JPH01291834A - X-ray ct scanner - Google Patents

Abstract

PURPOSE:To restrain a drift error accompanied with the use of an operation amplifier so as to reduce artifact in data correction by actuating a photodiode only by use of passive element composed of resistor and condenser with no operation amplifier. CONSTITUTION:An output from a photodiode PD is delivered to a direct change circuit DC composed of a signal band limiting resistor R1, a capacitor C1 for charging signal charge and a signal reading switch SW1. An output from this direct charge circuit DC is delivered to an analog/digital converter A/D-C through a charge/voltage converter Q/V composed of a charge/voltage converting operation amplifier A2, a charge/voltage converting capacitor C2 and a reset switch 2.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to an X-ray CT scanner device using a solid state detector as a multi-channel radiation detector.

(Prior Art) For example, if the X-ray CT scanner device is of the third generation, as shown in FIG.
and a multi-channel radiation detector 2 are arranged, and this X
A pair of the ray 1Fjl and the multi-channel radiation detector 2 is placed facing each other and rotates around the subject P while irradiating X-rays from the X-ray source 1, and transmits the X-rays that have passed through the subject P into multiple channels. It is received by each channel of type radiation detector 2, extracted as an electrical signal, and converted into a DA including an analog/digital converter.
S (Data Acquisltlon Syst
em ) 3 to the arithmetic unit 4 and performs reconstruction processing here, an image sliced in the X-ray emission direction of the X-ray source 1 and the multi-channel radiation detector 2 in the subject P is obtained. generated and displayed on the monitor 5.

Here, a solid-state detector is used as the multi-channel radiation detector 2, and a conventional example of a data collection system consisting of this type of solid-state detector and DAS will be described below with reference to FIG. Although Figure 4 shows one channel, the output current of the photodiode PD of the solid-state detector XD is 1/V
-C is converted into a voltage signal by an operational amplifier.

After passing through an integrator IC consisting of a capacitor, the signal is sent to an analog/digital converter A/DC via a multiplexer MUX which receives signals from all channels, and is output as digital data to the DAS.

As shown in FIG. 4, the photodiode PD
The operational amplifier of the voltage converter 1/V-C is configured to receive the signal at virtual ground, and the photodiode PD is operated with no bias (or reverse bias).

(Problem to be Solved by the Invention) In the conventional technology, the photodiode of the solid-state detector is received by the operational amplifier of the current/voltage converter, so the offset voltage of the operational amplifier and the temperature drift of the bias current This resulted in an error, which resulted in image artifacts. It is also necessary to suppress the noise of the operational amplifier itself to a level that does not contribute to image quality.

In this case, in a pulsed X-ray system, the above drift error can be suppressed by adding an auto-zero circuit etc. that operates during the data collection pause time, but continuous X-ray
Line systems collect data continuously, so
There is no time for correction, and means such as such an auto-zero circuit cannot be employed.

Therefore, an object of the present invention is to provide an X-ray CT scanner device that suppresses drift errors in a system using continuous X-rays and a solid-state detector, reduces artifacts in data collection, and obtains good images. do.

[Structure of the Invention] (Means for Solving the Problems) The present invention has a structure in which the following means are taken to solve the above problems and achieve the objects.

That is, the present invention provides an X-ray CT scanner device using a solid-state detector, in which the photodiode of the solid-state detector is
Connected between the inverting terminal and the output terminal of the operational amplifier, the output of the photodiode is charged to a capacitor, and then taken out via a signal readout switch, and a charge/voltage converter,
It is characterized by comprising a data collection system configured to be introduced into an A/D converter.

(Function) According to this configuration, since the photodiode is operated only by passive elements such as resistors and capacitors without using an operational amplifier, it is possible to suppress the drift error that occurs in the conventional example due to the use of operational amplifiers. Therefore, artifacts in data collection can be reduced and good images can be obtained.

(Embodiment) An embodiment of the X-ray CT scanner apparatus according to the present invention will be described below with reference to FIG. 1, which shows a data acquisition system which is a main part thereof.

As shown in FIG. 1, in this embodiment, the photodiode PD of the solid-state detector The output of the diode PD is introduced into a direct charge circuit DC including a resistor R1 for signal band limitation, a capacitor C1 for charging signal charge, and a switch SWI for signal readout.

Then, the output of this direct charge circuit DC is passed through a charge/voltage converter Q/VC including an operational amplifier A2 for charge/voltage conversion, a capacitor C2 for charge/voltage conversion, and a reset switch SW2. , and sent to an analog/digital converter ^/D-C.

Here, a capacitor CI is provided corresponding to each channel, but this may be done by using a capacitor array that collects a large number of capacitors, or by using a switch SWI for signal readout.
A high-speed analog switch array such as an FET element is used for this, which is provided corresponding to each ah. In addition,
In the data acquisition system, the circuit system from the detector
The section up to C is called the front end section.

Next, the operation of this embodiment configured as described above will be explained with reference to FIGS. 1 and 2. First, data collection in an X-ray CT scanner is a repetitive operation at a predetermined sampling rate. The basic cycle will be explained.

That is, in the basic cycle of data collection, the signal readout switch SWt of the direct charge circuit DC is switched from ON to OFF for each channel, as shown in FIGS. 2(a) and 2(b).
It starts at the timing to FF.

In the initial state of this data collection circuit system, capacitor CI
It is assumed that the signal charge charged at the previous acquisition timing is transmitted to the subsequent operational amplifier A2, and the switch SW2 is also OFF and in a reset state as shown in FIG. 2(C).

In this state, the non-inverting input of the operational amplifier AI is Ov, and the VO of the operational amplifier A1 is connected to both ends of the photodiode PD.
A potential difference equal to 3 is applied.

Here, vos is the offset voltage of the operational amplifier AI. Then, with the switch SWI in the OFF state, the optical output from the scintillator of the detector XD is transferred to the photodiode P.
D converts it into a signal current. Next, the foot diode PD
The signal current from is charged to the capacitor C1 through the resistor R1.

As the capacitor Ct is charged, the non-inverting input potential of the operational amplifier AI also rises, but the output of the operational amplifier At also follows, so the potential difference of Vos is maintained across the photodiode PD, allowing non-biased operation to continue. become.

When data collection is completed, switch SWI turns OFF → ON.
N, and the charge charged in the capacitor CI is transferred to the operational amplifier A2. At this time, since the signal current from the photodiode PD also flows to the operational amplifier A2, all the signal current from the photodiode PD can be collected, and as shown in FIG. You can now get output from . This is because in a continuous X-ray system, the photodiode PD is operated only with passive elements such as resistors and capacitors, without using an operational amplifier as in the past.
By eliminating radiation exposure, it is possible to suppress the drift errors associated with the use of operational amplifiers that occur in the conventional example, thereby reducing artifacts in data collection and obtaining good images.

Here, the output of the operational amplifier A2 is expressed as follows.

becomes.

Here, 1s(t) is the signal current from the photodiode PD (Fig. 2(d)), and vo is the operational amplifier At
is the output voltage (FIG. 2(d)), and T is the sampling period.

According to the above equation, the sensitivity is determined only by capacitor C2. This is because the capacitor C1 of the front end section does not affect the sensitivity characteristics, which has the advantage of facilitating component selection when manufacturing a multi-channel front end section.

Furthermore, the number of operational amplifiers A2 can be reduced by reading out the output of each CH of the front end section by sequentially turning on the switch SWI of each CH as a bus configuration.
A multi-channel data collection system can be mounted with high density by facilitating component selection. In this case, the number of operational amplifiers A2 may be determined in accordance with the required sampling rate in the configuration of the DAS.

The present invention is not limited to the above embodiments, but can be implemented with various modifications without departing from the gist of the present invention.

[Effects of the Invention] As described above, in the present invention, the photodiode of the solid state detector is connected between the inverting terminal and the output terminal of the operational amplifier, and after charging the output of the photodiode to the capacitor, the signal readout switch is connected. By having a data acquisition system configured to take out the data through the charge/voltage converter and introduce it into the charge/voltage converter and A/D converter, the photodiode can be operated using only passive elements such as resistors and capacitors without using an operational amplifier. Therefore, it is possible to suppress the drift error caused by the use of an operational amplifier, which occurs in the conventional example, thereby reducing artifacts in data collection and obtaining a good image.

Therefore, according to the present invention, it is possible to provide an X-ray CT scanner device that suppresses drift errors in a system using continuous X-rays and a solid-state detector, reduces artifacts in data collection, and obtains good images.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the configuration of a data collection system which is a main part of the X-ray CT scanner device according to the present invention, Fig. 2 is a waveform diagram showing the operation of the same embodiment, and Fig. 3 is the X-ray CT scanner device. FIG. 4 is a diagram showing a conventional example. XD... solid state detector, PD... photodiode,
At, A2... operational amplifier, R1... resistor, C1゜C2... capacitor, SWI... switch, DC...
...Direct charge circuit, Q/V-C...Charge/
Voltage converter, A/D-C...analog/digital converter. Figure 1 (C) 5W2 (d) Figure 2

Claims (1)

[Claims] In an X-ray CT scanner device using a solid-state detector, the photodiode of the solid-state detector is connected between the inverting terminal and the output terminal of an operational amplifier, and after charging a capacitor with the output of the photodiode, a signal readout switch is connected. 1. An X-ray CT scanner device comprising a data collection system configured to take data out through a charge/voltage converter and introduce it into a charge/voltage converter and an A/D converter.