JPH0515523A - X-ray computer tomography system - Google Patents

Abstract

(57) [Summary] [Object] An object of the present invention is to provide an X-ray CT apparatus capable of suppressing the occurrence of shower-like artifacts by sufficiently suppressing the vibration of the rotating gantry during data collection. The present invention collects multi-directional projection data of a subject P while a rotary gantry 1 having at least one of an X-ray tube 11 and an X-ray detector 12 rotates around the subject P at a predetermined angular velocity. In an X-ray computed tomography apparatus that obtains a tomographic image based on the multidirectional projection data, the angular acceleration of the rotary gantry during the first half of the acceleration period is increased with time, and the angle of the rotary gantry during the second half of the acceleration period is increased. A memory 3 storing a control signal for controlling the rotation of the rotary gantry so as to reduce the acceleration with time, and a servo motor 7 for rotating the rotary gantry 1 according to the control signal. And

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to both the X-ray source and the X-ray detector, or at least one of the X-ray source and the X-ray detector while rotating around the subject. The present invention relates to an X-ray computed tomography apparatus (hereinafter referred to as “X-ray CT apparatus”) of a type that collects projection data.

[0002]

2. Description of the Related Art An X-ray CT apparatus is an example of an apparatus for obtaining a tomographic image of a subject. This X-ray CT apparatus is classified according to its scanning type. Currently used X-ray CT apparatuses are so-called third-generation X-ray CT apparatuses in which an X-ray source and an X-ray detector rotate together, and so-called fourth-generation X-ray CT apparatuses in which only the X-ray source rotates. This is a generation X-ray CT system. The X-ray CT apparatus of the third generation has an X-ray source for blasting fan-shaped fan beam X-rays and a plurality of X-ray detection elements for detecting the X-rays arranged in parallel according to the spread width of the fan beam X-rays. And a multi-channel type X-ray detector composed of the X-ray detector and the X-ray detector, and the X-ray source and the X-ray detector are integrated and rotated at a predetermined angular velocity in a predetermined direction around the position of the test object. After collecting the X-ray projection data of the subject in multiple directions by performing X-ray bombardment at predetermined angles, image processing such as reconstruction processing is performed based on the multi-direction X-ray projection data to obtain a tomographic image. Obtainable.

However, various artifacts (pseudo images) such as a ring shape and a shower shape may occur in the tomographic image. One of the causes of generating shower-like artifacts among these artifacts is "vibration of the rotary mount". Most of the vibration of the rotary gantry occurs during the acceleration period and the deceleration period of the rotary gantry.

By the way, the rotary mount is sufficiently heavy by itself, but since it is equipped with an X-ray source and an X-ray detector, it is considerably heavy and its moment of inertia is very large. Therefore, vibration is generated during acceleration accompanied by abrupt acceleration or during deceleration accompanied by abrupt deceleration. Hereinafter, the generation of vibration of the rotary gantry will be described based on the relationship between the angular velocity change and the vibration level change during acceleration of the rotary gantry.

FIG. 5 is a diagram showing a displacement of a control voltage applied to a servo motor for rotationally driving the rotary gantry during a period from when the rotary gantry starts to rotate to when a predetermined angular velocity is reached. Thus, the control voltage is linearly increased in proportion to the passage of time. FIG. 6 is a diagram showing a change in the actual angular velocity of the rotary mount when the control voltage shown in FIG. 5 is applied. Here, the alternate long and short dash line indicates the change in the vibration level of the rotary mount caused by the change in the actual angular velocity of the rotary mount. As shown in FIG. 6, the vibration level of the rotary gantry increases when the rotary gantry is started and when a predetermined angular velocity is reached. The vibration level increased at the time of start-up and when reaching a predetermined angular velocity is not sufficiently small, although it is slightly attenuated, until the projection data acquisition period, and shower-like artifacts are generated.

When it is necessary to change the rotation direction of the rotary gantry for every predetermined number of rotations, the period from the predetermined angular velocity until the rotation is stopped once, and from the stopped state, the rotation is accelerated again for rotation in the other direction, and the rotation is fixed. In some cases, the vibrations generated during the period until reaching the angular velocity of ## EQU1 ## synergistically strengthen each other, and in that case, shower-like artifacts are strongly generated.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide an X-ray CT apparatus capable of suppressing the occurrence of shower-like artifacts by sufficiently suppressing the vibration of the rotary mount during data collection.

[0008]

In an X-ray CT apparatus according to the present invention, a rotary mount equipped with at least one of an X-ray tube and an X-ray detector is rotated around a subject at a predetermined angular velocity, and In an X-ray computed tomography apparatus that collects multi-directional projection data of a specimen and obtains a tomographic image based on the multi-directional projection data,

The angular acceleration of the rotary mount increases from the time of start-up to the first predetermined time with the passage of time, and the rotary mount from the second predetermined time after the first predetermined time to the time when the predetermined angular velocity is reached. A memory for storing a control signal for controlling the rotation of the rotary mount so as to decrease the angular acceleration of the rotary mount over time, and a unit for rotating the rotary mount according to the control signal. To do.

[0010]

According to the present invention, when the angular acceleration of the rotary gantry increases from the start up to the first predetermined time with the passage of time, and the predetermined angular velocity is reached from the second predetermined time after the first predetermined time. By controlling the rotation of the rotary gantry so that the angular acceleration of the rotary gantry decreases with time, it is possible to sufficiently suppress the vibration that occurs when the angular velocity of the rotary gantry changes. Vibration can be suppressed sufficiently.

[0011]

Embodiments will be described below with reference to the drawings.

FIG. 1 is a diagram showing a configuration of a main part to which the present invention is applied in an X-ray CT apparatus according to an embodiment. The X-ray source 11 bombards the subject P with a fan-shaped fan beam X-ray FX by a high voltage applied from a high-voltage generator (not shown). This bombardment is continuous bombardment or intermittent bombardment, and the timing of this bombardment is controlled by a system control unit (not shown) that controls the entire system. The multi-channel type X-ray detector 12 having a plurality of X-ray detection elements is
X-rays that have been bombarded from the radiation source 11 and transmitted through the subject P are detected, converted into projection data that is an electrical signal, and output to a reconstruction unit (not shown). The X-ray source 11 and the X-ray detector 12 are provided opposite to each other on the rotary mount 1, and integrally rotate in the direction of arrow a around the position of the subject P. Where X
The case where the radiation source 11 and the X-ray detector 12 are fixedly provided on the rotary gantry 1 and the case where the X-ray source 11 and the X-ray detector 12 maintain a predetermined distance from each other There are cases where the relative position is changed. The latter is generally called a shift mechanism and is smaller than the subject P.
When X'is imaged, X-rays spread too much, and there is an X-ray detection element that does not detect projection data, and the resolution cannot be utilized to the maximum extent. As a solution to this, the X-ray source 11 and the X-ray detector 12 are shifted in the direction of arrow c, and their positions are changed to the X-ray source 11 'and the X-ray detector 12' shown by the dotted lines. Then, the spread width of the fan beam X-rays FX radiated from the X-ray source 11 'can be matched with the size of the subject P', and all X-rays can be bombarded on the subject P ',
As a result, it is possible to obtain a clear tomographic image by making full use of the resolution of a small object to be imaged.
However, when the shift position is changed by this shift mechanism, the weight balance of the rotary gantry changes, the weight balance and the center of rotation do not match, and the vibration of the rotary gantry increases. X in FIG.
The shift position of the radiation source 11 and the X-ray detector 12 is shifted to A
Then, the shift position of the X-ray source 11 'and the X-ray detector 12' is referred to as shift B. The servomotor 7 rotates at a rotation speed according to the applied control voltage, and rotationally drives the rotary mount 1 using the belt 8 as a power transmission medium.

The console 5 has a shift position before the start of photographing.
It has various operation switches to set imaging conditions such as slice thickness, scan time, and angular velocity during data collection (however, the angular velocity during data collection may be automatically set depending on the scan time setting). The shift position information is output to the shift control unit 6, the shift position information and the angular velocity information are output to the rotation control unit 2, and the slice thickness information is output to a bed driving unit (not shown). The shift control unit 6 uses the X-ray source 11 and the X-ray detector 12 based on the shift position information input from the console 5.
To move. Here, the shift position may be set by previously storing some preset values and selecting a desired preset value from the preset values, or the operator may arbitrarily move the shift position to set the shift position. , Usually the former is often the case. The angular velocity is also set by selecting it from some preset values set in advance.

The memory 3 is composed of the angular velocity at every minute time from the time of starting and the angular velocity at the time of collecting the data so that the most vibration does not occur in the acceleration period from the time when the rotary gantry 1 is started to the time when the angular velocity at the time of data collection is reached. The angular velocity at each minute time elapsed from the start of deceleration is stored so that the most vibration is not generated during the deceleration period up to the stop. For example, in FIG. 2, the angular velocity after the lapse of t1 time from the start is S1 and the angular velocity after the lapse of t2 time from the start is S2. . The same applies to the deceleration period. FIG. 2 shows the angular velocities for each minute time such that the most vibration does not occur in the acceleration period from the start of the rotary gantry 1 to the angular velocity at the time of data collection. The angular velocity for each minute time (hereinafter referred to as "acceleration curve") is the rate of change of the angular velocity of the rotary gantry 1, that is, the angular velocity during the first half of the acceleration period from the time of startup until the angular velocity of the rotary gantry reaches a predetermined angular velocity. The displacement is such that the acceleration gradually increases with time, and further, the angular acceleration of the rotary gantry 1 gradually decreases with time during the latter half of the acceleration period. Further, since the acceleration curve differs depending on the shift position and the angular velocity at the time of data collection, it is preset and stored for each shift position and the angular velocity at the time of data collection. This acceleration curve is shown in FIG. 2 when the shift position is changed from shift A to shift B even if the angular velocity at the time of data collection is the same, that is, when the rotation center position of the rotary gantry and its center of gravity position are significantly deviated. As shown, the change in the angular velocity between the first half period and the second half period is made more gradual.

The vibration sensor 4 is arranged in a part of the rotary gantry 1 or a part of the housing of the gantry unit, detects an electric signal according to the level of vibration generated by the rotation of the rotary gantry 1, and detects the electric signal. The signal is output to the rotation control unit 2.

FIG. 3 shows the displacement of the control voltage applied from the rotation control unit 2 to the servo motor 7 according to the acceleration curve read from the memory 3. The rotation control unit 2 reads an acceleration curve corresponding to the shift position information and the angular velocity information input from the console 5 from the memory 3, and applies a control voltage according to the acceleration curve to the servo motor 7, thereby performing servo control. The number of rotations of the motor 7 is controlled. Here, the correction of the control voltage based on the electric signal input from the vibration sensor 4 will be described. Figure 4
Shows the change in angular velocity (solid line) according to the acceleration curve stored in the memory 3, the change in vibration level (dashed line), and the change in angular velocity after correction of the control voltage (dotted line). The rotation control unit 2 is applying the control voltage to the servo motor 7 while changing the control voltage (acceleration period,
When an electric signal indicating the vibration level is input from the vibration sensor 4 during the deceleration period), the control voltage set according to the acceleration curve of the memory 3 is corrected according to the vibration level,
The corrected control voltage is applied to the servo motor 7. The corrected control voltage (dotted line part in FIG. 4) is made smaller than the uncorrected control voltage (solid line part in FIG. 4). Therefore, the change in the actual angular velocity of the rotary gantry 1 is slower than the change in the angular velocity according to the acceleration curve, and the vibration level is reduced. And
The correction amount of the control voltage also decreases as the vibration level decreases, and the control voltage approximates the acceleration curve.

Next, the operation of the X-ray CT apparatus configured as described above will be described. First, before the start of imaging, the operator operates the console 5 to set imaging conditions such as shift position, slice thickness, scan time, and angular velocity at the time of data acquisition. For example, assume that the shift position is set to shift A shown in FIG. 1 and the angular velocity at the time of data collection is set to Sa. After that, when an instruction operation for starting the photographing is given, the rotation control unit 2 reads an acceleration curve corresponding to the information of the shift A and the angular velocity Sa supplied from the console 5, and a minute time elapses after the start shown by the acceleration curve. A control voltage V1 for generating an angular velocity S1 of t1 is applied to the servo motor 7 at t1. The control voltage V2 for generating the angular velocity S2 at the time point t2 after the minute time has elapsed is applied to the servo motor 7 at the time point t2. In this way, by displacing the control voltage every minute time from the start-up, the actual angular velocity of the rotary gantry 1 is gently changed in the same manner as the velocity curve stored in the memory 3 and is generated by the rapid acceleration. Vibration can be suppressed. Further, when vibrations are generated due to the speed curve of the memory 3 due to unpredictable reasons such as variations in the output characteristics of the servo motor, the control voltage may be reduced according to the vibration level obtained by the vibration sensor. The vibration can be suppressed by changing the angular velocity of the rotary mount 1 more gently. The above explanation has been focused on the acceleration period of the rotary gantry, but of course the deceleration period from the angular velocity at the time of data collection to the stop is the same as the acceleration curve, that is, the angle of the rotary gantry in the first half of the deceleration period. A deceleration curve is stored in the memory 3 so that the acceleration increases with time and the angular acceleration of the rotating platform decreases with time in the latter half of the deceleration period. The deceleration curve is rotated based on the deceleration curve during the deceleration period. It is possible to decelerate while suppressing the occurrence of frame vibration.

Next, another embodiment will be described. FIG. 5 shows a change in control voltage (solid line), a change in speed (dotted line), and a change in vibration level (dashed line) of the apparatus of this embodiment. The configuration of the apparatus of this embodiment is similar to that of the previous embodiment shown in FIG. However, the speed curve stored in the memory 3 is a speed curve that causes a change in the control voltage as shown in FIG. That is, the control voltage is controlled by the rotation control unit 2 according to the speed curve supplied from the memory 3, and rapidly changes from the time of startup until a predetermined time elapses (time tm) and reaches a desired angular speed from time tm. (Time ti
) Changes slowly. In response to this control voltage, the angular velocity of the rotary gantry 1 is at the beginning of the acceleration period (time t0
From time to tm), and then gradually changes in the latter half of the acceleration period (from time tm to time ti) to reach the desired angular velocity Sa.

Next, the change of the vibration level due to the change of the angular velocity of the rotary mount will be described. In the initial stage of the acceleration period in which the rotary gantry 1 is rapidly accelerated, strong vibration causes strong vibration. Of course, the level of vibration generated at the beginning of this acceleration period is stronger than the level of vibration generated at the beginning of the acceleration period in the previous embodiment. However, this strongly generated vibration level decays during the latter part of a sufficiently long acceleration period until it becomes weak enough to not generate artifacts at the start of data collection. The reason why the latter half of the acceleration period can be made sufficiently long is as follows.
This is because the steeper the angular acceleration at the beginning of the acceleration period, the longer the period that can be applied to the latter part of the acceleration period.

As described above, according to this embodiment, similarly to the previous embodiments, the vibration level of the rotary gantry at the start of data collection can be sufficiently weakened to the extent that no artifact is generated.

The present invention is not limited to the above embodiment. The X-ray CT apparatus according to the above-described embodiment is a so-called third generation X-ray CT apparatus of a type in which the X-ray tube and the X-ray detector are integrally rotated around the subject. The so-called fourth generation X of the type that only rotates around the subject
It may be a line CT device.

[0022]

As described above, according to the present invention, there is provided an X-ray CT apparatus capable of sufficiently suppressing the vibration generated when the angular velocity of the rotary mount changes, and suppressing the occurrence of shower-like artifacts. be able to.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an X-ray CT apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram showing a change in angular velocity of a rotary mount of the X-ray CT apparatus shown in FIG.

FIG. 3 is a diagram showing changes in control voltage applied to the servo motor shown in FIG.

FIG. 4 is a diagram showing a change in angular velocity of the rotary mount of the X-ray CT apparatus shown in FIG. 1, a change in vibration level, and a change in angular velocity when the change in angular velocity is corrected according to the level of vibration.

FIG. 5 is a diagram showing changes in the control voltage applied to the servo motor of the X-ray CT apparatus according to another embodiment of the present invention, changes in the angular velocity of the rotary mount, and changes in the vibration level.

FIG. 6 is a diagram showing changes in control voltage applied to a servomotor in a conventional X-ray CT apparatus.

FIG. 7 is a diagram showing a change in angular velocity and a change in vibration of a rotary mount of a conventional X-ray CT apparatus.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Rotary mount, 2 ... Rotation control part, 3 ... Memory, 4 ... Vibration sensor, 5 ... Console, 6 ... Shift control part, 7 ... Servo motor, 8 ... Belt, 11a ... X-ray tube, 12a ... X-ray detection vessel.

Claims (2)

[Claims] 1. A multi-directional projection data collection system for collecting multi-directional projection data of a subject while a rotary gantry equipped with at least one of an X-ray tube and an X-ray detector rotates around the subject at a predetermined angular velocity. In an X-ray computed tomography apparatus for obtaining a tomographic image based on data, the angular acceleration of the rotary mount from the time of start-up to a first predetermined time is increased with time, and a second predetermined time after the first predetermined time is increased. To a predetermined angular velocity, the memory stores a control signal for controlling the rotation of the rotary gantry so as to decrease the angular acceleration of the rotary gantry with time, and the rotation according to the control signal. An X-ray computed tomography apparatus, comprising: a means for rotating a gantry. 2. A multi-directional projection data of the subject is collected while a rotary mount equipped with at least one of an X-ray tube and an X-ray detector is rotated around the subject at a predetermined angular velocity, and the multi-directional projection is performed. In an X-ray computed tomography apparatus for obtaining a tomographic image based on data, the angular acceleration of the rotary mount from the time when the predetermined angular velocity is reached to a first predetermined time is increased with time,
A memory that stores a control signal for controlling rotation of the rotary gantry so as to decrease the angular acceleration of the rotary gantry from a second predetermined time after the first predetermined time to a stop time with time; An X-ray computed tomography apparatus, comprising: means for rotating the rotary mount according to a control signal.