JP2000030891A - X-ray automatic exposure control device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an X-ray automatic exposure control device preventing the density from being affected by the length of a high-voltage cable connected to an X-ray tube. SOLUTION: The X-rays from an X-ray tube 4 transmits a subject 3, scattered rays are removed by an X-ray grid 10, and a transmitted X-ray image is received by a cassette 1 (X-ray film, intensifying screen). An automatic exposure control X-ray detector 2 detects the X-ray dose entering the cassette 1 with a photoelectron multiplier tube 8 as the current signal ip proportional to the dose rate. The length of a high-voltage cable 5 is stored in the memory 18 of a CPU 17 in advance, and the reference voltage value Vr of a phototimer is corrected in response to the length of the high-voltage cable 5 and the values of radiographic voltage/current. The signal proportional to the X-ray intensity detected by the photoelectron multiplier tube 8 during radiographing is time- integrated by an integrator 12. When the output Vc becomes the said corrected reference voltage value Vr, a high-voltage cutoff signal is sent to an X-ray switch 16 from a comparison detector 13 via a switch 14, and X-ray radiation is automatically completed, thereby the density error of an X-ray photograph can be eliminated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic X-ray exposure controller, and more particularly to an automatic X-ray exposure controller for automatically setting X-ray conditions using a photo timer.

[0002]

2. Description of the Related Art A tube voltage, a tube current, and a photographing time, which are photographing conditions of an X-ray photograph, are determined by knowing a part of a subject and a degree of X-ray absorption. However, since it is difficult to measure a part each time imaging is performed, a high-voltage device that sets imaging X-ray conditions (tube voltage and tube current) from information on X-ray absorption of a subject obtained during fluoroscopy prior to imaging. And a photo-timer mechanism for terminating the signal proportional to the X-ray intensity detected by the sensor during imaging by an integrator and terminating the X-ray irradiation when the output reaches a certain reference voltage value (imaging time determination) ) Automatically sets the X-ray conditions.

FIG. 2 shows an X-ray automatic exposure control device using a conventional phototimer mechanism. For X-ray imaging, tube voltage and tube current are manually set according to the purpose of imaging in the case of general imaging, but the output of the image intensifier during fluoroscopy is used for gastrointestinal use. The tube voltage and tube current are automatically set for both fluoroscopy and imaging.

[0004] X set by the control circuit 15 of the high-voltage device.
A high voltage is generated by the high voltage transformer 6 under the line condition, and the high voltage is applied to the X-ray tube 4 via the high voltage cable 5. X-rays emitted from the X-ray tube 4 are transmitted through the subject 3 and then scattered by the X-ray grid 10 to remove the scattered rays.
(X-ray film, intensifying screen). At this time,
X incident on the image receiver by the automatic exposure control X-ray detector 2
The dose is converted by the photomultiplier tube 8 into a current signal i proportional to the dose rate.
Detected as p. This ip is integrated by the integrating circuit 12 into the current /
The voltage is converted and amplified to obtain a voltage signal Vp, and this detection signal is integrated. Here, if ip is completely proportional to the rate of increase in image density D due to X-ray energy absorption of the image receiver, then dD / dtｄip∝Vp, that is, V
c = {Vp.dt} D (where t is the shooting time)
The output voltage Vc of the integration circuit 12 is proportional to the image density D.
Therefore, the output voltage Vc and the reference voltage source 11
Is compared with the reference voltage Vr by the comparison detector 13,
When they match, an X-ray cutoff signal is output from the switch 14 to the X-ray switch 16 in the control circuit 15 of the X-ray high-voltage device, and the output of the high-voltage transformer 6 is stopped. As a result, an X-ray image having a constant density can be obtained regardless of the subject.

[0005]

The conventional X-ray automatic exposure control device is constructed as described above. FIG. 3 shows the X-ray tube current waveform, and FIG. 4 shows the relationship between the reference voltage of the photo timer and the integrated value. As shown in the figure, the X-ray switch 16 is turned on, the X-ray tube current rises, and flows constantly at the mA value set by the control circuit 15 of the high-voltage device.
Since the detection signal current ip from the automatic exposure control X-ray detector flows uniformly from to the switch OFF, the integrated value of the detected electric signal increases in proportion to time as shown in FIG. When the integrated value Vc of the detected electric signal becomes equal to the comparison reference voltage value Vr, the switch is turned off.
Due to the discharge of the electric charges charged in the stray capacitance Cs of the high-voltage cable 5 connecting the X-ray high-voltage transformer 6 and the X-ray tube 4, even after the switch is turned off as shown in FIG. X-ray tube current flows. The X-rays caused by the discharge of the residual charges in the high-voltage cable 5 usually cause extra exposure for the subject, and also cause a problem of a density error during automatic exposure control. In particular, when two or more X-ray tubes are connected to the X-ray high-voltage device, there is a problem that the effect is large when the lengths of the high-voltage cables for the respective X-ray tubes are significantly different.

[0006] The present invention has been made in view of such circumstances, and the density is not affected by the length of the high-voltage cable 5 connected to the X-ray tube 4 when the automatic exposure control is performed. An object of the present invention is to provide a line automatic exposure control device.

[0007]

In order to achieve the above object, an automatic X-ray exposure control apparatus according to the present invention comprises a mechanism for determining an X-ray imaging condition from information on X-ray absorption of a subject; A mechanism that integrates a signal proportional to the X-ray intensity detected by the sensor during time by an integrator and terminates X-ray irradiation when the output reaches a certain reference voltage value, and automatically sets the X-ray conditions In the automatic exposure control device, the reference voltage value is corrected according to the length of a high-voltage cable connected to the X-ray tube and the value of the imaging tube voltage.

The X-ray automatic exposure control device of the present invention is configured as described above, sets an imaging voltage and an imaging current in a control circuit of the high-voltage device, and sets the length of the high-voltage cable connected to the X-ray tube. Is stored in the memory of the control circuit of the high-voltage device, the reference voltage value of the photo timer is corrected in advance in accordance with the length of the high-voltage cable and the value of the imaging voltage, and the X-ray detected by the sensor during the imaging. A signal proportional to the intensity is time-integrated by an integrator, and the X-ray irradiation is automatically terminated when the output reaches the corrected reference voltage value.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the X-ray automatic exposure control device of the present invention will be described with reference to FIG. This device is a CPU,
Inverter type high voltage device control circuit with memory 1
5, a high-voltage transformer 6 for generating a high voltage, an X-ray tube 4 connected to the high-voltage cable 5, and an X-ray grid 10 for removing scattered X-rays transmitted through the subject 3. Cassette 1 for receiving transmitted X-ray image (X-ray film, intensifying screen)
And an X-ray detector 2 for automatic exposure control for detecting the amount of X-rays incident on the image receiver as a current signal ip proportional to the dose rate by a photomultiplier tube 8, and a phototimer control circuit 7.

When performing X-ray imaging, a high voltage is generated by the high-voltage transformer 6 under the conditions of the imaging voltage and current set by the control circuit 15 of the high-voltage device. A high pressure is applied to 4. The X-rays emitted from the X-ray tube 4 are transmitted through the subject 3 and then scattered by an X-ray grid 10 and imaged by a cassette 1 (X-ray film, intensifying screen) as an image receiver. You. At this time, the X-ray dose incident on the image receiver is detected by the automatic exposure control X-ray detector 2 as a current signal ip proportional to the dose rate by the photomultiplier tube 8. This ip is subjected to current / voltage conversion and amplification by an integration circuit 12 to obtain a voltage signal Vp, and this detection signal is integrated. The integral value of the detected electric signal increases in proportion to time as shown in FIG. Integrated value V of detected electric signal
When c becomes equal to the comparison reference voltage value Vr, the switch is turned off. However, the electric charge charged in the stray capacitance Cs of the high-voltage cable 5 connecting the X-ray high-voltage transformer 6 and the X-ray tube 4 is changed. Switch is turned off as shown in FIG.
An X-ray tube current corresponding to a shaded portion flows later. (A stray capacitance of 150 to 250 PF / m exists between the conductor core of the high-voltage cable 5 and its protective shield.)

At this time, the amount of the shaded portion, that is, the residual charge of the high-voltage cable 5 is determined by the cable length and the kV value. That is, assuming that the length Q of the high-voltage cable 5 is L, the stray capacitance per unit length of the high-voltage cable is Cs, and the tube voltage is V, Q = L × Cs × V (1) Become. The relationship between the charge and mAs is mAs = mA × sec (2), and from the charge equation Q = I × T (I: current, T: time), Q = mA × sec (mA: tube current, sec) : Shooting time)
... (3). From equations (1), (2) and (3), mA
s = L × Cs × V (4)

According to the present invention, the length L of the high-voltage cable 5 is determined by the C in the control circuit 15 of the X-ray high-voltage device for each X-ray tube.
It is registered in the memory 18 of the PU 17. First, examinee 3
A high-voltage device control circuit 15
X-ray tube 4 selected on the operation panel of
Choose Control circuit 1 for high-voltage equipment by selecting X-ray tube 4
X-ray tube 4 stored in the memory 18 of the CPU 17 of FIG.
Of the high voltage cable 5 is selected. Then, the imaging tube voltage and the tube current are set on the operation panel of the control circuit 15 of the high-voltage device. (Generally, if the subject's imaging region is specified on the panel, the optimal tube voltage and the tube current determined from the load characteristics of the X-ray tube 4 are determined.) Next, the subject 3 is placed on the imaging table (X The X-ray detector 10 is mounted on the X-ray detector 2 for automatic exposure control, the X-ray film and the cassette 1) containing the intensifying screen. Then, an X-ray imaging button (not shown) on the operation panel of the control circuit 15 of the high-voltage device is pressed.

The X-rays that have been irradiated and transmitted through the subject 3 enter the X-ray detector 2 for automatic exposure control after the scattered rays are removed by the X-ray grid 10, and the current signal ip is proportional to the X-ray dose.
Flows from the photomultiplier tube 8 and enters the integration circuit 12. While compensating for the dark current from the photomultiplier tube 8, the integration circuit 12 performs current / voltage conversion and amplifies it into a voltage signal Vp, and integrates this detection signal. The integral value of the detected electric signal increases in proportion to time as shown in FIG. On the other hand, the CPU 17 determines the length L of the high-voltage cable of the X-ray tube 4 registered in the memory 18, the stray capacitance Cs per unit length of the high-voltage cable, and the imaging tube set by the control circuit 15 of the high-voltage device. From the voltage, the mAs value due to the residual charge is calculated from equation (4), and the value is divided by the tube current to reduce the reference voltage corresponding to the value of the imaging time s due to the residual charge. (The coefficient for reducing the comparison reference voltage when the switch is turned off from FIG. 4 so that the decrease value of the photographing time is equal to the photographing time s due to the residual charge) is converted to a voltage value. A correction operation for subtracting from the value of the voltage Vr is performed, and the photo timer control circuit 7
Is set as the corrected reference voltage in the reference voltage source 11 of FIG.

Then, the output voltage Vc of the integrating circuit 12 and the corrected reference voltage Vr are compared by a comparison detector 13, and when they match, the control circuit 1 of the X-ray high voltage device is controlled.
At 5, the switch 14 outputs an X-ray cutoff signal to the X-ray switch 16, and stops the output of the high-voltage transformer 6. By this correction, the imaging time is shortened by an amount corresponding to the excessive exposure time due to the residual charge of the high-voltage cable, the density error with respect to the output set value is eliminated, and an X-ray image of a constant density is obtained regardless of the subject. Can be.

In the above embodiment, the front lighting system in which the automatic exposure control X-ray detector 2 is provided between the cassette (X-ray film, intensifying screen) and the subject has been described. The invention can also be applied to a rear-lighting type built in an intensifier optical system.

Although a phototimer using a photomultiplier tube 8 has been described as a phototimer detector, an X-ray detector such as a semiconductor detector or an ion chamber may be used instead.

[0017]

The X-ray automatic exposure control apparatus according to the present invention is configured as described above. The control panel sets the voltage and current for radiography and stores the length of the high-voltage cable in the memory of the control circuit. Depending on the length and the values of the shooting voltage and tube current,
Correct the reference voltage value of the photo timer in advance,
A signal proportional to the X-ray intensity detected by the sensor during imaging is time-integrated by an integrator, and when the output reaches the corrected reference voltage value, the X-ray irradiation is terminated, so that excessive exposure is avoided. As a result, there is no density error with respect to the output set value, and an X-ray image having a constant density can be obtained regardless of the subject.

[0018]

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of an X-ray automatic exposure control device of the present invention.

FIG. 2 is a diagram showing a conventional X-ray automatic exposure control device.

FIG. 3 is a diagram showing an X-ray tube current waveform.

FIG. 4 is a diagram showing a reference voltage and an integrated value of a phototimer control circuit.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Cassette 2 ... X-ray detector for automatic exposure control 3 ... Subject 4 ... X-ray tube 5 ... High voltage cable 6 ... High voltage transformer 7 ... Photo timer control circuit 8 ... Photomultiplier tube 10 ... X-ray Grid 11 Reference voltage source 12 Integrator 13 Comparison detector 14 Switch 15 Control circuit of high-voltage device 16 X-ray switch 17 CPU 18 Memory Cs Stray capacitance of high-voltage cable

Claims (1)

[Claims] 1. A controller of a high-voltage device for setting X-ray imaging conditions, and an integrator integrates a signal proportional to the X-ray intensity detected by a sensor during imaging by an integrator and outputs the signal to a reference voltage value. A mechanism for terminating X-ray irradiation when
An automatic exposure control device capable of automatically setting X-ray conditions, wherein the reference voltage value is corrected according to the length of a high-voltage cable connected to an X-ray tube and the value of a shooting tube voltage. Automatic line exposure control device.