JPH08168482A - Ct scanner - Google Patents

Abstract

PURPOSE: To obtain a desired dynamic range at a low cost by enabling the prevention of possible overflow. CONSTITUTION: A charge amplifier comprises an operational amplifier 3, a first capacitor, a second capacitor 5 and an electronic switch 6. A comparator 9 and a latch circuit 11 are so arranged to perform an ON controlling of the electronic switch 6 when an output voltage of the operational amplifier 3 exceeds a reference voltage to be supplied from an auxiliary power source 10. A switching/non-switching control section 12 is provided to cut a control signal to the electronic switch 6 from the latch circuit 11 when a non-switching control signal from a DAS is already inputted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is a CT which detects an X-ray transmitted through an object to be photographed by an X-ray detector equipped with a scintillator and a photoelectric conversion element, and amplifies and outputs an electric signal from the X-ray detector. Regarding scanners.

[0002]

2. Description of the Related Art FIG. 5 shows a conventional D / S (data) at a front stage of a CT scanner, in which a charge is accumulated and amplified according to an electric signal from an X-ray detector composed of a scintillator and a photodiode. FIG. 3 is a circuit diagram showing a main part of a portion for outputting to a collecting device).

A photodiode 52 constituting the X-ray detector 51 receives light from a scintillator (not shown) and converts it into an electric signal. The anode terminal of the photodiode 52 is connected to the power supply V, and the cathode terminal thereof is connected to the inverting input terminal of the operational amplifier 53. A capacitor 54 and a switch 55 are connected in parallel between the input terminal and the output terminal of the operational amplifier 53. The non-inverting input terminal of the operational amplifier 53 is connected to the ground.

That is, the operational amplifier 53 and the capacitor 54 form an integrator. The switch 55 is on / off controlled by a reset signal from a control unit (not shown), and by turning on (closing) the switch 55, the electric charge accumulated in the capacitor 54 is discharged.

The output terminal of the operational amplifier 53 is an FPA.
It is connected to the input terminal of (floating point amplifier, amplifier) 56. The output signal (voltage signal) amplified by the FPA 56 is A / D (analogue / digi
tal) conversion unit 57. The digital data converted and obtained by the A / D converter 57 is output as OUT and supplied to a DAS (not shown).

Although not shown here, the CT scanner has an X-ray generator (X-ray tube) and an X-ray drive control section for driving and controlling the X-ray generator, and further the above-mentioned X-ray drive controller.
A plurality of line detectors 51 are provided, and the A / D converter 57 is provided.
The digital signal output from the D is not directly output to the DAS, but is integrated into a serial signal through a circuit such as a sample hold circuit or a multiplexer, and is converted into a D signal.
Supplied to AS.

Here, a description will be given of how the electric signal output from one X-ray generator is converted into a digital signal by the A / D converter. The transmitted X-rays emitted from the X-ray generator to the imaging target are incident on the scintillator of the X-ray detector. The scintillator converts the energy of the incident X-rays into light energy and radiates it, and upon receipt of this light, the photodiode 52 causes a current to flow according to the magnitude of the light energy. Therefore, at the cathode terminal of the photodiode 52, a voltage corresponding to the X-ray transmission dose is generated.

This cathode terminal, that is, the operational amplifier 5
Since the switch 55 is normally in the open state in response to the voltage increase of the second inverting input terminal, electric charge is accumulated in the capacitor 54.

The output level of the output terminal of the operational amplifier 52 is amplified by the FPA 56 and converted into a digital signal by the A / D converter 57. Data collection timing occurs at a predetermined time interval in the DAS, and the A / D conversion unit 57
Capture the digital signal output from.

Immediately after the completion of the data collection, the switch 55 is turned on by the reset signal to be in the closed state, and the electric charge accumulated in the capacitor 54 is discharged. After that, when the accumulated charge of the capacitor 54 is sufficiently discharged, the switch 55 is turned off to be in the open state. From this time, the next X-ray measurement is started.

[0011]

As described above, the conventional CT scanner has a problem in that the gain is fixed and the dynamic range (sensitivity) is insufficient when the X-ray dose is small in the front end. There is a problem that overflow occurs when the dose of X-ray is large.

Usually, in order to realize a dynamic range of 20 bits or more in a circuit that processes an electric signal output from an X-ray detector, the signal is amplified in order to suppress the noise generated on the circuit to a low level. It is necessary to set a high gain to increase the power supply voltage.

However, the reason why the gain is set high and the dynamic range is widened is that the electric signal level output from the X-ray detector differs depending on the X-ray dose, the object to be imaged, and the scanning conditions, and the low-level output. There is no problem in the case of a signal, but overflow may occur in the case of a high level output signal.

Further, the power supply voltage is increased to increase the dynamic range because there is a circuit for performing digital processing in the subsequent stage of the circuit of the signal processing system.
Since the number of logic circuits is limited, problems such as cost may occur.

Therefore, an object of the present invention is to provide a CT scanner which can prevent overflow and can obtain a desired dynamic range at low cost.

[0016]

The invention according to claim 1 is
The X-ray detector irradiates X-rays on the object to be photographed, and detects the X-ray detector equipped with a scintillator that converts the X-rays that have passed through the object to be photographed into light energy and a photoelectric conversion unit that converts the light from the scintillator into an electric signal, In a CT scanner that accumulates, amplifies and outputs electric charge by an electric signal from the X-ray detector, a charge amplifier that accumulates electric charge according to an electric signal output from the X-ray detector, and an output voltage of the charge amplifier And a gain switching means for switching the gain of the charge amplifier to a low value according to the rise of the charge amplifier.

According to a second aspect of the invention, in the first aspect of the invention, the charge amplifier is composed of an integrating circuit composed of a capacitor and an operational amplifier, and the gain switching means responds to an increase in the output voltage of the charge amplifier. The capacity of the capacitor of the charge amplifier is switched to a larger capacity.

According to a third aspect of the present invention, a scintillator for irradiating an X-ray to an object to be imaged and converting the X-ray transmitted through the object to be imaged into light energy, and a photoelectric conversion means for converting light from the scintillator into an electric signal. In a CT scanner which detects with an X-ray detector equipped with, accumulates and amplifies electric charges by an electric signal from the X-ray detector and outputs the electric charge to a data acquisition device, according to an electric signal output from the X-ray detector. A charge amplifier consisting of a capacitor and an operational amplifier for accumulating electric charge, a gain switching means for switching the capacity of the capacitor to a larger capacity in response to an increase in the output of the integrating circuit, and a predetermined time of data acquisition timing in the data acquisition device. The gain switching prohibition means for prohibiting the switching of the capacitance of the capacitor by the gain switching means is provided from the time from the front to the data collection. Than it is.

[0019]

In the invention according to claim 1, the gain switching means switches the gain of the charge amplifier to a low value in accordance with an increase in the output voltage value of the charge amplifier. Therefore, for example, if the gain of the charge amplifier is initially set high, a sufficient dynamic range can be obtained even if the dose of X-ray is small. Further, even if the dose of X-rays is large, the gain of the charge amplifier is switched to a low value according to the increase of the output voltage value of the charge amplifier, so that the occurrence of overflow is prevented, and the output voltage value of the charge amplifier is once lowered. Start rising again.

According to the second aspect of the present invention, the capacitance of the capacitor of the charge amplifier is increased to a larger capacitance by the gain switching means in response to the increase of the output voltage of the charge amplifier which is composed of the integrating circuit including the capacitor and the operational amplifier. Can be switched.

As a result, the gain of the charge amplifier is switched to a low level. In the invention corresponding to claim 3, the capacitance of the capacitor of the charge amplifier is switched to a larger capacitance by the gain switching means in response to the rise of the output voltage of the charge amplifier which is composed of the integrating circuit including the capacitor and the operational amplifier.

However, the gain switching prohibiting means prohibits the switching of the capacitance of the capacitor by the gain switching means during a period from a predetermined time before the data collecting timing in the data collecting device to the data collecting.

As a result, the time from the predetermined time before the data collection timing by the data collection device to the data collection,
Since the capacitance switching of the capacitor is not performed by the gain switching means, data collection by the unstable output voltage of the charge amplifier immediately after switching the capacitance of the capacitor is prevented, and data collection by the stable output voltage of the charge amplifier is performed. .

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described below with reference to FIGS.
Will be described with reference to. Although not shown, the CT scanner of the first embodiment and the CT scanner of the second embodiment to be described later is an X-ray tube and an X that supplies electric power to the X-ray tube, like a general X-ray CT apparatus. It has a wire tube drive control unit,
The X-ray emitted from the X-ray tube is applied to the object to be imaged, and the X-ray transmitted through the object is detected by an X-ray detector including a scintillator and a photodiode. The detection signal from the X-ray detector is supplied to the DAS (data collection device) via the front end section.

FIG. 1 is a circuit diagram showing a main portion of a front end portion of a CT scanner. The photodiode 2 forming the X-ray detector 1 of this CT scanner is a scintillator.
The light from (not shown) is received and converted into an electric signal.
The anode terminal of the photodiode 2 is connected to the power supply V, and the cathode terminal thereof is connected to the inverting input terminal of the operational amplifier 3. A switch 7 and a series circuit including a first capacitor 4, a second capacitor 5 and an electronic switch 6 are connected in parallel between the input terminal and the output terminal of the operational amplifier 3. The non-inverting input terminal of the operational amplifier 3 is connected to the ground.

That is, the operational amplifier 3, the first capacitor 4, the second capacitor 5, and the electronic switch 6 constitute an integrator as a charge amplifier.

The switch 7 is on / off controlled by a reset signal from a control unit (not shown), and when the switch 7 is turned on (closed), it is stored in the first capacitor 4 and the second capacitor 5. The generated charge is discharged.

The output terminal of the operational amplifier 3 is FPA (
It is connected to the input terminal of a floating point amplifier (amplifier) 8 and to the inverting input terminal of a comparator 9. A reference voltage is supplied to the non-inverting input terminal of the comparator 9 from the auxiliary power supply 10.

The reference voltage of the auxiliary power supply 10 is, as shown in FIG. 2, the upper limit level L of the operational region of the operational amplifier 3.
It is set to a level L2 which is lower than 1 and the output of the operational amplifier 3 does not immediately exceed the upper limit level L1 by the signal from the photodiode 2.

The output of the comparator 9 is latched by a latch circuit 11, and the latch output of the latch circuit 11 controls the electronic switch 6 to be turned on / off. The second capacitor 5, the electronic switch 6, the auxiliary power supply 10, the comparator 9 and the latch circuit 11 constitute a gain switching means.

The latch circuit 11 and the electronic switch 6
A switchability control unit 12 as a gain switching prohibition unit is interposed between the switch and the switchability control unit 12 before and after the predetermined time (this predetermined time will be described later) of the DAS data collection timing (not shown). The switch-disable control signal (high-level signal) generated at the time causes the disconnection state / conduction state between the latch circuit 11 and the electronic switch 6.

The output signal amplified by the FPA8 (
The voltage signal), the A / D (analog / digital) converter 13
Is supplied to. The digital data converted and obtained by the A / D converter 13 is output as OUT and supplied to DAS.

FIG. 3 is a diagram showing various timings in the main circuit of the front end section. These various timings are generated based on the timing (data collection timing) for collecting the data generated in the DAS.

The reset signal for controlling ON / OFF of the switch 7 changes from the low level to the high level (ON control) when a time t1 sufficient to complete the data collection has elapsed after the data collection timing has occurred. Then, when a time t2 sufficient to complete the discharge of the charges accumulated in the first capacitor 4 and the second capacitor 5, the high level is changed to the low level (OFF control).

Therefore, at the transition point (edge) of the reset signal from the high level to the low level, the integration of the output signal from the photodiode 2 of the X-ray detector 1 (charge to the capacitor 4 or the capacitors 4 and 5) is performed. Accumulation) is started. That is, X-ray measurement is started. Since integration is performed until the data acquisition timing occurs thereafter, the time t3 from the edge of the reset signal from the high level to the low level until the next data acquisition timing occurs is the X-ray measurement time ( Data accumulation time).

The latch release timing for resetting the latch circuit 11 (signal input to the reset terminal of the latch circuit 11) is at least to turn on / off the switch 7.
This occurs when the level of the output terminal of the operational amplifier 3 is sufficiently lowered before the reset signal for off control changes from the high level to the low level.

As shown in FIG. 2, since the electric charge is discharged from the first capacitor 4 to the second capacitor 5 at the moment when the electronic switch 6 is turned on, until a predetermined time t4 elapses thereafter. The level of the output terminal of the operational amplifier 3 becomes unstable.

Therefore, the switching prohibition control signal output from the DAS or the like and input to the switching propriety control unit 12 permits the switching permission before the time t5 slightly longer than the unstable time t4 from the occurrence of the data collection timing. When the data collection timing is changed from the low level shown to the high level indicating that switching is not possible, the high level is changed to the low level immediately after that.

In the first embodiment having such a structure,
The scintillator on which the X-rays are incident converts the X-ray energy into light energy and radiates it. The emitted light is received by the photodiode and converted into an electric signal. Electric charges are accumulated in the integrator including the operational amplifier 3 and the capacitor 4 by this electric signal.

When the dose of X-rays is small, the output voltage from the operational amplifier 3 does not rise near the overflow upper limit. When the X-ray dose is high, the output voltage of the operational amplifier 3 approaches the upper limit L1 at which it overflows, and exceeds the voltage L2 of the auxiliary power supply 10 first.
Then, the output voltage of the comparator 9 changes from high level to low level.

When the output voltage of the comparator 9 changes to low level, the latch circuit 11 holds this low level signal. By holding the low level signal, the latch circuit 11 outputs a control signal for turning on the electronic switch 6. This ON control signal is input to the switchability control unit 12 and is output from the switchability control unit 12 to the electronic switch 6 unless the switchability control signal (high level signal) from the DAS is input.

Then, the electronic switch 6 is turned on, and the second capacitor 5 is connected in parallel to the first capacitor 4, so that the gain of the charge amplifier composed of the operational amplifier 3 and the like is switched to a lower level. At this time, the output of the operational amplifier 3 drops and becomes unstable as shown in FIG. 2, and becomes stable again after a predetermined time t4.

However, the switching disable control signal (
When the high level signal is input to the switchability control unit 12, the control signal from the latch circuit 11 for turning on the electronic switch 6 is cut off by the switchability control unit 12 and is not output to the electronic switch 6. Therefore, the latch circuit 1
Unless the latched state by 1 is released, when the non-switchable control signal from DAS is turned off (low level), the control signal to be turned on from the latch circuit 11 is the electronic switch 6
Output to

Then, the electronic switch 6 is turned on, and the second capacitor 5 is connected in parallel with the first capacitor 4, so that the gain of the charge amplifier composed of the operational amplifier 3 and the like is switched to a lower level.

As described above, according to the first embodiment, the charge amplifier including the operational amplifier 3, the first capacitor, the second capacitor 5, and the electronic switch 6 and the output voltage of the operational amplifier 3 are supplied from the auxiliary power supply 10. When the comparator 9 and the latch circuit 11 for controlling the ON state of the electronic switch 6 when the reference voltage is exceeded and the non-switchable control signal from the DAS are input, the switchable / non-switchable state of interrupting the control signal from the latch circuit 11 to the electronic switch 6 is inputted. Since the control unit 12 is provided, it can be configured at low cost, and further, even when the gain of the charge amplifier is increased in the initial state and the X-ray dose detected by the X-ray detector is low, desired dynamics can be obtained. When the range can be obtained and the X-ray dose detected by the X-ray detector is large, the comparator 9 and Switching acceptance control unit from the latch circuit 11 1
If the control signal for turning on the electronic switch 6 is output via 2 and the non-switchable control signal is not output from the DAS, the gain of the charge amplifier can be automatically switched to a low level, thus preventing the occurrence of overflow. can do.

Further, when a switch disable control signal is input to the switch enable / disable control unit 12 from a predetermined time before data acquisition from the DAS to the end of data acquisition, the switch enable / disable control unit 12 causes the latch circuit 11 to output an electronic signal. By cutting off the control signal to the switch 6, the gain switching of the charge amplifier can be prohibited, so that data collection due to unstable output of the charge amplifier at the time of gain switching can be prevented.

A second embodiment of the present invention will be described with reference to FIG. In contrast to the second embodiment, in which the first embodiment has a single series circuit including an electronic switch for adjusting the gain of the integrating circuit by an electronic switch and a capacitor,
A plurality of (two in the second embodiment) series circuits for gain adjustment are provided.

FIG. 4 is a circuit diagram showing a main portion of the front end portion of the CT scanner. The anode terminal of the photodiode 22 forming the X-ray detector 21 is connected to the power supply V, and the cathode terminal thereof is connected to the inverting input terminal of the operational amplifier 23. Between the input terminal and the output terminal of the operational amplifier 3, a series circuit including a first capacitor 24, a second capacitor 25 and a first electronic switch 26, a third capacitor 27 and a second capacitor are provided in parallel. Electronic switch 28
And the switch 29 are connected to each other. The non-inverting input terminal of the operational amplifier 23 is connected to the ground.

That is, the operational amplifier 23, the first
Capacitor 24, a series circuit including the second capacitor 25 and the first electronic switch 26, and the third circuit
An integrator is constituted by a series circuit including the capacitor 27 and the second electronic switch 28.

The switch 29 is on / off controlled by a reset signal from a controller (not shown). The output terminal of the operational amplifier 23 is connected to the input terminal of the FPA 30 and the control circuit 31.

The control circuit 31 turns on the electronic switches in the order of the first electronic switch 26 and the second electronic switch 28 in accordance with the rise of the voltage level output from the output terminal of the operational amplifier 23. Output a control signal. Further, the control circuit 31 receives a switch disable control signal generated from a DAS (not shown) generated a predetermined time before the data collection timing. When disable is designated by the switch disable control signal, the control circuit 31 operates. The output of the control signal for turning on the respective electronic switches 26 and 28 described above is not started.

Output signal amplified by the FPA 30
The (voltage signal) is supplied to the A / D converter 32. The digital data converted by the A / D converter 32 is O
It is output as UT and supplied to DAS.

In the second embodiment having such a structure,
Two circuits, a series circuit including a second capacitor 25 and a first electronic switch 26 for changing the gain and a series circuit including a third capacitor 27 and a second electronic switch 28, are provided in parallel with the operational amplifier 23. Is connected to the first electronic switch 2 by the control circuit 31.
The sixth and second electronic switches 28 are on / off controlled.

Therefore, according to the output of the operational amplifier 23,
By sequentially controlling the second electronic switch 26 and the third electronic switch 28 to be turned on, the gain of the charge amplifier composed of the operational amplifier 23 and the like is switched in three stages.

Further, when the DAS designates the disable by the switch disable control signal, the control circuit 31
Is the second electronic switch 26 and the third electronic switch 2
The output of the control signal for turning on 8 is not started.

As described above, according to the second embodiment, it is possible to obtain the same effect as that of the first embodiment described above, and further, the first embodiment sets the gain of the charge amplifier in two stages.
In contrast to switching to the stage, in the second embodiment, 3
Since the stages can be switched, the gain can be switched more appropriately according to the output of the charge amplifier, and a more appropriate dynamic range can be obtained at each output level of the charge amplifier.

In the second embodiment, the charge amplifier gain is switched in three stages, but the present invention is not limited to this, and a series circuit composed of a capacitor and an electronic switch is used. It is also possible to connect three or more circuits in parallel to the operational amplifier and switch the gain of the charge amplifier in four or more steps.

[0058]

As described above in detail, according to the present invention,
It is possible to provide a CT scanner that can prevent overflow and can obtain a desired dynamic range at low cost.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a main part of a front end section of a CT scanner according to a first embodiment of the present invention.

FIG. 2 is a diagram showing an output waveform of an operational amplifier when the gain of the front end portion of the CT scanner of the embodiment is switched.

FIG. 3 is a diagram showing timings of various signals in the front end section of the CT scanner of the embodiment.

FIG. 4 is a circuit diagram showing a main part of a front end portion of a CT scanner according to a second embodiment of the present invention.

FIG. 5 is a circuit diagram showing a main part of a front end section of a conventional CT scanner.

[Explanation of symbols]

 2, 22 ... Photodiode, 3, 23 ... Operational amplifier, 4, 24 ... 1st capacitor, 5, 25 ... 2nd capacitor, 6 ... Electronic switch, 8, 30 ... FPA, 9 ... Comparator, 10 ... Auxiliary power supply , 11 ... Latch circuit, 12 ... Switchability control unit, 26 ... First electronic switch, 27 ... Third capacitor, 28 ... Second electronic switch, 31 ... Control circuit.

Claims (3)

[Claims] 1. An X-ray detector comprising a scintillator for irradiating an X-ray on an object to be imaged and converting the X-ray transmitted through the object to be light energy, and a photoelectric conversion unit for converting light from the scintillator into an electric signal. In a CT scanner that detects electric energy from an X-ray detector, accumulates and amplifies and outputs electric charges by the electric signal from the X-ray detector, and a charge amplifier that accumulates electric charges according to the electric signal output from the X-ray detector, A CT scanner comprising: a gain switching means for switching the gain of the charge amplifier to a low value in accordance with an increase in the output voltage of the charge amplifier. 2. The CT scanner according to claim 1, wherein the charge amplifier is composed of an integrating circuit composed of a capacitor and an operational amplifier, and the gain switching means is arranged to charge the charge amplifier in response to an increase in the output voltage of the charge amplifier. A CT scanner characterized in that the capacity of the condenser of is switched to a larger capacity. 3. An X-ray detector comprising a scintillator for irradiating an X-ray on an object to be imaged, converting the X-ray transmitted through the object to be converted into light energy, and a photoelectric conversion means for converting light from the scintillator into an electric signal. In a CT scanner which is detected by an X-ray detector, accumulates and amplifies electric charges by an electric signal from the X-ray detector and outputs the electric charges to a data acquisition device, the electric charges are accumulated in accordance with an electric signal outputted from the X-ray detector. A charge amplifier consisting of a capacitor and an operational amplifier, a gain switching means for switching the capacity of the capacitor to a larger capacity in response to an increase in the output of the integrating circuit, and data from a predetermined time before the data acquisition timing in the data acquisition device. A gain switching prohibiting unit that prohibits the switching of the capacitance of the capacitor by the gain switching unit during the time until collection is provided. And CT scanner.