KR20150027340A - X-ray image apparatus and control method for the same - Google Patents

Abstract

One embodiment of a method of controlling an x-ray imaging device includes: setting a first threshold voltage of a photon coefficient detector at a first time; Obtaining reference x-ray data corresponding to the first threshold voltage; And correcting the first threshold voltage to a second threshold voltage with reference to the reference X-ray data at a second time point.
According to one aspect of the X-ray imaging apparatus and the control method thereof, the accuracy of the X-ray image can be improved by correcting the error caused by the change of the threshold energy and the threshold voltage with time.

Description

X-RAY IMAGE APPARATUS AND CONTROL METHOD FOR THE SAME

Ray imaging apparatus for generating an x-ray image by transmitting an x-ray to a target object, and a control method thereof.

An X-ray imaging apparatus is an apparatus that can acquire an internal image of an object by irradiating the object with the X-rays and using the X-rays transmitted through the object. Since the transmittance of the X-rays differs depending on the characteristics of the material constituting the object, the intensity or intensity of the X-rays transmitted through the object can be detected to image the internal structure of the object.

Specifically, when an X-ray is generated in the X-ray generator and irradiated to a target object, the X-ray detector detects an X-ray transmitted through the target and converts the detected X-ray into an electrical signal. Since the electrical signal conversion is performed on a pixel-by-pixel basis, a single x-ray image can be obtained by combining electrical signals corresponding to each pixel.

In the past, a method of accumulating electrical signals for a predetermined period of time has been mainly applied. Recently, a photon counting detector has been developed that counts photons over a certain energy and separates the detected X-rays by energy.

The photon counting detector is capable of separating a specific substance from the x-ray image, and has the advantage of less exposure to the x-ray and less noise. However, since the physical characteristics of the light receiving element or the readout circuit change with time, The relationship between the voltages can be changed. For this reason, even if the same energy is incident, a different counter value may be output, causing noise to be generated in the image.

One aspect of the X-ray imaging apparatus and its control method provides an X-ray imaging apparatus and a control method thereof capable of correcting an error of a photon coefficient detector according to an irradiation time of an X-ray.

One embodiment of a method of controlling an x-ray imaging device includes: setting a first threshold voltage of a photon coefficient detector at a first time; Obtaining reference x-ray data corresponding to the first threshold voltage; And correcting the first threshold voltage to a second threshold voltage with reference to the reference X-ray data at a second time point.

One embodiment of the x-ray imaging apparatus includes an x-ray generator for generating an x-ray and irradiating the x-ray to a target object; An x-ray detector for detecting the x-ray and counting the number of photons having an energy greater than a threshold energy among the photons contained in the detected x-ray; And a controller for setting a first threshold voltage corresponding to the threshold energy at a first time point and correcting the first threshold voltage to a second threshold voltage at a second time point.

According to one aspect of the X-ray imaging apparatus and the control method thereof, the accuracy of the X-ray image can be improved by correcting the error caused by the change of the threshold energy and the threshold voltage with time.

1 is an external view illustrating a typical X-ray imaging apparatus.
2 is a control block diagram of an embodiment of an x-ray imaging apparatus.
3 is a view showing a configuration for an embodiment of the x-ray tube.
4 is a graph showing the relationship between the energy and the damping coefficient for each substance in the object.
5A is a diagram schematically showing the structure of an X-ray detector in an embodiment of the X-ray imaging apparatus.
FIG. 5B is a view schematically showing a single pixel region of the X-ray detecting unit shown in FIG. 5A.
6 is a graph showing the relationship between the energy of the photons included in the X-ray input to the X-ray detecting unit and the voltage output from the X-ray detecting unit.
FIG. 7 is a graph showing a change in attenuation coefficient with respect to energy of an X-ray irradiated to gold according to an embodiment of a K-edge filter.
8 is a graph showing the change in keV-mV relationship with time.
9 is a graph showing the relationship between the threshold voltage measured at different times and the normalized x-ray intensity
10 is a control block diagram for one embodiment of an x-ray imaging device.
11 is a diagram for explaining a method of correcting the first threshold voltage in the correcting unit.
12 is a diagram showing a method of updating the keV-mV relation based on the second threshold voltage.
13 is a flowchart of a method of correcting a threshold voltage according to an irradiation time of an X-ray.
14 is a flowchart specifically for explaining a method of correcting a first threshold voltage at a second time point.

Hereinafter, embodiments of an X-ray imaging apparatus and a control method thereof according to an aspect of the present invention will be described in detail with reference to the accompanying drawings.

The X-ray imaging apparatus may be structurally different from the X-ray imaging apparatus depending on the imaging site, the type of X-ray image, or the purpose of imaging. Specifically, a general X-ray imaging apparatus for photographing a chest, an arm, a leg, an X-ray imaging apparatus using mammography, an X-ray imaging apparatus using fluoroscopy, an X-ray using an angiography, An X-ray imaging apparatus for cardiography, an X-ray imaging apparatus using tomography, and the like. The X-ray imaging apparatus according to an embodiment of the disclosed invention may be any one of the X-ray imaging apparatuses, Two or more x-ray imaging devices may be combined.

1 is an external view illustrating a typical X-ray imaging apparatus.

Referring to FIG. 1, a typical X-ray imaging apparatus may include an X-ray generator 110, an X-ray detector 120, and a host device 170.

The X-ray generator 110 may generate an X-ray to acquire an X-ray image of the object 35 and irradiate the inspected object 30 with the generated X-ray.

Here, the subject 30 may be a living body of a human being or an animal, but the present invention is not limited thereto. The subject 30 may be a subject 30 if the internal structure thereof can be imaged by the X- have.

In addition, the object 35 means a part to be diagnosed in the subject 30 using the X-ray imaging apparatus 100, that is, an X-ray imaging site. Accordingly, in the case of a general X-ray imaging apparatus as shown in FIG. 1, the object 35 may be a chest, arm, leg, or the like.

The X-ray generating unit 110 may be connected to the ceiling through a holder 102 mounted on the X-ray generating unit 110. The position of the X-ray generating unit 110 may correspond to the position of the object 35 by adjusting the length of the holder 102. The holder 102 may be vertically adjustable in length.

The X-ray detecting unit 120 is disposed on the opposite side of the X-ray generating unit 110 with the object 35 interposed therebetween so as to detect the X-rays transmitted through the object 35 irradiated by the X- In addition, the X-ray detector 120 can convert the detected X-rays into an electrical signal.

One end of the X-ray detector 120 may be mounted on the support 101 so as to be movable up and down. Therefore, the position of the X-ray detecting unit 120 can be moved corresponding to the position of the target object 35. [

1, the subject 30 is laid on a table, the X-ray generating unit 110 is mounted movably in the ceiling of the table in the longitudinal direction of the table, the X-ray detecting unit 120 is moved in the table longitudinal direction It is also possible to mount it movably.

The host apparatus 170 is provided with an input unit 171 for receiving commands from a user and a display unit 172 for displaying x-ray images, thereby providing a user interface. Here, the user may be a medical staff including a doctor, a radiologist, a nurse, and the like as a person who performs diagnosis of the target object 35 using the X-ray imaging apparatus 100, but the present invention is not limited thereto, It is assumed that the user can be all users who use the service.

The input unit 171 may include at least one of a switch, a keyboard, a trackball, and a touch screen, but is not limited thereto.

The display unit 172 can be applied to a cathode ray tube (CRT), a liquid crystal display (LCD), an organic light emitting diode (LED), or the like, but is not limited thereto .

2 is a control block diagram of an embodiment of the X-ray imaging apparatus.

2, an embodiment of the X-ray imaging apparatus 100 includes an X-ray generating unit 110 for generating an X-ray to irradiate the X-ray to a target object 35, an X- And a controller 130 for correcting the X-ray data and the X-ray data according to the characteristics of the light receiving element or the readout circuit for each pixel.

The X-ray generator 110 generates an X-ray to irradiate the object 35. The X-ray generator 110 receives power from a power supply unit (not shown) to generate X-rays. The energy of the X-rays can be controlled by the tube voltage, and the X-ray intensity or dose is controlled by the tube current and the X- .

The X-ray generating unit 110 may irradiate a monochromatic X-ray or a polychromatic X-ray, but in this embodiment, the X-ray generating unit 110 irradiates a multi-color light ray having a certain energy band , The energy band of the irradiated X-rays is defined by the upper limit and the lower limit.

The X-ray generating unit 110 includes an X-ray tube 111 for generating an X-ray.

3 is a view illustrating the configuration of an X-ray tube.

3, the X-ray tube 111 may be a bipolar tube including an anode 111c and a cathode 111e, and the tube may be a glass tube 111a made of a material such as a hard silica glass .

The cathode 111e includes a filament 111h and a focusing electrode 111g for focusing electrons and the focusing electrode 111g is also called a focusing cup. The inside of the glass tube 111a is made to a high vacuum state of about 10 mmHg and the filament 111h of the cathode is heated to a high temperature to generate thermoelectrons. As an example of the filament 111h, a tungsten filament can be used, and the filament 111h can be heated by applying an electric current to the electric conductor 111f connected to the filament. However, the embodiment of the disclosed invention is not limited to the use of the filament 111h in the cathode 111e, and it is also possible to use a carbon nano-tube that can be driven by a high-speed pulse as a cathode.

The anode 111c is mainly made of copper and a target material 111d is coated or disposed on the side facing the cathode 111e and the target material is a high resistance material such as Cr, Fe, Co, Ni, Can be used. The higher the melting point of the target material, the smaller the focal spot size.

When a high voltage is applied between the cathode 111e and the anode 111c, the thermal electrons are accelerated and collide with the target material 111d of the anode to generate X-rays. The generated X-rays are irradiated to the outside through the window 111i, and a beryllium (Be) thin film can be used as the material of the window.

The target material 111d can be rotated by the rotor 111b and when the target material 111d is rotated, the heat accumulation rate can be increased by 10 times or more per unit area as compared with the case where the target material 111d is fixed, and the focus size is reduced .

The voltage applied between the cathode 111e and the anode 111c of the X-ray tube 111 is referred to as a tube voltage, and its magnitude can be expressed by the peak value kvp. As the tube voltage increases, the speed of the thermoelectrons increases and consequently the energy (photon energy) of the x-ray generated by collision with the target material increases. The current flowing through the X-ray tube 111 is referred to as a tube current and can be expressed as an average value mA. When the tube current increases, the dose of the X-ray (the number of the X-ray photons) increases.

Accordingly, the energy level of the X-ray can be controlled by the tube voltage, and the intensity or the dose of the X-ray can be controlled by the tube current and the exposure time of the X-ray. And strength can be controlled.

The X-ray generating unit 110 generates an X-ray using the X-ray tube 111 described above and irradiates the generated X-ray to the subject 30, more precisely, the target 35.

When the object 35 is irradiated with the X-rays from the X-ray generator 110, the degree of attenuation of the X-rays varies depending on the material inside the object 35 and the energy level of the irradiated X-rays. Here, the numerical representation of the degree of attenuation of the x-ray is called the attenuation coefficient.

First, the damping coefficient varies depending on the material inside the object 35.

For that example, reference is made to FIG. 4 is a graph showing the relationship between the energy and the damping coefficient for each substance in the object. The x-axis represents the photon energy irradiated to the object 35, and the y-axis represents the attenuation coefficient.

4, the curve representing the damping coefficient of the bone is located above the curve representing the damping coefficient of the soft tissue (muscle, fat). Specifically, X-rays for example in the same energy level as when E ₁ investigation, the damping coefficient of the bone (B ₁₎ is greater than the attenuation coefficient of the muscle (M _1), the attenuation coefficient of the muscle (M ₁₎ is a local Is greater than the damping coefficient (F ₁ ).

That is, the different materials in the object 35 have different attenuation coefficients, and the damping coefficients increase as the material properties become stiffer.

Second, the damping coefficient depends on the energy level of the irradiated X-rays.

In the graph of FIG. 4, when x-rays having energy levels E ₁ and E ₂ are irradiated to bones, which are substances inside the object 35, the attenuation coefficient (B ₁ ) at E ₁ , Is larger than the damping coefficient (B ₂ ) at a high E ₂ . The attenuation coefficient (M ₁ ) when the energy level E ₁ is irradiated, F ₁ ) is the attenuation coefficient when the energy level E ₂ is irradiated even when the substance inside the object 35 is muscle or fat (M ₂ , F ₂ ), respectively.

That is, the damping coefficient increases as the energy level of the X-rays irradiated on the object 35 becomes lower.

Such a damping coefficient can be expressed by the following equation (1).

[Equation 1]

Where I ₀ is the intensity of the x-ray irradiated to the substance, I is the intensity of the x-ray transmitted through the substance, and μ (E) is the attenuation coefficient of the substance to the x- T is the thickness of the material through which the x-rays are transmitted.

According to Equation (1), it can be seen that as the damping coefficient increases (i.e., the harder the material property or the lower the energy level of the irradiated x-ray), the thicker the material, have.

Referring again to FIG. 2, the X-ray detector 120 detects the X-rays transmitted through the object 35, and converts the detected X-rays into electrical signals to acquire X-ray data.

In general, the X-ray detecting unit 120 can be classified according to a material construction method, a method of converting the detected X-rays into an electrical signal, and a method of acquiring X-ray data. Hereinafter, To an electrical signal to obtain x-ray data.

First, the X-ray detecting unit 120 is divided into a case where the X-ray detecting unit 120 is composed of a single type device and a case where the X-ray detecting unit 120 is composed of a horn molding device.

In the case of a single-element device, it corresponds to a case where a part for detecting an X-ray to generate an electric signal and a part for reading and processing an electric signal are made of a single material semiconductor or manufactured by a single process. For example, (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor), which are devices.

In the case of a horn-forming device, a portion for detecting an X-ray to generate an electrical signal and a portion for reading and processing an electrical signal are formed of different materials or manufactured by different processes. For example, when an X-ray is detected using a light receiving element such as a photodiode, a CCD, or a CdZnTe and an electrical signal is read and processed using a CMOS ROIC (Read Out Integrated Circuit), the X- When reading and processing electrical signals using ROIC, and when using an a-Si or a-Se flat panel system.

The X-ray detector 120 is divided into a direct conversion method and an indirect conversion method according to a method of converting an X-ray into an electrical signal.

In the direct conversion system, when an X-ray is irradiated, an electron-hole pair is temporarily generated inside the light-receiving element, and electrons move to the anode and the holes move to the cathode due to the electric field applied to both ends of the light- Into an electrical signal. In the direct conversion method, the materials used for the light receiving element are a-Se, CdZnTe, HgI ₂ , and PbI ₂ .

In the indirect conversion method, a scintillator is provided between a light receiving element and an X-ray generating portion, and when the X-ray emitted from the X-ray generating portion reacts with the scintillator to emit a photon having a wavelength in a visible light region, And converts it into an electrical signal. In the indirect conversion method, a-Si is used as a light-receiving element. As the scintillator, a GADOX scintillation thin film, a micro-columnar or needle-structured CSI (T1) is used.

The X-ray detector detects a charge accumulation mode (Charge Integration Mode) in which a charge is stored for a predetermined time and acquires a signal therefrom in accordance with a method of acquiring X-ray data, and a threshold energy and a photon counting mode for counting photons having an energy equal to or higher than a threshold energy.

An embodiment of the X-ray imaging apparatus 100 uses a photon counting method in which the X-ray exposure amount of the object 35 and the noise of the X-ray image are small as compared with the charge accumulation method. Accordingly, the X-ray detector 120 is implemented as a photon counting detector.

There is no limitation on the material construction method and the electrical signal conversion method of the X-ray detector 120, but in the following embodiments, for convenience of explanation, the direct conversion method for directly acquiring the electric signal from the X- A hybrid method in which a device and a read circuit chip are combined is applied to explain a specific embodiment.

The X-ray detector 120 detects the X-rays transmitted through the object 35, converts the detected X-rays into electrical signals, and outputs the electrical signals.

5A is a view schematically showing the structure of the X-ray detecting unit in the embodiment of the X-ray imaging apparatus, and FIG. 5B is a view schematically showing a single pixel region of the X-ray detecting unit shown in FIG. 5A.

5A, the X-ray detecting unit 120 includes a light receiving element 121 for detecting an X-ray and converting the X-ray into an electrical signal, and a read circuit chip 122 for reading an electrical signal. Here, the read circuit chip 122 is formed in the form of a two-dimensional pixel array including a plurality of pixel regions. As the material of the light receiving element 121, a single crystal semiconductor material can be used to secure high resolution, fast response time, and high dynamic range at low energy and small dose. The single crystal semiconductor material used here includes Ge, CdTe, CdZnTe, GaAs and the like.

The light receiving element 121 can be formed in the form of a PIN photodiode by bonding a p-type layer 121b in which a p-type semiconductor is arranged in a two-dimensional pixel array structure to a lower portion of the n-type semiconductor substrate 121a having a high resistance, The reading circuit chip 122 using the CMOS process is coupled to the light receiving element 121 for each pixel. The CMOS readout circuit 122 and the light receiving element 121 can be coupled by a flip chip bonding method to form a bump 123 such as solder PbSn or indium In and then reflow, And they can be joined together by pressing and pressing. However, the above-described structure is only an embodiment of the x-ray detector 120, and the structure of the x-ray detector 120 is not limited thereto.

Referring to FIG. 5B, when a photon of an X-ray is incident on the light receiving element 121, the electrons in the current collecting band receive the energy of the photon and are excited to the conduction band beyond the band gap energy difference. This results in electron-hole pairs in the depletion region.

When a metal electrode is formed on each of the P-type layer and the n-type substrate of the light receiving element 121 and reverse bias is applied thereto, electrons in the electron-hole pairs generated in the depletion region are attracted to the n-type region and holes to the p- . Then, the holes drawn into the p-type region are inputted to the readout circuit 122 through the bump bonding 123 so that the electric signal generated by the photon can be read. However, it is also possible that electrons are input to the reading circuit 122 in accordance with the structure of the light-receiving element 121 and the voltage applied thereto to generate an electric signal.

The reading circuit 122 may be formed of a two-dimensional pixel array structure corresponding to the p-type semiconductor of the light receiving element 121, and reads an electric signal for each pixel. When charges are input from the light receiving element 121 to the read circuit 122 through the bump bonding 123, the input charge generated from one photon at the pre-amplifier 122a of the read circuit 122 And outputs a corresponding voltage signal.

The voltage signal output from the preamplifier 122a is transmitted to the comparator 122b. The comparator compares the externally controllable threshold voltage with the input voltage signal and outputs a '1' or ' 0 ", and the counter 122c counts how many times '1' has occurred and outputs the X-ray data in digital form. The X-ray image of the object 35 can be obtained by combining the X-ray data for each pixel.

Here, the threshold voltage corresponds to a threshold energy, and when a photon having energy of E or more is to be counted, a threshold voltage corresponding to the threshold energy E is input to the comparator 122b. The reason why the threshold energy and the threshold voltage can be matched is that the magnitude of the electrical signal (voltage) generated in the light receiving element changes depending on the energy of the photon. Therefore, a threshold voltage corresponding to a desired threshold energy can be calculated using a relational expression between the energy of the photon and the generated voltage.

6 is a graph showing the relationship between the energy of the photons included in the X-ray input to the X-ray detecting unit and the voltage output from the X-ray detecting unit. The x-axis represents the energy of the photons incident on the x-ray detector 120, and the y-axis represents the voltage generated corresponding to the incident photons. Hereinafter, the relationship between the energy of the photons included in the X-ray input to the X-ray detecting unit and the voltage output from the X-ray detecting unit is referred to as keV-mV. In this case, keV is a photon energy unit and mV is a unit of voltage output from the x-ray detector 120.

The X-ray detector 120 outputs an electrical signal corresponding to the input photon energy. Therefore, when the input photon energy increases, the voltage output corresponding thereto also increases. That is, the keV-mV relation is a linear relationship in which the output voltage increases in proportion to the amount of incident photon energy.

As can be seen from the keV-mV relationship, the photon energy input to the x-ray detector 120 and the voltage output from the x-ray detector 120 correspond to each other at a ratio of 1: 1. Accordingly, when a specific energy of the photon energy input to the X-ray detector 120 is determined as a threshold energy, the corresponding output voltage becomes a threshold voltage. As described above, the determination of the threshold energy of the x-ray detector 120 may be equivalent to setting a threshold voltage corresponding to the threshold energy.

For example, as shown in FIG. 6, when it is desired to acquire an x-ray image by counting only E _th or more photons from the object, the threshold energy can be determined as E _th . If the threshold energy is determined as Eth, refer to the point P where the x coordinate is E _th in the keV-mV relationship. At this time, the y coordinate mV ₀ of the point P means the threshold voltage corresponding to the threshold energy E _th . Accordingly, the user sets the threshold voltage of the X-ray imaging apparatus 100 to mV ₀ to obtain a desired x-ray image.

However, in order to understand the threshold energy setting and the threshold voltage setting in the same sense, the keV-mV relationship must be measured very accurately. There are various methods for measuring the keV-mV relationship. Hereinafter, a method using a radioactive isotope, a method using an x-ray tube, and a method using a K-edge filter will be described.

First, there is a method using a radioactive isotope. The same term refers to different atomic nuclei with the same number of neutrons. Of these isotopes, those carrying radioactivity are called radioisotopes. These radioactive isotopes have different decay types depending on the species, emit radiation with specific energy, and collapse into stable isotopes. The energy released at this time is a value that does not change because it is a physical characteristic of the radioisotope.

The keV-mV relationship can be measured using the physical characteristics of the radioisotope. For example, when the radioactive isotope A is supposed to emit an X-ray of 30 keV at the time of collapse, an X-ray is incident from the A to the X-ray detecting unit 120. A voltage is generated in the X-ray detecting unit 120 by the incident radiation, and the generated voltage is measured. The keV-mV relationship can be measured by mapping the measured voltage to photon energy of 30 keV.

The keV-mV relationship can also be determined using an x-ray tube. As mentioned above, the voltage applied between the cathode and anode of the X-ray tube is called the tube voltage, and its size can be expressed as the peak value kVp. Since the X-ray tube is a device for converting the kinetic energy of the electron into the energy of the X-ray, as the tube voltage increases, the speed of the thermoelectron increases. As a result, the energy of the X-ray generated by collision with the target material increases. In particular, the tube voltage corresponds to the highest energy in the energy spectrum of the generated x-rays. That is, the tube voltage becomes the upper limit value of the generated x-ray energy, and the keV-mV relation can be measured by confirming the voltage corresponding to the x-ray energy.

There is also a method using a K-edge filter. FIG. 7 is a graph showing a change in attenuation coefficient with respect to energy of an X-ray irradiated to gold according to an embodiment of a K-edge filter. The x-axis represents the x-ray energy to be irradiated and the y-axis represents the attenuation coefficient. The damping coefficient is an energy function of the x-ray.

The lower the damping coefficient, the higher the transmittance. In the case of gold, the transmittance increases as the energy of the x-ray is increased. However, when the energy of the X-ray irradiated to gold is 80 keV, the damping coefficient greatly changes. This point is called K-edge. Since this is the physical property of a substance, it has a unique value for each individual substance. In the case of gold, K-edge is known to occur at 80 keV.

The keV-mV relationship can be obtained by using the physical properties inherent to the material. For example, gold can be used as a K-edge filter. When the X-ray detection unit 120 detects the X-ray in gold, a point at which the intensity value of the X-ray detected at a certain point changes greatly occurs. The voltage value at that time corresponds to 80 keV, and the keV-mV relation can be obtained through this.

In addition to the above-mentioned methods, there are many ways to obtain the keV-mV relationship, so that it is not limited to the above example. In addition, the keV-mV relationship can be obtained by selecting a plurality of methods out of the methods exemplified above.

In principle, the relationship keV-mV thus obtained should be independent of time. The same X-ray generating unit 110 irradiates the same object 35 and the X-ray detecting unit 120 detects the same object. However, the keV-mV relationship may change with time due to changes in physical properties due to aging of the light receiving element of the X-ray detecting unit 120, temperature changes in the light receiving element and the readout circuit, and changes in characteristics of the interface between the reading circuit and the light receiving element have. Therefore, if this change is not corrected, the energy separation characteristic of the x-ray detector 120 is changed and an error occurs in the measured value.

8 is a graph showing the change in keV-mV relationship with time. 8, the x-axis represents the x-ray energy irradiated on the object 35, and the y-axis represents the voltage generated by the x-ray detecting unit 120. Graph J represents the keV-mV relationship at the first time t ₀ , and graph K represents the keV-mV relationship at the second time t ₁ .

As shown in FIG. 8, when only photons whose photon energy is equal to or greater than E _th are to be counted, the threshold energy can be determined as E _th . When the threshold energy is determined, a voltage corresponding to the threshold energy E _th is found in the graph J showing the keV-mV relationship at the first time point, and this voltage should be set to the threshold voltage of the X-ray imaging apparatus 100. The reason is that the actual x-ray detector 120 does not count the photon based on the energy of the input photon but measures the output voltage corresponding to the photon energy and counts photons that output an output above the threshold voltage. Therefore, by setting the voltage mV ₀ corresponding to the threshold energy E _th to the threshold voltage with reference to the graph J in FIG. 8, only the photons above the threshold energy E _th can be counted.

However, due to the various reasons mentioned above, the keV-mV relationship may vary depending on the time of irradiation of the x-ray. Referring to FIG. 8, when the keV-mV relation is measured (graph K) with the viewpoint being t ₁ , the result of measuring the keV-mV by irradiating the x-ray at t ₀ (graph J) may be different.

When the threshold voltage set at the first point of time is used at the second point of time without reflecting the changed keV-mV relation, an error occurs in counting the photons having the energy of the threshold energy or more. This is because the threshold voltage set at the first time point is not equal to the threshold energy at the second time point.

For example, when following graph J in Fig. 8, the threshold voltage mV ₀ corresponds to the threshold energy E _th at the first time point. However, in the keV-mV relation graph K at the second time point, the threshold energy E _th 'corresponds to the threshold voltage mV ₀ . Therefore, when the threshold voltage mV ₀ is set at the second point in time, the photon actually obtained has an energy equal to or greater than E _th ', so that a result different from the first point is obtained.

It is necessary to determine a proper threshold energy for counting the photons by separating the photons of the x-ray input from the x-ray detector 120 into energy bands, and to count the photons having energy of the threshold energy or more. However, if the threshold energy as shown in FIG. 8 in accordance with the change of the time change to _th E 'from E _th it is difficult to obtain an accurate x-ray images due to such an error.

9 is a graph showing the relationship between the threshold voltage measured at different times and the normalized x-ray intensity. In FIG. 9, the x-axis represents the threshold voltage and the y-axis represents the normalized x-ray intensity I ₀ / I. Where I ₀ is the intensity of the x-ray irradiated on the object 35 and I is the intensity of the x-ray after passing through the object 35. The graph M is the curve measured at the first point in time t ₀ , and the graph N is the curve measured at the second point in time t ₁ .

Similarly to FIG. 8, in FIG. 9, when the threshold energy is determined as E _th at the first time (graph M), the corresponding threshold voltage can be set to mV ₀ . As a result, photon counting is performed according to the set threshold voltage, and as a result, the standardized X-ray intensity becomes a.

When the X-ray is irradiated to the object 35 at the second time point without changing the X-ray generating unit 110, the X-ray detecting unit 120 and the object 35, in principle, the standardized X- Should be. This is because, when the keV-mV relationship and the configuration and thickness of the object 35 are determined, the standardized X-ray intensity is also determined. However, the relationship of keV-mV varies with time due to a change in characteristics of the x-ray detector 120 and the like. The standardized x-ray intensity obtained as the keV-mV relationship is varied is also measured differently from the first point.

Referring to graph N measured at the second point of time t ₁ in FIG. 9, when the threshold voltage mV ₀ set at the first point in time at the second point of time t ₁ is used equally, b is obtained as the normalized X-ray intensity. In this case, an error c of a to b occurs. This error is due to, as mentioned in Fig. 8, the threshold energy E _th byeonhaetgi as' from E _th. Therefore, it is necessary to correct the first threshold voltage mV ₀ to the second threshold voltage corresponding to the threshold energy at the second time point.

10 is a control block diagram for one embodiment of an x-ray imaging device. The control unit may set the first threshold voltage corresponding to the threshold energy at the first time point and correct the first threshold voltage to the second threshold voltage at the second time point. The control unit may include a first threshold voltage setting unit 131, a reference x-ray data obtaining unit 132, and a correction unit 133. In addition, the control unit may include a keV-mV relation update unit 134.

The first threshold voltage setting unit 131 sets a first threshold voltage corresponding to the threshold energy at a first time point. The first threshold voltage setting unit 131 determines the relationship between the energy of the photons included in the X-ray input to the X-ray detecting unit 120 at the first time point and the voltage output from the X-ray detecting unit 120, that is, the keV- the first threshold voltage reference unit 131b may refer to the first threshold voltage corresponding to the threshold energy in the determined keV-mV relationship with the keV-mV relation confirmation unit 131a.

Concretely, the keV-mV relation confirming unit 131a can confirm the keV-mV relation at the first time point, and can provide the reference information when correction of the threshold voltage is required after a lapse of time. For example keV-mV relation confirmation unit may determine a relationship between a first X-ray energy and in response to this generated voltage to the X-ray irradiation detection unit 120 at the time point t _0, the 2 keV- a previously measured at the time t ₁ the threshold voltage can be corrected based on the mV relationship.

In order to correct the threshold voltage that varies with the irradiation time of the X-ray, it is necessary to accurately measure the keV-mV relationship at the first time point. Whenever an x-ray is irradiated, the relationship of keV-mV is measured, and when the threshold voltage is set to a voltage corresponding to the threshold energy in this relation, time and cost are constrained. Therefore, the keV-mV relationship is measured once at the first time point, and the relationship is referred to every time the X-ray is examined. Therefore, it is necessary to accurately measure the keV-mV relationship measured at the first time point, and correct values can be obtained when the threshold voltage is corrected at a later time point.

As the measurement method, there are a method using the radioisotope described above, a method using an X-ray tube, or a method using a K-edge filter. However, the measurement method is not limited to the above embodiment, and can be used to measure the keV-mV relationship if it can emit a constant photon energy as a physical property of the material, or if the exact amount of photon energy is known in advance.

The first threshold voltage reference unit 131b can refer to the first threshold voltage based on the keV-mV relationship determined by the keV-mV relation confirmation unit 131a. To refer to the graph in Fig. 8 J, the output voltage corresponding to ₀ mV from the check-keV mV relationship at a first time t ₀ to the threshold energy E _th can be referred to as a first threshold voltage. Since the threshold voltage mV ₀ is referred to based on the keV-mV relation identified at the first time point, it can not be used as the threshold voltage at the second time point, and it should be corrected to an appropriate value.

When obtaining x-ray images of different energy bands, the threshold energies can be set to a plurality. Accordingly, a plurality of threshold voltages can be referred to at a first time point corresponding to the plurality of threshold energies. A plurality of reference x-ray data can be obtained based on the plurality of threshold voltages referred to above, and the relationship between the threshold voltage and the x-ray data can be confirmed at the first point of view based on the plurality of reference x-ray data. This will be described later.

The reference x-ray data obtaining unit 132 may receive the first threshold voltage from the first threshold voltage reference unit 131b and obtain reference x-ray data corresponding to the first threshold voltage. At this time, the x-ray data may be the ratio of the intensity of the x-rays irradiated to the object 35 to the intensity of the x-rays transmitted through the object 35, that is, the standardized x-ray intensity. The reference x-ray data corresponding to the first threshold voltage ultimately corresponds to the threshold energy. Since the threshold energy and the reference X-ray data correspond to each other regardless of time, it is possible to correct the threshold voltage which varies with time.

Referring to the graph M of FIG. 9, the x-ray data a corresponding to the first threshold voltage mV ₀ becomes the reference x-ray data. In the first point, it is that setting the threshold voltage of ₀ mV, so that the same meaning as the threshold energy E _th to, the x-ray data is a response that is obtained when the threshold energy E _th to. Since the X-ray data corresponding to the threshold energy is constant irrespective of the time, the X-ray data a obtained from the threshold energy E _th becomes the reference X-ray data. The reference X-ray data a can be used to correct the threshold voltage mV ₀ according to the X-ray irradiation timing in the future.

When there are a plurality of first threshold voltages referenced by the first threshold voltage reference unit 131b, the reference x-ray data obtaining unit 132 may obtain a plurality of reference x-ray data corresponding to the first threshold voltage. The plurality of reference X-ray data thus obtained are shown on the coordinate plane, and the relationship between the threshold voltage and the X-ray data can be confirmed at the first time point. In this case, the relationship between the threshold voltage and the X-ray data means a change in the X-ray data obtained when the threshold voltage changes at the first time point, and can be expressed as a graph M in FIG.

The correcting unit 133 can correct the first threshold voltage to the second threshold voltage by referring to the reference x-ray data at the second time point. The correcting unit 133 refers to the reference x-ray data in the relationship confirmed by the x-ray data confirming unit 133a and the x-ray data confirming unit 133a that confirms the relationship between the threshold voltage and the x- And a first threshold voltage correction unit 133b that corrects the first threshold voltage to a second threshold voltage.

Hereinafter, each configuration of the correction unit 133 will be described with reference to FIG.

11 is a diagram for explaining a method of correcting the first threshold voltage in the correcting unit. In FIG. 11, the x-axis represents the threshold voltage and the y-axis represents the normalized x-ray intensity I ₀ / I. Where I ₀ is the intensity of the x-ray irradiated on the object 35 and I is the intensity of the x-ray after passing through the object 35. The graph M is the curve measured at the first point in time t ₀ , and the graph N is the curve measured at the second point in time t ₁ .

The X-ray data verifying unit 133a can measure the X-ray data obtained when the threshold voltage is changed at the second time point and confirm the relationship between the threshold voltage and the X-ray data at the second time point. This relationship can be expressed as graph N in FIG. Referring to the graph N, when the threshold voltage is maintained at mV ₀ at the second time point, the obtained x-ray data becomes b and an error occurs by c. This has been confirmed in the same manner in FIG.

The first threshold voltage corrector 133b finds the second threshold voltage with reference to the reference x-ray data in the relationship confirmed by the x-ray data verifying unit 133a. Here, the second threshold voltage can be a threshold voltage corresponding to the reference x-ray data in relation to the threshold voltage and x-ray data identified at the second time point.

Referring to FIG. 11, a point R having a reference x-ray data a is found as a y coordinate value in a graph N showing a relationship between threshold voltage and x-ray data identified at a second point in time. At this time, the voltage mV _{1, which} is the x coordinate value of the point R, means the second threshold voltage. The first threshold voltage correction section (133b) may correct the first threshold voltage of ₀ mV to a second threshold voltage ₁ mV.

The keV-mV relation updating unit 134 updates the relation between the energy of photons included in the x-ray input to the x-ray detector 120 at the second time point and the voltage output from the x-ray detector 120, that is, the keV-mV relation . Since the keV-mV relationship is linear, it is possible to renew it if only two points are known. Therefore, if there are a plurality of first threshold voltages referred to in the first threshold voltage reference portion 131b, there are a plurality of reference x-ray data corresponding thereto. In the graph of the relationship between the threshold voltage and the X-ray data confirmed at the second time point, when a plurality of reference X-ray data are found as y-coordinates, a plurality of second threshold voltages, which are the x-coordinates of the points, are also present. It is possible to update the keV-mV relation by converting the corresponding threshold voltage for the same threshold energy from the first threshold voltage to the second threshold voltage.

12 is a diagram showing a method of updating the keV-mV relation based on the second threshold voltage. The x-axis represents the x-ray energy irradiated on the object 35, and the y-axis represents the voltage generated by the x-ray detecting unit 120. Graph J is the keV-mV relationship measured at the first time t ₀ , and graph K is the updated keV-mV relationship at the second time t ₁ .

The relationship between threshold energy and x-ray data is constant regardless of time. First, a point P, in which the x-coordinate is the threshold energy E, is checked in the graph k, that is, the keV-mV relationship graph at the first point of time t ₀ . The y coordinate mV ₀ at the identified point P means the first threshold voltage corresponding to the threshold energy E _th . Since the threshold voltage is corrected from the first threshold voltage to the second threshold voltage in correspondence with the same threshold energy, the y coordinate of the point P identified above is changed to the second threshold voltage mV ₁ in order to update the keV-mV relation. Therefore, the point P moves to the position of the point Q. This means that the threshold voltage corresponding to the threshold energy E _th has been changed from mV ₀ to mV ₁ . As described above, when the process of changing the y coordinate of the point is performed plural times, the keV-mV relation can be updated.

13 is a flowchart of a method of correcting a threshold voltage according to an irradiation time of an X-ray.

First, the relationship between the photon energy input to the x-ray detector 120 and the voltage output from the x-ray detector 120, that is, the relationship keV-mV, is checked at the first point of time. (200) Is a photon coefficient detector. Since the keV-mV relationship changes according to the irradiation time of the X-ray, the following correction process is performed based on the relationship acquired at the first time point.

As described above, since the keV-mV relation acquired at the first time point is referred to at the time of correcting the threshold voltage at a later time point, the method of measuring the keV-mV relation should have high accuracy.

Examples of the method for obtaining the keV-mV relationship include a method using a radioactive isotope, a method using an x-ray tube, or a method using a K-edge filter. However, this is merely an example for obtaining the keV-mV relation, and it is also possible to use other methods as long as the accurate keV-mV relationship can be measured.

(210) Since the keV-mV relation is linear, the photon energy and the output voltage correspond to each other in a ratio of 1: 1 (keV-mV). . Therefore, if the threshold energy is determined according to the x-ray image of the desired energy band, a threshold voltage corresponding to 1: 1 can be set to the determined threshold energy. With the threshold voltage thus set, only the photons above the threshold energy can be counted, thereby obtaining desired x-ray images.

When a plurality of x-ray images of different energies are to be obtained, a plurality of threshold energies can be set. In this case, a plurality of first threshold voltages can be set corresponding to a plurality of threshold energies.

After setting the first threshold voltage, reference x-ray data corresponding to the set first threshold voltage is obtained (220). At this time, the x-ray data may be standard x-ray intensity. The reference x-ray data refers to x-ray data acquired by counting only photons having an energy greater than or equal to a threshold energy corresponding to the first threshold voltage.

If the threshold energy is constant, the same x-ray data is acquired regardless of the x-ray irradiation time. Accordingly, the X-ray data obtained corresponding to the first threshold voltage can be used as the reference X-ray data, and the threshold voltage can be corrected by referring to the reference X-ray data at the second time.

When a plurality of first threshold voltages are set, a plurality of X-ray data is acquired. The relationship between the threshold voltage and the X-ray data can be confirmed at the first time point based on the plurality of X-ray data thus obtained. The relationship between the threshold voltage and the X-ray data at the first time point means a change in the X-ray data obtained when the threshold voltage changes at the first time point. This relationship can be expressed in the form of a graph on a coordinate plane.

Finally, referring to the reference X-ray data at the second time point, the first threshold voltage is corrected to the second threshold voltage. (230) As described above, the X-ray data corresponding to the threshold energy is used as the reference X-ray data, Thereby correcting the threshold voltage. A more detailed description will be given with reference to FIG.

14 is a flowchart specifically for explaining a method of correcting a first threshold voltage at a second time point.

First, the relationship between the threshold voltage and the X-ray data is checked at a second time point. (231) At this time, the X-ray data may be standard X-ray intensity. When the X-ray data is measured while changing the threshold voltage at the second time point, the relationship between the threshold voltage and the X-ray data can be confirmed at the second time point. Changing the threshold voltage means that the threshold energy is different, so that the number of photons counted by the x-ray detector 120 varies with the change of the threshold energy, and the x-ray data also changes.

The relationship between the threshold voltage and the x-ray data at the second time point can be represented by a graph on the coordinate plane, where the x-axis can be the threshold voltage and the y-axis can be the x-ray data.

(232) Since the relation between the threshold energy and the reference X-ray data is constant irrespective of the timing of the X-ray irradiation, the threshold voltage corresponding to the reference X-ray data at the second time point .

When the relationship between the threshold voltage and the X-ray data at the second time point is expressed graphically on the coordinate plane as described above, a point having the reference X-ray data as the y coordinate is searched. The x-coordinate of this point becomes the threshold voltage corresponding to the reference x-ray data.

The second threshold voltage is set to the second threshold voltage 233 and the first threshold voltage is corrected to the second threshold voltage 234. Since the second threshold voltage corresponds to the threshold energy at the second time point, So that the same x-ray data can be obtained by correcting the threshold voltage to the second threshold voltage.

100: X-ray imaging device
110: X-ray generator
120: X-ray detector
130:
131: first threshold voltage setting unit
131a: keV-mV relation confirmation unit
131b: first threshold voltage reference portion
132: reference x-ray data acquisition unit
133:
133a: X-ray data verification unit
133b: a first threshold voltage corrector
134: keV-mV relation update unit

Claims (19)

Setting a first threshold voltage of the photon coefficient detector at a first time;
Obtaining reference x-ray data corresponding to the first threshold voltage; And
And correcting the first threshold voltage to a second threshold voltage with reference to the reference X-ray data at a second time point. The method according to claim 1,
Wherein the step of setting the first threshold voltage comprises:
Confirming a relationship between the energy of photons included in the X-ray input to the photon coefficient detector at the first time point and the voltage output from the photon coefficient detector; And
And setting a first threshold voltage corresponding to a threshold energy in the identified relationship. 3. The method of claim 2,
The relationship between the energy of the photons included in the x-ray input to the photon coefficient detector and the voltage output from the photon coefficient detector is determined by a method using a radioisotope, an x-ray tube or a K-edge filter, Control method. The method of claim 3,
The step of correcting the first threshold voltage
Confirming a relationship between the threshold voltage and X-ray data at the second time point;
And correcting the first threshold voltage to a second threshold voltage with reference to the reference x-ray data in the confirmed relationship. 5. The method of claim 4,
Wherein the step of correcting the first threshold voltage comprises setting the threshold voltage corresponding to the reference x-ray data to the second threshold voltage in the relationship between the threshold voltage and the x- . 3. The method of claim 2,
Wherein the x-ray data is expressed by a ratio of the intensity of the x-ray irradiated to the object to the x-ray intensity transmitted through the object. 3. The method of claim 2,
Wherein the first threshold voltage is set in the step of setting the first threshold voltage. 8. The method of claim 7,
Wherein the obtaining of the reference x-ray data comprises determining a relationship between a threshold voltage and x-ray data at a first time point based on the plurality of first threshold voltages. 9. The method of claim 8,
And updating the relationship between the energy of the X-ray input to the photon coefficient detector and the voltage output from the photon coefficient detector at the second time point. An x-ray generator for generating an x-ray and irradiating the object with the x-ray;
An x-ray detector for detecting the x-ray and counting the number of photons having an energy greater than a threshold energy among the photons contained in the detected x-ray; And
And a controller for setting a first threshold voltage corresponding to the threshold energy at a first time point and correcting the first threshold voltage to a second threshold voltage at a second time point. 11. The method of claim 10,
Wherein,
A first threshold voltage setting unit setting a first threshold voltage corresponding to the threshold energy at the first time point;
A reference x-ray data obtaining unit for obtaining reference x-ray data corresponding to the first threshold voltage; And
And a correction unit for correcting the first threshold voltage to a second threshold voltage by referring to the reference X-ray data at a second time point. 12. The method of claim 11,
Wherein the first threshold voltage setting unit includes:
A keV-mV relation confirming unit for confirming the relationship between the energy of photons included in the X-ray input to the X-ray detecting unit at the first time point and the voltage output from the X-ray detecting unit; And
And a first threshold voltage reference unit for referring to a first threshold voltage corresponding to the threshold energy in the determined relation. 13. The method of claim 12,
Wherein the keV-mV relation checking unit uses any one of a method using a radioactive isotope, an x-ray tube, or a K-edge filter. 13. The method of claim 12,
Wherein,
An X-ray data verifying unit for verifying a relationship between a threshold voltage and X-ray data at the second time point; And
And a first threshold voltage correcting unit for correcting the first threshold voltage to a second threshold voltage by referring to the reference x-ray data in the determined relation. 15. The method of claim 14,
Wherein the first threshold voltage corrector sets the threshold voltage corresponding to the reference x-ray data as the second threshold voltage in the relationship between the threshold voltage and the x-ray data identified at the second time point. 13. The method of claim 12,
Wherein the x-ray data is expressed by a ratio of the intensity of the x-ray irradiated to the object and the x-ray intensity transmitted through the object. 13. The method of claim 12,
And a plurality of first threshold voltages referenced by the first threshold voltage reference unit. 18. The method of claim 17,
And the acquiring unit acquires the reference X-ray data by checking the relationship between the threshold voltage and the X-ray data at a first time point based on the plurality of first threshold voltages. 19. The method of claim 18,
And the control unit includes a keV-mV relation updating unit for updating a relation between the energy of photons included in the X-ray input to the X-ray detecting unit at the second time point and the voltage output from the X-ray detecting unit.

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