Patent Application
20130223593
The present invention relates to an X-ray sensor, a method for testing the X-ray sensor, and an X-ray diagnostic device equipped with the X-ray sensor.
An X-ray diagnostic device includes an X-ray source and an X-ray sensor. The X-ray sensor is located at a predetermined distance from the X-ray source so as to be opposed to the X-ray source. Specifically, the X-ray source and the X-ray sensor are located such that X-rays from the X-ray source are emitted to a human body and then are incident on the X-ray sensor through the human body. The X-ray diagnostic device irradiates the human body with X-rays to visualize the internal condition of the human body. Since the human body is irradiated with X-rays in the X-ray diagnostic device, a minimum amount of X-ray irradiation is preferable. Thus, the sensitivity of the X-ray sensor needs to be improved in the X-ray diagnostic device.
In a method for improving the sensitivity of the X-ray sensor, the X-ray sensor may be an imaging device including a photoconductive target portion having a blocking structure with a charge-multiplying function. Such an imaging device is proposed in, for example, Patent Literature 1.
Specifically, the imaging device includes a hole injection blocking layer, an electron injection blocking layer, and a photoconductive sensitive layer interposed between the blocking layers. Light incident on the photoconductive sensitive layer generates charge in the layer. The sensitive layer is made of a material that multiplies charge when being subjected to high fields. Patent Literature 1 proposes a sensitive layer composed of an amorphous semiconductor that consists principally of Se. The imaging device can capture an image with high sensitivity even in the case of a small light quantity.
Thus, the sensitivity of an X-ray sensor can be improved by forming, in the X-ray sensor, a photoconductive film including the hole injection blocking layer, the electron injection blocking layer, and the photoconductive sensitive layer having the charge-multiplying function between the blocking layers.
In the known technique, however, scratches or irregularities on the hole injection blocking layer may lead to insufficient blocking of hole injection. In other words, holes from a high voltage power supply pass through scratches or irregularities on the hole injection blocking layer and then reach the sensitive layer, causing white scratches on a captured image in the known technique.
Scratches or irregularities on the hole injection blocking layer are mainly caused by small dust remaining on a light-transmissive substrate. Specifically, dust on the light-transmissive substrate may cause irregularities transferred to a thin light-transmissive electrode, resulting in scratches or irregularities on the hole injection blocking layer.
An X-ray sensor in an X-ray diagnostic device requires quite a large area. For example, in an X-ray diagnostic device for mammography, an X-ray sensor includes a rectangular sensor surface having a side length of about 200 mm to 250 mm in plan view. In an X-ray diagnostic device for a chest, an X-ray sensor includes a rectangular sensor surface having a side length of about 450 mm in plan view. In the fabrication of the X-ray sensor having a large area, dust cannot be completely removed from a light-transmissive substrate, causing scratches or irregularities partially on a photoconductive film.
In view of the problem, an object of the present invention is to provide an X-ray sensor that can reduce the number of white scratches caused by scratches or irregularities on a hole injection blocking layer, a method for testing the X-ray sensor, and an X-ray diagnostic device equipped with the X-ray sensor.
In order to attain the object, an aspect of an X-ray sensor according to the present invention includes: a light-transmissive substrate; a light-transmissive electrode formed on a first surface of the light-transmissive substrate; and a photoconductive film including a hole injection blocking layer, a field buffer layer, a hole trap layer, a photoconductive sensitive layer having a charge-multiplying function, and an electron injection blocking layer, the layers being sequentially provided on the first surface of the light-transmissive substrate having the light-transmissive electrode, wherein the field buffer layer is larger in thickness than a layer composed of the light-transmissive electrode and the hole injection blocking layer.
According to the X-ray sensor of the present invention, even if scratches or irregularities occur on the hole injection blocking layer, holes from a high voltage power supply for supplying a high voltage to the light-transmissive electrode can be trapped in the hole trap layer. Furthermore, the trapping of the holes can buffer an electric field between the light-transmissive electrode and the hole trap layer. According to the present invention, the field buffer layer can expand a field buffer region. Furthermore, according to the present invention, the field buffer layer is larger in thickness than the layer composed of the light-transmissive electrode and the hole injection blocking layer, so that the field buffer layer can be larger in thickness than the field buffer layer of a visible light sensor, thereby more effectively buffering an electric field than in the visible light sensor. Thus, the present invention can reduce the number of white scratches caused by scratches or irregularities on the hole injection blocking layer.
Another aspect of the X-ray sensor according to the present invention further includes a bias light source that irradiates a second surface of the light-transmissive substrate with light, the second surface being located opposite to the first surface so as to receive X-rays. With this configuration, an electric field can be more effectively buffered by the hole trap layer using the bias light source, thereby reliably reducing the number of white scratches caused by scratches or irregularities on the hole injection blocking layer. Furthermore, the field buffer effect of the hole trap layer can be stably obtained over the sensor surface of the X-ray sensor by using the bias light source, thereby preventing the field buffer effect in the sensor surface of the X-ray sensor from varying with the history of X-ray irradiation. Thus, the X-ray sensor can be obtained with constant sensitivity.
An aspect of a method for testing an X-ray sensor according to the present invention is a method for testing the X-ray sensor of the present invention, the method including: a first step of, in a state in which the light-transmissive electrode is fed with a voltage so as to apply an electric field to the photoconductive film, emitting red light to the second surface of the light-transmissive substrate, the second surface being located opposite to the first surface so as to receive X-rays; and a second step of collecting an electric signal from the photoconductive film according to the amount of charge generated on the sensitive layer of the photoconductive film by the irradiation of the red light.
According to the method for testing the X-ray sensor of the present invention, red light having a wavelength of 620 nm that is visual light can be used instead of X-rays when the quality of the X-ray sensor is evaluated. Specifically, since red light can pass through the field buffer layer having a thickness of 6 μm or less, the quality of the X-ray sensor can be evaluated using red light when the field buffer layer has a thickness of 6 μm or less. Thus, a defect (black scratch or white scratch) on the photoconductive film of the X-ray sensor can be evaluated by irradiating the X-ray sensor with red light.
An X-ray diagnostic device according to the present invention is equipped with the X-ray sensor according to the present invention, the X-ray sensor being located at a predetermined distance from an X-ray source so as to be opposed to the X-ray source.
The X-ray sensor according to the present invention can reduce the number of white scratches caused by scratches or irregularities on the hole injection blocking layer.
According to the method for testing the X-ray sensor of the present invention, the quality of the X-ray sensor can be evaluated using red light. Thus, the quality of the X-ray sensor can be more easily evaluated than in a testing method using X-rays.
An embodiment of an X-ray sensor, a method for testing the X-ray sensor, and an X-ray diagnostic device equipped with the X-ray sensor according to the present invention will be described below with reference to the accompanying drawings. The accompanying drawings are schematically or conceptually illustrated for understanding. The same constituent elements are indicated by the same reference numerals and may not be further described herein.
Referring to
More specifically, the X-ray source 1 is provided on one end of an arm 4, which is substantially semicircular in side view, while the X-ray sensor 2 is provided on another end of the arm 4. In this way, the X-ray source 1 and the X-ray sensor 2 are opposed to each other at the predetermined distance. Thus, the arm 4 allows the X-ray source 1 and the X-ray sensor 2 to oppose each other with the subject 3 laid on a bed 5 between the X-ray source 1 and the X-ray sensor 2, allowing the X-ray sensor 2 to detect X-rays having passed through the subject 3 from the X-ray source 1.
The arm 4 is rotatably attached to a main unit case 6. The main unit case 6 contains a drive unit (not shown) for rotating the arm 4. This configuration makes it possible to visualize the internal condition of the subject 3 from various angles.
As shown in
The X-ray sensor 2 is connected to an image processing unit 9. The image processing unit 9 detects an electric signal (photoelectric conversion signal) collected from the X-ray sensor 2 and performs image processing on the signal. The signal processed by the image processing unit 9 is transmitted to the controller 8. The controller 8 displays an image on a monitor 10 according to the light quantity (intensity) of X-rays detected by the X-ray sensor 2. The image displayed on the monitor 10 shows the internal condition of the subject 3.
The X-ray sensor 2 is also connected to an electron source control unit 11. The electron source control unit 11 is connected to the controller 8. The electron source control unit 11 controls the irradiation of electron beams from an electron source, which will be described later, and the amount of irradiation of electron beams from the electron source in response to a command signal from the controller 8.
The X-ray sensor 2 is also connected to a bias light-source control unit 12. The bias light-source control unit 12 is connected to the controller 8. The bias light-source control unit 12 controls the irradiation of light (bias light) from a bias light source, which will be described later, and the amount of irradiation of light from the bias light source in response to a command signal from the controller 8.
The drive unit (not shown) contained in the main unit case 6 to rotate the arm 4 is connected to a movement control unit 13 that is connected to the controller 8. The movement control unit 13 controls the rotating operation of the arm 4 in response to a command signal from the controller 8.
The controller 8 is also connected to an input unit 14. The controller 8 controls the overall operation of the X-ray diagnostic device in response to a command inputted from the input unit 14.
Referring to
As shown in
Specifically, the light-transmissive substrate 17 on the opposite side from the mounting substrate 15 has a first surface including a light-transmissive electrode (not shown) and a second surface that is opposite to the first surface including the light-transmissive electrode, the second surface receiving X-rays (X-ray incidence surface). Thus, the light-transmissive substrate 17 is disposed on the opposite side from the mounting substrate 15 such that the first surface including the light-transmissive electrode faces the electron source 16. Furthermore, a photoconductive film 18 is provided on the first surface of the light-transmissive substrate 17 including the light-transmissive electrode.
As illustrated in
As illustrated in
The light-transmissive electrode 21 is made of materials such as indium tin oxide (ITO) and tin oxide. The hole injection blocking layer 22 is made of ceric oxide (CeO2), the field buffer layer 23 is made of amorphous selenium (a-Se), the hole trap layer 24 is made of amorphous selenium (a-Se) and lithium fluoride (LiF), the sensitive layer 25 is made of amorphous selenium (a-Se), and the electron injection blocking layer 26 is made of antimony trisulfide (Sb2S3).
In the present embodiment, the photoconductive film 18 includes the field buffer layer 23 and the hole trap layer 24 in order to reduce the number of white scratches caused by scratches or irregularities on the hole injection blocking layer 22.
Specifically, if scratches or irregularities occur on the hole injection blocking layer 22, a high voltage applied to the light-transmissive electrode 21 from a high voltage power supply connected to the light-transmissive electrode 21 allows holes to pass through the scratches or irregularities on the hole injection blocking layer 22 into the photoconductive film 18. Hence, in the case where the hole injection blocking layer 22 and the sensitive layer 25 are adjacent to each other, holes flowing in the photoconductive film 18 are accelerated and multiplied by the internal field of the photoconductive film 18, causing white scratches. To address this problem, the hole trap layer 24 is provided between the hole injection blocking layer 22 and the sensitive layer 25. This configuration makes it possible to trap holes in the hole trap layer 24, and the trapping of holes can buffer an electric field between the light-transmissive electrode 21 and the hole trap layer 24. Hence, even if the hole injection blocking layer 22 has scratches or irregularities, holes flowing from the high voltage power supply are unlikely to reach the sensitive layer 25, reducing the number of white scratches on a captured image.
Moreover, in the present embodiment, the field buffer layer 23 is disposed between the hole injection blocking layer 22 and the hole trap layer 24. This configuration makes it possible to increase a field buffer region between the light-transmissive electrode 21 and the hole trap layer 24, allowing the field buffer layer 23 to further reduce the number of occurrences of white scratches.
Furthermore, in the present embodiment, the field buffer layer 23 is larger in thickness than a layer composed of the light-transmissive electrode 21 and the hole injection blocking layer 22. With this configuration, the field buffer layer 23 can be larger in thickness than the field buffer layer of a visible light sensor (about 30 nm). Thus, an electric field can be more effectively buffered than in the visible light sensor. Also in the visible light sensor, a light-transmissive electrode is made of materials such as indium tin oxide (ITO) and tin oxide, a hole injection blocking layer is made of ceric oxide (CeO2), a field buffer layer is made of amorphous selenium (a-Se), a hole trap layer is made of amorphous selenium (a-Se) and lithium fluoride (LiF), a sensitive layer is made of amorphous selenium (a-Se), and an electron injection blocking layer is made of antimony trisulfide (Sb2S3).
In the present embodiment, the bias light source 20 is provided to allow the hole trap layer 24 to effectively buffer an electric field. Specifically, the X-ray incidence surface of the light-transmissive substrate 17 is irradiated with light (bias light) from the bias light source 20 before X-ray irradiation. Thus, holes generated by light from the bias light source 20 can be trapped in the hole trap layer 24 so as to buffer an electric field. This can reduce the influence of scratches or irregularities on the sensitive layer 25.
As shown in
As shown in
The light-transmissive electrode 21 is connected to a high voltage source 31 for supplying a high voltage Vharp. The high voltage Vharp applies a high electric field to the sensitive layer 25 of the photoconductive film 18. The light-transmissive electrode 21 is connected to the image processing unit 9. As has been discussed, the image processing unit 9 detects a photoelectric conversion signal from the light-transmissive electrode 21 and performs image processing on the signal. The level of the photoelectric conversion signal varies with the quantity (intensity) of light incident on the X-ray incidence surface of the light-transmissive substrate 17.
The operation of the X-ray diagnostic device will be described below according to the present embodiment. In the present embodiment, before the subject 3 undergoes radiography, the overall X-ray incidence surface of the light-transmissive substrate 17 is uniformly irradiated with light (bias light) from the bias light source 20.
The initial operation is performed to more effectively buffer an electric field by the hole trap layer 24. Specifically, the X-ray incidence surface of the light-transmissive substrate 17 is irradiated with bias light to trap a sufficient amount of holes in the hole trap layer 24, effectively buffering an electric field before radiography.
For example, as illustrated in
For example, as illustrated in
In an initial state, as shown in
From the initial state to first X-ray irradiation, bias light is uniformly emitted at least to the overall region in which the photoconductive film 18 is projected on the X-ray incidence surface of the light-transmissive substrate 17. In this case, as indicated by solid lines in
In the case where bias light is not emitted, as indicated by broken lines in
After the first X-ray irradiation, that is, before second X-ray irradiation, a sufficient amount of holes is trapped in the hole trap layer 24 in the A part by the first X-ray irradiation, thereby buffering an electric field in the A part as shown in
In the B part, however, an insufficient amount of holes is generated by X-ray irradiation and then is trapped by the hole trap layer 24. Thus, the absence of bias light irradiation results in imperfect field buffer. In the B part, as indicated by a broken line in
The absence of bias light irradiation causes a difference between the A and B parts in the amount of holes that are generated by X-ray irradiation and are trapped by the hole trap layer 24, leading to variations in potential from the light-transmissive electrode 21 to the hole trap layer 24. In other words, the field buffer effect of the hole trap layer 24 varies between the A and B parts. Consequently, a voltage fluctuation from the hole trap layer 24 to the electron injection blocking layer 26 varies between the A and B part, leading to a difference between the A and B parts in the field intensity of the sensitive layer 25 disposed between the hole trap layer 24 and the electron injection blocking layer 26. The field intensity of the sensitive layer 25 is closely associated with the sensitivity of the sensitive layer 25. Thus, a difference between the A and B parts in the field intensity of the sensitive layer 25 causes a difference in signal level between the A and B parts even in the radiography of the same subject. In other words, the absence of bias light irradiation leads to the influence of the history of previous subjects.
In contrast, bias light as in the initial X-ray irradiation is emitted before the second X-ray irradiation, achieving the field buffer effect both in the A and B parts. Thus, as indicated by solid lines in
As has been discussed, bias light irradiation before X-ray irradiation allows the hole trap layer 24 to have an enhanced field buffer effect. Furthermore, an X-ray sensor with constant sensitivity can be obtained without being affected by the history of irradiation. In particular, as the field buffer layer 23 increases in thickness, the effect of bias light is improved. Specifically, as shown in
Holes are unstably trapped in the hole trap layer 24. Thus, at least before X-ray irradiation, the bias light source 20 is desirably turned on to stably buffer an electric field over the sensor surface of the X-ray sensor 2.
The thickness of the field buffer layer 23 will be described below. As has been discussed, the field buffer layer of the visible light sensor has a thickness of about 30 nm, whereas in the present embodiment, the thickness of the field buffer layer 23 is larger than that of the layer composed of the light-transmissive electrode 21 and the hole injection blocking layer 22, so that the field buffer layer 23 is larger in thickness than the field buffer layer of the visible light sensor.
In other words, the field buffer layer of the visible light sensor has a thickness of about 30 nm because visible light needs to completely reach the sensitive layer in the visible light sensor, whereas in the X-ray sensor, the field buffer layer 23 can be larger in thickness than the field buffer layer of the visible light sensor because X-rays are more penetrable than visible light. In the present embodiment, the light-transmissive electrode 21 is about 30 nm in thickness and the hole injection blocking layer 22 is about 30 nm in thickness, so that the layer composed of the light-transmissive electrode 21 and the hole injection blocking layer 22 is about 60 nm in thickness. Thus, in the present embodiment, the thickness of the field buffer layer 23 is preferably set at 60 nm or larger.
In the present embodiment, also in the case where the field buffer layer 23 is larger in thickness than the field buffer layer of the visible light sensor, the use of the bias light source 20 can achieve an X-ray sensor with smaller variations in sensitivity without being affected by the history of previously captured subjects.
As has been discussed, the thickness of the field buffer layer 23 is larger than that of the layer composed of the light-transmissive electrode 21 and the hole injection blocking layer 22, so that the field buffer layer 23 is larger in thickness than the field buffer layer of the visible light sensor. This can improve the field buffer effect more than that of the visible light sensor. More preferably, the thickness of the field buffer layer 23 is set sufficiently larger than that (about 60 nm) of the layer composed of the light-transmissive electrode 21 and the hole injection blocking layer 22.
Thus, the field buffer layer 23 has a thickness of at least 1.5 μm, which is sufficiently larger than that (about 60 nm) of the layer composed of the light-transmissive electrode 21 and the hole injection blocking layer 22, thereby more effectively reducing the influence of scratches or irregularities on the hole injection blocking layer 22 on the sensitive layer 25.
In the present embodiment, bias light is emitted. Thus, the thickness of the field buffer layer 23 is desirably set so as not to transmit light (bias light) from the bias light source 20 to the sensitive layer 25. Since light from the bias light source 20 does not reach the sensitive layer 25, bias light does not affect the photoelectric conversion signal even if the bias light source 20 is continuously turned on. In other words, the timing for turning on the bias light source 20 does not need to be controlled. Specifically, the bias light source 20 does not need to be turned on before X-ray irradiation or in each blanking period of X-ray radiography. The thickness of the field buffer layer 23 for preventing bias light from reaching the sensitive layer 25 varies with the wavelength of light emitted from the bias light source 20.
Thus, as has been discussed, the number of occurrences of white scratches can be sufficiently reduced by setting the thickness of the field buffer layer 23 at 1.5 μm or larger. For example, light having a wavelength selected from the range of 440 nm to 540 nm (blue light or green light) is selected as bias light, thereby sufficiently preventing light from the bias light source 20 from reaching the sensitive layer 25.
For example, in the case where red light having a wavelength of 620 nm is emitted from the bias light source 20 as shown in
As has been discussed, the field buffer layer 23 having a sufficiently large thickness can reduce the number of occurrences of white scratches and prevent bias light from reaching the sensitive layer 25. If the field buffer layer 23 has an extremely large thickness, visible light cannot pass through the field buffer layer 23. Thus, testing of the X-ray sensor 2 requires X-rays, resulting in large testing equipment including an X-ray shield mechanism. Thus, in order to facilitate testing of the X-ray sensor 2, the thickness of the field buffer layer 23 needs to be limited to allow testing with visible light.
In the present embodiment, the thickness of the field buffer layer 23 is limited to 6 μm or less, and the bias light source 20 is, for example, a light source capable of emitting visible light having a wavelength of 440 nm (blue) or 540 nm (green). As shown in
Referring to
First, a voltage is applied to the light-transmissive electrode 21 so as to apply an electric field of 50 V/μm to 100 V/μm to the photoconductive film 18. In this state, red light having a wavelength of 620 nm is evenly emitted to the overall X-ray incidence surface of the light-transmissive substrate 17 from a light source 32 (first step).
The red light is incident on the sensitive layer 25 and thus generates charge on the sensitive layer 25. Subsequently, the electron source 16 is controlled by the X scanning driver 27 and the Y scanning driver 28 so as to scan electron beams on the surface of the electron injection blocking layer 26 of the photoconductive film 18. By scanning electron beams, an electric signal corresponding to the amount of charge generated on the sensitive layer 25 by the irradiation of red light, that is, an electric signal corresponding to the intensity of the red light incident on the sensitive layer 25 is read to the image processing unit 9 (second step).
The read electric signal is subjected to image processing by the image processing unit 9 and then is displayed as a video signal on the monitor 10 by the controller 8. Consequently, white scratches and black scratches are displayed as they are on the display screen of the monitor 10. Thus, scratches on the photoconductive film 18 can be evaluated using red light that is visible light.
As has been discussed, according to the present embodiment, the field buffer layer 23 is sufficiently larger in thickness than the field buffer layer of the visible light sensor. Thus, even if scratches or irregularities occur on the hole injection blocking layer 22, an electric field can be effectively buffered to reduce the number of white scratches (the number of occurrences of white scratches) on a captured image.
Moreover, according to the present embodiment, even if the field buffer layer 23 has a larger thickness, the use of the bias light source 20 can achieve an X-ray sensor with smaller variations in sensitivity without being affected by the history of previously captured subjects.
Furthermore, according to the present embodiment, red light having a wavelength of 620 nm that is visible light can be used when the quality of the X-ray sensor 2 is evaluated. Thus, the X-ray sensor 2 can be more easily evaluated than in the use of X-rays.
Another embodiment of the X-ray sensor will be described below. As shown in
The foregoing embodiments can be changed in various ways without substantially departing from the scope of the effect of the present invention.
An X-ray sensor and an X-ray diagnostic device equipped with the X-ray sensor according to the present invention can reduce the number of white scratches caused by scratches or irregularities on a hole injection blocking layer. Moreover, a method for testing the X-ray sensor according to the present invention can evaluate the quality of the X-ray sensor by using red light having a wavelength of 620 nm that is visible light. Thus, the quality of the X-ray sensor can be more easily evaluated than in a testing method using X-rays. For this reason, the X-ray sensor, the method for testing the X-ray sensor, and the X-ray diagnostic device equipped with the X-ray sensor according to the present invention are useful for devices for observing the internal condition of a test subject with X-rays.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-256365 | Nov 2010 | JP | national |
| 2010-256366 | Nov 2010 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2011/006368 | 11/16/2011 | WO | 00 | 5/2/2013 |
