WO1990006538A1

WO1990006538A1 - Apparatus for reading a transparent photostimulable luminescent screen

Info

Publication number: WO1990006538A1
Application number: PCT/US1989/005297
Authority: WO
Inventors: Jerald Dana Lee; Jeffrey Griffith Yorker
Original assignee: E.I. Du Pont De Nemours And Company
Priority date: 1988-11-25
Filing date: 1989-11-22
Publication date: 1990-06-14
Also published as: CA2003499A1

Abstract

Apparatus (10) for reading a transparent PSL screen (12) includes a mask (60) with a slit (62) for preventing secondary emissions (S) from reaching the photodetector (40).

Description

TITLE

Apparatus For Reading A Transparent Photostimulable Luminescen

Screen

Field of the Invention

The present invention relates to an apparatus for reading a transparent photostimulable luminescent screen and. in particular, to a reading apparatus which includes a mask to prevent the impingement of secondary radiation on a photodetector.

Background of the Invention

Photostimulable luminescent imaging systems are known. Such systems utilize a photostimulable phosphor sheet that is exposed to image producing x-ray radiation to produce a latent image pattern in the phosphor sheet. The latent Image Is read out by scanning the phosphor screen with a relatively long wavelength beam of interrogating radiation, such as the beam generated from a helium-neon laser. The interrogating radiation stimulates the phosphor to emit radiation from the phosphor, typically light at a blue wavelength. The emitted radiation is detected, typically with a photomultiplier tube, and thereby converted into an electrical signal. The signal may be converted to a series of digital picture element values and stored in such form in a suitable memory for later retrieval and display.

Two types of photostimulable phosphor screens are known, a scattering type screen and a transparent screen. The first type, the so-called scattering type, is exemplified by United States Patent Re. 31,847 (Luckey). A scattering type screen is one which scatters the stimulating radiation, resulting In relatively high stimulation efficiency. Although it is the most common screen in use today, the scattering type suffers from limited spatial resolution because, when manufactured thick enough to absorb an appreciable fraction of the x-rays, scattering causes Image degradation. Spatial resolution is the ability to resolve small, closely spaced details in the Image. Spatial resolution is usually expressed In terms of line- palrs-per-unit-dislance.

Apparatus for reading scattering type photostimulable luminescent screens is also well known. Representative of such devices are those disclosed in United Stales Patent 4,346.295 (Tanaka et al.) or United Stales Patent 4,742.225 (Chan). When reading a photostimulable luminescent screen the signal to noise ratio (S/N) of the reading system is important, it being recognized that high efficiency for collecting the emitted light is desirable.

Chan recognizes that a high percentage of the stimulating beam may be reflected from the photostimulable phosphor screen and then be re-reflected, as by the collection optics, back to some other area of the photostimulable screen, thus stimulating the phosphor to emit from this other location. This unwanted stimulating light is termed "flare" and the flare induced emission of light is termed "preslimulation." Prestimulation degrades the quality of the obtainable image. Accordingly, the device shown in the Chan patent incorporates a mask having a slit aperture in the collection system to reduce the effect of flare.

The second type of photostimulable luminescent screen is a so-called transparent type, as exemplified in United States Patent 4,316,817 (Cusano et al.). Although the phosphor disclosed in this last mentioned patent is not photostimulable, the chemical activation process to make this phosphor photostimulable is well known. See for example United Slates Patents 4,608,190 (Brixner) or 4,261,854 (Kotera, et al.) This type of screen is, as its name implies, substantially transparent to the stimulating radiation, although it may be somewhat scattering to the emitted radiation, and offers the potential for better spatial resolution than the scattering type screen.

Although the signal to noise ratio of the reading system for a transparent screen is important, another factor has been Identified that limits the spatial resolution achievable with transparent photostimulable luminescent screens. It Is recognized that the mean free pathlcngth of the stimulating radiation in the photostimulable phosphor Is quite long. Since no material can be perfectly transparent, a small portion of the stimulating beam is scailered into the phosphor and may travel a significant distance before stimulating emission. Theoretical analyses as well as experimental results have shown that about one-half of the total light emitted from a transparent screen is stimulated by this small scattered portion of the interrogating radiation beam regardless of the degree of transparency of the screen.

In very transparent screens few photons are scattered away from the spot of interrogating radiation. But, because the mean free path is very long, these photons can travel a large distance In the screen and therefore have a large probability of stimulating a luminescent site. In less transparent screens the mean free path is shorter, but more photons are scattered from the spot of interrogating radiation so the probability of stimulating luminescence away from the beam is approximately the same. This behavior is markedly different from that of scattering screens In which all the stimulated luminescence arises from the Immediate neighborhood of the spot of Interrogating radiation.

It is therefore believed advantageous to prevent emitted radiation from the small scattered portion of the stimulating radiation beam from reaching and being detected by the

photodelector.

Summary of the Invention

The present Invention relates to apparatus for reading a transparent photostimulable luminescence (PSL) phosphor screen having a first and a second major surface tliereon and having image Information stored therein. The apparatus Includes a platen for supporting the screen and a source of interrogating radiation disposed adjacent to the first major surface or the screen. Means is provided for focussing the radiation to a diffraction limited spot. Means is also provided for scanning the spot of radiation along a predetermined path across the first major surface of the screen, the scanning radiation exciting the phosphor in the screen along the path to emit primary radiation In accordance with the image stored therein. The primary radiation exits the screen from both the first and the second major surface thereof. The phosphor in the screen causes a small portion of the interrogating radiation to scatter and thereby to emit secondary radiation from other portions of the screen, the secondary radiation also exiting from both major surfaces of the screen. A photodelector responsive to both primary and secondary emitted radiation is disposed adjacent to the second major surface of the screen.

In accordance with a first embodiment of the present invention the apparatus includes a slit mask disposed between the second major surface of the screen and the photodelector, the silt in the mask being aligned with the path to permit only the primary emitted radiation from the second major surface to impinge the photodelector.

Since maximizing the collection efficiency to Improve signal to noise Is important to achieve high image quality. It has been found that in accordance with a more detailed embodiment of the present invention a second light collection system to collect the emitted light transmitted through the screen is desirable. To this end the use of a slit aperture in a mask to prevent the

collection of light emitted from other than the instantaneous scanning point improves the spatial resolution achievable with the transparent screen.

Brief Description of the Drawings

The invention will be more fully understood from the following detailed description thereof, taken in connection with the accompanying drawings, which form a part of this application and in which: Figure 1 is a stylized pictorial representation of an apparatus for reading a transparent photostimulable luminescence screen having a mask positioned In accordance with the present invention;

Figure 2 is an enlarged view of a portion of Figure 1 illustrating the generation of primary and secondary radiation; and

Figure 3 Is a view similar to Figure 1 illustrating the apparatus with a second mask positioned in accordance with a second embodiment of the present invention.

Detailed Description of the Invention

Throughout the following detailed description, similar reference numerals refer to similar elements in all Figures of the drawings.

With reference to Figure 1 shown is a stylized pictorial representation, partially in section, of an apparatus generally indicated by reference character 10 for reading a transparent photostimulable luminescence (PSL) screen 12. Suitable for use as the screen 12 is that disclosed in United States Patent 4,316.817 (Cusano et al.) although other suitable screens may be used. This last mentioned patent is hereby incorporated by reference herein. The screen 12 has first and second major surfaces 12A, 12B respectively thereon. The screen has image information previously stored therein.

The apparatus 10 includes a platen or support platform 14 which form means to support the screen 12 in a predetermined position. As will be developed any suitable screen support

arrangement may be utilized and remain within the contemplation of the invention.

The apparatus 10 includes a source 18 of interrogating radiation, such as a helium neon laser such as that manufactured by Jodon Lasers of Ann Arbor. Michigan and sold by model HN20G. The source 18 Is disposed adjacent to the first major surface 12A of the screen 12. The source 18 produces a beam 24 of stimulating radiation which is focussed by optical means 26 to produce an interrogating beam of radiation 28 that Is focussed to a diffraction limited spot.

Scanning means 30 directs the focussed Interrogating beam 28 along a predetermined path across the first major surface 12A of the screen 12 The scanning means 30 includes a mirror 32 driven by a drive motor 34 powered by a drive-source 36.

When operating in a transmissive mode, as Illustrated In Figures 1 and 2, a photodelector 40 is disposed adjacent to the second major surface 12B of the screen 12. Suitable for use as the photodelector 40 is the photomultiplier sold under model number 9635B by EMI Industrial Electronics of Ruisllp.. Middlesex,

England.

As is believed best seen In the diagrammatic view of Figure 2 the interrogating radiation, as it scans the screen 12, excites the phosphor therein and causes the same to emit primary radiation schematically Indicated by reference character P. The intensity of the primary emission P Is In accordance with the Image stored in the screen 12. A portion of the primary radiation P exits the screen 12 from both the first and the second major surfaces 12A. 12B thereof, as shown at 44 and 46, respectively. The phosphor in the screen 12 causes a small portion of the

interrogating radiation to scatter, as at 48. The scattered radiation travels through die screen 12 and causes the phosphor in other regions thereof to emit secondary emissions S. The secondary emissions S are depicted by dolled lines. A portion of the

secondary emissions S also exits from both the first and second major surfaces 12A, 12B of the screen, as at 52, 54, respectively.

The secondary emissions S, if permitted to reach the photodelector 40, degrades the Image obtainable therefrom and, for this reason, is believed to be disadvantageous. In particular, the spatial resolution of the apparatus Is compromised. In accordance with the present Invention the apparatus 10 further includes a mask 60 having a slit 62 therein is disposed between the second major surface 12B of the screen 12 and the photodetector 40. The mask 60 is mounted on suitable support abutments 64 in any convenient fashion in the position between the second surface 12B of the screen 12 and the photodelector 40. The slit 62 in the mask 60 is aligned with the path of the

interrogating radiation 28 to permit only the emitted primary radiation P exiting at 46 from the second major surface 12B to impinge upon the photodetector 40. The presence of the mask 60 prevents secondary emissions S exiting at 54 from the second surface 12B of the screen 12 from reaching and being detected by the photodetector 40. As a result improved spatial resolution of the image is obtained.

In some instances It is desirable, to increase the collection efficiency of the apparatus and thereby further Improving the signal to noise ratio, to position a second photodetector 40' proximal to the first major surface 12A of the screen 12. The outputs of both the first and the second photodetectors 40 and 40', respectively, are connected to a suitable image storage device, such as an image memory 80. The second photodetector 40' is

conveniently mounted in any suitable manner. In accordance with a second embodiment of the invention a second mask 70 having a slit 72 therein is mounted on suitable support abutments 74 in any convenient fashion to position the second mask 70 between the first major surface 12A of the screen 12 and the second

photodetector 40'. The slit 72 in the second mask 70 is also aligned with the path of the interrogating radiation 28. The second mask 70 functions to permit only the emitted primary radiation exiting at 44 from the first major surface 12A to reach the second photodetector 40'.

It may thus be appreciated that the presence of the second mask 70 prevents secondary emissions S exiting at 52 from the first surface 12A of the screen 12 from reaching and being delected by the photodelector 40'. As a consequence the spatial resolution of the image obtainable from photodetector Is also Improved.

Those skilled in the art, having the benefit of the present Invention as set forth herein may effect numerous modifications thereto. It should be understood, however, that such modifications should be construed as lying within the scope of the present Invention, as defined by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. Apparatus for reading a transparent photostimulable luminiscence phosphor screen having image Information stored therein, the apparatus having

a platen for supporting a transparent photostimulable luminescence phosphor screen having image information stored therein, the screen having a first and a second major surface thereon,

a source of Inlerrogating radiation disposed adjacent to the first major surface of the screen;

means for focussing the radiation to a diffraction limited spot;

means for scanning the spot of radiation along a predetermined path across the first major surface of the screen, the scanning radiation exciting the phosphor In the screen along the path to emit primary radiation in accordance with the image stored therein, the primary radiation exiting the screen from the second major surface thereof, the phosphor in the screen causing a portion of the interrogating radiation to scalier and thereby to emit secondary radiation from other portions of the screen, the

secondary radiation also exiting from the second major surface of the screen; and

a photodetector responsive to both primary and secondary emitted radiation, the photodetector being disposed adjacent to the second major surface of the screen;

wherein the improvement comprises:

a slit mask disposed between the second major surface of the screen and the photodetector, the slit in the mask being aligned with the path to permit only the primary emitted radiation from the second major surface to Impinge the photodelector.

2. The apparatus of claim 1 wherein the primary radiation also exits the screen from the first major surface, wherein the apparatus further includes

a second photodelector disposed adjacent to the first major surface of the screen, the improvement further comprising:

a second slit mask disposed between the first major surface of the screen and the second photodetector, the slit in the second mask being aligned with the path to permit only the primary emitted radiation exiting from the first major surface to impinge the second photodelector.