Radiant energy – Source with recording detector – Using a stimulable phosphor
Reexamination Certificate
1998-11-16
2001-07-03
Epps, Georgia (Department: 2873)
Radiant energy
Source with recording detector
Using a stimulable phosphor
C250S586000
Reexamination Certificate
active
06255667
ABSTRACT:
The invention relates to a method for the read-out of a latent image recorded in a stimulable phosphor sheet by X-ray radiation or the like, in which method a separate phosphor sheet of a flexible material, which carries a latent image, is positioned in a curved shape along the circumference of a cylinder; this curved phosphor sheet is rotated around the axis of the cylinder circumference; a stimulating beam of light obtained from a source of light is aimed at the phosphor surface of the sheet carrying the latent image, the light emitted from the phosphor surface under the effect of this beam being directed to a photodetector; and the stimulating beam of light and the phosphor sheet are moved relative to each other in a direction parallel to the said axis for the read-out of the phosphor sheet. The invention also relates to an apparatus for the read-out of such a phosphor sheet while it is positioned in a curved shape.
Phosphor sheets in which a latent image is recorded by X-radiation are commonly used in radiography. After the exposure of the radiograph, this latent image is read out by aiming at the sheet, point by point, a stimulating beam of light under the effect of which there is produced an emission light activated by the latent image, and this emission in turn is detected by a detector and is converted into an electronic form for further processing. Numerous different read-out methods and read-out apparatuses have been proposed for the read-out of the latent image in such a phosphor sheet. For introducing the emitted light to a detector or detectors there is used, for example, a light collector, which is made up of an inwardly reflective tube extending over the entire width of the phosphor sheet, an inwardly reflective conical piece extending over the entire width of the phosphor sheet or an optical fiber bundle extending over the entire width of the phosphor sheet, which collector is moved in a direction parallel to the length of the phosphor sheet, this direction being perpendicular to the said width of the light collector. At the same time the stimulating beam of light is scanned along the whole width of the light collector, either through a slot in the tubular light collector or via a mirror surface positioned in a massive light collector based on fiber optics or in a similar manner on total reflection, the stimulating beam scanning the light collector from one end to the other. In this manner the entire phosphor plate is covered by the movement, the scanning traces forming a zig-zag pattern over the surface area of the phosphor sheet. In all of these described embodiments, the phosphor sheet is maintained flat during the read-out. Such methods and apparatuses suitable for the read-out of a phosphor sheet are described in publications U.S. Pat. No. 4,629,890, U.S. 4,742,225, U.S. 4,743,759 and U.S. 4,829,180. The apparatuses and methods of the type described above have a number of disadvantages. First, the manufacture of light collectors which extend over the entire width of the image is very complicated and expensive. The manufacture of the stimulating-beam scanning device inevitably associated with them requires top precision because it is at a distance from the phosphor plate, and also because it is necessary to know the mutual locations of each position of the turning light beam and the corresponding image point in the phosphor sheet, in order to avoid errors in the final image pattern. This part is thus also expensive. Since in such apparatuses the angle of incidence of the stimulating beam against the phosphor surface of the phosphor sheet varies, in which case the beam at the edge of the phosphor sheet is at a considerable angle as compared to being perpendicular to the phosphor sheet, this causes imprecision in the final read-out image, especially in the area of the edges of the image. In addition, in such large-sized light collectors, which are truly large as compared with the cross section of the stimulating beam and to the photosensitive area of the detector, the losses of light are very large, in which case there tends to be noise and other interference in the obtained signal.
Publication U.S. Pat. No. 4,922,102 describes a phosphor sheet read-out method in which the phosphor sheet is rotated in its plane while it is being read by means of a point-like reading head which is, between rotations, moved in a direction parallel to the rotational radius of the phosphor sheet, which remains flat. In this case the phosphor sheet is read one circular trace at a time, one inside another. Non-public patent application FI-950048 describes a phosphor sheet read-out method in which the phosphor sheet is moved in its plane in one direction while a read-out apparatus having a point-like reading head is rotated around an axis perpendicular to the plane of the phosphor sheet. In this case the phosphor sheet is read in successive curved traces. The rotating of the phosphor sheet in a plane and the use of a rotating reading head are relatively poor in the efficiency of their use of the read-out time, since for a considerable proportion of the total read-out time the reading beam is outside the rectangular surface area of the phosphor sheet. These structures presuppose of the rotating apparatus a precision which is difficult to implement in order that the distance between the reading head made up of a stimulating beam and a detector on the one hand and the phosphor sheet on the other should remain precisely the same at each point of the phosphor sheet.
Publication U.S. Pat. No. 5,416,336 describes an apparatus in which a stimulating beam is scanned in one direction on a mirror inside an emitted radiation collector, while the phosphor plate is being moved in another direction. By this arrangement, indeed a high read-out precision and low losses of light are achieved, but the apparatus is best suited for use for the read-out of relatively small-sized images.
Publication U.S. Pat. No. 4,829,180 also describes an arrangement in which the phosphor sheet is positioned on a cylinder with the phosphor surface outwards, and the phosphor sheet is read by means of a point-like reader which comprises a stimulating beam and a detector for emitted radiation. During the read-out the cylinder is rotated around its axis, whereby one scanning direction of the phosphor sheet is accomplished, whereas another scanning direction, parallel to the axis of the cylinder, is accomplished by moving either the reading head or the cylinder in a direction parallel to the rotation axis. This publication does not at all describe how the phosphor sheet is attached to the outer surface of the cylinder. Article Hildebolt, Vannier: “PSP-Photostimulable Phosphor Dental Radiography”—Washington University School of Medicine, St. Louis, Mo. describes a corresponding phosphor sheet read-out method, but it describes additionally that the backing of the phosphor sheet is made up of a thin metal sheet which adheres magnetically to the outer surface of the cylinder. The reliability of such magnetic adhesion is not very good, at least if the cylinder is rotated at even a moderate speed. In addition, the metal backing of the phosphor sheet renders it very rigid, in which case its shaping around the cylinder is uncertain and the risk of its becoming detached during the rotation of the cylinder further increases. Thus these described arrangements have the disadvantage of a high risk that the phosphor sheet becomes detached, and for this reason it is most likely necessary to maintain a low rotation speed of the cylinder, which in turn leads to the read-out of the phosphor sheet being slow.
An object of the present invention is therefore to provide a method and apparatus wherein the phosphor sheet is, in a manner known per se, moved during its read-out, but wherein the phosphor sheet, in spite of this movement, can be maintained precisely in the required shape. This means that the distance between the read-out apparatus, which includes a source of stimulating light and a detector for emitted radiation, and the phosphor shee
Banner & Witcoff Ltd
Epps Georgia
Hanig Richard
Orion Corporation
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