JPH01135331A - X-ray tomographic diagnostic apparatus - Google Patents

Abstract

PURPOSE:To enhance X-ray sensitivity and accuracy, by converting one sucked X-ray quantum to one electric signal pulse. CONSTITUTION:The X-rays generated from an X-ray tube bulb 4 are molded as fanwise beam by a slit to transmit through an object and subsequently absorbed by each element 3 of a semiconductor X-ray detector. Each element of said semiconductor X-ray detector absorbs one X-ray quantum to output one electric signal pulse and the intensity of X-rays is identified as the number of the electric signal pulses outputted from each element by a pulse amplifying/ counting circuit system 5. As the material of each element 3, cadmium telluride (CdTe) or gallium arsenide (GaAs) is used.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to an X-ray detection method for a computer tomography (hereinafter abbreviated as CT) device that performs X-ray tomography of a human body or other body. It provides images.

(Prior Art) Unlike conventional X-ray photography, the X11iACT device
It is used to obtain cross-sectional images of subjects such as the human body.
It was developed by G, N, ) lounsfield et al. of Company I [Reference 1): G, N, H
unsfield: Br1tish J, of
Radiology, vol, 46. pp, 101
6-1022, 1973]. Initially, a single pencil beam and an X-ray measuring device facing it were used, and these were mechanically moved as a pair, and the screen was constructed based on the intensity of the X-rays measured by the Xi measuring device. For this reason.

It took a long time, from several minutes to several tens of seconds, to create one screen.

For this reason, a method has been developed that uses a plurality of measuring instruments arranged on a circumference or an arc and a fan-shaped X-ray beam to improve the speed of parallel measurement. This is the current C
It is a standard type of T.

These X-ray measuring instruments use a scintillation counter or a xenon ionization chamber array [for example, Reference 2]: Edited by Yoshinori Iwai: CT Scanner, published by Corona Publishing], or a sensitive part of a scintillation counter. is a combination of a scintillator and a photomultiplier tube, and a large number of these are arranged in a row. When one xii particle is incident on and absorbed by a scintillator, it emits a fluorescent pulse, which is amplified by a photomultiplier tube and converted into an electric pulse. In this way, scintillation counters are capable of pulse counting of X-ray quanta in principle, but in actual CT, this pulse counting is not performed and the integrated photocurrent based on many fluorescent pulses is calculated using direct current. have been measured [for example, Reference 2].

P, 122), scintillators generally have some afterglow, and even those with a narrow emission pulse width are 1 to 0.1 μs. If you try to count these pulses, if you take in too many X-ray quanta in a short period of time, the pulses will overlap, and if you take them in slowly to avoid overlapping, then one CT
It takes a long time to create a screen. Therefore, in actual CT, direct current measurement of photocurrent has been performed without pulse counting.

In addition, the xenon ionization chamber array was, of course, based on the direct current measurement method.

(Problem to be solved by the invention) The surface resolution of the screen of a conventional CT device is not very large;
Normal (250X 250) pixels/screen.

On a high-end machine (soo x 5oo) pixels/screen. The reasons for this are that the scintillation counter array and xenon ion chamber array mentioned above are 1) structurally incapable of becoming smaller;

2) The measurement accuracy of each measurement element is insufficient. It is in. These will be explained in detail below.

First, regarding point 1) above, the radiation sensitive part of a scintillation counter consists of a scintillator and a photomultiplier tube, and the photomultiplier tube is made especially small for CT use. , still diameter 5
~10 yen. In addition, the xenon ionization chamber has a depth of about 30 cm to increase the absorption efficiency of X-rays, and therefore,
The thickness cannot be reduced to less than several I, and there is a problem in that the structural dimensions are restricted.

Next, the above point 2) will be discussed. FIG. 2 is a diagram showing the principle of object measurement in CT. Although the X-rays 1 pass through the human body 2, they are attenuated in the process and are measured by the measuring device 3. Then, by calculation by a computer, the attenuation coefficient μ8 of each pixel along the X-ray path is calculated. μ2
．． ......,μ. It will make you healthy. If one attempts to increase the number of pixels n here, it is necessary to increase the measurement accuracy of the measuring device 3. That is, each μ,
This is because as n becomes larger, higher accuracy is required for the change in Δμ in the measuring instrument.

However, as is generally said, the DC measurement method has problems with stability and high accuracy cannot be obtained (for example, Reference 2), p. 122) - The DC measurement method has problems with noise and instability factors. is large.

Semiconductor X-ray detectors have never been used in CT scanners, nor were they ever intended to be used. The scintillation counter and xenon ionization chamber described above each have a self-multiplying effect as an X-ray detection element. In the former case, the secondary electron multiplier has a multiplication effect of 1OS to 106 times, and the xenon ionization chamber also has a gas multiplication effect.

Therefore, the output current due to X-ray quanta is large.However, since semiconductor X-ray detectors do not have a multiplication effect in the element itself, the output current is small.Even if you try to DC-amplify the output current due to X@quanta, , which is completely impossible in terms of SZN ratio, and has been excluded from consideration in the past.

SUMMARY OF THE INVENTION An object of the present invention is to provide a compact and efficient semiconductor X-ray tomographic diagnostic apparatus that can perform measurements in a short time.

(Means for Solving the Problems) In order to solve the above problems, the present invention has an array in which a large number of X-ray measuring devices are arranged in a line, and a fan-shaped beam emitted from an X-ray tube. An X-ray tomographic diagnostic apparatus that measures the amount of X-rays by irradiating the measuring device array through the object, repeating this irradiation measurement in each fixed angular direction, and obtaining an image of the cross section of the object from the measured values. A semiconductor X-ray detector array is used as a radiation measuring device, a pulse amplifier and a pulse counter are arranged for each element of this detector, and these sets are arranged in an arc shape or a circumferential shape and detected by the above detector. The pulse signals of X-ray quanta are simultaneously subjected to coefficient processing in parallel, and a tomographic image of the object is constructed based on the number of coefficient pulses obtained by each element and its circuit system.

(Function) A semiconductor X-ray detector converts one absorbed X-ray quantum into one electric signal pulse, so it theoretically has the highest X-ray sensitivity. Furthermore, the pulse counting method has higher accuracy than the direct current method, making it possible to solve the aforementioned r', , 'jM points.

(Example) FIG. 1 is a diagram showing the configuration of the main parts of an X-ray tomography diagnostic apparatus according to the present invention. X-rays generated from the XM tube 4 are shaped into a fan-shaped beam 1 by the slit. After passing through the object, it is absorbed by each probe 3 of the semiconductor X-ray detector. The element material is cadmium telluride (CdT
e) or gallium arsenide (GaAs) is used. These single crystals are cut into small cubes of 0.1 to 0.5 wo cubes, electrodes are attached to opposing surfaces, and a high electric field is applied to these cubes. When one X-ray quantum enters this crystal, it emits one high-speed secondary electron due to the photoelectric effect, and the xis quantum is absorbed. High-speed secondary electrons run through the crystal, excite many electron-hole pairs along their range, and stop after traveling several tens of micrometers. A high electric field is applied inside the crystal, and excited electrons run toward the anode and holes toward the cathode.

The movement of these electron holes generates one electric signal pulse in the external circuit.

In this way, one X-ray quantum is absorbed by each element of the semiconductor X-ray detector to obtain one electrical signal pulse.

This electrical signal pulse occurs irregularly. Because X
This is because the incident absorption of line quanta is stochastic and does not occur regularly. What is the X-ray intensity at this time?

It is recognized as the number of detection signal pulses. FIG. 3 shows an example of observing such a detection signal pulse.

Both the pulse interval and wave height are irregular. -Generally,
In electronics, digital code signals are used, in which one pulse is given for a certain physical quantity. On the contrary, one pulse is clearly a significant amount. However, one pulse signal corresponding to the above-mentioned xi quantum is
It only means that the amount of X-rays corresponding to 1 exists stochastically, and 1 as a digital quantity in electronics.
It is different from. CT devices handle pulse signals as digital quantities when composing images.

Due to the need for arithmetic processing, it is necessary to convert the above-mentioned stochastic pulse signals into electronic digital quantities.

Needless to say, as the intensity of the X-rays increases, the number of pulses increases, but the reliability also increases. In other words, when n observations are made, the probability is within the range nfv"ir6. For example, n = 100
, then the X11Aik of that point lies between 100+vTQT, ie between 90 and 110.

Since this is an error of 10%, 100 counts is regarded as the minimum unit amount of X-rays that is reliable, and is considered to be 1 digit, that is, 100 counts is 1 digit. Of course, 1000 counts may be used as 1 digit, and the conversion is performed by determining the promise (reliability) of 1 digit. One image calculation is performed using the new quantities converted in this way.

A design example of an X-ray CT device with approximately 1000 x 1000 pixels will be described. The imaging time for one screen of CT equipment is
It is desirable to have the shortest possible time, and it is necessary to design it at 1 to 10 seconds/screen. In FIG. 2, if each element 3 and circuit system 5 of the X-ray detector has 1000 channels, the screen will have higher resolution than the conventional one. The rotational exposure is also done in xoooi steps.

If these design conditions are set, one exposure time will be 1
It becomes ~10 fertilizer. In order to obtain highly accurate measurement data,
As mentioned above, it is necessary to capture 10' to 10' counts in one exposure measurement, but for this purpose, 3
A pulse count rate of X10' to 10' cps is required.

Although this value is very large, C
This was achieved with a semiconductor X-ray detector using dTe and GaAs. In this case, the pulse width needs to be 100 to 0.3 ns. FIG. 4 is a diagram illustrating the relationship between pulse width and pulse overlap. The pulses in Figure (A) do not overlap, but in Figure (B), where the pulse width is longer, pulses overlap. Even if two X@ quanta are incident and absorbed, only one pulse can be generated, resulting in an error. Pulse count rate is 3×10
'~10'cps, pulse width is 100~0.3
If it is ns, the overlap rate is low.

FIG. 5 is a structural diagram of an array including a semiconductor crystal 6 that makes such characteristics possible and a field effect transistor 8 as a first-stage pulse amplifier.

This is an example in which electrodes 7 are attached above and below an elongated single crystal 6.

In order to extend the frequency characteristics to a high frequency range, it is preferable that the electrode spacing of the single crystal 6, that is, the crystal thickness, be small. For CdTe, it is 0.5 mm or less, and for GaAs, it is 2 mm or less. This shortens the transit time of electron-hole pairs. Also,
In order to reduce stray capacitance, it is necessary to make the length of the wiring between the single crystal 6 and the first stage amplifier 8 as short as possible. That is, the crystal and the first stage amplifier are placed as close as possible and connected using thin lead wires. These structures make it possible to obtain output pulses with narrow pulse widths.

Figure 6 shows a 24"
This is an example of measurement of 60 k eV γ rays of Am. Pulses of various wave heights are observed irregularly. Among these, one with a low wave height value catches the eye. This is a pulse of scattered rays generated when 60 k eV γ rays are scattered by an object or the element itself. In actual image measurement, this low-wavelength pulse does not contain image data, so it is better to remove it. this is,
This can be achieved by discriminating the wave height using a discriminator circuit and selecting only those whose wave height is larger than the discriminator level.

(Effects of the Invention) As described above, according to the present invention, since a semiconductor Xi detector is used, it is possible to realize an X-ray CT device with a larger number of pixels and better surface resolution than before. Ta.

Furthermore, according to the present invention, since a semiconductor X-ray detector is used, it is possible to realize a compact X-ray CT apparatus with short measurement time.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an X-ray tomography diagnostic apparatus according to an embodiment of the present invention, FIG. 2 is a diagram explaining the principle of X-ray CT, and FIG. 3 is an example of observation of X-ray pulses in a semiconductor X-ray detector. FIG. 4 is a diagram showing examples of pulses with excellent time resolution and pulses with poor temporal resolution. FIG. 5 is a diagram showing an example of the structure of a semiconductor X-ray detector array. FIG. 6 is a diagram showing an example of a semiconductor Iil with ray detector
! FIG. 3 is a diagram showing an example of observation of many pulses caused by measured X-rays. DESCRIPTION OF SYMBOLS 1... X-ray beam, 2... Human body, 3... Each element of semiconductor X-ray detector, 4... X-ray tube, 5...
Pulse amplification/counting circuit system. Figure 1 1 °X beam 3°°°Ushi Hayabusa (see
°゛Pyinto Masanobu・↓Yako Urohe 501 ”°゛ λ
71st period hashle included) - Width L 502... Palmata Yagura @
J Tun 503... N\゛S Meter & Kyu Takashi Fig. 2 Fig. 3 □ Closing time Fig. 4 Closing Fig. 5 Fig. 6 □ Screaming threshold

Claims (5)

[Claims] (1) It has an array in which a large number of X-ray measuring devices are arranged in a line, and measures the X-ray dose by irradiating a fan-shaped beam emitted from an X-ray tube to the measuring device array through the subject; In an X-ray tomography diagnostic device that repeats this irradiation measurement in each fixed angular direction and obtains a cross-sectional image of the object from the measured values, a semiconductor X-ray detector array is used as the X-ray measuring device, and each element of this detector A pulse amplifier and a pulse counter are arranged for each element, these sets are arranged in an arc shape or a circumferential shape, and the pulse signals of X-ray quanta detected by the above detector are simultaneously coefficiented in parallel, and each element and its circuit system are 1. An X-ray tomographic diagnostic apparatus, characterized in that a tomographic image of a subject is constructed based on the number of coefficient pulses obtained by. (2) When the number of X-ray quanta counted by each element of the semiconductor X-ray detector and its circuit system reaches a certain value, convert this into 1-bit binary digital code data,
The X-ray tomographic diagnostic apparatus according to claim 1, wherein a tomographic image is obtained by calculating a series of these code data. (3) X-ray tomographic diagnosis according to claim (1), characterized in that low pulse heights below a certain value are removed from the output pulses of each element of the semiconductor X-ray detector through a pulse height discriminator. Device. (4) A patent characterized in that the pulse width of the output pulse of each element of the semiconductor X-ray detector is in the range of 0.3 ns to 100 ns, and the counting rate is in the range of 3 mega cps to 1000 mega cps at maximum. X described in claim (1)
Linear tomography diagnostic device. (5) The X according to claim (1), wherein the material of each element of the semiconductor X-ray detector is cadmium telluride (CdTe) or gallium arsenide (GaAs).
Linear tomography diagnostic device.