Stimulable phosphor radiation image storage screen having an anti-reflection layer

A stimulable phosphor luminescent storage screen of the type used for the latent storage of X-ray images, wherein read-out of the stored image ensues by exciting the phosphor with radiation of a first wavelength thereby causing radiation of a second wavelength to be emitted by the phosphor, has at least one optical surface coat layer for reducing reflections. The stimulable phosphor is transparent.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is directed to a luminescent storage screen having a 
stimulable phosphor for the latent storage of x-ray images. 
2. Description of the Prior Art 
Luminescent storage screens are known in the art wherein a latent x-ray 
image is stored using a stimulable phosphor, with read-out of the x-ray 
image being achieved by exciting the phosphor with radiation of a first 
wavelength (stimulating radiation), which causes the phosphor to emit 
radiation of a second wavelength, which is acquired by a detector. Such a 
luminescent storage screen is disclosed, for example, in European 
Application 0 174 875. 
A luminescent storage screen of this type employed in an image pick-up 
device is described in U.S. Pat. No. 3,859,527. In such an x-ray 
diagnostics installation, a luminescent storage screen, consisting of a 
luminescing stimulable phosphor which is irradiated with x-rays, is used 
as a radiation-sensitive transducer. Electronic holes are generated in the 
stimulable phosphor by the incident radiant intensity, these holes being 
stored in traps having a energy level, so that the latent x-ray image is 
stored in the screen. 
For read-out, the entire area of surface of this screen, used as a master, 
is caused to luminesce pixel-by-pixel by an additional radiation source, 
which may be a laser. Due to the stimulating radiation, the energy of the 
holes stored in the traps is boosted and they can fall back into lower 
energy levels, whereby the energy difference is radiated in the form of 
light quanta. The stimulable phosphor thereby emits light dependent on the 
energy stored in the phosphor. The light emitted as a result of this 
stimulation is detected and rendered visible, so that the x-ray image 
which was latently stored in the screen can be read out. 
A problem in the read-out of such conventional screens is that the 
stimulable phosphor is not sufficiently transparent for the laser light. A 
minimum thickness of the stimulable phosphor is required to be able to 
achieve adequate x-ray quantum absorptions. In the case of a 
non-transparent, tightly compressed or sintered phosphor, the laser beam 
is so greatly attenuated by the phosphor that the penetration depth of the 
laser beam is too small. Because the energy is no longer adequate for 
boosting the holes to the energy level required for quantum emission, the 
information stored in the deeper levels cannot be read out. 
The storage screen disclosed in the European Application 0 174 875 has 
phosphor grains which are applied on a substrate enveloped by a binder. 
The binder serves the purpose of fixing the phosphor grains. A 
light-transmissive carrier material is usually employed as the binder, 
which is transparent both for the exciting laser light and for the emitted 
luminescent light. A problem associated with screens of this type, 
however, is that the laser beam spreads increasingly with increasing 
penetration depth, due to scattering of the beam at the phosphor grains, 
so that the modulation transfer function of the overall system is 
degraded. A storage screen in binder technology also has poorer quantum 
x-ray quantum absorption, compared to a layer of comparable thickness of 
the stimulable phosphor. 
It is preferable, however, to vapor-deposit the stimulable phosphor onto a 
carrier in a high vacuum, and to temper the phosphor in a protective gas 
atmosphere, or in the vacuum, or to compress the phosphor under vacuum 
and/or heating, as disclosed in European Application 0 369 049. It is also 
possible to reshape single crystals of stimulable phosphor to the large 
area required for medical diagnostics by compressing such crystals. The 
latter methods yield transparent stimulable phosphor panels. The advantage 
of the transparency is that the stimulating laser beam cannot be spread in 
the storage medium due to scattering at the grains of the material. Such 
spreading of the read-out beam, as noted above, considerably degrades the 
modulation transfer function of the overall system. Spreading of the laser 
beam upon transirradiation of the storage medium is greatly diminished by 
using a transparent stimulable phosphor manufactured, for example, by 
compressing the phosphor powder. 
The problem of reflection of the exciting, electromagnetic radiation having 
a first wavelength at the back side of the stimulable phosphor layer 
arises to a far greater degree than in the case of non-transparent layers. 
This problem is explained in detail with reference to FIG. 1. For 
pixel-by-pixel read-out of the x-ray image, the stimulating beam 6, having 
a first wavelength, penetrates the luminescent storage screen 1 which, for 
example, may be composed of a carrier and a binder having a stimulable 
phosphor applied thereon, or may be composed of a single crystal. The 
radiation is incident on the stimulable phosphor causing the phosphor to 
emit radiation 9, at a second wavelength, as a result of the 
above-described excitation. Upon emergence of the beam 6 from the storage 
screen 1, radiation 7, which in turn are incident or phosphor particles, 
are reflected back into the screen 1, and thus emit radiation 8, having 
the second wavelength, again due to excitation. The radiation 8 and the 
radiation 9 emerge from the storage screen 1, and are acquired by a 
detector (not shown). As a result, the detector also receives the 
radiation 8, which are allocated to different locations of the storage 
screen 1 than those which are to be read, the radiation 8 arriving at the 
detector either earlier or later than the radiation 9. Information in the 
form of radiation, which does not belong to the pixel currently being 
scanned by the beam 6, thereby degrades the resolution of the resultant 
image because the radiation 8 degrade the signal-to-noise ratio as 
background radiation. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a luminescent storage 
screen of the type described above which has a high x-ray quantum 
absorption and high imaging sharpness and a good modulation transfer 
function, and wherein the disturbing influences of reflections are 
avoided. 
The above object is achieved in accordance with the principles of the 
present invention in a storage screen having at least one optical surface 
coating layer. As a result of this layer, reflections which occur given 
the emergence of the read-out radiation, having a first wavelength, from 
the storage medium are either reduced or entirely prevented. 
The action of the surface coating layer takes full effect when the 
stimulable phosphor is transparent in the region of both wavelengths. 
Specifically, the reflections of the exciting radiation are prevented when 
the surface coating layer is effective at least for the radiation having 
the first wavelength. It is preferable that the anti-reflection layer be 
applied to that side of the storage screen facing away from the detector. 
The light yield can be increased if a wavelength-selective mirror is used 
as the anti-reflection layer, the wavelength-selective mirror being 
applied to the side of the storage screen facing away from the detector. 
The wavelength-selective mirror forms an anti-reflection layer for the 
radiation having a first wavelength, and forms a reflection layer for the 
radiation having a second wavelength. Thus, the exciting radiation emerges 
from the storage luminescent screen without reflections, whereas the 
excited radiation is reflected in the direction of the detector, so that 
this reflected radiation can contribute to the detected signal. 
The wavelength-selective mirror is applied to that side of the storage 
screen from which the radiation of the first wavelength emerges. A 
complete reception of the excitation radiation into the storage screen, 
and an optimally complete emergence of the excited radiation, ensues when 
the front side of the storage screen is provided with an anti-reflection 
coating. 
Acquisition of the read out reflected radiation can ensue when the 
wavelength-selective mirror is applied to that side of the luminescent 
storage screen from which the radiation having the first wavelength 
emerges. A complete entrance of the excitation radiation into the storage 
screen, and an optimally complete emergence of the excited radiation, 
ensues when the front side of the storage screen is provided with an 
anti-reflection coat. 
Acquisition of the excited radiation in transmission can ensue when the 
wavelength-selective mirror is applied to that side of the storage screen 
into which the radiation of the first wavelength enters, and when an 
anti-reflection coat is disposed at the opposite side.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A luminescent storage screen 1 constructed in accordance with the 
principles of the present invention is shown in FIG. 2 into which an 
exciting (stimulating) beam 6 penetrates, having a first wavelength. An 
anti-reflection coat 2 is applied to the back side of the storage 
luminescent screen 1, i.e., to that side at which the beam 6 emerges from 
the storage screen 1. The anti-reflection coat 2 prevents reflection of 
the beam at the transition boundary. As a result, the beam 6 emerges 
unimpeded and without reflection. Within the storage 1, the beam 6 excites 
the phosphor pixel-by-pixel, causing the emission of radiation 9 and 10. 
A detector which receives the emitted light can thus be disposed either at 
the front side of the storage screen 1, i.e., the side of the entry of the 
beam 6 into the storage screen 1, to receive the radiation 9, or can be 
arranged at the back side of the storage screen 1 for detecting the 
radiation 10. It is also possible to use two detectors respectively 
disposed at both sides of the storage screen 1. A broadband 
anti-reflection coat 3 can be additionally provided at the front side of 
the storage screen 1, so that the exciting beam 6 can be coupled into the 
storage screen 1 as completely as possible, and the emitted radiation 9 
can emerge as completely as possible. 
A further embodiment of a luminescent storage screen 1 constructed in 
accordance with the principles of the present invention is shown in FIG. 
3, which is read out in reflection. A wavelength-selective mirror 4, which 
forms an anti-reflection coat for the radiation 6 having the first 
wavelength and forms a reflection coat for the radiation 11 having a 
second wavelength, is applied to the back side of the storage screen 1. As 
a result, not only the radiation 9, but also the radiation 11, proceed to 
that side of the storage screen 1, so that only one detector is needed to 
acquire all of the emitted radiation 9, 10 and 11. In this embodiment as 
well, the storage screen 1 is provided with an anti-reflection coat 3. 
A further embodiment of a storage screen 1 constructed in accordance with 
the principles of the present invention is shown in FIG. 4, wherein 
read-out ensues in transmission, i.e. at the back side of the storage 
screen 1. In this embodiment, the storage screen 1 is provided with a 
wavelength-selective mirror 5 at the entry side of the beam 6, the 
wavelength-selective mirror 5 acting as anti-reflection coat for the beam 
6 and as a reflection coat for the emitted radiation 12. The back side of 
the storage screen 1 is provided with an anti-reflection coat 2, so that 
both the beam 6 and the radiation 10 and 12 emerge unreflected from the 
storage screen 1, and the radiation 10 and 12 can be completely acquired 
by a detector. 
A transparent panel of, for example, rubidium bromide (RbBr) can be used as 
the stimulable phosphor for the storage screen 1, being doped with 
thallium bromide (TlBr) in a ratio of 0.01 through 1 molecular percent. 
The read-out of the stored information can ensue with a beam 6 of a HeNe 
laser having a wavelength of 633 nm. The emitted radiation 8 through 12 
will thereby have a wavelength of 400 through 420 nm. The laser beam 6 is 
focused, for example, to a width of 50 .mu.m. The detector and the laser 
are situated at the same side of the storage screen 1, so that read-out 
ensues in reflection. The other side of the storage screen 1 is 
vapor-deposited with a wavelength-selective mirror 4 in a high-vacuum, the 
wavelength-selective mirror 4 having a high transmission for 
electromagnetic radiation having the wavelength 633 nm (for example, 
&gt;99%), and simultaneously having a high reflection for a wavelength range 
from 400 through 420 nm (for example, &gt;90%). Such a beam splitter, for 
example, may be composed of a multi-layer system of cryolite Na.sub.3 
AlF.sub.6 and ZnS. The number and the grid layers must be optimized to the 
wavelengths of the electromagnetic radiation to be separated. 
A luminescent storage screen is obtained in accordance with the above 
which, due to the use of a transparent stimulable phosphor, has a high 
x-ray quantum absorption and good imaging sharpness and a good modulation 
transfer function, and disturbing influences of reflections are avoided by 
employing the surface coat layers 2 through 5. 
Although modifications and changes may be suggested by those skilled in the 
art, it is the intention of the inventors to embody within the patent 
warranted hereon all changes and modifications as reasonably and properly 
come within the scope of their contribution to the art.