JPS5835481A - Measuring device for radiation - Google Patents

Abstract

PURPOSE:To enable direct count of the number of light quantums according to conditions, by counting output pulses by using a double-integrating digital transducing system when the dose rate of incident radiation is high, and by using a direct counting system when the rate is low. CONSTITUTION:When the incidence of the radiation having a dose rate higher than a level in a radiation detecting element D is expected, a controller CNT makes a selection switch SWc operate to select an output pulse of an AND gate AND2. In this case, the output of the element D is stored in an integrator INT1 through the intermediary of a filter FTR. When a prescribed time passes, an electric charge stored in an integrating capacitor C of INT1 is discharged at a constant velocity by an output of a constant current source, and during a time period until the completion of the discharge, a counter CTR counts clock pulses of AND2. On the other hand, when the dose rate of the incidence is expected to be low, the switch SWc is shifted to the output side of a pulse-height discriminating circuit PHA, and the output pulses of the element D, discriminated from noise by the circuit PHA, is given to the counter CTR to be counted.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a radiation needle-side device used in a so-called combination tomography device and the like and for transmitting radiation dose as digital information to the needle side.

The so-called Combiator Tomograph 4 (Comput
＠riz＠d tomography' (hereinafter referred to as a CT apparatus) scissor holder; An xI made by installing multiple vx@detecting elements in parallel to detect xox*! The X-ray source 1 and XII detector 2 are placed opposite to each other with the subject P in between, and the X-ray source 1 and the XII detector 2 are rotated in the same direction and at the same angular velocity around the subject P, so that the After collecting X-ray projection data in various directions and collecting sufficient data, this data is analyzed by a computer to calculate the xIs absorption coefficient corresponding to each position in the cross section of the object. The image information in the cross section of the object is reconstructed by giving a gradation level according to the absorption rate, and a clear tomogram can be obtained from soft tissue to hard tissue.

The X-ray detector 2 is configured, for example, as a radiation detector in which a high-pressure gas of Xs (xenon), which is filled with a large number of radiation detection cells each forming an ionization chamber, is transmitted through a cross section of the subject P. X1! ! The energy is detected as an ionization current, and this is output as detection data by X-ray projection.

In other words, in collecting this X-ray projection data, X-rays incident on the path that connects each radiation detection cell making up the ionization chamber and the X-ray source 1 (this is called the "X-ray/4 rus") are used to collect the X-ray projection data. Photons collide with high-pressure gas and are ionized. By extracting the ionized charge, the energy of the incident X-ray is detected as an ionization current, which is integrated over a predetermined time, and the integrated value is applied to a discharge circuit with a predetermined time constant. Discharge at
# This is XII projection data for one pass.

When data collection for all X fg /# beams at one angular position is completed, data collection for each X-ray beam at the next angular position is started.

By the way, conventionally, as mentioned above, in the measurement of radiation in a CT apparatus, as shown in FIG. 2, an amplifier for amplifying the output is connected to each of a large number of radiation detection elements.
Photons of X-rays arriving at each radiation detection element are detected by the radiation detection element, amplified, and then integrated by an integrator INT connected to the subsequent stage of the Angoo.

Then, the output of each integrator INT is selectively extracted by the multi-bleph sensor MP, which sequentially selects and extracts the output of each integrator INT, and after converting it into a digital quantity by feeding it to the ψ converter ADO, this is converted into a digital quantity for each channel. The data is sent as X-ray absorption data to a host system such as a computer for processing.

In this case, the system from the radiation detection element to the output side of the integrator INT is an analog system.

In the circuit shown in Figure 2, the fan beam X for one pulse is
Each time data collection by FX exposure is completed, the integrating capacitor C of the integrator INT is short-circuited and discharged by the switch SW to prepare for the next data collection.

That is, in the case of FIG. 3, a double integration type integrator INT1 is used as the integrator INT, the output of this integrator INTI is compared by a comparator CMP, and the comparison output is multiplied by an AND gate ANDI. While controlling and extracting the radiation, the extracted output is used as a gate signal to extract clock pulses while controlling the AND gate AND 2, which is counted by the counter CTR, thereby obtaining a count value corresponding to the detected radiation dose and converting it into a digital quantity. *X* absorption data.

That is, the integrator INT1 is provided with a switch SWa for opening and closing the input signal at its input stage, and during the X-ray exposure period for one pulse, this switch 8Wa is closed and the output of the radiation detection element is connected to the integrating capacitor C. Accumulate in. This accumulated charge of the next capacitor is given to the comparator CMP, and the reference voltage vrvf is
The comparator CMP generates an output for a period during which the charge exceeds the reference voltage vr*f.

This comparator output is XI! During the exposure pause period, one pulse of
Immediately after the radiation exposure is completed, the comparator output is taken out via this AND1.

On the other hand, the integrator INT 1 is connected to a constant current source consisting of a DC power supply V which applies charges of opposite polarity to the integrating capacitor C, a resistor 8 and a switch gwb, and the output of the ANDZ is connected to the integrator INT1. Therefore, as soon as the X-ray exposure is finished, the switch n is closed and a constant current is passed to the integrator INT10.
It is applied to the input side, and the accumulated charge in the integrating capacitor C is discharged at a constant rate 111'lIr1L.

Therefore, the output of the integrator INT_I decreases and eventually stops below the level of the reference voltages vr, f. Then, the comparator commercial
The P output disappears, and the output of the AND gate ANDZ also disappears.

During this period, the output of the AND gate AND1 is given as the gate control signal of the AND gate AND2, so this AND/DIRT AND:I extracts the clock pulse that is separately applied during the input period of this dart control signal. Give to counter CTR. For this, counter C
In order to count the count value corresponding to the accumulated charge of the integrator INT1, TRFi calculates the digital value corresponding to the amount of incident X-rays on the radiation detection element. It can be obtained as I absorption data.

The above is the measurement method using double integration. None of these methods directly count X@photons, but instead indirectly count incoming X-ray photoelectrons by replacing them with current values. For this reason, an analog system is involved, so the subject is extremely When the radiation dose is large, or when the radiation dose is significantly lowered to reduce the exposure dose, the output signal of the detector becomes extremely small, and the influence of the electrical noise of the analog system becomes significant. There remain drawbacks such as the fact that the physical limit can no longer be achieved and the S//'N ratio of the obtained signal is slightly inferior to the S/N ratio determined by the number of arriving photons due to the broadening of the energy spectrum p of the arriving photons.

Furthermore, the fact that the energy spectrum of the arriving photon is broadened, the fact that in analog measurement methods the detector output is generally a function of the energy E of the arriving photon, and the fact that CT
In the case of the device, due to the fact that the X-ray absorption coefficient of the subject is also a function of the incoming photon energy E, there are two methods: one is to measure the X-ray photons indirectly by replacing them in the form of an electric current, and the other is to directly measure the X-ray photons. The image quality information obtained differs depending on the measurement method that counts light quanta, and according to Shimemi 2, there are cases where objects with image contrast that are difficult to see with the former measurement become more visible when using the latter measurement method that directly counts light quanta. I found out that there are some.

However, if we try to apply the method of counting each incoming photon one by one to a CT device, it will be difficult to
It is actually impractical to require a circuit with a response speed of several GHz and a radiation detection element as a measurement system, and therefore, in the past, measurement methods using analog systems have been unavoidable.

The present invention has been developed in view of the above-mentioned circumstances, and includes the following features:
a double-integration type digital conversion circuit that double-integrates the output of the radiation detection element and generates a pulse corresponding to the number of photons based on the product value; It consists of a /4' pulse wave height discrimination circuit, a circuit that selects the output pulses of both of these circuits, and a counter that counts the selected output pulses. By counting the output of the pulse height discrimination circuit or, when imaging at other radiation dose rates, the output of the double integral type digital conversion circuit and using this as radiation absorption data of the subject, the light quanta can be dispersed. When imaging at a low radiation dose rate, where it is relatively easy to count the light quanta individually, the photons are counted directly, and when imaging at a relatively high radiation dose rate, where it is difficult to directly count the light quanta individually, it is possible to count the photons directly. A multi-integral type digital conversion circuit generates pulses corresponding to the number of incident photons and collects radiation absorption data corresponding to the number of incident photons, making it possible to directly count the number of photons depending on the situation. The purpose is to provide a relatively inexpensive radiation measurement device.

An embodiment of the present invention will be explained below with reference to Figure WJ4.

#! In Fig. 4, D is a radiation detection element, and this radiation detection element is capable of generating a Norlus output corresponding to each arriving light quantum when radiation is incident at a low radiation dose rate. There is.

FTR is a low/arm type filter that smoothes the output of this radiation detection element, and INT1 is this filter FTR.
Double integrate the output. This integrator is the same as the integrator INT1 shown in Figure 3 above.

CMP is the integrator INT based on the reference voltage ■1°.
A comparator that compares the outputs of 1 and generates an output while the comparison input exceeds the reference voltage vref, AND r Id is output by a control system (not shown) during the exposure pause period after radiation exposure within the imaging period. Pass the control signal of the comparator AND I as a gate control signal, and pass the output of AND1 as a dart control signal. The second andf-
), integrator INT 11 comparator CMP, and dart AND 1 +AND: The part consisting of I is the third
The configuration is exactly the same as that shown in the figure. Therefore, in this device as well, the constant current source for constant current discharge of the integrating capacitor C, which is composed of the DC power supply V and the switch of the resistor R1 provided in the integrator INT1, is the first AND.

The configuration is such that the dart AND 1 output closes the switch and discharges a constant current.

The above filter FR1, integrator IN'r1, comparator CMP
, AND gates AND 1 and AND j constitute a light amount pre-detection system using an analog system.

The PHA receives the output of the radiation detection element as an input, discriminates the wave height, and as a result of the discrimination, if the input is within a predetermined level range, it passes through the Nolles wave height discrimination circuit, gee.
Fi This I arm pulse height discrimination circuit PHA output or the AND f-) ANDJ output pulse is a selection switch that selects one output, and CNT is selected depending on the magnitude of the radiation dose rate used. A controller for switching and controlling the switch 8Wc, CTRFi, is a counter that counts pulses applied via the selection switch SWe.

Next, the operation of this apparatus having the above configuration will be explained.

Normally C) Final scan of the radiation a that enters the radiation detection element in the X@CT device (imaging, i.e., data collection)
It is predictable to some degree before the start of. This is because, before scanning, the host duo knows the slice width of the cross-sectional view to be photographed under the X@ exposure conditions, the size of the scan field (photographing region), etc. The size of the scan field is indicative of the size of the object and therefore
Line attenuation can also be estimated to some extent.

Furthermore, it is often known which part of the subject to scan. Otherwise, the operator may know the information as to whether the scan is a high dose rate scan or a low dose rate scan and may input that information into the system manually or by other means.

Based on this information, when the radiation detector is expected to be exposed to radiation at a dose rate higher than a certain level, the controller CNT operates the selection switch c to select the output pulse of the second AND j. Switch as follows. Radiation detection elements are used that have a fast enough response to produce a pulse output corresponding to each incoming X-ray photon at a certain low incident dose rate, but images are taken using radiation at a high dose rate. In one case, substantially continuous detected objects are output from the radiation detection element. This output is smoothed through a filter FTR and then passed through an integrator IN.
T1 is stored in
The charge stored in the integrating capacitor C of TZ is discharged at a constant rate by the current from the constant current source, and the counter CTR counts clock pulses until the discharge is completed. The low-pass filter has an integrating circuit that handles high frequency components.
This was added out of concern that it would cause errors, and is essentially unnecessary.

That is, the integrator INT 1 is provided with a switch SWa for opening and closing the input signal at its input stage, and for one pulse of X @ exposure time, this switch 8Wa is closed and the radiation detection element applied via the filter FTR is The output of is stored in the integrating capacitor C. The accumulated electric charge of the capacitor is applied to the comparator CMP, and the basic electric power F! , vr, f, and during a period in which the charge exceeds the reference voltage vr*f, the comparator CMP generates an output.

The output of this comparator is added to an AND gate AND J which is controlled by a control signal (not shown in the control system) during the X-ray exposure pause period, and XI! Immediately after the exposure ends, this and goo) AN
The comparator output is taken out via DI.

On the other hand, the integrator INT1 is connected to a constant current source consisting of a DC power supply V, a resistor R, and a switch (T), which provides a charge of opposite polarity to the integrating capacitor CK, and the above AND1. At the output, the switch SW
Since the configuration is such that the switch swb is closed, the switch swb is immediately closed when the X-ray exposure is finished, and a constant current is supplied to the integrator IN.
When applied to the input side of T I, the accumulated charge in the integrating capacitor C is discharged with a constant discharge current.

Therefore, the output of the integrator INT1 decreases and eventually becomes below the level of the reference voltage r*f. Then, the comparator commercial
There is no P output, and the ANDZ output (Cal, ANDG) also disappears.

During this period, the output of AND1 is given to AND2 as a dart control signal, so this AND gate AND2 extracts the clock pulses and pulses that are applied separately during the input period of this f-) control signal. and gives it to counter CTH. As a result, the counter CTRd counts a count value corresponding to the accumulated charge of the integrator INT I, and a digital value corresponding to the amount of incident xII on the radiation detection element can be obtained as X-ray absorption data.

The above is the needle side method of measuring the output of the radiation detection element as a digital value using double integration during XII irradiation at a high dose rate.

On the other hand, when the incident dose rate to the radiation detection element is expected to be lower than a certain level, the controller CTR switches the selection switch 8We to the /# pulse height discrimination circuit PEA output value. In this case, the number of incident photons is small due to the low dose rate of XII exposure, and therefore, the incident photons are discrete, so the output of the radiation detection element is in this case individual xII
This results in a Norse sequence corresponding to the arrival of light quanta. This is discriminated from noise by the pulse height discriminator PHA and shaped to the OA pulse height enough to drive the counter CTR. This /gus is given to counter CTR, counter CT
R counts this /4 rus.

At the end of counting within a sampling period, the counting result, which corresponds to the number of arriving photons within that period, is sent to the host combination.

In this way, we have designed two circuits, a double-integral type digital conversion system and a direct counting system that uses the I-quantity pulse height discrimination circuit as the counting output to determine the magnitude of the incident radiation dose rate. When the incident radiation dose rate is high, a double integral type digital conversion system is used, and when the incident radiation dose rate is low, a direct counting system is used to calculate the output/frus. Since the radiation dose incident on the radiation detection element is measured by counting, in the case of the conventional method, when the incident dose rate to the detector is extremely low as mentioned above, the electrical noise of the analog system and the characteristic triad However, since we count each photon individually, this problem can be avoided, and at high dose rates, conventional analog systems, i.e.,
Since a double integral type digital conversion system is used, it is sufficient that the radiation detection element/high-speed discrimination circuit and the counter have high-speed response characteristics that can cope with the incident frequency of incident photons at a spread dose rate. N9. Therefore, this makes it possible to directly reduce the number of photons in the X@CT@ configuration, and furthermore, since the counter can be shared by the two circuit systems mentioned above,
In addition to reducing costs and saving space, it is also possible to use both conventional scans and scans by direct photon counting when the quantity rate is low. It is possible to provide a radiation measuring device having excellent features such as the possibility of obtaining information.

[Brief explanation of the drawing]

111(a)(b) C. An example of a CT device O1 [in 11'
2 and 3 are block diagrams showing a conventional example of a device for directing the incident radiation dose to the needle side. Fig. 4 is a block diagram showing an embodiment of the present invention. It is a diagram. I...X@source, 2...Radiation detector, D...Radiation detection element, FTR...Filter, lNTl...Integrator, CMP...Comparator, AND Z, AND
! ...AND gate, P)IA...Rigulus wave height discrimination circuit, swC...selection switch, CNT...controller, CTR...counter.

Claims (1)

[Claims] In a device that measures the amount of incident radiation by the output of a radiation detection element that generates an output corresponding to the number of photons of incident radiation, the output of the radiation detection element is integrated, and a number of nofs corresponding to the integral value is obtained. A conversion circuit for outputting, a discrimination circuit for discriminating the wave height of the output of the radiation detection element and outputting an output within a predetermined level range, a switch for selecting one of the outputs of these two circuits, and counting the selected output. and a counter that outputs the count value as incident radiation detection data, and by selecting the output of the discrimination circuit when the dose rate of the incident radiation is low, it is possible to directly count the photons of the incident radiation. Radiation measuring device.