JP5697656B2 - X-ray imaging device - Google Patents

Description

  The present invention relates to alignment control of an X-ray focal position.

  In an X-ray imaging apparatus such as an X-ray CT apparatus, an X-ray tube used as an X-ray source accelerates thermoelectrons generated in a filament at a high voltage, as described in, for example, Patent Document 1, It converges at the focal point and collides with the rotating anode target to generate X-rays. Due to the heat generated at this time, the focal point becomes high temperature, the temperature of the rotating shaft or the like that supports the X-ray target rises, and thermal expansion occurs, and the focal position changes. After that, the rotation axis of the X-ray target is cooled and contracted by radiation and a cooler, and the focus position changes again. In many X-ray CT apparatuses, the direction of the rotation axis of the X-ray target in the X-ray tube is arranged so as to coincide with the direction of the rotation axis of the gantry rotating unit, and this direction is the slice direction of the X-ray detector. Matches. Therefore, when expansion or contraction of the X-ray target rotation axis or the like occurs, the X-ray focal point, that is, the X-ray irradiation range changes in the slice direction. Such a change in the X-ray irradiation range may cause image quality deterioration such as generation of artifacts in the reconstructed image and deterioration of quantitativeness.

  Therefore, in order to prevent the fluctuation of the X-ray irradiation range due to the focus movement as described above, for example, in Patent Document 2, the focus position detection X-ray irradiation (pre-exposure) is performed separately from subject measurement immediately before imaging. The position is detected, and the X-ray position incident on the X-ray detector is changed using this result. In addition, for example, as described in Patent Document 1 described above, the focal position at the next X-ray irradiation is estimated using the focal position measured by the focal position detector and the cooling characteristic data when scanning is performed. This moves the X-ray collimator and moves the irradiation range.

JP 2000-51209 A Japanese Patent Laid-Open No. 10-211199

  However, when pre-exposure as shown in Patent Document 2 is used, it takes time to perform pre-exposure after the start of imaging, and there is a problem that imaging cannot be performed immediately. This is a particular problem when taking an image at a required moment of a moving subject such as a contrast imaging of the heart.

  In addition, immediately before the X-ray irradiation as shown in Patent Document 1, the focal position is estimated by estimation from the position found by the previous X-ray irradiation, and when correcting, the position moves greatly compared to the previous time, etc. If it fluctuates out of the fluctuation amount characteristic of the focal point, accurate estimation cannot be performed, so that the focal position cannot be corrected to an appropriate position, and an artifact is generated on the tomographic image. In addition, it is necessary to obtain the function used for estimation of the cooling function and its parameters and store them in the apparatus in advance, so that data for deducing the focal position for each imaging condition can be obtained or In order to perform various controls according to the situation, a lot of hardware and software are required, which is complicated.

  The present invention has been made in view of the above-described problems, and allows the X-ray irradiation range to be appropriately moved with simple control, thereby allowing the X-ray focus to be moved without delaying the imaging timing. An object of the present invention is to provide an X-ray imaging apparatus capable of removing and suppressing image quality deterioration such as generation of artifacts and deterioration of quantitativeness.

  In order to achieve the above-described object, the present invention provides an X-ray source that irradiates X-rays, an X-ray detector that faces the X-ray source and converts the X-rays into an electrical signal, and the X-ray detector. An X-ray imaging apparatus comprising: a moving mechanism that is driven to move an irradiation range of incident X-rays; and a focal position detection unit that detects a focal position of the X-ray, and the focal position detection unit A first movement for moving the focal position to the ideal focal position based on the focal position detected by the step, and after the first movement, the moving mechanism is further moved at a constant speed until the focal position reaches a predetermined reference position. An X-ray imaging apparatus comprising: a second movement for driving; and a control means for controlling to perform the second movement.

  According to the present invention, it becomes possible to move the X-ray irradiation range to an appropriate position with simple control, thereby generating artifacts caused by moving the focal point of X-rays and reducing the quantitativeness without delaying the imaging timing. It is possible to provide an X-ray imaging apparatus capable of removing and suppressing image quality degradation such as the above.

Hardware block diagram of X-ray CT system 1 The figure which shows an example of the focus position detection apparatus 4 The figure which shows the relationship between the difference value of the output value from each detection elements 42 and 43 of the shift detector 40, and an X-ray focus position. Flowchart explaining the flow of irradiation range movement processing A graph showing the time variation 125 of the distance between the origin and the focal position due to cooling and the time variation 128 of the distance from the origin of the X-ray tube 2 due to the second movement (control virtual focal position) (the movement amount due to thermal expansion is the maximum) (When starting control from the position where A graph showing the time variation 125 of the distance between the origin and the focal position due to cooling and the time variation 128 of the distance from the origin of the X-ray tube 2 due to the second movement (control virtual focal position) (the amount of focal movement due to thermal expansion is (When control is started from a position that becomes L) Graph (thermal) showing an example 136 of the time change 135 of the distance between the origin and the focal position due to cooling (ideal focal position) and the time change of the distance from the origin of the X-ray tube 2 due to the second movement (control virtual focal position). (Specific example when starting control from the position where the focal point shift due to elongation becomes maximum) FIG. 7 is a graph showing the amount of deviation 141 (control virtual focus position−ideal focus position) of the control virtual focus position 136 with respect to the ideal focus position 135 of FIG. A graph showing the amount of deviation between the ideal focus position and the control virtual focus position with respect to the assumed focal movement amount L, where the curve 172 is the maximum deviation amount, and the curve 173 is the maximum deviation value (deviation in the second direction). The absolute value, the curve 174, shows the absolute value of the minimum deviation (deviation in the first direction). The graph shows the change in the maximum deviation amount between the ideal focal position and the control virtual focal position, and the curve 142 is the focal position moving speed V when the focal position moving speed V _const is V _{high. When const is set} to V _low For the assumed focus movement amount L, the deviation amount between the ideal focus position and the control virtual focus position is the second direction (absolute value 173 of the maximum deviation value) and the first direction (absolute value 174 of the minimum deviation value). A graph showing the amount of deviation when the moving speed V _const of the focal position is determined so that it becomes the same size at A graph for explaining the case where the reference position 151 of the control virtual focus position is set by shifting in the first direction by a predetermined distance A graph showing the shift amount between the virtual control focus position and the ideal focus position with respect to the assumed focus movement amount L, where the solid line 145 is based on the reference position 151 (F _conv = 0 μm), and the solid line 146 is the reference When the position 151 is shifted in the first direction by 20 μm from the origin (F _conv = 20 μm), the dotted line 147 indicates the magnitude of the deviation in the first direction, the magnitude of the deviation in the second direction, and the origin of the reference position. When the distance F _conv is equal This is a graph showing the magnitude 173 of the deviation in the second direction and the magnitude 174 of the deviation in the first direction for the assumed focal distance L, and the distance F _conv between the reference position and the origin position is ideal. When optimized to match the amount of deviation in the first direction and the amount of deviation in the second direction between the focal position and the control virtual focal position The figure explaining application of this invention with respect to another example of a focus position fluctuation | variation The magnitude of deviation in the second direction (absolute value of the maximum deviation value) 173 and the magnitude of deviation in the first direction (absolute value of the minimum deviation value) with respect to the assumed focal shift amount L in the case of FIG. ) Graph showing 174 Hardware configuration diagram of X-ray CT apparatus 1 equipped with collimator moving mechanism 302 17 is a cross-sectional view showing the positional relationship between the X-ray tube 2, the collimator 3, and the X-ray detector 5 at the position 327 in FIG. Hardware configuration diagram of X-ray CT apparatus 1 equipped with X-ray detector moving mechanism 200 19 is a cross-sectional view showing the positional relationship between the X-ray tube 2, the collimator 3, and the X-ray detector 5 at the position 327 in FIG. Sectional drawing which shows the positional relationship of the X-ray tube 2, the collimator 3, and the X-ray detector 5 in the case of moving both the collimator 3 and the X-ray detector 5 in order to change the X-ray irradiation range

  Hereinafter, an X-ray CT apparatus 1 according to the present invention will be described in detail with reference to the accompanying drawings.

  First, a configuration of an X-ray CT apparatus 1 that is an embodiment of an X-ray imaging apparatus according to the present invention will be described with reference to FIG.

  As shown in FIG. 1, the X-ray CT apparatus 1 includes an X-ray tube (X-ray source) 2, a collimator 3, a focal position detection device 4, an X-ray detector 5, a rotating body 7, a rotating body driving device 8, and a drive The transmission system 9, the signal collection device 12, the control device 10, the X-ray tube moving mechanism 11, the central processing device 20, the display device 21, the input device 22, and the bed 30 are configured.

  The X-ray tube 2 is an X-ray source and is controlled by the control device 10 to irradiate the subject 33 with X-rays continuously or intermittently. The control device 10 controls the X-ray tube voltage and the X-ray tube current applied or supplied to the X-ray tube 2 according to the X-ray tube voltage and the X-ray tube current determined by the central processing unit 20.

  As the X-ray tube 2, for example, a rotary anode X-ray tube is used. In this rotating anode X-ray tube, thermoelectrons emitted from the cathode collide with the target, and X-rays are irradiated with the collision point as the X-ray focal point. The target is supported by a rotating shaft, and this rotating shaft is provided so as to coincide with the slice direction. In the X-ray tube 2 configured as described above, when the target becomes high temperature, thermal expansion occurs in the rotating shaft, and the X-ray tube 2 contracts due to cooling. Thus, the X-ray tube focal position moves in the slice direction by the heat generated by the X-ray irradiation, and the X-ray irradiation range changes.

  The collimator 3 irradiates the subject 33 with X-rays radiated from the X-ray tube 2 as X-rays such as a cone beam (conical or pyramidal beam), and the aperture width is controlled by the control device 10 Is done. X-rays transmitted through the subject 33 enter the X-ray detector 5.

  The focal position detection device 4 is configured by combining a focal position measurement detector 40 (hereinafter referred to as a shift detector 40) and a focal position measurement slit 41, for example. These are provided between the X-ray tube 2 and the X-ray detector 5, for example.

  Further, for example, as shown in FIG. 2 (a), the shift detector 40 includes X-ray detection elements 42 and 43 for focus detection arranged in parallel on the X-ray irradiation surface, and these X-ray detection elements 42 and 43. The connector 44 for outputting the output signal to the central processing unit 20 is provided. The X-ray detection elements 42 and 43 are arranged in the slice direction.

  The slit 41 for measuring the focal position is a metal piece made of a metal having a large X-ray absorption such as tungsten, molybdenum, lead, or brass, for example, and is provided with a cut portion as shown in FIG. . The slit 41 is disposed between the shift detector 40 and the X-ray tube 2. When X-rays are irradiated, shadows 51 of the slits 41 are formed on the X-ray detection elements 42 and 43 of the shift detector 40 as shown in FIG. The position of the shadow 51 moves in the slice direction (the direction of the arrow 110 in FIG. 2) with the movement of the focal position. Since the output values of the detection elements 42 and 43 of the shift detector 40 are different depending on the position of the shadow 51, the central processing unit 20, for example, uses the difference value of the output values from the detection elements 42 and 43 of the shift detector 40. Can measure the amount of movement of the X-ray focal point.

  FIG. 3 shows an example of the relationship between the difference value between the output values from the detection elements 42 and 43 of the shift detector 40 and the X-ray focal position. In the example of FIG. 3, the focal position moves as a straight line 122 according to the difference value, and when the difference value is Sb, compared to the case where the difference value is Sa, the focal position is Fb in the slice direction. Shift.

  In the example shown in FIG. 1, the focal position detection device 4 is disposed in the vicinity of the X-ray tube 2, but is not limited thereto, and may be, for example, in the vicinity of the X-ray detector 5, or the X-ray source 2 and It may be between the X-ray detector 5 or a position farther from the X-ray source 2 than the X-ray detector 5. Further, for example, one or a plurality of X-ray detection elements provided at one or both ends in the channel direction of the X-ray detector 5 may be used as the focus position detection element. In this case, adjustment of the positional relationship between the shift detector 40 and the X-ray detector 5 is not necessary. Further, the focus position may be detected by using both the output from the shift detector 40 and the output from the focus position detection element provided in the X-ray detector 5.

  Returning to the description of FIG. The X-ray detector 5 is, for example, about 1000 X-ray detection element groups configured by a combination of a scintillator and a photodiode in the channel direction (circumferential direction), for example, 1 to 1 in the column direction (slice direction, body axis direction). About 320 are arranged and arranged to face the X-ray tube 2 with the subject 33 interposed therebetween. The X-ray detector 5 detects X-rays emitted from the X-ray tube 2 and transmitted through the subject 33, and outputs the detected transmitted X-ray data to the signal collection device 12.

  The signal collection device 12 is connected to the X-ray detector 5, collects transmission X-ray data detected by individual X-ray detection elements of the X-ray detector 5, and outputs it to the central processing unit 20.

  The central processing unit 20 includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and the like. The central processing unit 20 controls the control device 10 and controls the bed control device in the bed 30.

  Further, the central processing unit 20 acquires the transmission X-ray data collected by the signal acquisition device 12, performs image reconstruction processing based on the transmission X-ray data by the image reconstruction unit, and reconstructs a tomographic image.

  Further, the central processing unit 20 executes processing for moving the X-ray irradiation range to a predetermined position (irradiation range moving processing; see FIG. 4).

  The X-ray irradiation range is the range of the X-ray main shadow incident on the X-ray detector 5, and includes the position of the focal point of the X-ray tube 2, the position of the collimator 3, and the position of the X-ray detector 5. , Determined by the relative positional relationship. In the following description, the X-ray focal position or X-ray irradiation range will be described as a relative position with respect to the X-ray detector 5.

  In addition, the X-ray tube 2, the collimator 3, or the X-ray detector 5 can move the position (absolute position) in the X-ray CT apparatus 1 by a moving mechanism.

  In this embodiment, in order to move the X-ray irradiation range or the X-ray focal position, the positions of the X-ray detector 5 and the collimator 3 are fixed, and the absolute position of the X-ray tube 2 is moved by the X-ray tube moving mechanism 11. An example of moving is described. However, the present invention is not limited to this, and the X-ray irradiation range or the X-ray focal point may be moved by moving the position of the X-ray detector 5 or the position of the collimator 3. Details will be described later.

  Further, the position of the X-ray tube 2 where the center of the X-ray main shadow is incident on the center of the X-ray detector 5 in the state where the X-ray tube 2 is not thermally stretched is set as the origin.

  The X-ray tube 2 has a structure in which the position of the X-ray focal point is moved to one side in the slice direction by heat generated when X-rays are irradiated and moved in the opposite direction by cooling. Hereinafter, the direction in which the X-ray focal point moves due to thermal elongation is referred to as a first direction, and the direction in which the X-ray focal point moves due to cooling is referred to as a second direction.

  The X-ray tube moving mechanism 11, which is a mechanism for moving the X-ray irradiation range, is composed of, for example, a stepping motor or a hydraulic cylinder, and is driven and controlled by the control device 10 to move the absolute position of the X-ray tube 2.

  The control device 10 drives and controls the X-ray tube moving mechanism 11 according to the moving amount and moving speed calculated by the central processing unit 20, and moves the position of the X-ray tube 2 in the slice direction, thereby causing X-ray irradiation. Move the range. The control device 10 also functions as a means for acquiring the amount of movement of the X-ray tube moving mechanism 11. Specifically, for example, when the X-ray tube moving mechanism 11 is moved by a stepping motor and the movement amount and direction are determined by two types of forward and backward pulse numbers, the total number of each pulse number is recorded. deep. From this number of pulses, the amount of movement of the X-ray tube 2, that is, the amount of movement of the focal position in the control can be measured. The movement amount of the past X-ray tube moving mechanism 11 acquired in this way is regarded as the previous focal position, and is used as a virtual focal position when there is no X-ray irradiation.

  As a method for detecting the amount of movement of the X-ray tube moving mechanism 11, an optical distance measuring means such as a laser displacement meter may be used. In this case, the distance between the reference position of the X-ray tube moving mechanism 11 and the X-ray tube 2 may be measured, and the measured value may be sent to the central processing unit 20 as the amount of movement of the X-ray tube 2.

  An X-ray tube 2, a collimator 3, a focus position detection device 4, an X-ray detector 5, and a signal collection device 12 are mounted on the rotating body 7. The rotating body 7 is rotated by a driving force transmitted through the drive transmission system 9 from the rotating body driving device 8 controlled by the control device 10.

  The display device 21 includes a display device such as a liquid crystal panel and a CRT monitor, and a logic circuit for executing display processing in cooperation with the display device, and is connected to the central processing device 20. The display device 21 displays an image reconstructed by the central processing unit 20, imaging conditions to be set, various processing results, and the like.

  The input device 22 includes, for example, a keyboard, a pointing device such as a mouse, a numeric keypad, various switch buttons, and the like, and outputs various instructions and information input by the operator to the central processing unit 20. The operator interactively operates the X-ray CT apparatus 1 using the display device 21 and the input device 22.

  The bed 30 includes a top plate on which the subject 33 is placed, a bed control device, a vertical movement device, and a top plate driving device. The bed 30 controls the vertical movement device by control of a bed control device (not shown). Make the height of the appropriate. Further, the top plate driving device is controlled to move the top plate back and forth in the body axis direction, or to move in a direction perpendicular to the body axis and parallel to the top plate (left and right direction). Thereby, the subject 33 is carried into and out of the X-ray irradiation space.

  Next, the irradiation range moving process according to the present invention will be described with reference to FIG.

  The central processing unit 20 reads a program and data related to the irradiation range processing from a storage device (not shown), and executes processing based on the program and data.

  First, after the power of the X-ray CT apparatus 1 is turned on (step S101), the central processing unit 20 determines whether or not the immediately preceding X-ray irradiation is present (step S102). Here, the determination of the presence or absence of the immediately preceding X-ray irradiation is performed based on, for example, the time from when this process was performed to the current time, for example, X-rays within a predetermined time interval such as an interval of 1 second. The determination is based on the presence or absence of irradiation.

  If X-ray irradiation has been performed immediately before (step S102; present), the focus position is first detected by the focus position detection device 4 (step S103). Based on this result, the central processing unit 20 calculates a movement amount for the focal position to reach the ideal focal position (step S104). The ideal focal position is a position where the center of the X-ray main shadow is incident on the center of the X-ray detector 5. In this example, as a means for moving the irradiation range, the focal position is moved by moving the absolute position of the X-ray tube 2, and as a result, the irradiation range is moved.

The moving direction of the X-ray tube 2 is the opposite direction of the focal movement due to thermal elongation, that is, the second direction.
Specifically, when the detected focal position is Fb, the X-ray tube 2 is moved by −Fb.

  The central processing unit 20 sends the calculated movement amount and movement direction to the control device 10 and sends a movement instruction for the X-ray tube 2. The control device 10 moves the X-ray tube 2 to the ideal focal position according to the instruction from the central processing unit 20 (first movement; step S105). During the X-ray irradiation, the processes in steps S103 to S105 are repeated.

  On the other hand, when the X-ray irradiation is stopped and it is determined in step S102 that the X-ray irradiation is not performed (step S102; none), the central processing unit 20 uses the focal position detection device 4 to determine the focal position. Instead of this detection, the focal position is obtained from the absolute position of the X-ray tube 2 that has been cumulatively moved up to the time of the previous X-ray irradiation. Here, the focal position obtained from the absolute position of the X-ray tube 2 is referred to as “virtual focal position”. In the present embodiment, since the X-ray tube 2 is moved to move the focal position (X-ray irradiation range), the X-ray tube moving mechanism 11 actually sets the virtual focal position described above. Calculated from the amount by which the tube 2 is moved.

  The central processing unit 20 determines whether or not the acquired virtual focus position is deviated from a predetermined stop position (step S106). The stop position is a position where the X-ray tube 2 finally arrives by a second movement described later.

  Since various modes can be considered for the stop position, details will be described later, but for the sake of simplicity, the origin (the position where the focal point coincides with the center of the X-ray detector 5 in the absence of thermal expansion) is stopped here. Position.

  If the acquired virtual focus position is not at the stop position (step S106; Yes), the central processing unit 20 reaches the stop position at a constant moving speed (until the virtual control focus position described later reaches the reference position) X. The tube 2 is moved (second movement; step S107).

  Although the movement speed in the second movement will be described later, it is assumed that an appropriate value is obtained and held in advance.

  Normally, immediately after the X-ray irradiation is stopped, the X-ray tube 2 is thermally stretched by the heat generated by the previous X-ray irradiation, and in order to correct this, the X-ray tube 2 is moved in the direction to cancel the thermal expansion by the first movement. And adjusted to the ideal focal position. Further, in the subsequent second movement, the X-ray tube 2 is moved in the first direction in order to reduce the influence of the focal movement due to cooling over time (the movement of the control virtual focal position described later is the same direction as the cooling). Is).

  Here, the reference position is a position obtained by converting the stop position of the X-ray tube 2 described above into a focus position in a state where there is no thermal expansion. That is, when only the X-ray tube 2 is moved as in this example, if the stop position of the X-ray tube 2 is X, the focal position is -X, and therefore the reference position is -X.

  Since the X-ray tube 2 is stopped when the focal point reaches a certain reference position, the position of the X-ray tube 2 at which the focal point becomes the reference position is set as the stop position. A limiter detection means is provided at the stop position, and the central processing unit 20 determines whether or not the X-ray tube 2 has reached the stop position. If the X-ray tube 2 reaches the stop position, the movement is stopped assuming that the focal position has reached the reference position. To do. Further, without performing such a stop position determination process, for example, a stopper may be installed at the stop position, and the structure may move only to the stop position.

In this way, after the focal position (the focal position moved by control; the controlled virtual focal position) is moved to the ideal focal position and the fluctuation of the X-ray focal point due to the thermal elongation caused by the previous X-ray irradiation is corrected. Furthermore, it is moved in the same direction as the focal point movement accompanying cooling at a predetermined speed V _const . For this reason, the movement of the X-ray focal point due to thermal elongation and cooling is reduced, and defocusing from the ideal position can be always suppressed.

  With reference to FIGS. 5 and 6, an example of movement by cooling of the focal position and movement by control will be described.

  In the following description, the control virtual focus position is a focus position moved by control, and in this example, is the position of the origin viewed from the absolute position of the X-ray tube 2. That is, when the absolute position of the X-ray tube 2 is at the position −Fb, the control virtual focus position is Fb. When considering the control virtual focus position, it is assumed that there is no thermal expansion of the X-ray tube 2.

  In FIG. 5, the vertical axis indicates the focal position (the relative position between the focal point of the X-ray tube 2 and the X-ray detector 5), and the horizontal axis indicates the time after the X-ray irradiation is stopped. A solid line 125 indicates a temporal change in the focal position after the thermal expansion of the X-ray tube 2 is maximized, and a solid line 128 indicates a temporal change in the control virtual focal position realized by this control.

As shown in FIG. 5, the focal position is at point C when the maximum focal shift is caused by heat.
Thereafter, the focal position changes as shown by the solid line 125 by cooling.

On the other hand, in the focus control by the irradiation range moving process described above, the position of the X-ray tube 2 moved by the previous actual imaging (X-ray irradiation) in step S106 is detected as the virtual focus position, as shown by the solid line 128 in FIG. Then, after moving the control virtual focus position to the point C which is the ideal focus position, the control virtual focus position is moved to the reference position at a predetermined speed V _const . After the control virtual focus position reaches the reference position, the focus movement control is stopped.

That is, when the control virtual focus position is between the reference position and the focus position at the maximum thermal expansion, the focus position at the predetermined speed V _const is moved.

  If it is determined in step S106 that the acquired virtual focal position matches the reference position (step S106; No), no particular processing is performed. It is determined whether or not an instruction to turn off the power is input from the input device 22 (step S108). If there is no instruction to turn off the power (step S108; No), the process returns to step S102. If there is an instruction to turn off the power (step S108; Yes), the virtual control focus position is moved to the reference position (step S109), and then the power is turned off (step S110).

  In this way, when the power is turned off, the X-ray tube 2 is moved to a predetermined stop position so that the control virtual focal position is at a predetermined reference position. The positional deviation can be reduced.

Next, the moving speed V _const of the focal position by the second movement in step S107 will be specifically described.

  In the example of FIG. 5, when the control virtual focus position is on the solid line 125, that is, when the distance between the focus position changed by cooling and the origin coincides with the distance from the origin of the X-ray tube 2 by the second movement. The X-ray irradiation range becomes the ideal irradiation range.

Accordingly, it is desirable that the moving speed V _const of the focal position by the second movement is set so as to satisfy the relationship of the following expression (1).

ここで、
V_lowは、焦点位置が熱伸び最大時の位置Cから基準位置Ｄまで、冷却によって移動する場合の平均移動速さ(図5の点線127)であり、
V_highは、熱伸び最大時における冷却による単位時間当たりの焦点位置の変化量(図5の点線126)である。

here,
V _low is the average moving speed (dotted line 127 in FIG. 5) when the focal position moves from the position C when the thermal expansion is maximum to the reference position D by cooling.
V _high is the amount of change in the focal position per unit time due to cooling at the maximum thermal elongation (dotted line 126 in FIG. 5).

Also, as shown in FIG. 6, when performing the first movement and the second movement by the main irradiation range movement process in a state where the thermal elongation is not maximum, the central processing unit 20 first performs the control virtual focus by the first movement. The position is moved to the ideal focal position E, and then moved to the reference position at a predetermined speed V _const in the second movement thereafter.

  Next, focus control by irradiation range movement processing will be described using specific numerical values.

  For example, consider a case where the actual focal position moves to a maximum of 300 μm due to thermal elongation due to X-ray irradiation, and then the focal position F (t) moves according to equation (2).

ここで、F(t)の単位は［μm］であり、tはX線照射後の時間で単位は［分］とする。

Here, the unit of F (t) is [μm], t is the time after X-ray irradiation, and the unit is [minute].

  The focal point movement due to cooling in this example is as indicated by a dotted line 135 in FIG.

At this time, the speed V _const of the second movement may be set so as to satisfy the relational expression (1) V _low <V _const <V _high .

Specifically, when it is considered that the reference position has been reached when the focal point approaches a position about 10 μm away from the reference position, the distance 290 μm is moved in time 34 minutes. That is, V _low is 8.5 μm / min.

V _high is a differential value at time 0 in equation (2), and is 30 μm / min.

For example, when the value of the moving speed V _const of the focal position by the second movement is an average value of V _low and V _high , V _const is 19 μm / min. In this case, the virtual control focus position is a solid line 136 in FIG.

  A curve 141 shown in FIG. 8 shows a change over time in the amount of deviation of the control virtual focus position 136 from the ideal focus position 135 in FIG. The shift amount is calculated as control virtual focus position 136−ideal focus position 135.

  As shown in FIG. 8, the magnitude of the deviation amount between the ideal focal position 135 and the control virtual focal position 136 is a maximum value of 22 μm and a minimum value of −62 μm. That is, it is 22 μm in the direction (second direction) in which the focal position is moved by cooling, and 62 μm in the opposite first direction. That is, the maximum deviation from the ideal focal position is 62 μm.

  Further, when the shift amount between the ideal focal position and the control virtual focal position when the thermal expansion is not the maximum and the focal point is moved by the distance L, it is as shown in FIG. The vertical axis in FIG. 9 represents the deviation amount between the ideal focal position and the control virtual focal position, the horizontal axis represents the focal movement amount L, the curve 172 is the maximum deviation amount, and the curve 173 is the maximum deviation value (in the second direction). The absolute value of the deviation, and the curve 174 is the absolute value of the minimum value of the deviation (shift in the first direction). As shown in FIG. 9, the curve 172 and the curve 174 overlap. Thus, it can be seen that when the focal point movement amount is about 190 μm, the control virtual focal point position has a maximum deviation amount of about 70 μm from the ideal focal point position. Therefore, by performing the focus control of the present invention, it is possible to realize the accuracy that the maximum deviation amount is about 70 μm or less.

In the examples of FIGS. 7 to 9, the moving speed V _const of the focal position in the second movement is an average value of V _high and V _low , but the present invention is not limited to this. As the moving speed V _const of the focal position, any value between V _high and V _low may be used.

For example, when the movement speed V _const of the focal position is V _high , the change in the maximum value of the deviation amount between the ideal focal position and the control virtual focal position with respect to the focal movement amount L is as shown by a curve 142 in FIG. Also, for example, when the moving speed V _const of the focal position is set to V _low , the change in the maximum value of the deviation amount between the ideal focal position and the control virtual focal position with respect to the focal movement amount L is as shown by a curve 143 in FIG. .

According to Fig. 10, when the focus slide speed V _{const is set} to V _high , the control virtual focus is accurate with a maximum deviation amount of about 110 µm, and when V _low is set to an accuracy of about 103 µm. The position can be brought close to the ideal focus position.

In order to further improve accuracy, the deviation amount between the ideal focus position and the control virtual focus position in the range of the assumed focal movement amount L is the first direction (absolute value of the maximum deviation value) and the second direction. It is desirable to determine the moving speed V _const of the focal position so as to be the same size as (the absolute value of the minimum deviation).

  This is when the maximum absolute value 173 of the maximum deviation value and the maximum absolute value 174 of the minimum deviation value in FIG. 9 are the same.

The moving speed V _const of the focal position satisfying this is 14.2 μm / min in this example. FIG. 11 shows the absolute value 173 of the maximum deviation (deviation in the second direction) and the absolute value 174 (minimum deviation in the first direction) when the moving speed V _const is 14.2 μm / min. The maximum deviation amount in each direction is about 52 μm. Therefore, in this example, it is possible to control the control virtual focus position so as to be close to the ideal focus position with an accuracy of setting the maximum deviation amount to about 52 μm or less.

  Next, the reference position will be described.

  In the above example, for the sake of explanation, the reference position, which is the stop position of the control movement, is set to the origin (the X-ray tube when the center of the X-ray main shadow coincides with the center of the X-ray detector in the absence of thermal expansion). However, the accuracy can be further improved by providing the reference position (reference position of the control virtual focus position) at a position shifted by a predetermined distance in the direction of focus movement (first direction) due to thermal expansion. In this example, moving the reference position in the first direction means that the X-ray tube 2 moves in the second direction opposite to the first direction.

As shown in FIG. 12, the central processing unit 20 moves the control virtual focal point 136 at a predetermined moving speed V _const and then moves away from the origin by a predetermined distance in the first direction (the direction in which the focal point moves due to thermal elongation). It stops at the reference position 151.

Amount of deviation between the virtual control focus position and the ideal focus position when the reference position 151 moves in the direction in which the focus moves when the thermal extension is 0 μm or 20 μm from the origin and the moving speed V _const is 17 μm / min. Is shown in FIG. In FIG. 13, the solid line 145 indicates the maximum deviation when the reference position 151 (F _conv ) is the origin (0 μm), and the solid line 146 is the first direction by 20 μm from the origin to the reference position 151 (F _conv ). The maximum shift amount when shifted to is shown. The horizontal axis of the graph of FIG. 13 is the assumed focal shift amount L.

As can be seen from the solid line 145 in FIG. 13, the maximum deviation when the reference position (F _conv ) is the origin (0 μm) is about 60 μm. On the other hand, as shown by the solid line 146, when the reference position (F _conv ) is shifted in the first direction by 20 μm, the maximum shift amount can be improved to about 48 μm.

Further, in the range of the assumed focus movement amount L, the magnitude of the deviation between the control virtual focal position and the ideal focal position in the first direction, the magnitude of the deviation in the second direction, and the first from the origin of the reference position. It has been found that better results are obtained when the offset distance F _{conv in the} direction is equal.

In other words, this is when the magnitude of the maximum value of the deviation between the control virtual focus position and the ideal focus position is equal to the offset distance F _conv of the reference position and the minimum value of the deviation is −F _conv .

Thus, when the distance F _conv between the reference position and the origin position is optimized so as to match the deviation amount in the first direction and the deviation amount in the second direction between the ideal focus position and the virtual control focus position, For the assumed focus shift amount L, the magnitude of the deviation is as shown in FIG. In FIG. 14, a solid line 173 indicates the magnitude of displacement 173 in the second direction, and a solid line 174 indicates the magnitude of displacement 174 in the first direction. Further, a change in the maximum deviation amount at this time is shown by a dotted line 147 in FIG.

The moving speed V _const of the focal position by the second movement is 16.3 μm / min, and the reference position F _conv is 38 μm. As a result, as shown in FIG. 14, the maximum value of the deviation (size of deviation in the second direction) 173 and the size of the minimum value of deviation (size of deviation in the first direction) 174 It can be seen that the maximum value of each is 38 μm. This coincides with the offset value F _conv of the reference position. Therefore, as shown in FIG. 13, it can be seen that the control can be performed with the accuracy that the deviation amount of the focal position from the ideal focal position is about 38 μm or less with respect to the assumed focal movement amount L.

  As described above, the X-ray CT apparatus 1 of the present embodiment includes an X-ray source 2 that irradiates X-rays, and an X-ray detector 5 that faces the X-ray source 2 and converts X-rays into electrical signals. The central processing unit includes a moving mechanism 11 that is driven to move the irradiation range of the X-rays incident on the X-ray detector 5, and a focal position detection device 4 that detects the focal position of the X-rays. 20 is a first movement for moving the focal position to the ideal focal position based on the focal position detected by the focal position detection device 4, and a constant speed after reaching the predetermined stop position after the first movement. To perform the second movement for driving the movement mechanism.

  That is, after the focal position is moved to the ideal focal position by the first movement described above, the movement mechanism 11 is further moved to the second position to move the focal position, thereby reducing the influence of the movement of the focal position due to cooling. Further, since the second movement is realized by a simple control of movement at a predetermined constant speed, it is not necessary to predict the focal position.

  Therefore, the focus position can be moved to an appropriate position where the influence of cooling is reduced by simple control, and image quality deterioration such as generation of artifacts and deterioration of quantitativeness due to focus fluctuation can be removed and suppressed. . Furthermore, simplification of the control method makes it possible to simplify the hardware and software, and the amount of pre-data to be acquired can be reduced, so that the cost of the apparatus can be reduced and the development / production time can be shortened.

  Further, when the X-ray focal point moves in the first direction due to the thermal expansion of the X-ray tube 2 and moves in the second direction opposite to the first direction due to cooling, the stop position described above is the X-ray tube 2 It is desirable that the focal position when there is no thermal expansion is set at a position that is a predetermined distance away from the origin in the first direction. In this case, the amount of focus shift described above can be reduced, and accurate focus control is possible.

Furthermore, the distance F _conv between the reference position and the origin position, which is a position obtained by converting the stop position of the X-ray tube 2 to the focus position in a state where there is no thermal expansion, is the ideal focus position and the focus position moved by the second movement. When the setting is made so as to coincide with the shift amount in the first direction and the shift amount in the second direction with respect to the (virtual control focal position), the accuracy is further improved, which is preferable.

In addition, when the focal position moves from the position at the maximum thermal expansion to the reference position by cooling, the average moving speed is V _low , and the amount of change in the focal position per unit time due to cooling at the maximum thermal expansion is V _high . In this case, the moving speed V _const of the focal position by the second movement is set so as to satisfy the relationship of Expression (1) V _low <V _const <V _high .

  For this reason, the appropriate range of the moving speed of the X-ray tube moving mechanism 11 can be easily calculated.

Further, the focal position is moved by the second movement so that the deviation amount between the ideal focal position and the focal position moved by the second movement (control virtual focal position) matches in the first direction and the second direction. When the speed V _const is set, the above-described deviation amount can be reduced, and accurate focus control is possible.

  Further, the central processing means 20 determines the presence or absence of X-ray irradiation when detecting the focal position, and if it is determined that there is no X-ray irradiation, the central processing means 20 has been moved cumulatively until the immediately preceding X-ray irradiation. The current focal position is acquired from the moving amount of the moving mechanism.

  Therefore, it is not necessary to perform pre-exposure for detecting the focal position, and the movement control of the focal position can be started without missing a desired photographing timing.

  In the above-described embodiment, the case where the focal shift due to heat is in the slice direction has been described, but this is an example and does not limit the present invention. However, in the present invention, the control movement of the X-ray focal point is a movement in one direction of the slice direction regardless of the focal position. Therefore, even when the direction of control movement changes depending on the focal position in a direction other than the slicing direction, the fluctuation of one main factor is large compared to the other factors, and it can be regarded as a movement in one direction substantially. It is desirable to apply.

  Further, in the above-described embodiment, the case where the movement of the focal position by cooling follows the relationship shown in Expression (2) is shown, but the present invention is not limited to this. The expression indicating the moving position of the focal point may be various functions such as a polynomial. As an example of another function, for example, a function such as Expression (3) may be used.

ここで、a，bは定数で、tは時間(単位［分］)とする。

Here, a and b are constants, and t is time (unit [minute]).

The dotted line 148 in FIG. 15 shows the temporal change of the focal position when a = 300 μm and b = 10 minutes in the equation (3). At this time, if the moving speed V _const of the focal position by the second movement is 10.2 μm / min and the distance F _conv from the origin of the reference position is 51 μm, the control virtual focal position is as shown by a solid line 149. Further, the magnitude of the deviation in the second direction (absolute value of the maximum deviation) 173 and the magnitude of the deviation in the first direction (the minimum deviation) with respect to the focal shift amount L assumed in the case of FIG. (Absolute value) 174 is shown in FIG. From this result, it is possible to bring the control virtual focus position closer to the ideal focus position with an accuracy that the maximum deviation amount is about 51 μm.

Furthermore, instead of the relational expressions as shown in the above formulas (2) and (3), the focal position moving speed V _const and the reference position value F _conv may be obtained from the curve obtained by the measurement. Good.

  Further, a function F (t) representing the focus movement due to cooling may be obtained by calculation. For example, a standardized function G (t) is calculated from the cooling function of the temperature of the X-ray tube 2, and F (t) is determined as shown in Equation (4) using the actually measured maximum moving focal position A. Also good.

更に、最大移動焦点位置Aに、シミュレーション等で決定した値を用いてもよい。また関数F(t)が、その他のG(t)の関数であっても構わない。このように計算によって理想焦点位置を求める場合には、事前に推定用データを取得する必要が無く、手間、時間、及び工数が低減でき、開発・作製時間の短縮が可能となる。

Furthermore, a value determined by simulation or the like may be used for the maximum moving focal position A. The function F (t) may be another G (t) function. In this way, when the ideal focal position is obtained by calculation, it is not necessary to obtain estimation data in advance, and labor, time, and man-hours can be reduced, and development / production time can be shortened.

  In the above-described embodiment, the determination of the presence / absence of X-ray irradiation (step S102 in FIG. 4) is performed every predetermined time. However, the present invention is not limited to this. You may go to Furthermore, the time until the next determination (step S102) may be changed according to the presence or absence of X-ray irradiation and the amount of movement of the X-ray tube 2.

  In the above-described embodiment, the example in which the focal position is controlled and moved by moving the X-ray tube 2 by the X-ray tube moving mechanism 11 is shown, but the present invention is not limited to this. For example, in the case where the X-ray tube 2 is a mechanism that generates an X-ray using an electron beam, the focal position may be moved by an electric field, a magnetic field, or the like as in the flying focus technique.

  Further, in the above-described embodiment, the case where the X-ray tube 2 has one focal point is exemplified, but the present invention is not limited to this and may have a plurality of focal points. In this case, the plurality of focal points may be moved in the same manner as in the above embodiment, or the respective focal points may be moved separately. When the X-ray tube 2 is moved as in the present embodiment, each focal point moves in the same manner.For example, when the focal point position is moved by an electric field, a magnetic field, or the like, the electric field or magnetic field is separately applied to each focal point. By generating the above, the same movement or different movements are possible.

  In the above-described embodiment, the X-ray tube 2 is moved to move the X-ray irradiation range, but the position of the collimator 3 may be moved.

  An example of moving the X-ray irradiation range by moving the collimator 3 will be described with reference to FIG. 17 and FIG.

  In this case, as shown in FIG. 17, the X-ray CT apparatus 1 shown in FIG. The X-ray tube moving mechanism 11 in FIG. 1 is not necessary.

  The collimator moving mechanism 302 is driven and controlled by the control means 10.

  Also in this case, the focal position is moved in the same manner as the irradiation range moving process in FIG. However, the steps (S105, S107) corresponding to “X-ray tube movement” are replaced with movement of the collimator 3.

  Further, the position Y of the collimator 3 is controlled so as to satisfy the relationship of the following formula (5) with the position X of the X-ray tube 2.

ここでX線管2の焦点位置Xとコリメータ3の位置Yは、原点を基準とした位置である。

Here, the focal position X of the X-ray tube 2 and the position Y of the collimator 3 are positions based on the origin.

  In Equation (5), T is the distance from the X-ray detector 5 to the focal position of the X-ray tube 2, and S is the distance from the X-ray detector 5 to the collimator 3 (see FIG. 18). ).

  18 is a cross-sectional view showing the positional relationship between the X-ray tube 2, the collimator 3, and the X-ray detector 5 at the position 327 in FIG. A region indicated by hatching in FIG. 18 is an X-ray irradiation region. 18 (a) shows the positional relationship before moving the focal point, and the straight line 328 is a straight line connecting the focal point position 333-1 and the slice center of the X-ray detector 5 when the X-ray tube 2 is not thermally stretched. is there. FIG. 18B shows the positional relationship after moving the focus.

  The focal position 333-2 after the focal movement is at a position away from the straight line 328 by a distance 326 with respect to the slice direction 107. When the X-ray tube 2 is moved by the X-ray tube moving mechanism 11 as in the above-described embodiment, the X-ray tube 2 is moved in the direction opposite to the moving direction of the focal point 333-2. -X.

  In FIG. 18 (b), in order to obtain the ideal focal position similar to that in FIG. 18 (a), the slice center of the X-ray detector 5 in the actual irradiation region may be set to the position 334, and the focal position X If the collimator 3 is moved by a distance multiplied by the difference in magnification due to the difference in the focal distance T between the X-ray detector 5 and the X-ray tube 2 and the distance S from the X-ray detector 5 to the collimator 3, It will be good.

  Further, as shown in Expression (5), the position Y and the position X are in opposite directions. Accordingly, the collimator 3 is moved in a direction opposite to the direction in which the X-ray tube 2 is moved in the above-described embodiment. That is, in the second movement, the collimator 2 may be moved in the second direction that is the same as the movement direction due to the cooling of the focal point.

  Further, the position of the X-ray detector 5 may be moved in order to move the X-ray irradiation range.

  An example in which the X-ray irradiation range is moved by moving the X-ray detector 5 will be described with reference to FIGS.

  In this case, as shown in FIG. 19, the X-ray CT apparatus 1 shown in FIG. The X-ray tube moving mechanism 11 in FIG. 1 is not necessary.

  The X-ray detector moving mechanism 200 is driven and controlled by the control means 10.

  Also in this case, the focal position is moved in the same manner as the irradiation range moving process in FIG. However, the step (S105, S107) corresponding to “movement of the X-ray tube” is replaced with movement of the X-ray detector 5.

  Further, the position Z of the X-ray detector 5 is controlled so as to satisfy the relationship of the following formula (6) with the position X of the X-ray tube 2.

ここで、X線管2の焦点位置XとX線検出器5の位置Zは、原点を基準とした位置である。

Here, the focal position X of the X-ray tube 2 and the position Z of the X-ray detector 5 are positions based on the origin.

  In Equation (6), T is the distance from the X-ray detector 5 to the focal position of the X-ray tube 2, and S is the distance from the X-ray detector 5 to the collimator 3 (see FIG. 20). ).

  20 is a cross-sectional view showing the positional relationship between the X-ray tube 2, the collimator 3, and the X-ray detector 5 at the position 327 in FIG. An area indicated by hatching in FIG. 20 is an X-ray irradiation area. FIG. 20 (a) shows the positional relationship before moving the focal point, and the straight line 328 is a straight line connecting the focal point position 333-1 and the slice center of the X-ray detector 5 in a state where the X-ray tube 2 is not thermally stretched. is there. FIG. 20B shows the positional relationship after moving the focus.

  In FIG. 20 (b), in order to obtain the ideal focal position similar to that in FIG. 20 (a), the slice center position 334 of the X-ray detector 5 in the actual irradiation region is a point 337 where the straight line 328 and the collimator 3 intersect. On the other hand, the position may be symmetrical with respect to the focal point 333-2. Therefore, the X-ray detector is the distance obtained by multiplying the focal position X by the ratio of the distance S from the collimator 3 to the X-ray detector 5 and the distance (TS) from the collimator 3 to the focal position 333 of the X-ray tube 2. Move 5 to do that. That is, the X-ray detector 5 may be moved to the position Z that satisfies the equation (6).

  Further, as shown in Expression (6), the position Z and the position X are in the same direction. Therefore, the X-ray detector 5 is moved in the same direction as the direction in which the X-ray tube 2 is moved in the above-described embodiment. That is, in the second movement, the X-ray detector 5 may be moved in the same first direction as the direction in which the focal point moves due to thermal elongation.

  Further, the focal point may be moved by combining any two or all of the X-ray tube 2, the collimator 3, and the X-ray detector 5.

  For example, in FIG. 21, both the collimator 3 and the X-ray detector 5 are moved to change the X-ray irradiation range. At this time, the position Y of the collimator 3 and the position Z of the X-ray detector 5 need only be moved by the same position as the focal position X, and the moving direction is also the same. Therefore, the same amount of movement as in the example of moving the X-ray tube 2 may be performed in the opposite direction.

  Thus, when both the collimator 3 and the X-ray detector 5 are moved corresponding to the movement of the focal point, it is possible to prevent the X-rays from entering the X-ray detector 5 at a large angle. It is possible to suppress the generation of artifacts and the deterioration of quantitativeness that occur when X-rays are incident on the X-ray detector obliquely.

  Although the preferred embodiments of the X-ray imaging apparatus according to the present invention have been described above, the present invention is not limited to the above-described embodiments. For example, non-destructive CT equipment, X-ray cone beam CT equipment, dual energy CT equipment, X-ray imaging equipment, X-ray imaging equipment, X-ray fluoroscopy equipment, mammography, digital subtraction equipment, nuclear medicine screening equipment, radiation The present invention may be applied to a treatment apparatus or the like.

In addition, it is obvious for those skilled in the art that various changes or modifications can be conceived within the scope of the technical idea disclosed in the present application, and these naturally belong to the technical scope of the present invention. It is understood. For example, in the second movement, not only the movement to the reference position but at a predetermined speed V _const may be performed at two different speeds V _const1 and V _const2 . In this case, when a V _const1> V _const2, it is preferable to move V _const2 after moving at V _const1.

  1 X-ray CT device, 2 X-ray tube, 3 Collimator, 4 Focus position detection device, 40 Shift detector, 41 Focus position detection slit, 42, 43 Focus position detection X-ray detection element, 44 connector, 5 X Line detector, 7 rotating body, 8 rotating body drive device, 9 drive transmission system, 10 control device, 11 X-ray tube moving mechanism, 12 signal acquisition device, 20 central processing unit, 21 display device, 22 operation device, 30 bed , 33 subjects

Claims (8)

To move the X-ray source that irradiates the X-ray, the X-ray detector that opposes the X-ray source and converts the X-ray into an electrical signal, and the irradiation range of the X-ray incident on the X-ray detector An X-ray imaging device comprising a driven moving mechanism,
A focal position detecting means for detecting a focal position of the X-ray;
Based on the focal position detected by the focal position detecting means, a first movement for moving the focal position to the ideal focal position, and after the first movement, the movement at a constant speed until reaching a predetermined stop position. Control means for controlling to perform the second movement for moving the mechanism;
An X-ray imaging apparatus comprising: The origin is the ideal focal position in a state where there is no thermal expansion of the X-ray source, the focal position moves substantially in the first direction due to thermal expansion of the X-ray source, and the first direction opposite to the first direction due to cooling. When it can be considered to move in two directions
2. The X-ray imaging apparatus according to claim 1, wherein the stop position is set at a position away from the origin by a predetermined distance in the first direction. The distance between the stop position and the origin position is
The ideal focal position and the focal position moved by the second movement are set so as to coincide with both the deviation amount in the first direction and the deviation amount in the second direction or the larger deviation amount. The X-ray imaging apparatus according to claim 2, wherein: When the focal position moves from the maximum thermal expansion position to the stop position by cooling, the average moving speed is V _low ,
When the amount of change of the focal position per unit time due to cooling at the maximum thermal elongation is V _high ,
The moving speed V _const of the focal position by the second movement is
V _low <V _const <V _high
2. The X-ray imaging apparatus according to claim 1, wherein a moving speed of the focal position by the second movement is set so as to satisfy the relationship. The moving speed V _const of the focal position by the second movement is set so that the deviation amount between the ideal focal position and the focal position moved by the second movement matches in the first direction and the second direction. 5. The X-ray imaging apparatus according to claim 4, wherein A determination means for determining the presence or absence of X-ray irradiation is provided,
The control means includes
When the determination means determines that there is X-ray irradiation, the first movement is performed, and when the determination means determines that there is no X-ray irradiation, the second movement is controlled. 2. The X-ray imaging apparatus according to claim 1, wherein:   The moving means moves in combination of at least one position or a plurality of positions among the X-ray tube, the X-ray detector, or a collimator that limits the X-ray irradiation range irradiated from the X-ray tube. 2. The X-ray imaging apparatus according to claim 1, wherein the focal position is moved accordingly.   2. The X-ray imaging apparatus according to claim 1, wherein the X-ray imaging apparatus is an X-ray CT apparatus.

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