JPH0441021B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an X-ray tomographic imaging apparatus, and more particularly, to improvement of measurement errors.

(Prior Art) FIG. 3 is a block diagram of the main parts of a pulse irradiation type X-ray tomographic imaging apparatus. In the figure, 1 is an X-ray tube, 2
is an X-ray detector row consisting of a large number of X-ray detectors (502 in this example) that detects X-rays emitted from the X-ray tube 1, and these X-ray tubes 1 and X-ray detector row 2
is arranged rotatably around the subject T. A part of the X-rays emitted from the X-ray tube 1 is absorbed and attenuated by the subject T, and the X-rays forming the X-ray detector array 2 are
It is detected by line detectors 2 ₂ to 2 ₅₀₁ and converted into a current signal. On the other hand, X-rays that do not pass through the subject T are detected by reference X-ray detectors A (2 ₁ ) and B (2 ₅₀₂ ) provided at both ends of the X-ray detector array 2, and converted into a current signal. Each of these X-ray detectors 2 ₁ to _{2 5}
The output terminal of ₀₂ is connected to a common potential point via capacitors 3 ₁ to 3 ₅₀₂ and is connected to the first switch 4 ₁
~4 ₅₀₂ are commonly connected to the amplifier 5. The input terminal of the amplifier 5 is connected to a common potential point via a second switch 6, and the output terminal is connected to the A/D
It is connected to the converter 7. The output terminal of the A/D converter 7 is connected to a computer 8. 9 is the first
This is a control circuit that sends out control signals for on/off control of the switches 4 ₁ to 4 ₅₀₂ and the second switch 6, and the output terminal of an external clock signal source 10 is connected to the control circuit 9. . That is, the first
The switches 4 ₁ to 4 ₅₀₂ , the second switch 6 , and the control circuit 9 selectively control the voltage charged in each of the capacitors 3 ₁ to 3 ₅₀₂ by a measurement circuit composed of an amplifier 5 and an A/D converter 7. It constitutes an input selection circuit for inputting to the .

In such a configuration, each capacitor 3 ₁ to
3 ₅₀₂ is charged with the detection current of the X-ray detectors 2 ₁ to 2 ₅₀₂ , and the voltage charged in each of the capacitors 3 ₁ to 3 ₅₀₂ is passed through the first switch 4 ₁ to 4 ₅₀₂ and the amplifier 5 to the A/ The signal is added to a D converter 7 and converted into a digital signal. Then, the digital signal converted by the A/D converter 7 is applied to a computer 8, where predetermined arithmetic processing is performed. Also, the first switches 4 ₁ to 4 ₅₀₂
By turning on the second switch 6 with the capacitors 3 1 to 3 502 turned on, each capacitor 3 ₁ to 3 ₅₀₂ is reset (discharged).

FIG. 4 is a time chart showing a conventional driving example of the apparatus shown in FIG. 3, and shows an example in which X-rays are irradiated in a pulsed manner. In FIG. 4, a indicates the detection current I, b to d indicate the operation of the first switches 4 ₁ to 4 ₅₀₂ , respectively, and e indicates the operation of the second switch 6.

The first switches 4 ₁ to 4 ₅₀₂ are turned on simultaneously at time t ₁ when there is no detection current I.
At time _t2 when there is no time, the power is turned off at the same time. The detection current I rises at time _t3 after the first switches ₄₁ to ₄₅₀₂ are turned off, and rises at time _t4 after a certain period of time has elapsed. On the other hand, the second switch 6 operates at time t ₅ after the detection current I falls.
It turns off at . As a result, each capacitor 3 ₁ to 3 ₅₀₂ is reset by the first switch 4 ₁ to 4 _50
2 and the second switch 6 are turned on at time _t1 , and the first switches ₄₁ to ₄₅₀₂
is completed by time t ₂ when it turns off. Thereafter, each of the capacitors 3 ₁ to 3 ₅₀₂ is charged by the detection current I until time t ₅ when the second switch 6 is turned off. When charging of each capacitor 3 ₁ to 3 ₅₀₂ is completed in this way, the first switches 4 ₁ to 4 ₅₀₂ are turned on for a certain period of time in a predetermined order as shown in FIG. Capacitor 3 ₁ ~
A scanning measurement of the voltage charged to 3 ₅₀₂ is taken.
That is, at time _t5 , the second switch 6 is turned off, and at the same time, for example, the first switch ₄₂₄₉ is turned on, and the voltage charged in the capacitor ₃₂₄₉ is measured, and at time _t6 , the first switch ₄₂₄₉ is turned off. At the same time as the capacitor 3 250 is turned off, the second switch 4 ₂₅₀ is turned on and the voltage charged in the capacitor 3 ₂₅₀ is measured. Such voltage measurement operation is performed sequentially up to capacitor 3 ₅₀₂ .

In addition, X-ray detector 2 located near the center of rotation ₂₄₉
From 2 ₂₅₀ →2 ₂₄₈ →2 ₂₅₁ →…→A(2 ₁ )→B(2 _50
2 ) An example was shown in which scanning measurements are performed in a direction that spreads from side to side in the order of 2), but this is because measurements of adjacent X-ray detectors are performed at times as close as possible.

Now, let's consider the case of continuous irradiation of X-rays and observation using such a pulsed X-ray irradiation system. According to the drive shown in Fig. 4,
Since the time from the completion of reset of each capacitor 3 ₁ to 3 ₅₀₂ to the start of voltage measurement is different, if this drive method is applied as is in the case of continuous irradiation, a difference will occur in the charging time of each capacitor 3 ₁ to 3 ₅₀₂ . Measured voltage becomes inaccurate.

For example, in the case of an X-ray tomographic imaging device configured to continuously generate X-rays using high voltage pulsating waves obtained by full-wave rectification of commercial power supply,
Due to waveform distortion and the like existing in the commercial power supply, the X-rays become pulsating waves that fluctuate every two cycles, making it impossible to accurately measure the detected current. Another drawback is that the efficiency of using the detection current that contributes to measurement is reduced. Therefore, as a measure to solve this problem, the applicant has realized a driving system as shown in FIG.

That is, the first switch of each channel is turned on at the period To of the clock CL, and its on time is kept constant, and the second switch is turned on for the latter half of the on time Trs to reset the capacitor. As a result, each capacitor has a uniform charging time, although there is a difference in timing.
Trs, which enables accurate measurement of detection current.

(Problems to be Solved by the Invention) However, when aiming for high speed, high-speed sampling of the detection current is required, but if the amplifier installed in front of the A/D converter is simply replaced with a high-speed response type, This poses a problem in that noise increases and measurement errors occur.

Also, after sampling the input and performing A/D conversion, the A/D converter is in a waiting state until the next channel is sampled and A/D converted.
The use efficiency of the D converter is poor, and it takes a long time to A/D convert the input of all channels.
This is a factor that hinders speeding up.

The present invention has been made in view of the above points, and an object of the present invention is to provide an X-ray tomographic imaging apparatus that continuously irradiates X-rays without using a high-speed response amplifier, and to provide an A/D A/D for all inputs with small measurement errors without increasing the number of converters
An object of the present invention is to provide an X-ray tomography device that requires a short time for conversion.

(Means for Solving the Problems) The present invention, which solves these problems, consists of an X-ray tube that continuously irradiates the subject with X-rays, and an X-ray tube that is arranged opposite to the X-ray tube with the subject as the center of rotation. A row of X-ray detectors that detects X-rays that have passed through the subject, a number of capacitors each charged by the detection current of each X-ray detector in the row of X-ray detectors, and a voltage charged to the capacitors. In the X-ray tomographic imaging apparatus, the input selection circuit includes a first switch that selects and takes out the outputs of the plurality of capacitors, respectively. , the plurality of capacitors are divided into a plurality of groups, one amplifier is provided for each group, and the output of each amplifier is selectively selected and inputted to an A/D converter. A third switch, a second switch connected between the input of each amplifier and a common potential point, and a third switch sequentially alternatively select the outputs of the multiple amplifiers, and the outputs are selected. After A/D conversion of the selected amplifier, the input is reset by the second switch until the next selection, and then the first switch samples and settles the input of the next channel and waits. The device is characterized in that it is comprised of a control circuit that controls each switch.

(Example) Hereinafter, an example of the present invention will be described in detail using the drawings.

FIG. 1 is a configuration diagram of an embodiment of the present invention, and particularly only the configuration of the portions that are characteristic of the present invention are shown. Therefore, the parts not shown are the same as the conventional example shown in FIG. FIG. 2 is a time chart for explaining the operation. FIG. 1 shows an example in which the output of the X-ray detector is divided into four groups and the capacitor voltage is amplified. The first group transfers the charging voltage of the capacitors C ₁₁ to C ₁ n to the first switch S 11 to S ₁ n via the first switch S ₁₁ to S 1 n.
input to the amplifier A1, amplified, and the third switch
The signal is led to the A/D converter 7 via _SS1 . Note that the input terminal of amplifier A1 is connected to the second switch.
Connected to common potential point via SR ₁ . The same configuration applies to the other groups, that is, the second to fourth groups.

Each switch is appropriately turned on and off by a control circuit (not shown). The operation in such a configuration will be explained next. At time _t1 shown in FIG. 2, sufficient time has elapsed since the first switch _S11 was turned on. The charging voltage signal of the capacitor _C11 is amplified by the amplifier A1 via the switch _S11 , and is led to the A/D converter ₇ via the third switch SS1, which is turned on during the period _t1 - _t3 . /D conversion and measurement.

During the next period t ₃ - _{t 5} , switch SS ₁ is turned off and switch SS ₂ is turned on. At time _t3 , the signal on capacitor _C21 has been amplified by amplifier A2 over a sufficient period of time. And t ₃ − _{t 5}
It is A/D converted and measured within the period of .

On the other hand, at time t ₃ , the second switch SR ₁ is turned on, the charge of the capacitor C ₁₁ is discharged during the period t ₃ - t ₄ , and at time t ₄ , the switch S ₁₁ is turned off, and the switch SR ₁ resets the input of amplifier A1 over the period t ₄ -t ₅ .

Next, at time _t5 , the first switch _S12 is turned on and remains on until time _t11 , and the signal from the capacitor _C12 is amplified by the amplifier A1 and output. Therefore, the signal from capacitor _C12 only needs to be stabilized by time _t9 . This signal is A/D in the period t ₉ - t ₁₁
converted. Similarly, at t ₇ − _{t 9} , the switch is
SS ₄ is turned on and the signal from C ₄₁ is A/D converted and measured.

Thereafter, the same operation is repeated and the signal due to C ₄ n of the last channel is measured, and the scanning is completed.

In this way, the A/D converter 7 A/D converts the capacitor voltages of all channels without requiring any waiting time. Amplifiers A1 to A in this case
4 does not require a high-speed response type at all.

Although the embodiment shows an example in which the channel is divided into four groups and four amplifiers are used, the present invention is not limited to this, and the channel may be divided into m groups and configured using m amplifiers. , the purpose is achieved as well. In this case, all channels are A/
The time for D conversion is the same regardless of the value of m.

Also, if m is increased, the settling time will be longer, and therefore a slower, less noisy amplifier can be used. As a result, accurate detection current measurement with small measurement errors becomes possible.

(Effects of the Invention) As explained above, according to the present invention, the A/D
Conversion latency can be eliminated and the following advantages can be obtained with respect to the amplifier:

That is, in the conventional system, as shown in Fig. 6,
It is necessary for the amplifier to settle sufficiently within the time periods t ₂ - _{t 1} , t ₄ - t ₃ , and t ₇ - t ₆ , and in reality, measurements were taken at the very edge of the amplifier's settling time, but the present invention According to the method, it is possible to shorten the time for measuring all inputs compared to the conventional method, and at the same time, it is possible to take sufficient settling time, and it is possible to obtain data with small errors even with a low-speed amplifier.

Further, since the gain of the amplifier can be made larger than that of the conventional method, there is an advantage that the dynamic range of the data acquisition device can be expanded.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of an embodiment of the main part of the present invention, Fig. 2 is a time chart for explaining the operation, Fig. 3 is a block diagram showing an example of a conventional device, and Fig. 4 is a diagram explaining the operation of the conventional device. FIG. 5 is a diagram explaining the scanning order, and FIG. 6 is a time chart showing other operations in the apparatus shown in FIG. 3. 1...X-ray tube, 2...X-ray detector row, C ₁₁ to C ₄
n...Capacitor, _S11 to _S4 n...First switch, _SR1 to _SR4 ...Second switch, A1 to A4
...Amplifier, SS ₁ to SS ₄ ...Third switch, 7
... A/D converter, 8 ... Calculator, 9 ... Control circuit, 10 ... External clock signal source.

Claims (1)

[Claims] 1. An X-ray tube that continuously irradiates the subject with X-rays,
an X-ray detector array that is arranged opposite to the X-ray tube with the subject as a rotation center and detects X-rays that have passed through the subject;
It includes a large number of capacitors each charged by the detection current of each X-ray detector in the X-ray detector array, and an input selection circuit that selects the voltage charged to the capacitor and inputs the voltage to the A/D converter. In the X-ray tomographic imaging apparatus, the input selection circuit includes a first switch that selects and takes out the outputs of the plurality of capacitors, and a first switch that divides the plurality of capacitors into a plurality of groups, one switch for each group. A large number of amplifiers provided in correspondence with each other, a third switch that selectively selects the output of each amplifier and inputs it to the A/D converter, and a connection between the input of each amplifier and a common potential point. A second switch and a third switch selectively select the outputs of a large number of amplifiers in sequence, and the amplifier whose output is selected is
After D conversion, the second switch resets the input until the next selection, and then the first switch controls each switch so that it samples and settles the input of the next channel and waits. An X-ray tomographic imaging device comprising a circuit.