Radiant energy – Source with recording detector – Using a stimulable phosphor
Reexamination Certificate
2000-06-19
2003-03-04
Hannaher, Constantine (Department: 2878)
Radiant energy
Source with recording detector
Using a stimulable phosphor
C250S581000, C250S584000
Reexamination Certificate
active
06528812
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and a system of reading a radiation image that has been stored in a photostimulable phosphor screen, wherein the photostimulable phosphor screen can be re-used. The invention further relates to a re-usable radiation detector.
BACKGROUND OF THE INVENTION
Radiation image recording systems wherein a radiation image is recorded on a photostimulable phosphor screen by exposing said screen to image-wise modulated penetrating radiation are widely used nowadays.
The recorded image is reproduced by stimulating the exposed photostimulable phosphor screen by means of stimulating radiation and by detecting the light that is emitted by the phosphor screen upon stimulation and converting the detected light into an electrical signal representation of the radiation image.
In such a system it is preferred, in view of economy, that the stimulable phosphor screen can be used in many imaging cycles.
The reuse of the stimulable phosphor screen is possible when the previously stored radiation image is erased to a sufficient extent.
When reading out an image by scanning a phosphor screen that has been exposed to penetrating radiation, less than 90% of the stored energy is released. Thus there arises a problem that, upon reuse, part of the radiation image is still stored in the phosphor screen and can appear in the subsequent image as a so-called ghost image.
In general medical radiography, images are made with widely differing X-ray doses.
To make images of extremities, like e.g. fingers, doses are used of the order of 1 mR. On the other hand, images of internal organs, like the stomach are made with X-ray doses that may be as high as 300 mR.
To avoid ghosting, when making a 1 mR image immediately after a 300 mR image, the signal of the first image must be reduced by more than a factor of 300.
As a matter of fact, a dynamic range is desired in the second image of at least 100. This implies that the signal created by the first irradiation must be reduced by a factor of at least 3.10
4
, which is equivalent to requiring an erasure depth of 1/(3.10
4
)=3.3.10
−5
.
According to U.S. Pat. No. 3,859,527 (column 4, lines 5-7) the phosphor can be reduced to neutral state by actions like a uniform illumination, irradiation or heating.
In commercial systems, the phosphor screen is erased by illumination with visible light.
Commonly incandescent lamps are used because they are cheap, high power light sources. High power light sources are selected, because in order to guarantee a high through-put scanning system, the phosphor screen must be erased in a short time. High power lamps, however, generate a lot of heat, which may destabilise the scanner to read out the storage phosphor screens. In order to sufficiently remove the heat generated by the high power lamps the size of the erasing unit has to be rather large.
Furthermore, in case incandescent lamps are used in an erasure unit of a phosphor read out apparatus, the dimensions of the phosphor erasure unit are determined by the dimensions (more specifically the diameter) of the incandescent lamps.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide a method and a system for reading a radiation image on a photostimulable phosphor screen wherein the screen is erased in between successive recordings to an adequate extent so as to permit re-use of the screen.
It is a further object of the present invention to provide such a system that is compact and has at the same time a high throughput.
Still another object is to provide a compact re-usable radiation detector.
Further objects will bercome apparent from the description given below.
SUMMARY OF THE INVENTION
The inventors have found that the above mentioned objects are realised by a method of reading a radiation image that has been stored in a photostimulable phosphor screen comprising the steps of
(1) stimulating said phosphor screen by means of stimulating radiation,
(2) detecting light emitted by the phosphor screen upon stimulation and converting the detected light into a signal representation of said radiation image,
(3) erasing said phosphor screen by exposing it to erasing light, wherein
(4) said phosphor is a divalent europium activated cesium halide phosphor wherein said halide is at least one of chloride and bromide, and
(5) said erasing light is emitted by at least one electroluminescent lamp.
In this document the term “radiation” has to be understood as any penetrating radiation and includes irradiation originating from a radioisotope (e.g. Co60, Ir192, Se75, etc.), radiation created by an X-ray generator of any type, radiation and high energy particles created by a high energy radiation generator (e.g. Betatron), radiation from a sample labelled with a radioisotope as is the case in e.g. autoradiography.
An electroluminescent lamp can be in the form of an electroluminescent film based on an inorganic electroluminescent phosphor, e.g. ZnS:Mn, or an electroluminescent film based on organic light-emitting diodes (OELDs).
The use of electroluminescent lamps in the present invention is advantageous in that these lamps are ideal for uniform illumination applications.
While eliminating the need for sockets, bulbs, diffusers and reflectors, these lamps provide uniform lighting across the entire lamp surface. The lamps are a cold light source, so little heat is added to the assembly.
Most light emitting devices vary in luminance according to the direction. Electroluminescent lamps have essentially the same luminance independent of angle, i.e. an electroluminescent lamp is a Lambertian emitter.
Moreover, electroluminesent lamps can be made with an emitting surface that is of the same size as the surface of the phosphor screen that must be erased. This makes it possible to erase phosphor screens in a very homogeneous way.
Nevertheless a prejudice exists against the use of electroluminescent lamps as erasing light source in a photostimulable phosphor read out system because the power of the light source is low and consequentially these lamps are thought to be inadequate for obtaining a sufficient erasure depth so as to enable re-use of the photostimulable phosphor screen.
The inventors have found that by using a specific phosphor, more specifically a divalent europium activated caesium halide phosphor, wherein said halide is at least one of chloride and bromide, erasure to a sufficient extent can be obtained with a low power and compact electroluminescent lamp. In this way the erasure device can be made very compact without implying a longer erasure time and consequentially a lower throughput.
Another aspect of the present invention relates to a radiation image read out apparatus as set out in claims 8 to 12.
The compactness of an erasure unit which comprises an electroluminescent lamp makes it appropriate for integration in a radiation detector according to the present invention.
Still another aspect thus relates to a radiation detector as set out in claim 13 and following claims.
Specific features for preferred embodiments of the invention are disclosed in the dependent claims.
In a first embodiment of the method, of the system and of the detector according to the present invention (an) electroluminescent lamp(s) is(are) used that is(are) based on inorganic electroluminescent phosphors as e.g. ZnS:Mn, ZnS:Cu, CaS:Eu, CaS:Ce.
These electroluminescent lamps can have an optical power of upto ca. 0.3 mW/cm
2
.
Furthermore, by adjusting the phosphor composition and the operating frequency of the lamp, the light spectrum emitted by the electroluminescent lamp can be matched with the erasure spectrum of the phosphor. Hence, electroluminescent lamps lead to higher erasure depth as incandescent lamps at equal optical power while having much smaller dimensions and dissipating less heat.
It will further be explained that an optical energy of 10 mJ/cm
2
is needed to erase a CsBr:Eu
2+
phosphor to a sufficient extent. This implies that the CsBr:Eu
2+
phosphor can be erased with an electroluminescent lamp in a
Leblans Paul
Struye Luc
Agfa-Gevaert
Hannaher Constantine
Hoffman, Warnick & D'Alessandro
Merecki John A.
Moran Tim
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