Facsimile and static presentation processing – Facsimile – Picture signal generator
Reexamination Certificate
2000-03-23
2003-06-17
Coles, Edward (Department: 2622)
Facsimile and static presentation processing
Facsimile
Picture signal generator
C358S445000, C358S451000, C358S448000, C358S003210, C358S486000, C358S496000, C358S412000, C358S409000, C358S497000, C358S471000, C250S584000, C250S585000, C250S586000, C250S587000, C250S472100, C382S132000
Reexamination Certificate
active
06580525
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of and a system for reading out image signal, and more particularly to a method of and a system for reading out image signal in which the density of picture elements of the read-out image signal can be changed.
2. Description of the Related Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, &agr;-rays, &bgr;-rays, &ggr;-rays, cathode rays or ultraviolet rays, they store a part of the radiation. Then when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is generally referred to as “a stimulable phosphor”. In this specification, the light emitted from the stimulable phosphor upon stimulation thereof will be referred to as “stimulated emission”. There has been known a radiation image recording and reproducing system in which a sheet provided with a layer of the stimulable phosphor (will be referred to as “a stimulable phosphor sheet”, hereinbelow) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored on the stimulable phosphor sheet, a stimulating light beam such as a laser beam is caused to scan the stimulable phosphor sheet so that the stimulable phosphor sheet emits stimulated emission as signal light bearing thereon information on the radiation image, the stimulated emission is photoelectrically detected, thereby obtaining an analog image signal, the analog image signal is sampled at predetermined intervals and quantized, thereby obtaining a digital image signal at a predetermined picture element density, and the radiation image of the object is reproduced as a visible image on the basis of the digital image signal on a recording medium such as a photographic film or a display such as a CRT. See, for instance, Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 56(1981)-11395 and 56(1981)-11397.
This system is advantageous over a conventional radiography system using silver salt film in that an image can be recorded over a very wide radiation exposure range.
As a system for reading out the stimulated emission, there has been proposed a radiation image read-out system in which a photoelectric read-out means is disposed on each side of the stimulable phosphor sheet, stimulating light is projected onto one side or both sides of the stimulable phosphor sheet and the stimulated emission from both sides of the stimulable phosphor sheet is detected by each of the photoelectric read-out means. See, for instance, Japanese Unexamined Patent Publication No. 55(1980)-87970. In such a radiation image read-out system, since a single radiation image is stored in a stimulable phosphor sheet and a pair of photoelectric read-out means are disposed to detect the stimulated emission from both sides of the stimulable phosphor sheet, light collecting efficiency is improved, and by adding the obtained two image signals at a predetermined ratio of addition, positions in which noise components are detected differ by the sides of the stimulable phosphor sheet and accordingly, an addition signal which is improved in S/N ratio as compared with an image signal obtained from only one side can be obtained.
Further there has been proposed a method of superposing images in which an addition image signal is obtained after carrying out filtering processing, using a filter having frequency response properties such as will increase the S/N ratio of an image signal (including an addition signal), on a single image signal obtained from one side of the stimulable phosphor sheet or two image signals obtained from opposite sides of the same. (Japanese Unexamined Patent Publication No. 7(1995)-287330) In accordance with this method, since the amount of exposure to the radiation to which the object is exposed upon taking the radiation image is obtained and the parameter (the coefficient of filter) which is used for carrying out filtering processing is obtained on the basis of the amount of exposure to the radiation, an image signal representing an image of optimal quality or an addition image signal representing a superposed image of optimal quality can be obtained according to the amount of exposure to the radiation. Further, since processing to change the frequency characteristics is carried out on the overall image signal, it becomes unnecessary to effect frequency transformation such as Fourier transformation and the amount of computation can be reduced.
There has been a demand toward changing the density of picture elements of a single image signal or an addition signal in the image read-out section of the aforesaid radiation image recording and reproducing system or in the radiation image read-out system.
For example, when a large number of radiation images are taken as in a group examination, there is a demand toward increasing the radiation image read-out speed while the quality of the images to be reproduced need not be so high provided that whether reexamination is necessary can be judged. In such a case, the images need not be read out at a high picture element density. Conversely, there is a case where the image should be read out at a high picture element density so that the image can be reproduced at a high quality even if the read-out speed is lowered.
To read out the image at a picture element density other than the preset picture element density may be realized by simply changing the main scanning speed and the sub-scanning speed of the stimulating light beam. The main scanning speed of the stimulating light beam can be changed by changing the driving speed of the scanning optical system for causing the stimulating light beam to scan the stimulable phosphor sheet (e.g., a polygonal mirror or a galvanometer mirror).
However, when the driving speed of the scanning optical system is changed, it takes a certain time for the driving to be stabilized due to inertia of the optical system, and the optical system cannot be constantly stably driven over the entire driving speed range. Accordingly, it is preferred that the picture element density be changed without changing the main scanning speed of the stimulating light beam. Further when the picture element density is to be changed, it is necessary to change the picture element density in the sub-scanning direction in the same proportion as the picture element density in the main scanning direction.
Further when the picture element density is to be changed, it is preferred that the picture element density changing processing be carried out at a speed as high as possible.
Further, when the picture element density is simply changed, there is a possibility that the following problems arise.
That is,
1) When the picture element density is changed, energy of signal light emitted from the stimulable phosphor sheet per one picture element differs from that for the original picture element density. Accordingly, when the difference between the changed picture element density and the original picture element density is large, the density (or brightness) of the overall image to be reproduced can be changed, which can adversely affect diagnostic performance of the image.
2) When shading correction is to be carried out on the analog image signal, properties of shading to be corrected varies before and after the picture element density change and the shading sometimes cannot be properly corrected.
3) When the analog image signal is to be logarithmically amplified, the frequency transfer properties can vary before and after the picture element density change.
4) When filtering for removing aliasing noise is to be carried out prior to sampling the analog image signal, aliasing noise sometimes cannot be properly cut since Nyquist frequency varies before and after the picture element density change.
These problems arise not only when an image signal representing a radiation image is read out from a st
Arakawa Satoshi
Iwakiri Naoto
Coles Edward
Sherrill Jason
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