14: imaging surface, 16: high voltage generator, 17: X-ray control unit, 18: mechanism control unit, 19: data processing unit, 20: display Unit, 21: scan control unit, 22: shift calculation / control unit, 23: rotation center finding unit, 24: reconstruction unit, 30: center line, 31: offset line, 40: reverse conversion, 45: original sinogram , 46: recalculation unit, 47: sinogram after reverse processing, 50: movement locus of rotation center C, 51: movement locus of detection center D, 53: calibration point, 101: X-ray tube, 102: radiographed X-ray beam , 103: detector, FCD: shooting distance, DD: detection distance, C: rotation center, Cn: rotation center, Cof: rotation center, D: detection center, F: focus of the X-ray beam 2, θ: set angle of the detection channel, φ: rotation angle of the detection channel, An: scan area (normal scan), Aof: scan area (offset scan), δy: mechanical error, L _w: effective half width of the detector 3, Lof: offset position, r: radius of the normal scan area, r ': mechanical error Radius, θ0: set angle of rotation center, h: table height, ro: maximum radius of subject 4, dc: rotation center position.

Claims (1)

An X-ray source that emits an X-ray beam;
A rotating table on which the subject is placed,
An X-ray detector for detecting a fan-shaped X-ray beam orthogonal to a rotation axis of at least the rotary table from the X-ray source transmitted through the subject,
A computer tomography apparatus for obtaining a cross-sectional image of the subject from a plurality of transmission data of the subject detected by the X-ray detector at a plurality of rotation positions of the rotary table,
On a sinogram created by the plurality of transmission data of the subject, an extended sinogram having a larger distance between the rotation center of the rotary table and both edges of the X-ray beam on both sides of the rotation center of the rotary table. Is set and the transmission data of the X-ray path in the opposite direction to the X-ray path of each point is adopted for at least the expanded area, thereby completing the expanded sinogram and reconstructing a cross-sectional image from the expanded sinogram. A computed tomography apparatus characterized by comprising structural means.

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