System for digital radiography and dosimetry

Details System for digital radiography and dosimetry System for digital radiography and dosimetry

2001-04-05

2003-06-24

Hannaher, Constantine (Department: 2878)

Radiant energy

Source with recording detector

Using a stimulable phosphor

C250S583000

Reexamination Certificate

active

06583434

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a radiography system of the type having means for irradiation of an object in order to form an image of the said object or in order to detect irradiation of said object as a dosimetric application, wherein captured energy has to be stored for a long time.
BACKGROUND OF THE INVENTION
A well known use of phosphors is in the production of X-ray images. In a conventional radiographic system an X-ray radiograph is obtained by X-rays transmitted image-wise through an object and converted into light of corresponding intensity in a so-called intensifying screen (X-ray conversion screen) wherein phosphor particles absorb the transmitted X-rays and convert them into visible light and/or ultraviolet radiation to which a photographic film is more sensitive than to the direct impact of X-rays.
According to another method of recording and reproducing an X-ray pattern disclosed e.g. in U.S. Pat. No. 3,859,527 a special type of phosphor is used, known as a photostimulable phosphor, which being incorporated in a panel, is exposed to incident pattern-wise modulated X-ray beam and as a result thereof temporarily stores energy contained in the X-ray radiation pattern. At some interval after the exposure, normally a beam of visible or infra-red light scans the panel in order to stimulate the release of stored energy as light that is detected and converted to sequential electrical signals which can be processed to produce a visible image. For this purpose, the phosphor should store as much as possible of the incident X-ray energy and emit as little as possible of the stored energy until stimulated by the scanning beam. This is called “digital radiography” or “computed radiography”.
In the production method of high quality storage screens or panels the best results are obtained when phosphor crystals with high crystal symmetry and simple chemical composition are used. Phosphors having complicated crystal structures as, e.g., alkaline earth fluorohalides, tend to decompose (partially) under physical vapor deposition and the production of screens in this way while using phosphors with complicated crystal structure is quasi impossible and leads to sub-optimal results. The use of alkali metal halide phosphors in storage screens or panels is well known in the art of storage phosphor radiology and the high crystal symmetry of these phosphors makes it possible to provide structured screens and binderless screens.
So in e.g. U.S. Pat. No. 5,055,681 a storage phosphor screen comprising an alkali metal phosphor in a pile-like structure is disclosed. In U.S. Pat. No. 5,736,069 an alkali metal storage phosphor is disclosed corresponding to the formula:
M
1+
X.aM
2+
X′
2
BM
3+
X″
3
:cZ
wherein: M
1+
is at least one member selected from the group consisting of Li, Na, K, Cs and Rb,
M
2+
is at least one member selected from the group consisting of Be, Mg, Ca, Sr, Ba, Zn, Cd, Cu, Pb and Ni,
M
3+
is at least one member selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Bi, In and Ga,
Z is at least one member selected from the group Ga
1+
, Ge
2+
, Sn
2+
, Sb
3+
and As
3+
,
X, X′ and X″ can be the same or different and each represents a halogen atom selected from the group consisting of F, Br, Cl, I and 0≦a≦1, 0≦b≦1 and 0≦c≦0.2.
In EP-A 0 174 875 amongst other alkali metal stimulable phosphors a CsBr:Eu phosphor is disclosed, wherein the Eu is incorporated in the CsBr by firing CsBr with Europium oxide.
The alkali metal phosphors according to the disclosures mentioned above, make it indeed possible to produce structured screens and binderless screens.
A common characteristic of the phosphors mentioned above however is that they have a quick dark-decay, in the order of minutes up to about 1 hour. Absence of such a quick dark-decay, although permitting to read-out the phosphors with visible or infrared light as low energy sources may however lead to loss of stored energy, even at room temperature, and as a consequence such phosphor panels are not suitable for use in dosimetric applications or in applications wherein a long time has to be expected between exposure of an object which has to be examined and (digital) processing of the energy stored during said exposure. Although a quick response is normally expected, very specific applications require such a long time as set forth hereinbefore, more particularly, in security systems wherein detection of natural (or even artificial) radiation sources is desired. So, e.g., possible emission of X-rays in rocky mountains after vulcanic eruptions and detection of the direction wherein said emission proceeds, makes installation over a longer period (e.g. several months or even one year) of a storage phosphor plate in the vicinity thereof at different sites a very useful tool, wherein readout of the phosphor plates may proceed after such long time periods. A “pocket image” may also be detected in a dosimeter having a stimulable storage phosphor plate in order to get more information about the dose, but also, e.g. over the direction of undesired radiation sources: part of the detector may be covered with a grill the dimensions and size of which is perfectly known and which permits to determine the said direction from information offered by the image obtained on the said storage phosphor panel.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide a radiation image storage panel, containing a storage phosphor having a slow dark-decay in order to make it suitable for use for dosimetric applications.
It is a further object of the invention to provide a method for recording and reproducing images of objects in specific applications wherein a long time is expected between exposure and read-out.
It is still a further object of the invention to provide a dosimeter detecting amounts of high energy radiation stored therein for a long time (even up to at least one month or even more)
Further objects and advantages of the invention will become clear from the detailed description hereinafter.
SUMMARY OF THE INVENTION
In order to reach the objects of the present invention a (high energy) radiography system is disclosed, wherein said system is capable to form an image of an object or to detect irradiation of the said object, said radiography system comprising a sensor having an exposure response function, wherein the said sensor is in form of a stimulable storage phosphor plate, comprising stimulable storage phosphors wherein energy of stimulation radiation is higher than energy of emission radiation upon stimulation of said stimulable storage phosphors. In a preferred embodiment for dosimetric applications said stimulable storage phosphors having a dark-decay of more than 24 hours.
Specific features for preferred embodiments of the invention are disclosed in the dependent claims.
Further advantages and embodiments of the present invention will become apparent from the following description.
DETAILED DESCRIPTION OF THE INVENTION
In this document the term “X-ray” has to be understood as any penetrating high energy radiation and includes irradiation originating from a radioisotope (e.g. Co
60
, Ir
192
, Se
75
, 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 labeled with a radioisotope as is the case in e.g. autoradiography. When the term “high energy” is used, any irradiation having a shorter wavelength than energy of visible light is mentioned (thus inclusive for “ultraviolet” rays).
We have unexpectedly found now phosphors having storage properties in order to store “X-ray” energy and photostimulation properties in that photostimulation proceeds by irradiation with a wavelength shorter than the wavelength of the emitted radiation caused by said photostimulation. Until now no examples of phosphors having such characteristics

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