JPH0373840B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image conversion method in a radiographic imaging system, and more specifically to a method for recording a radiographic image on a stimulable phosphor material (hereinafter simply referred to as "phosphor"), The present invention relates to an image conversion method in a radiation image system that reads and reproduces this radiation image and records it on a recording material as a final image.

Traditionally, so-called radiography, which uses silver salts, has been used to obtain radiographic images, but in recent years, due to problems such as the depletion of silver resources on a global scale, radiographic images have been developed without using silver salts. A new method has become desirable.

As an alternative to the radiographic method described above, the radiation transmitted through the object is absorbed by a phosphor, and then this phosphor is excited with a certain type of energy to release the radiation energy stored in the phosphor. A method has been considered in which the fluorescent light is emitted and the fluorescent light is detected and imaged. Specific methods include, for example, U.S. Pat.
No. 12144 proposes a radiation image conversion method using a photostimulable phosphor as a phosphor and electromagnetic radiation selected from visible light and infrared rays as excitation energy. This conversion method uses a panel in which a photostimulable phosphor layer is formed on a support, and the photostimulable phosphor layer of this panel absorbs the radiation that has passed through the subject. A device that accumulates energy and then scans this photostimulable phosphor layer with photostimulation excitation light, extracts the accumulated radiation energy as a light signal, and obtains an image by varying the intensity of this light. It is.

In such a conversion method, after the radiation energy is accumulated in a radiation image conversion panel (hereinafter simply referred to as a panel), it is not necessary to read it directly, but it is possible to store it as a latent image in the panel. This is mentioned as one of the characteristics of the conversion method. That is, there is no restriction that recording and reproduction of radiographic images be performed at the same time and place, and the recording speed is not limited by the reproduction speed.

Ideally, it is also desirable that the image information recorded in the phosphor used in the panel does not change over time.

However, with the phosphors known to date, the strength of the reproduced signal deteriorates over time, and even for those with a small degree of fading, the strength of the reproduced signal deteriorates by more than 10% in a day, and in general, It is more than 30% on a daily basis.

Therefore, when radiographic images reproduced from a panel are reproduced at arbitrary intervals over time, homogeneity over time cannot be expected between the reproduced images.

Decreased reproducibility of the image (loss of homogeneity over time)
This is a flaw in the advantageous feature of the radiation image conversion method using a photostimulable phosphor, which is the spatiotemporal arbitrariness of both image recording and image reproduction operations, and its improvement is desired. There is.

In relation to the above-mentioned problems, an object of the present invention is to solve the problem of reducing image reproducibility due to latent image fading in the photostimulable phosphor in the radiation image conversion method using the photostimulable phosphor. It is an object of the present invention to provide a conversion method that avoids loss of homogeneity over time (loss of homogeneity over time) and provides images that are homogeneous over time.

Another object of the present invention is to provide a radiation image conversion apparatus that easily implements the conversion method of the present invention.

An object of the present invention is to irradiate a radiation image conversion panel made of a stimulable phosphor with a radiation image, thereby stimulating the energy of the irradiation rays imagewise accumulated on the panel with excitation light. A method for converting a radiation image in which the panel is irradiated with a radiation image and is heated to 50 to 200°C before photostimulation excitation. This can be achieved by

As a result of research on latent image fading in radiation images generated on panels made of photostimulable phosphors, the present inventors found that (1) the fading is large immediately after the latent image is generated, and it reaches equilibrium after a certain period of time. reach the level. (The certain period of time is called the decay period.) Regarding (2), we have found that the decay period differs depending on the type of stimulable phosphor.

Therefore, if the image is reproduced from the panel after the decay period has elapsed, it is possible to guarantee the homogeneity of the reproduced image over time. However, the decay period can be as short as about 1 hour, or as long as more than 50 hours, which poses a significant practical problem.

As a result of further research on fading, we found that by heating the panel prior to photostimulation for image reproduction, the total decay period or residual decay period passes quickly and reaches an equilibrium level. Furthermore, we found that by heating the panel during panel radiation irradiation, the attenuation period disappears and an equilibrium level appears when a latent image is generated.

That is, radiation incident on the panel in an imagewise manner is absorbed into the stimulable phosphor layer of the panel in proportion to the imagewise radiation dose, and a number of electrons and/or holes are created therein in proportion to the absorbed radiation dose. is captured by the trap level of the phosphor, forming a kind of latent image for the radiation image. At this time, there is not a single trap level that captures electrons and/or holes, but there are various deep and shallow levels. Electrons, etc. captured in shallow levels are emitted according to the characteristics of the phosphor through thermal processes during natural storage such as aging, and the total amount generated when photostimulated is excited depending on the degree of aging. This is recognized as a phenomenon of latent image fading. On the other hand, electrons captured in deep levels are stable against thermal processes over time,
It is thought that the electrons captured in the shallow levels are exhausted and only the levels stable in the thermal process remain, and this group of levels forms the equilibrium level described above. Note that the period during which the shallow level is exhausted corresponds to the decay period.

Note that the depletion of electrons and the like in the shallow level exhibits remarkable temperature dependence, and as the temperature rises, the depletion is rapidly accelerated and an equilibrium level appears immediately.

Therefore, in the present invention, the radiation image conversion panel made of a photostimulable phosphor is exposed to the photostimulable phosphor during a recording period from the time of recording when the radiation image conversion panel is irradiated with radiation to the time of photostimulation excitation for image reproduction. The attenuation period is eliminated by heating to a temperature that corresponds to the characteristics of the radiation, and images with uniformity over time are reproduced by photostimulation regardless of the length of time since radiation irradiation.

Next, the radiation image conversion method of the present invention will be explained using a block diagram of an apparatus used to implement the method.

In FIG. 1, 10 is a heating device for the panel 13, 11 is a radiation generator, 12 is a subject, and 13
14 is an excitation light source for emitting the radiation latent image of the panel as stimulated light; 15 is a photoelectric conversion device for detecting the fluorescent light emitted from the panel; 16 is a device that reproduces the photoelectric conversion signal detected by the photoelectric conversion device 15 as an image; 17 is a device that displays the reproduced image; and 18 is a device that cuts reflected light from the light source 14 and is emitted from the panel 13. This is a filter that allows only light to pass through. From 15 onwards, it is sufficient that the optical information from 13 can be reproduced as an image in some form, and is not limited to the above.

Alternatively, to cut off the reflected light from the light source 14, the filter 18 may not be used, but a separation method using delay in light emission as shown in Japanese Patent Application No. 124744/1986 may be used. Furthermore, during the recording of radiation generated by the radiation generating device 11 and transmitted through the subject 12 on the panel 13, the heating of the panel 13 by the heating device 10, and the reproduction of the latent image formed on the panel 13 by 14 to 18, the panel 13 is separated. It goes without saying that the device may be of the type or portable type, and does not need to be fixed at the same installation location as other devices.

As shown in FIG. 1, when the subject 12 is placed between the radiation generator 11 and the panel 13 and irradiated with radiation, the radiation passes through each part of the subject 12 as the radiation transmittance changes. An image (that is, an image of the intensity of radiation) is incident on the panel 13. This incident transmitted image is absorbed by the phosphor layer of the panel 13, thereby generating a number of electrons and/or holes in proportion to the amount of radiation absorbed in the phosphor layer, which is then absorbed by the phosphor layer. is accumulated at the trap level. That is, an accumulated image (latent image) of a radiographic image is formed. By heating this panel to 50-200℃ with a heating device, it accelerates the decay over time.
Allow equilibrium to be reached.

This latent image is then excited with light energy to become visible. That is, the light source 14 irradiates the phosphor layer with electromagnetic waves containing long-wavelength visible light and/or infrared radiation of 500 nm or more to expel the electrons and/or holes accumulated at the trap level, and radiates the accumulated image as fluorescent light. urge The intensity of this emitted fluorescent light is proportional to the number of accumulated electrons and/or holes, that is, the intensity of the radiation energy absorbed by the phosphor layer of the panel 13, and this optical signal can be used, for example, by photoelectron multiplication. A photoelectric conversion device 15 such as a tube converts the signal into an electrical signal, an image reproducing device 16 reproduces it as an image, and an image display device 17 displays this image.

In this way, images with good temporal quality can be obtained regardless of the length of time from recording to reproduction.

FIG. 2 is a schematic diagram showing a heating method for the panel 13 in the method of the present invention. The air sent out by the blower 21 is turned into hot air by the heater 22, and the panel 13 is uniformly heated. The temperature of the hot air is measured by a temperature sensor 24 connected to the heater controller 23 and kept constant. Further, the heating time is controlled by a timer 25.

In addition to the above-mentioned examples, the heating device necessary for the present invention may be any device that can finally uniformly heat the panel 13 by contact, radiation, or heat conduction. plate heater,
An infrared lamp or the like is possible.

If the heating temperature is below 50°C, the effect of imparting uniformity over time to the image will be small, and if it is above 200°C, the electrons and/or holes accumulated in the deep traps that form a stable latent image will also be released. I ended up
Radiographic images will be lost.

Next, the effects of the present invention will be shown by the results of measuring the fading over time from recording to reproduction. FIG. 3 shows the change over time in the intensity of a reproduced signal read from a panel on which X-rays were recorded under the same conditions. The vertical axis is relative photostimulated light intensity, and the horizontal axis is time (time). FIG. 3a shows an example in which no heat treatment is performed, and FIG. 3b shows an example in which heat treatment according to the present invention is performed. As is clear from this result,
According to the present invention, a homogeneous radiographic image can be obtained regardless of the time elapsed from recording to reproduction. The heating temperature and heating time have an optimum range depending on the phosphor, and this is related to the depth and amount of the shallow trap.

The radiation image conversion panel used in the radiation image conversion method of the present invention and the excitation light source for emitting the accumulated image as fluorescent light will be described in detail below.

As shown in FIG. 4a, the structure of the radiation image storage panel consists of a support 41 and a phosphor layer 42 formed on one side of the support 41. This phosphor layer 42 is made of a so-called photostimulable phosphor.

Examples of such phosphors include, for example, Japanese Patent Application Laid-open No. 1986-
BaSO ₄ described in No. 80487; Ax (where A is at least one of Dy, Tb and Tm, and X is
0.001≦X<1 mol%. ), MgSO ₄ described in JP-A No. 48-80488; Ax (where A is at least one of Ho and Dy, and X is 0.001≦X≦1 mol%). A phosphor represented by SrSO ₄ described in JP-A-48-80489;
A phosphor represented by Ax (where A is at least one of Tm, Tb and Dy, and X is 0.001≦X<1 mol%), Na ₂ described in JP-A-51-29889 Mn, SO ₄ , CaSO ₄ and BaSO ₄ etc.
Fluorescent material doped with at least one of Dy and Tb, BeO, LiF described in JP-A No. 52-30487,
Fluorescent materials such as Mg ₂ SO ₄ and CaF ₂ , JP-A-53-
Li ₂ B ₄ O ₇ described in No. 39277: Fluorescent material such as CuAg, Li ₂ O.(B ₂ O ₂ ) x: Cu (where X is 2<X≦3) described in JP-A-54-47883 , and _Li2O ( _B2O3 ) _x :
Fluorescent material such as Cu, Ag (where X is 2<X≦3), SrS described in U.S. Patent No. 3859527: Ce, Sm, SrS:
Eu, Sm, La ₂ O ₂ S: Eu, Sm and (Zn, Cd)
S: A phosphor represented by Mn, X (where X is halogen). ZnS: Cu, P ₆ described in JP-A-55-12142
Fluorescent material, general formula is BaO・XAl ₂ O ₃ :Eu (however, 0.8≦
A barium _aluminate phosphor represented by
is Mg, Ca, Sr, Zm, Cd, or Ba, and A
are Ce, Tb, Eu, Tm, Pb, TI, Bi, and Mn
At least one of the following, and X is 0.5≦X≦2.5
It is. ) is an alkaline earth metal silicate phosphor. The general formula described in JP-A-55-12143 is (Ba _1-xy Mg _x Ca _y )FX:eEu ² + (where X is at least one of Br and Cl, and x, y, and e are each 0<x+y≦
0.6, xy≠0 and 10 ⁻⁶ ≦e≦5×10 ⁻² . ), the general formula of the alkaline earth fluorohalide phosphor described in JP-A-55-12144 is LnOX:xA (Ln represents at least one of La, Y, Gd, and Lu, and X represents Cl and/or Br, A is Ce and/or
or Tb, and x represents a number satisfying 0<x<0.1. ), the general formula described in JP-A-55-12145 is (Ba _1-x M〓 _x )FX:yA (where M〓 is Ma, Ca, Sr, , Zn and Cd) X is at least one of Cl, Br and I; A is Eu, Tb, Ce,
At least one of Tm, Dy, Pr, Ho, Nd, Yb and Er, x and y are 0≦x≦0.6 and 0
Represents a number that satisfies the condition ≦y≦0.2. ), the general formula described in JP-A-55-84389 is BaFX, xCe, yA (where X is Cl, Br and I
At least one of them A is Im, Tl, Gd, Sm
and Zr, and X and y are 0<x≦2×10 ^-1 and 0<y
≦5×10 ^-2 . ), and the general formula described in JP-A-55-160078 is M〓FX XA:yLn (where M〓 is Ba, Ca, Sr, Mg, Zn and Cd
At least one of the following, A is BeO, MgO,
CaO, SrO, BaO, ZnO, Al ₂ O ₃ , Y ₂ O ₃ ,
La ₂ O ₃ , In ₂ O ₃ , SiO ₂ TiO ₂ , ZrO ₂ , GeO ₂ , SnO ₂ ,
At least one of Nb ₂ O ₅ , Ta ₂ O ₅ , and ThO _{2 ,} Ln is Eu, Tb, Ce, Tm, Dy, Pr,
At least one of HO, Nd, Yb, Er, Sm and Gd, X is at least one of Cl, Br and I, x and y are each 5 x 10
This is a number that satisfies the conditions of ≦x≦0.5 and 0<y≦0.2. ) and a rare earth element-activated divalent metal fluorohalide phosphor with the general formula ZnS:
A, CdS: A; (Zn, Cd) S: A, ZnS: A, X
and CdS: A, X (where A is Cu, Ag, Au, or Mn, and X is a halogen), and the like. However, the phosphor used in the radiation image conversion method of the present invention is not limited to the above-mentioned phosphor, but any phosphor that exhibits stimulated luminescence when irradiated with radiation and then irradiated with excitation light. Needless to say, it may be a fluorescent material.

Next, an example of a method for manufacturing the panel is shown below.

First, a phosphor is mixed with a binder solvent solution such as polyvinyl butyral or nitrified cotton (solvents such as cyclohexane, acetone, acetic acid, ethyl, and acetic acid/butyl mixture) to prepare a coating solution with a viscosity of 50 centistokes. Next, this coating solution is uniformly applied onto a horizontally placed polyethylene terephthalate film (support), and left to dry naturally overnight to form a phosphor layer with a thickness of preferably about 300 μm, and then the panel shall be.

For example, a transparent glass plate or a thin metal plate such as aluminum may be used as the support.

The panel is constructed by sandwiching a phosphor between two transparent substrates 43 and 44 such as glass plates as shown in FIG. It may also have a sealed structure.

In the radiation image conversion method of the present invention, the light source for the light energy that excites the phosphor layer of the panel described above has a band spectrum distribution in the long wavelength visible region of 500 nm or more, preferably 1100 nm or less, and/or in the infrared region. In addition to light sources that emit light, He−
A light source that emits light of a single wavelength is used, such as Ne laser light (633 nm), YAG laser light (1064 nm), ruby laser light (694 nm), argon laser, or semiconductor laser. Particularly when using laser light, high excitation energy can be obtained.

As explained above, in the present invention, it is possible to obtain an image with good temporal homogeneity regardless of the elapsed time between recording and reproducing a radiation image on the radiation image conversion panel. As described above, the present invention solves the constraints on the temporal and spatial relationship between recording and reproduction in a radiation image conversion method using a photostimulable phosphor, and provides a method and a simple method for carrying out the method. The present invention improves the practicality of the device and has great industrial utility value.

Next, the present invention will be specifically described with reference to examples.

Example 1 A radiation image conversion panel was prepared by dispersing 8 parts by weight of a phosphor made of BaFBr:Eu and 1 part by weight of polyvinyl butyral using a solvent (cyclohexanone).
This was applied uniformly onto a polyethylene terephthalate substrate, left overnight, and air-dried to form a phosphor layer of about 300 μm. X-rays with a tube voltage of 80KV are applied to this radiation image conversion panel.
Immediately after 10 millimeter X-ray irradiation or after being left for a certain period of time, the sample was heated with hot air at 120°C for 1 minute and then excited with a 10 mW argon laser to measure stimulated luminescence.

Further, for comparison, the same measurement was carried out without heat treatment, and the results shown in FIG. 3 are cited. As shown in FIG. 3a, if heat treatment is not performed, the magnitude of the obtained signal changes with the passage of time from recording to reproduction. but,
As shown in FIG. 4B, in an example in which the above-described heat treatment was performed immediately after recording, a constant signal was obtained regardless of the passage of time. In the case of a sample after a certain period of time has elapsed after recording, the residual decay period from that point in time disappears, and an equilibrium level appears.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an apparatus used in the radiation image conversion method of the present invention, FIG. 2 is a schematic diagram of a heating device used in the radiation image conversion method and apparatus of the present invention, and FIGS. FIGS. 4a and 4b, which are graphs showing changes over time in the intensity of stimulated light depending on the presence or absence, are cross-sectional views showing the structure of the radiation image conversion panel used in the above method of the present invention. 11...Radiation generating device, 12...Subject, 1
3... Radiation image conversion panel, 14... Stimulating excitation light source, 15... Photoelectric conversion device, 16... Image reproduction device, 17... Image display device, 18... Filter, 21... Air blower, 22... ...Heater, 23...
...Heater controller, 24...Temperature sensor, 25
...Timer, 41... Support, 42... Fluorescent layer, 43 and 44... Transparent support.

Claims (1)

[Claims] 1 By irradiating a radiation image conversion panel made of a stimulable phosphor with a radiation image, the energy of the irradiated radiation imagewise accumulated on the panel is stimulated with excitation light to produce photostimulation. In the radiation image conversion method of converting the image to
A radiation image conversion method characterized by heating to 200℃.