JPH0728861B2 - X-ray CT system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application] The present invention relates to an X-ray CT apparatus for reconstructing an image based on X-ray transmission data of a subject.

(Prior Art) As shown in FIG. 13, for example, an X-ray CT apparatus is configured such that an X-ray tube 2 and a detector 3 are opposed to each other with a subject 1 interposed therebetween, and the X-ray tube 2 is an X-ray tube. The X-ray transmission data of the subject 1 is collected by the detector 3 by rotating around the subject 1 integrally with the detector 3 while irradiating the radiation. As shown in FIG. 14, for example, the detector 3 is formed integrally with a plurality of channels by a pair of a scintillator 4 for converting X-rays into light and a photodiode 5 for converting light into current. . Reference numeral 6 is a collimator for guiding X-rays so that each channel is not affected by scattered rays.

FIG. 15 shows an X-ray detection circuit (for one channel) including a photodiode 5, in which the current detected by the photodiode 5 is applied to a current-voltage conversion circuit 8 including an operational amplifier 7 and a resistor R. After being converted into a voltage, this voltage is integrated by an integrating circuit including an operational amplifier 9 and a capacitor C.
Added to 10 and integrated. Subsequently, this voltage is converted into a digital value by the A / D converter 11 and sent to the reconstruction unit 12 as X-ray transmission data. The reconstruction unit 12 reconstructs the image of the subject based on the X-ray transmission data.

Here, the current Id flowing in the photodiode 5 is amplified by the operational amplifier 7 by the conversion magnification determined by the resistor R, and is converted into a voltage of V = Id × R and output from the current-voltage conversion circuit 8. The value of resistance R is usually several 100Ω
To several MΩ are selected.

(Problems to be Solved by the Invention) By the way, in the conventional X-ray CT apparatus, the current of the X-ray detection circuit −
Since not only the current detected by the photodiode 5 in the voltage conversion circuit 8 but also noise is amplified, there is a problem in that the quality of the reconstructed image is deteriorated due to the influence of this noise.

The present invention has been made to address the above problems,
It is an object of the present invention to provide an X-ray CT apparatus in which the control is not complicated and the influence of noise is prevented.

[Configuration of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention subtracts the voltage input to the inverting input terminal from the voltage applied to the non-inverting input terminal, and amplifies the subtracted result. And output from the output terminal and having an offset voltage between the non-inverting input terminal and the inverting input terminal, a second capacitor holding the offset voltage, and a transmission X of the subject. An X-ray detection element that outputs a charge corresponding to a dose, a first capacitor that accumulates the charge output from the X-ray detection element, and that applies a voltage generated by the accumulation to the non-inverting terminal; A first mode in which the first capacitor is discharged and the input offset voltage is applied to the second capacitor; and the second capacitor is connected between the output terminal and the inverting terminal. Canceling the offset voltage is connected, it is characterized in that the output of the X-ray detecting elements; and a switch group for switching the second mode to be accumulated in the first capacitor.

(Operation) According to the invention of the above configuration, when the switch group is switched to the first mode, the input offset voltage is applied to the second capacitor while discharging the first capacitor, so that the input offset voltage is canceled. Therefore, the semiconductor can perform the detection operation based on the normal temperature characteristic. Therefore, when the first mode is switched to the second mode by the switch group, only the charges output from the semiconductor can be accumulated and the influence of noise can be prevented.

Embodiments Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a connection diagram showing a first embodiment of an X-ray CT apparatus of the present invention, showing an X-ray detection circuit for one channel. The integrating circuit 10 is connected to the operational amplifier 13, the resistor R ₁ connected to the non-inverting input terminal of the operational amplifier 13, the first capacitor C ₁ connected to this, and the inverting input terminal of the operational amplifier 13. A second capacitor C ₂ , a photodiode 5 connected between the non-inverting input terminal and the output terminal of the operational amplifier 13,
First switch SW connected in parallel with the capacitor C ₁ of
₁ , the second switch SW ₂ connected between the inverting input terminal and the output terminal of the operational amplifier 13, and the third switch connected in parallel to the second switch SW ₂ via the second capacitor C _2. SW ₃ , a fourth switch SW ₄ connected between the inverting input terminal and the non-inverting input terminal of the operational amplifier 13 via the second capacitor C ₂ , and these first to fourth switches SW _{1 to} SW
It is composed of a switch switching control unit 14 that performs switching of _four . I ₁ and I ₂ are inverters. The output of the integrating circuit 10 is applied to the reconstructing unit 12 (not shown) via the A / D converter 11.

The integrating circuit 10 is basically composed of a combination of a voltage follower circuit 15 as shown in FIG. 9 and an integrating circuit 16 as shown in FIG. 10 (a). As shown in FIG. 9, when the inverting input terminal and the output terminal are short-circuited, the operational amplifier 13 has an input impedance Zi≈∞ and an output impedance Zo≈0. Therefore, in this state, when the voltage Vi is applied to the non-inverting input terminal, the current of the input signal is changed to the operational amplifier 13
Since it does not flow into the non-inverting input terminal of
An output of Vo≈Vi can be obtained. That is, an accurate detection operation (integration operation) of the input voltage becomes possible with such a circuit configuration having a high Zi.

On the other hand in series with the photodiode 5 with a capacitor C ₁ as FIG. 10 (a), when connecting the switch SW ₁ in parallel with the capacitor C _1, and stored is integrated from the photodiode 5 to the capacitor C ₁ Charge switch SW ₁
Each time the switch is turned on, it is discharged and reset as shown in FIG. 10 (b). t ₁ , t ₃ ... Indicates the timing from switch off to on, and t ₀ , t ₂ ... Indicates the timing from switch on to off.

In the circuit as shown in FIG. 10 (c), the operational amplifier 13
There is an unavoidable offset voltage V _OS , and when this V _OS is applied to the photodiode 5, the temperature characteristic in the voltage-current characteristic of the photodiode 5 increases exponentially as shown in FIG. Therefore, it becomes difficult to perform an accurate detection operation. A is a low load straight line, B is a high load straight line, and C is a load straight line when a bias is applied. However, the offset voltage V _OS can be canceled by alternately switching the integration operation and the reset operation as shown in FIG. 10 (b).

Therefore, by combining the circuit of FIG. 9 and the circuit of FIG. 10 (a) to form the integrating circuit 10 of the present embodiment of FIG. 1, an accurate X-ray dose can be detected without being affected by the offset voltage. It becomes possible.

Next, the operation of the first embodiment of the present invention will be described.

The operation of the circuit of the present embodiment shown in FIG. 1 is roughly divided into two modes based on the time chart of FIG. 4; (A) reset period (first mode) and (B) integration period (second mode). The switch switching control unit 14 alternately switches this.

(A) Reset period: As shown in the time chart of FIG.
Thus at time t ₁ , the first, second and fourth switches SW
The circuit shown in FIG. 2 is configured by turning on ₁ , SW ₂ , and SW ₄ . As the electric charge from the photodiode 5 stored in the first capacitor C ₁ until then is discharged,
The output e ₀ of the operational amplifier 13 also decreases. Further, the offset voltage V _OS of the operational amplifier 13 is applied to the second capacitor C ₂ .

(B) Integration period: Next, by the switch changeover control unit 14, at the time t ₂
The circuit shown in FIG. 3 is constructed by turning on the switch SW ₃ of. The electric charge in the _first capacitor C ₁ which has been lowered by the discharge increases in accordance with the amount of light incident on the photodiode 5. The output e ₀ of the operational amplifier 13 also increases accordingly. Further, at this time, since the voltage having the opposite polarity to the offset voltage V _OS is applied to the second capacitor C ₂ , the offset voltage V _OS is canceled. That is, the voltage between both terminals of the photodiode 5 is almost OV.

As described above, according to this embodiment, the switch switching control unit 14 controls the switching timing of the _{first to} fourth switches SW _{1 to} SW ₄ as shown in the time chart of FIG. In the circuit, two modes of (A) reset period and (B) integration period are alternately performed, and the operational amplifier 13
Since the offset voltage V _OS is canceled, no voltage is applied to the photodiode 5.

Note that the offset voltage V _OS is canceled during the reset period, and this cancel so-called auto-zero function does not have to be performed independently. FIG. 12 shows the relationship between the auto-zero period and the reset period. The time between the start time of the reset period and the end time of the auto-zero period is selected to be several μS or less. By thus performing the auto zero period at the same time as the reset period without specially providing it, the offset voltage V _OS can be canceled apparently without the auto zero period, and therefore the circuit configuration can be simplified. Further, by simultaneously performing the time for charging the second capacitor C ₂ for offset cancellation and the reset time, it is possible to perform a high-speed detection operation.

Since the offset voltage V _OS is canceled and is not applied to the photodiode 5 in this way, the temperature characteristic in the voltage-current characteristic of the photodiode 5 becomes like the low load line A in FIG. Therefore, since a linear detection operation with respect to temperature can be performed, a complicated correction circuit for temperature change is not required. Further, according to the present embodiment, since the current-voltage conversion circuit is not required as compared with the conventional one, the number of operational amplifiers can be reduced accordingly, and the circuit configuration can be simplified. Further, since the operation of the integrating circuit 10 is controlled so as to be divided into two modes, that is, a reset period and an integrating period, and alternately switched, the control becomes easy.

FIG. 5 shows a second embodiment of the present invention, in which the first capacitor C ₁₀ connected to the inverting input terminal of the operational amplifier 130.
And a second capacitor C ₂₀ connected to the non-inverting input terminal
And a photodiode 50 connected between the non-inverting input terminal and the inverting input terminal, and a first switch connected between the inverting input terminal and the non-inverting input terminal of the operational amplifier 130 via the second capacitor C _20. SW ₁₀ , a second switch SW ₂₀ connected in series with the first switch SW ₁₀ , and a second capacitor C
_A third switch SW ₃₀ connected in parallel to the second switch SW ₂₀ via ₂₀ ; a fourth switch SW ₄₀ connected between the inverting input terminal and the output terminal of the operational amplifier 130;
To a fourth switch SW _{10 to} SW ₄₀ and a switch switching control unit 140.

Next, the operation of the second embodiment of the present invention will be described.

The operation of the circuit of this embodiment shown in FIG. 5 is roughly divided into two modes, that is, (A) reset period and (B) integration period, based on the time chart of FIG.
It is switched alternately by 0.

(A) Reset period: As shown in the time chart of FIG.
The first and third switches S at time t ₁ by 0
By turning on W ₁₀ and SW _30, and turning on the fourth switch SW ₄₀ at t ′ _{1 which} is slightly delayed from this, the sixth switch SW 6 is turned on.
The circuit shown is constructed. As the electric charge from the photodiode 5 stored in the first capacitor C ₁₀ until then is gradually discharged, the output e ₀ of the operational amplifier 130 also gradually decreases. Further, the offset voltage V _OS of the operational amplifier 130 is applied to the second capacitor C ₂₀ . The fourth switch S
When W ₄₀ is turned on at the timing of t ′ ₁ , when the switch is turned on at t ₁ , the first capacitor C ₁₀ is not completely discharged through SW ₄₀ , and a part of the charge is transferred to the second side through the photodiode 50. This is a consideration to prevent the capacitor C _{20 from} being charged. In order to turn on the SW ₄₀ at a delayed timing in this way, a delay circuit may be provided on the line that controls the SW ₄₀ of the switch switching control unit 140. However, it is not necessary to delay SW ₄₀ , in principle
It can also be turned on at the same timing as SW ₁₀ and SW ₃₀ .

(B) Integration period: Next, the switch switching control unit 140 turns on the second switch SW ₂₀ at time t ₂ to configure the circuit shown in FIG. 7. The output e ₀ of the operational amplifier 130 gradually increases as the voltage of the first capacitor C ₁₀ that has dropped due to the discharge gradually increases due to the charge from the photodiode 50. At this time, the voltage having the opposite polarity to the offset voltage V _OS is maintained in the second capacitor C ₂₀ , so that the offset voltage V _OS is canceled.

As described above, according to the present embodiment as well, the two modes of (A) reset period and (B) integration period are alternately switched, so that the offset voltage V _OS of the operational amplifier 130 is canceled and the voltage applied to the photodiode 50 is cancelled. Becomes almost zero. Therefore, it is possible to obtain the same effect as that of the above embodiment.

Further, according to the present embodiment, by making the voltage applied to the photodiode 50 substantially zero in this way, the temperature characteristic in the voltage-current characteristic of the photodiode 50 becomes a normal linear characteristic, so that the temperature change of the shunt resistance thereof. It is also possible to obtain the effect that the output voltage fluctuation Δe ₀ of the operational amplifier 130 can be suppressed to a small _value . Therefore, it is not necessary to prepare a temperature controller for the photodiode 50.

[Effects of the Invention] As is apparent from the above description, according to the present invention, mode switching can be performed by a simple switching operation of a switch group, so control is not complicated and the output of a semiconductor is integrated. Before the second mode, the input offset voltage applied to the semiconductor is canceled so that only the charges output from the semiconductor can be stored, so that the influence of noise can be prevented.

[Brief description of drawings]

FIG. 1 is a connection diagram showing an embodiment of the X-ray CT apparatus of the present invention, FIG. 2 is a connection diagram when the reset operation is performed by the apparatus of this embodiment, and FIG. 3 is an integration operation by the apparatus of this embodiment. 4 is a time chart for controlling the apparatus of the present embodiment, FIG. 5 is a connection diagram showing another embodiment of the present invention, and FIG.
The figure is a connection diagram when performing the reset operation in the device of this embodiment,
FIG. 7 is a connection diagram when the integration operation is performed in the apparatus of this embodiment,
FIG. 8 is a time chart for controlling the embodiment apparatus,
FIG. 9 is a connection diagram showing a voltage follower circuit according to the principle of the present invention, and FIGS. 10 (a), (b), and (c) are connection diagrams, characteristic diagrams, and connection diagrams showing an integrating circuit according to the principle of the present invention. FIG. 11 is a current-voltage characteristic diagram of the photodiode, FIG. 12 is a time chart showing the relationship between the reset operation and the auto-zero operation in the present invention, and FIG. 13 is a schematic diagram showing the configuration of the X-ray CT apparatus.
14 is a schematic diagram showing the structure of the detector in FIG. 13, FIG.
The figure is a connection diagram showing a conventional example. 3 ...... detector, 5,50 ...... photodiode, 10 ...... integrating circuit, 13,130 ...... operational amplifier, 14, 140 ...... switch switching control unit, C _1, C ₁₀ ...... first capacitor (integrating capacitor) , C ₂ , C ₂₀ ... second capacitor (capacitor for offset voltage cancellation), V _OS ... offset voltage of operational amplifier, SW _{1 to} SW ₄ , SW _{10 to} SW ₄₀ ... switch.

Claims (1)

[Claims] 1. A method for subtracting a voltage input to an inverting input terminal from a voltage applied to a non-inverting input terminal, amplifying the subtracted result, and outputting the amplified result from the output terminal, and the non-inverting input terminal and the inverting input. An operational amplifier having an offset voltage between the terminal and the second capacitor, a second capacitor holding the offset voltage, an X-ray detection element outputting an electric charge corresponding to the transmitted X-ray dose of the subject, and an X-ray detection element A first capacitor that accumulates output charges and applies the voltage generated by the accumulation to the non-inverting terminal, and discharges the first capacitor and applies the input offset voltage to the second capacitor. The first mode and the second capacitor are connected between the output terminal and the inverting terminal to cancel the offset voltage.
An X-ray CT apparatus comprising: a switch group that switches between a second mode in which the output of the line detection element is stored in the first capacitor.