JP2006251690A - Imaging plate, radiological image information recording and reading device using the same, and radiological image information reading method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem of low spatial resolution phenomenon and fading (color fading) phenomenon in conventional BaFBr: Eu<SP>2+</SP>based imaging plates. <P>SOLUTION: The imaging plate has a fluorescent layer consisting of crystals or a thin film of lithium fluoride, or consisting of crystals or a thin film obtained by doping lithium fluoride with magnesium ions. The radiological image information recording and reading device, and the radiological image information reading method use this imaging plate. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an imaging plate, a radiation image information recording / reading apparatus using the imaging plate, and a radiation image information reading method.

  In recent years, for example, as a means for recording a radiographic image such as an X-ray diffraction image, the dynamic range can be increased compared to conventional photographic methods, and the computer analysis of the read image is easy. Therefore, so-called imaging plates have been used.

  This imaging plate has a phosphor layer on the surface that absorbs and accumulates radiation, and emits light according to the intensity of the accumulated radiation and erases the recording when irradiated with excitation light thereafter. A radiographic image is recorded as a latent image on the layer.

  The radiographic image recorded on the imaging plate is read by a radiographic image information recording / reading apparatus and used for computer analysis or the like. This radiation image information recording / reading apparatus irradiates excitation light while scanning each point of the phosphor layer of the imaging plate, and detects emitted light generated at each point by this irradiation.

Conventionally, the phosphor layer of the imaging plate ^{_{BaFBr: Eu 2+, BaFBr 0.85 I}} 0.15: Eu 2+ or the like of the BaFBr: ^{Eu 2+} based photostimulated luminescence phosphor is used and medical, as well as bio It is used in a wide range of application fields such as science, archeology, nondestructive inspection, measurement and analysis (for example, Patent Document 1).

An imaging plate using a BaFBr: Eu ²⁺ -based photostimulable phosphor has many advantages such as dynamic range, background noise, and detection quantum efficiency compared to conventional X-ray films. As the computer can handle images, it has shown rapid progress with the rapid spread of computers.

In general, a series of cycles of recording, reading, erasing and reusing a radiation image using an imaging plate is shown in FIG. In the imaging plate, a phosphor layer (150 μm) in which fine crystals of BaFBr: Eu ²⁺ -based stimulable phosphor having a particle diameter of about 5 μm and a binder are mixed is uniformly coated on a polyester film support (250 μm) A protective layer (10 μm) is attached (FIG. 2). The entire imaging plate is a flexible sheet having a thickness of about 0.5 mm.

First, electrons generated by X-ray irradiation are captured in a metastable state called a lattice defect (F center) in the phosphor, and an absorption band is formed as a concentration distribution corresponding to the X-ray irradiation dose. Next, when the imaging plate is scanned with the focused laser beam, the electrons trapped in the lattice defects pass through the conduction band and the excited state of Eu ²⁺ impurities doped in BaFBr, and the holes. Recombine and lose its energy. At this time, light corresponding to the energy difference is emitted as fluorescence. The process of exciting the lattice defect absorption band with a long wavelength and emitting light with a short wavelength corresponding to the Eu ²⁺ transition is called stimulated light emission (PSL). This fluorescence is converted into a digital quantity and taken into a computer in time series, and the original image is reproduced in the computer. Finally, the used imaging plate is completely in a state before use by irradiating an erasing light source, and can be reused. This is a measurement principle using an imaging plate using a BaFBr: Eu ²⁺ -based stimulable phosphor currently used.

The imaging plate using the BaFBr: Eu ²⁺ based photostimulable phosphor in the above process is reused. The problem with this method is that the photostimulable phosphor layer is coated on the support with a mixture of fine crystals of BaFBr: Eu ²⁺ based stimulable phosphor having a particle size of about 5 μm and a binder. In the laser beam scanning, scattering / diffusion or the like occurs due to the difference in refractive index between the microcrystal and the binder in the phosphor layer. This causes a decrease in the contrast of the image, and is a factor that decreases the spatial resolution.

The low spatial resolution of the BaFBr: Eu ²⁺ imaging plate is a serious problem in fields requiring high spatial resolution, such as mammography (breast diagnosis). In order to increase the probability of early cancer detection by mammography, the spatial resolution (about 25 μm) with the BaFBr: Eu ²⁺ imaging plate is insufficient, so that the other performances are significantly reduced. The current situation depends on conventional analog X-ray films that exhibit superior characteristics in terms of resolution. Furthermore, exposure to the breast in mammography imaging is 1 to 3 mGy (milli gray) for each imaging, and in order to greatly reduce the exposure dose, higher sensitivity and a high space of 1 μm or less New proposals are needed to resolve resolution and negligible fading phenomena.

Japanese Examined Patent Publication No. 6-14233

An object of the present invention is to solve the low spatial resolution and fading (fading) phenomenon in a conventional BaFBr: Eu ²⁺ imaging plate.

The present invention includes the following inventions.
(1) An imaging plate having a phosphor layer composed of a crystal or thin film of lithium fluoride or a crystal or thin film formed by doping lithium fluoride with magnesium ions.
(2) The imaging plate according to (1), wherein the phosphor layer is provided on a support.
(3) The imaging plate according to (2), wherein a surface protective layer is provided on the phosphor layer.
(4) The imaging plate according to any one of (1) to (3), a radiation source installed on one surface side of the imaging plate, and radiation passing through the subject from the radiation source to the imaging plate An image recording unit that accumulates and records a radiation transmission image of the subject on the imaging plate by irradiation, an excitation light source that emits excitation light that scans the imaging plate on which the radiation image is accumulated and recorded, and scanning by the excitation light Photoelectric reading means for reading the fluorescence emitted from the imaging plate and obtaining an image signal, and an image reading unit installed on the other surface side of the imaging plate, and image reading is performed in the image reading unit. Remaining on the imaging plate prior to image recording on the imaging plate Radiation image information recording and reading apparatus, comprising the erasing means to release the ray energy.
(5) In the method of reading radiographic image information using the radiographic image information recording / reading apparatus according to (4), a lithium fluoride crystal or thin film, or lithium fluoride is doped with magnesium ions by irradiation with radiation. A radiation image information reading method comprising: reading fluorescence emitted from an imaging plate by scanning an absorption band of F ₂ and / or F ₃ ⁺ center formed in a crystal or a thin film with a laser.

  According to the present invention, it is possible to provide a radiographic image information recording / reading technique that has a high spatial resolution and has greatly improved fading (fading) phenomenon. The present invention is particularly useful in fields requiring high spatial resolution such as mammography (breast diagnosis).

  The imaging plate of the present invention is characterized by using a crystal or thin film of lithium fluoride or a crystal or thin film obtained by doping lithium fluoride with magnesium ions as the material of the phosphor layer.

BaFBr: Eu ²⁺ crystals belong to the tetragonal system and have a layered structure (FIG. 3). Of course, in order to use the PSL phenomenon, Eu ²⁺ is substituted at an appropriate concentration on the Ba site. On the other hand, the lithium fluoride crystal used in the present invention belongs to a very simple equiaxed rock salt type as shown in FIG. In addition, the crystal formed by doping magnesium fluoride with lithium fluoride used in the present invention enters a monovalent lithium ion site by replacing divalent magnesium ions, so-called impurity-cation vacancy pairs or anion vacancies. The structure forms a hole.

By doping magnesium fluoride with lithium fluoride, the sensitivity of the imaging plate is improved. The amount of magnesium ions doped into lithium fluoride is preferably 10 to 150 wtppm, more preferably 70 to 100 wtppm. The doping of magnesium ions into lithium fluoride can be performed by charging an appropriate amount of magnesium fluoride (MgF ₂ ) powder into a crucible before crystal growth.

  Various polymer materials are used as the support used in the imaging plate of the present invention. In particular, a material that can be processed into a flexible sheet or web for handling as an information recording material is suitable. From this point, a cellulose acetate film, a polyester film, a polyethylene terephthalate film, a polyethylene naphthalate film, a polyamide film, a polyimide film, A plastic film such as a triacetate film or a polycarbonate film is preferred.

  The layer thickness of these supports varies depending on the material of the support used, but is generally 80 to 1000 μm, and more preferably 80 to 500 μm from the viewpoint of handling. The surface of these supports may be a smooth surface, or may be a mat surface for the purpose of improving the adhesion to the phosphor layer. Further, these supports may be provided with an undercoat layer on the surface on which the phosphor layer is provided for the purpose of improving the adhesion to the phosphor layer.

  As the surface protective layer provided on the phosphor layer, a polyester film, a polymethacrylate film, a nitrocellulose having an excitation light absorbing layer having a haze ratio measured by the method described in ASTM D-1003 of 5% or more and less than 60% Films, cellulose acetate films and the like can be used, but stretched films such as polyethylene terephthalate films and polyethylene naphthalate films are preferable in terms of transparency and strength. Furthermore, these polyethylene terephthalate films and polyethylene terephthalate films are preferred. From the viewpoint of moisture resistance, a vapor deposition film in which a thin film of metal oxide, silicon nitride or the like is vapor-deposited is more preferable.

  The haze ratio of the film used in the surface protective layer can be easily adjusted by selecting the haze ratio of the resin film to be used, and a resin film having an arbitrary haze ratio can be easily obtained industrially.

  The radiographic image information recording / reading apparatus of the present invention has the radiation source installed on one side of the imaging plate and the imaging plate, and irradiates the imaging plate with radiation from the radiation source through the subject. An image recording unit for accumulating and recording a radiation transmission image of the subject on the imaging plate, an excitation light source for emitting excitation light for scanning the imaging plate on which the radiation image is accumulated and recorded, and an imaging plate scanned by the excitation light An image reading unit installed on the other surface side of the imaging plate, and imaging after image reading is performed in the image reading unit. Remaining on the imaging plate prior to image recording on the plate Characterized by comprising the erasing means to release the ray energy.

  The apparatus of the present invention performs the reading operation and the subsequent erasing. The image signal obtained by the reading operation may be temporarily stored in a storage medium such as a magnetic tape or a magnetic disk, or may be a CRT or the like. It may be displayed on the display and immediately observed, or a hard copy may be recorded permanently. The playback device for this purpose may be directly joined to the device, or may be played once at a remote location via a storage device, or may be placed at a remote location and receive a signal wirelessly. And may be played back. In this way, for example, it is possible to reproduce an image taken with a mobile vehicle with a reception / reproduction device of a hospital, and a specialist can observe this and report a diagnosis result to the mobile vehicle wirelessly. In the radiographic image information recording / reading apparatus of the present invention having the above-described configuration, the imaging plate for recording the radiographic image is repeatedly used by being recorded, read and erased in the apparatus. The handling of the imaging plate is eliminated, the imaging plate is not damaged by the handling, the apparatus is easily operated and moved, and is easy to use in practice.

  Also, since one imaging plate is used repeatedly, if the quality of the reproduced image is lowered, the imaging plate in use may be replaced with a new one, and the quality control of the imaging plate is extremely easy. In addition, since one imaging plate is repeatedly used, an image with a constant image quality can be obtained without instability of quality due to variations in characteristics of each imaging plate.

  Further, since the imaging plate has a flat plate shape, and image recording is performed on the flat plate imaging plate, the image recording can be easily performed with high accuracy without distortion.

  Furthermore, since recording can be performed by repeatedly using one imaging plate, a large number of continuous photographing can be easily performed without preparing a large number of imaging plates, and the user needs to handle the imaging plate as described above. In addition, since the entire apparatus is extremely compact, it is optimal as an apparatus to be used, for example, when carrying out a mass examination in a remote place on an X-ray imaging vehicle.

In the present invention, when the imaging plate is irradiated with radiation (X-rays, α rays, β rays, γ rays, ultraviolet rays, etc.), the lithium fluoride crystal or thin film in the imaging plate or lithium fluoride is doped with magnesium ions. Since the F ₂ and F ₃ ⁺ centers are formed in the crystal or thin film, the fluorescence emitted from the imaging plate is excited by exciting the absorption band caused by the F ₂ and / or F ₃ ⁺ centers with a laser. Read it.

In the present invention, it is preferable that the wavelength region of the excitation light and the wavelength region of the emitted light (fluorescence) do not overlap in order to improve the S / N ratio, and the excitation light source and the phosphor are satisfied so as to satisfy such a relationship. Is preferably selected. Wavelength of crystals or thin films of lithium fluoride used in the present invention, or by doping magnesium ions into lithium fluoride crystal or a thin film of a light-emitting light (fluorescence) is usually, if used F ₂ Center, 600 Since it is 750 nm, it is preferable to select an excitation light source of 400 to 500 nm as the excitation light. Of course, the optimum fluorescence and excitation wavelengths are preferably light around 650 nm and 450 nm, which are the peak wavelengths, respectively.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the scope of the present invention is not limited to these examples.

[Example 1] Manufacture of an imaging plate An appropriate amount of MgF ₂ powder is added to pure (impurity-free) LiF powder, and it is pulled up using a vertical furnace in an argon atmosphere or vacuum, or a high-power excimer laser Etc. are irradiated to a LiF powder and deposited as a LiF thin film on a substrate heated to about 300 ° C. by a thermal evaporation method.

[Example 2]
(1) Using the experimental system shown in FIG. 5, the characteristics of the imaging plate of the present invention and an imaging plate using a BaFBr: Eu ²⁺ -based stimulable phosphor were compared.

As samples, the LiF crystals with and without Mg impurities produced in Example 1, BaFBr: Eu ²⁺ crystals, and a commercially available imaging plate (BAS-UR) were used. The stimulating light for these was condensed on the sample on the XY fine movement device using Ar ⁺ laser (457 nm) and He—Ne (632 nm) laser, respectively. The fluorescence (PL) (650 nm) and stimulated emission (PSL) (390 nm) intensities were detected by a photomultiplier tube with a filter (R928), and then taken into a personal computer through an amplifier and a data collection device.

After irradiation of the sample at room temperature with X-rays (35 kV, 25 mA, distance to sample 125 mm, 15 minutes), LiF crystal F ₂ band and BaFBr: Eu ²⁺ crystal, commercially available imaging plate F (Br ⁻ ) band FIG. 6 shows attenuation curves for standardized PL and PSL intensity continuous stimulation light (60-minute irradiation) when irradiated with 7.8 W / mm ² Ar + laser and 2.1 W / mm ² He—Ne laser, respectively.

(2) When irradiated with light having a wavelength of 450 nm corresponding to an absorption band having a wavelength of 450 nm that is laterally formed from the F ₂ and F ₃ ⁺ centers of the lithium fluoride crystal, as shown in FIG. Efficient (100%) release.

In this fluorescence spectrum, the peak at 530 nm is attributed to the F ₃ ⁺ center, and the peak at 650 nm is attributed to the F ₂ center. In FIG. 7, # 1, # 2, and # 3 indicate cases where the magnesium ion amount is 1 wtppm, 10 wtppm, and 100 wtppm, respectively. With magnesium ion doping, more holes are formed for charge compensation at the time of crystal formation, resulting in the formation of a larger absorption band, resulting in a highly sensitive imaging plate. The present invention uses a fluorescence process instead of a PSL process for reading out a radiation image as described above, and has a very high quantum efficiency of 100%.

(3) FIG. 8 shows the change in absorption intensity at each center of the lithium fluoride crystal when the X-ray irradiation time (proportional to the irradiation dose) is changed. M corresponds to an absorption band including F ₂ and F ₃ ⁺ centers, which are focused in the present invention. From this figure, it can be seen that M changes linearly. Further, FIG. 9 is a graph in which changes in absorption intensity are recorded after irradiating lithium fluoride crystals with X-rays at room temperature for 45 minutes and then storing in a dark place. Compared to other centers, it can be seen that the F ₂ and F ₃ ⁺ centers of interest in the present invention are hardly attenuated, that is, do not exhibit a fading phenomenon.

Further, from FIG. 10, the fluorescence lifetime at room temperature of the F ₂ and F ₃ ⁺ centers of the lithium fluoride crystal irradiated with X-rays was measured with a NAES-1100 manufactured by Horiba, which was 16.8 ns and 7.94 ns, respectively, very quickly. , BaFBr: Eu ²⁺ crystal, which is about 1/100 to 1/50 of 0.8 μs. This value is a factor that leads to shortening the readout time of a large image.

From the above experimental results, in the present invention, a crystal or thin film of lithium fluoride, or a crystal or thin film obtained by doping lithium fluoride with magnesium ions is used, and F ₂ and F ₃ are formed by X-ray irradiation on this. ⁺ Center (preferably, greater use of high F ₂ Center sensitivity) by scanning the absorption band of the laser, can be incorporated into a computer in a time series digital amount of the fluorescence. A series of these processes is the same cycle as in FIG. 1 except that an efficient PL is used instead of PSL at the time of reading. Furthermore, in this method, since fine particles such as BaFBr: Eu ²⁺ -based stimulable phosphor are not used, scattering and the like can be excluded as much as possible, and high-sensitivity measurement with high spatial resolution can be achieved. It is also advantageous that lithium fluoride materials are very stable chemically and physically and optically and have excellent characteristics.

It is a figure which shows a series of cycles of recording / reading / erasing / reusing of a radiation image using an imaging plate. It is sectional drawing of an imaging plate. It is a figure which shows the structure of BaFBr: Eu2 ⁺ crystal ^| crystallization. It is a figure which shows the structure of a lithium fluoride crystal. It is a figure which shows the experimental system used in the Example in order to compare the characteristic of the imaging plate of this invention and the imaging plate using BaFBr: Eu2 ⁺ type | stimulation light emission fluorescent substance. It is a figure which shows the time decay characteristic of PL (LiF crystal) and PSL (BaFBr: Eu2 ⁺ type ^| system ^| group imaging plate). It is a figure which shows the fluorescence spectrum at room temperature obtained by irradiating 450 nm light to the X-ray irradiated lithium fluoride crystal. # 1, # 2 and # 3 indicate cases where the magnesium ion amount is 1 wtppm, 10 wtppm and 100 wtppm, respectively. It is a figure which shows the change of the absorption intensity of each center of a lithium fluoride crystal when X-ray irradiation time (proportional to irradiation dose) is changed. M corresponds to an absorption band including F ₂ and F ₃ ⁺ centers, which are focused in the present invention. It is a figure which shows the change of the absorption intensity of each center of a lithium fluoride crystal after irradiating a X-ray to a lithium fluoride crystal for 45 minutes at room temperature. Is a diagram showing a result of the fluorescence lifetime was measured at room temperature of F ₂ and F ₃ ⁺ centers of the X-ray irradiated lithium fluoride crystals.

Claims (5)

  An imaging plate having a phosphor layer composed of a crystal or thin film of lithium fluoride or a crystal or thin film formed by doping lithium fluoride with magnesium ions.   The imaging plate according to claim 1, wherein the phosphor layer is provided on a support.   The imaging plate according to claim 2, wherein a surface protective layer is provided on the phosphor layer.   The imaging plate according to any one of claims 1 to 3, comprising a radiation source installed on one surface side of the imaging plate, and irradiating radiation from the radiation source to the imaging plate through a subject An image recording unit for accumulating and recording a radiation transmission image of the subject on the imaging plate, an excitation light source for emitting excitation light for scanning the imaging plate on which the radiation image is accumulated and recorded, and an imaging plate scanned by the excitation light Photoelectric reading means for reading the fluorescence emitted from the image reading unit to obtain an image signal, an image reading unit installed on the other surface side of the imaging plate, and after the image reading is performed in the image reading unit Prior to image recording on the imaging plate, residual radiation errors on the imaging plate are recorded. Radiation image information recording and reading apparatus, comprising the erasing means to release Energy. 5. A method of reading radiation image information using the radiation image information recording / reading apparatus according to claim 4, wherein the crystal is a lithium fluoride crystal or thin film, or a crystal or thin film obtained by doping lithium fluoride with magnesium ions. A radiographic image information reading method comprising: reading fluorescence emitted from an imaging plate by scanning a formed absorption band of F ₂ and / or F ₃ ⁺ center with a laser.

Images