JP2003167060A - X-ray plane detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an X-ray plane detector with improved resolution characteristic in the case of using a scintillator layer. <P>SOLUTION: This X-ray plane detector includes a plurality of pixel units 12 arrayed in two dimensions, and each of the pixel units 12 has a scintillator part 38, a photo diode 13 for converting light converted by the scintillator part 38 to a signal charge, a charge storage capacitor 15 for storing signal charges converted by the photo diode 13, and a TFT 14 for controlling readout of signal discharge. A reflecting member is disposed between the scintillator parts of the adjacent pixel units. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray flat panel detector for detecting an X-ray image.

[0002]

2. Description of the Related Art In a medical diagnostic apparatus, for example, X-rays are used for photographing a subject. An X-ray detector is used to detect an X-ray image of a subject. As an X-ray detector, an image tube has been widely used so far. In recent years, as a new generation X-ray detector, an X-ray flat panel detector in which a plurality of X-ray flattening elements are two-dimensionally arranged on a plane is drawing attention. The X-ray flat panel detector is configured to output an X-ray image taken by X-ray or a real-time X-ray image as a digital signal. Since the X-ray flat panel detector is a solid-state detector, it is expected to improve image quality performance and stability.

X-ray flat panel detectors for general radiography and chest radiography, which collect still images with a relatively large dose, have already been developed and commercialized. In the near future, with higher performance, real-time X-ray detection of 30 or more screens per second will be realized under fluoroscopy dose, and commercialization of products applied to the fields of cardiology and digestive organs will be realized. Is expected. For such detection of X-ray moving images, improvement of S / N and development of real-time processing technology for minute signals are required.

The X-ray flat panel detector is roughly classified into two types, a direct type and an indirect type. The direct method is a-Se
X-rays are directly converted into signal charges using a photoconductive film such as
This is a method of storing converted signal charges in a charge storage capacitor. The indirect method is a method in which X-rays are converted into visible light by a scintillator layer, the converted visible light is converted into signal charges by an a-Si photodiode or CCD, and the signal charges are stored in a charge storage capacitor.

[0005]

The X-ray flat panel detector of the direct type has a structure in which photoconductive charges generated by the incidence of X-rays are guided to a charge storage capacitor by utilizing a high electric field. In the case of this configuration, the resolution characteristic is almost defined by the pixel pitch. On the other hand, in the indirect X-ray flat panel detector, the visible light converted by the scintillator layer diffuses or scatters before reaching the photodiode, resulting in deterioration of resolution. Therefore, in the case of the indirect type X-ray flat panel detector, it is particularly necessary to improve the resolution characteristic.

Further, in the X-ray flat panel detector of the direct type, in order to improve the absorption rate of X-rays, a photoconductive film of, for example, a-Se is formed with a thickness of about 1 mm. And to increase the photoconductive charge generation rate per X-ray photon,
And, since the generated photoconductive charge is guided to the charge storage capacitor without being trapped in the defect level in the film,
Further, in order to suppress the diffusion of photoconductive charges in the direction perpendicular to the bias electric field, a strong bias electric field of 10 V / μm is applied to both ends of the a-Se photoconductive film, for example. Therefore, when the film thickness of the a-Se photoconductive film is about 1 mm, 10
A high voltage of about kV is applied.

When the direct method and the indirect method are compared, the direct method has excellent resolution characteristics. However, it is necessary to protect the TFT having a low operating voltage from a high voltage, which makes it difficult to secure reliability. In addition, there is a problem that a photoconductive material having low dark current characteristics, high sensitivity characteristics, thermal stability, etc. cannot be easily obtained.

The indirect X-ray flat panel detector uses a photo diode or CCD. Therefore, it is not necessary to apply a high voltage, and dielectric breakdown due to the high voltage does not occur.
Further, the scintillator material and the photo diode are known techniques and can be easily commercialized as compared with the direct method.

However, there is a problem that the resolution characteristic is inferior to that of the direct method. Further, if the scintillator layer is thickened to improve the sensitivity characteristics, there is a problem that optical diffusion or scattering occurs before reaching a photoelectric conversion element such as a photodiode, which deteriorates the resolution. Further, there is also a problem that the X-rays scattered by the scintillator layer enter the scintillator layer of the adjacent pixel and reduce the resolution.

An object of the present invention is to solve the above-mentioned drawbacks and to provide an X-ray flat panel detector having improved resolution characteristics when a scintillator layer is used.

[0011]

According to the present invention, a plurality of pixel units arranged two-dimensionally are provided, and each of the pixel units has a scintillator section for converting X-rays incident from a predetermined input surface into light. A photoconductive conversion unit that converts the light converted by the scintillator unit into a signal charge, and a charge storage unit that stores the signal charge converted by the photoconductive conversion unit,
In an X-ray flat panel detector having a switching unit that controls reading of the signal charges accumulated in the charge accumulation unit, between scintillator units of adjacent pixel units,
It is characterized in that an interruption region in which the scintillator portions are not continuous is provided, and a reflection member made of a material different from that of the scintillator portion is arranged in the interruption region.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described with reference to the schematic configuration diagram of FIG. Reference numeral 11 denotes a photoelectric conversion unit. In the photoelectric conversion unit 11, a plurality of pixel units 12 having the same structure are arranged in a row direction (for example, a horizontal direction in the drawing) and a column direction (for example, a vertical direction in the drawing) on an insulating substrate such as glass. Are formed two-dimensionally at a predetermined pitch L. In FIG. 1, for example, nine pixel units 12a to 12i are shown.

For example, the pixel unit 12i includes a photoelectric conversion unit for converting light into a signal charge, such as a photodiode 13,
Further, it is composed of a thin film transistor (hereinafter referred to as TFT) 14 which constitutes a switching portion, a charge accumulating portion for accumulating signal charges, for example, a storage capacitor 15. The TFT 14 has a gate electrode G, a source electrode S, and a drain electrode D. For example, the drain electrode D is electrically connected to the photodiode 13 and the storage capacitor 15.

Above the photoelectric conversion unit 11 having the above-mentioned structure,
A scintillator layer for converting X-rays incident from the outside into light is formed, but it is not shown in FIG.

A control circuit 16 for controlling the operating state of the TFT 14, for example, ON / OFF, is provided outside the photoelectric conversion unit 11. The control circuit 16 includes a plurality of control lines 1
7, for example, four first to fourth control lines 171-174 are provided in the figure. Each control line 17 has a TFT 14 which constitutes a pixel unit 12 in the same row.
Of the gate electrode G. For example, the first control line 171 is connected to the gate electrodes G of the pixels 12a to 12c, respectively.

In the column direction, a plurality of data lines 18, for example, first to fourth plurality of data lines 18 are shown.
1-184 are provided. The respective data lines 18 are connected to the source electrodes S of the TFTs 14 which form the pixel units 12 in the same column. For example, the first data line 181 is connected to the source electrodes S of the pixel units 12a, 12d, 12g. Each data line 17 is connected to a corresponding charge amplifier 19.

The charge amplifier 19 is composed of, for example, an operational amplifier, one input terminal a1 of which is connected to the data line 18, and the other input terminal a2 of which is grounded. A capacitor C is connected between the one input terminal a1 and the output terminal b to have an integration function. A switch S is connected in parallel with the capacitor C, and the switch S is closed to discharge the electric charge remaining in the capacitor C.

Each charge amplifier 19 is connected to a parallel / serial converter 20 which converts a plurality of electric signals input in parallel into serial signals. The parallel / serial converter 20 is
It is connected to an analog-digital converter 21 which converts an analog signal into a digital signal.

Next, the structure of each pixel will be described with reference to the schematic sectional view of FIG. FIG. 2 is a diagram in which one pixel unit portion is extracted, and portions corresponding to those in FIG. 1 are denoted by the same reference numerals and overlapping description will be partially omitted.

The TFT 14 is formed on the insulating substrate 31 such as glass.
And a storage capacitor 15 is formed. TFT1
Reference numeral 4 denotes a gate electrode G formed on the insulating substrate 31, an insulating film 32 covering the gate electrode G, a semi-insulating film 33 formed on the insulating film 32, a source electrode S provided on the semi-insulating film 33, It is composed of a drain electrode D and the like.

As shown in FIG. 1, the gate electrode G of the TFT 14 is connected to the common control line 17 together with the gate electrodes G of the other TFTs 14 located in the same row. The source electrode S is connected to the common data line 18 together with the source electrodes S of the other TFTs 14 located in the same column.

The storage capacitor 15 includes a lower electrode 34 formed on the insulating substrate 31, and a gate electrode G to the lower electrode 3.
4 and the upper electrode 35 provided on the insulating film 32. Upper electrode 35
Is electrically connected to the drain electrode D. TFT1
4 and the storage capacitor 15 is provided with an insulating layer 361, and the photodiode 13 is formed on the insulating layer 361.

The photodiode 13 is made of a-Si pn.
It is formed of a diode or a PIN diode. A through hole 37 is formed in a part of the insulating layer 36, and the first electrode 131 located below the photodiode 13 in the drawing is electrically connected to the drain electrode D of the TFT 14 through the through hole 37.

The second electrode 132 located above the photodiode 13 in the figure is formed by depositing an ITO transparent conductive film by, for example, a sputtering method, and the first electrode 131 is formed.
A bias voltage is applied between the second electrode 132 and the second electrode 132. Further, the scintillator layer 38 is formed on the second electrode 132.

The scintillator layer 38 is made of, for example, CsI:
A scintillator material such as Na or CsI: Tl is formed into a film by a vacuum evaporation method. In this case, the scintillator layer 38 is formed, for example, at a position displaced from the input electrode region of the photodiode 13. Further, a groove is formed, for example, in the depth direction at the boundary between the adjacent scintillator layers 381 in pixel units, so that the adjacent scintillator layers in pixel units, for example, the scintillator layer 38 and the scintillator layer 38.
An interruption area 39 is formed so that 1s do not continue.
Then, for example, the scintillator layer 3 is provided on the input surface 38a on the upper side of the drawing on which the X-ray is incident and on the interruption region 39, respectively.
Reflecting members 40a and 40b made of a material different from that of No. 8 are arranged. The reflecting members 40a and 40b are formed of a material that reflects visible light, such as TiO2, by a sedimentation method or the like.

An insulating layer 362 electrically separates the adjacent photodiodes 131 and 13 of the pixel unit.

Here, a groove is formed in the scintillator layer 38,
A method of forming the interruption region 39 in the scintillator layer 38 will be described.

A dicing method is used as one method. For example, using a diamond blade having a width of 20 μm, a cut is made in the scintillator layer 38 with a dicer. According to this method, a groove having a width W of 23 μm and a pixel pitch of 150 μm can be formed. Then, for example, TiO2 is filled in the groove and dried at 100 ° C.

There is also a method using a laser. For example, a third harmonic of a YAG laser is used to irradiate the scintillator layer with a pulse wave to form a hole. Alternatively, a method may be used in which the laser focus position is shifted by 1/4 of the hole diameter to irradiate a pulse wave to form a hole. For example, the focal position of the laser beam is moved linearly, and this process is repeated to form a groove and an interrupted region 39.

In the above structure, the X-ray 41 passes through the reflecting member 40a and enters from the input surface 38a to the scintillator layer 38.
And is converted into visible light 42. The visible light 42 is input to the photodiode 13, converted into a signal charge, and stored in the storage capacitor 15 via the through hole 37 and the drain electrode D.

On the other hand, the storage capacitor 1 of each pixel unit 12
The control circuit 16 controls the readout of the signal charges accumulated in the memory cell 5, and the readout is performed, for example, for each row of the pixel unit 12 in order. First, from the control circuit 16 to the first gate line 17
Pixel units 12a to 12c located in the first row through 1
An ON signal of, for example, 10 V is applied to the gate electrode G of
The pixel-unit TFTs 14 in the first row are turned on.

At this time, the pixel units 12a to 1 of the first row
The signal charges stored in the storage capacitor 15 of 2c are output as electric signals from the drain electrode D to the source electrode S, respectively. The electric signal output to the source electrode S is added in parallel to the plurality of charge amplifiers 19 and amplified.
After that, it is converted into a serial signal by the parallel / serial converter 19,
Further, it is converted into a digital signal by the analog-digital converter 20 and sent to a signal processing circuit (not shown) at the next stage.

When the reading of charges from the storage capacitors 15 in pixel units located in the first row is completed, the control circuit 16
Then, an OFF signal of, for example, −5V is applied to the gate electrode G of the pixel unit of the first row through the first gate line 171 to turn off the TFT 14 of the pixel unit of the first row.

The above-described operation is performed in the pixel unit 1 in the second and subsequent rows.
Steps 2 are also performed in order. Then, the signal charges stored in the storage capacitors 15 of all the pixel units 12 are read out, sequentially converted into digital signals and output, and electric signals corresponding to one X-ray screen are output from the analog-digital converter 20. Is output.

According to the above configuration, the interrupted region 39 in which the scintillator units are not continuous is provided between the adjacent scintillator units 38 and 381 of the pixel unit, and the interrupted region 3 is provided.
A reflecting member 40b is arranged at 9. The reflecting member 40a is also arranged on the input surface 38a of the scintillator unit 38. Therefore, the visible light 42 converted by the scintillator unit 38 is reflected by the upper surface 38 a and the side surface 38 b of the scintillator unit 38 and enters the photodiode 13. Therefore, the X-ray 4 in the scintillator unit 38
The conversion efficiency from 1 to light 42 is improved.

Light traveling in the lateral direction of the drawing, for example, in the direction of adjacent pixel units is reflected by the side surface 38b as indicated by arrows Y1 and Y2 in FIG. 2 and is not input to adjacent pixel units. Therefore, the resolution is also improved.

The upper surface 38a of the scintillator portion 38 and the reflecting members 40a and 40b arranged on the interruption area 39 may be made of the same material, or the material arranged on the upper surface 38a and the material arranged at the interruption area 39 may be different. Further, the reflection member 40b may be filled in the entire interruption region 39, or may be arranged only in a part of the interruption region 39, for example, a portion in contact with the side surface 38b.

In the case of the above configuration, the photodiode 1
5 is formed so as not to overlap the storage capacitor 13 and the TFT 14, for example. However, when increasing the light receiving area input to the photodiode 15,
It is also possible to provide an insulating layer on T14 and the storage capacitor 15 so that the input electrode surface of the photodiode 15 spreads over almost the entire area of each pixel, for example.

The scintillator layer 38 is made of CsI: N.
A scintillator material such as a or CsI: Tl is formed by a vacuum evaporation method. However, X-ray emitting phosphors such as Gd
A powder of 2 O2 S: Tb is used as a coating liquid with a resin binder and an organic solvent to form a scintillator-containing coating layer by a coating method using a dispenser, an inkjet, a spray, etc., and then the organic solvent is removed in a drying step, A method of solidifying the scintillator layer can also be used. According to this method, it is possible to obtain a scintillator layer having good characteristics that does not cause peeling or cracks.

In the case of the above embodiment, T is used as the reflecting member.
The interruption area is filled with i02. TiO2 has a small particle size of 1 to 2 .mu.m and can be filled in the interrupted region without any gap, so that light diffusion can be surely suppressed.

In general, when the reflecting member is formed of particles, if the average particle diameter of the particles is 1/3 or less of the width W of the interrupted region, light diffusion is suppressed and the resolution characteristics are improved.
It has been confirmed by experiments that the diffusion of light decreases when the average particle size becomes larger than that.

For the reflecting member, a material different from the material forming the scintillator layer or a material different in optical refractive index from the scintillator layer, such as titanium oxide, lead oxide, silicon, antimony sulfide, and sulfide. Cadmium, iron oxide, tungsten, diamond, platinum, KR
One or more of the high index materials such as S-5 can be used. Alternatively, an X-ray absorbing material that absorbs X-rays, such as aluminum, may be used as the reflecting member and placed in the interruption region. When the X-ray absorbing material is used, the resolution is improved. The X-ray absorbing material can be used by being included in another reflecting member.

It is also possible to use a gas such as the atmosphere or gas for the reflecting member, or to make it a vacuum, and to fill the space such as an interruption region with the gas, or to make it a vacuum.
In this case, assuming that the refractive index of gas or the like is ng and the refractive index of the scintillator layer is ns, the amplitude refractive index γ from the scintillator layer to the gas or the like is γ = (ns-ng) / (ns + ng)
Becomes The greater the difference between ns and ng, the greater the light reflection effect, and the light scattering and diffusion in the scintillator layer is prevented.

Generally, when the refractive index of the reflecting member is nr and the refractive index of the scintillator layer is ns, if the condition of nr / ns <1 is set, the fluorescent light converted by the scintillator layer is surrounded by the reflecting member. The scintillator layer is optically guided and efficiently input to the photodiode to improve the photoelectric conversion efficiency. Also, the resolution and sensitivity are improved.

The greater the difference between the refractive index nr of the reflecting member and the refractive index ns of the material forming the scintillator layer, the more suppressed the lateral scattering and diffusion of light generated in the scintillator layer.

Further, the metallic material film formed on the side surface of the scintillator layer can be used as the reflecting member.
In this case, the higher the light reflectance of the material of the metal film, the more the lateral scattering and diffusion of the light generated in the scintillator layer is suppressed, and the greater the effect. As the metal coating material, a metal containing aluminum, silver, gold, rhodium or the like, or a metal material containing a plurality of these metals as a main component is suitable.

Further, the reflecting member may be made of the same material as the scintillator layer. For example, when particles having an average particle size smaller than that of the material forming the scintillator layer are used, light is reflected on the side surface of the scintillator layer, and the same effect is obtained.

The depth of the interrupted region is preferably the same as the film thickness of the scintillator layer. However, even when the interrupted region is shallower than the thickness of the scintillator layer, the lateral scattering and diffusion of light is suppressed, and the effect is obtained.

In order to improve the conversion efficiency and resolution, the side surface of the scintillator layer may be flattened. When the side surface of the scintillator layer is flattened, irregular reflection of light on the side surface is suppressed, and conversion efficiency and resolution are improved.
The flattening of the side surface can be achieved by using a resin such as para-xylylylene as C.
There is a method of adhering to the side surface by the VD method, or a method of annealing the scintillator layer.

Next, in order to explain the effect of the present invention, C
The results of comparison of the TF characteristics with the related art will be described.
In this case, the CTF value (%) is 0.5 Lp / mm (0.5
It is the ratio of the contrast transmission ratio at line pair / millimeter) to the contrast transmission ratio at 2 Lp / mm.

In the conventional example, a coating liquid in which a phosphor powder (Gd2 O2 S: Tb) is dispersed in an epoxy resin is applied to a photoelectric conversion portion in a thickness of 400 μm, and then 100 ° C.
The scintillator layer is formed by heating the film to dry and cure it.

Inventive Example 1 has a structure in which grooves having a pixel pitch of 150 μm, a depth of 350 μm, and a width of 23 μm are formed in a grid pattern on the scintillator layer formed by the same method as the conventional example, in the interruption region. It is a structure that fills the atmosphere.

Inventive Example 2 has a structure in which the groove forming the interrupted region of Inventive Example 1 is filled with TiO 2 particles having a particle size of 1 μm.

In this case, the CTF values of Conventional Example, Invention Example 1 and Invention Example 2 were 20.6%, 24.3%, and 46.
8%, invention example 1 is about 20% of the conventional example, invention example 2
Is more than double that of the conventional example, and the invention has improved resolution characteristics.

[0055]

According to the present invention, it is possible to realize an X-ray flat panel detector having improved resolution characteristics when a scintillator layer is used.

[Brief description of drawings]

FIG. 1 is a schematic circuit configuration diagram for explaining an embodiment of the present invention.

FIG. 2 is a schematic cross-sectional view for explaining a pixel unit according to the embodiment of the present invention.

[Explanation of symbols]

11 ... Photoelectric conversion unit 12 ... Pixel unit 13 ... Photodiode 14 ... Thin film transistor (TFT) 15 ... Charge storage capacitor 16 ... Control circuit 17 ... Control line 18 ... Data line 19 ... Charge amplifier 20 ... Parallel / serial converter 21 ... Analog-to-digital converter 31 ... Insulating substrate 32 ... Insulating film 33 ... Semi-insulating film 37 ... Through hole 38 ... Scintillator layer 39 ... Suspended area 40a, 40b ... Reflective member 41 ... X-ray 42 ... visible light

Front page continuation (51) Int.Cl. ⁷ Identification code FI theme code (reference) H04N 5/32 H04N 5/335 U 5/335 H01L 27/14 K (72) Inventor Hiroyuki Soda Isogo-ku, Yokohama-shi, Kanagawa 8th Shinsugita-cho F-term in Toshiba Yokohama Works (reference) 2G088 EE01 FF02 GG19 JJ05 4M118 AA10 AB01 BA05 CA03 CA05 CB06 CB11 CB20 DD09 DD12 FB03 FB04 FB09 FB13 FB16 FB24 FB25 GA10 5C024 GX03 CX AX12 CX

Claims (8)

[Claims] 1. A plurality of pixel units arranged two-dimensionally, each pixel unit being a scintillator unit for converting X-rays incident from a predetermined input surface into light, and converted by the scintillator unit. A photoconductive conversion unit that converts the light into a signal charge, a charge storage unit that stores the signal charge converted by the photoconductive conversion unit, and a readout of the signal charge stored in the charge storage unit are controlled. In an X-ray flat panel detector having a switching unit, an interruption region in which the scintillator units are not continuous is provided between adjacent scintillator units of pixel units, and the reflection region is formed of a material different from that of the scintillator unit. An X-ray flat panel detector characterized in that 2. A plurality of pixel units arranged two-dimensionally, each of the pixel units being converted by the scintillator unit for converting X-rays incident from a predetermined input surface into light. A photoconductive conversion unit that converts the light into a signal charge, a charge storage unit that stores the signal charge converted by the photoconductive conversion unit, and a readout of the signal charge stored in the charge storage unit are controlled. In an X-ray flat panel detector having a switching unit, an interruption region in which the scintillator units are not continuous is provided between adjacent scintillator units of pixel units, and the interruption region is made of a material having an average particle size smaller than that of the scintillator unit. An X-ray flat panel detector characterized in that the formed reflecting member is arranged. 3. A plurality of pixel units arranged two-dimensionally, each pixel unit being converted by the scintillator unit for converting X-rays incident from a predetermined input surface into light, and the scintillator unit. A photoconductive conversion unit that converts the light into a signal charge, a charge storage unit that stores the signal charge converted by the photoconductive conversion unit, and a readout of the signal charge stored in the charge storage unit are controlled. In an X-ray flat panel detector having a switching unit, an interruption region in which the scintillator units are not continuous is provided between adjacent scintillator units of pixel units, and is located on the input surface of the scintillator unit and the adjacent pixel unit side. An X-ray flat panel detector in which a side surface of the scintillator portion is in contact with a reflecting member made of a material different from that of the scintillator portion. 4. The X-ray flat panel detector according to claim 1, wherein the reflecting member is gas, metal, or vacuum. 5. The X-ray plane according to claim 1, wherein nr / ns <1 when the refractive index of the reflecting member is nr and the refractive index of the scintillator layer is ns. Detector. 6. The X-ray flat panel detector according to claim 1, wherein the reflecting member contains an X-ray absorbing material. 7. L / W> 3, where L is the pitch between adjacent pixel units and W is the width of the interrupted region.
The X-ray flat panel detector according to claim 6. 8. The reflection member is formed of a collection of particles, and the average particle diameter of the particles is 1/3 or less of the width of the interrupted region. The X-ray flat panel detector according to any one of claims 6 and 7.