RADIATION IMAGE REPRODUCING METHOD AND APPARATUS

Abstract: In the radiation image recording and reproducing sys­ tem, a radiation image stored in a stimulable phosphor sheet at an n+1'th object image recording step is read out to obtain an electric image signal, and an electric image signal corresponding to radiation energy remaining in the stimul­ able phosphor sheet after an n'th image recording step is subtracted from the electric image signal obtained after the n+1'th image recording step. A visible image only of the radiation image recorded at the n+1'th image recording step is reproduced on the basis of an electric image signal obtained by the subtraction.

Description: This invention relates to a method of reproducing a radiation image in a radiation image recording and reproducing system using a stimulable phosphor sheet, and an apparatus for carrying out the method according to the preambles of claims 1 and 5, respectively, which method and apparatus are known from EP-A 0 079 751. Description of the Prior Art When certain kinds of phosphors are exposed to a radiation such as X-rays, α-rays, β-rays, γ-rays, cathode rays or ultraviolet rays, they store a part of the energy 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 by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Patent No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photographic film or on a display device such as a cathode ray tube (CRT). In the aforesaid radiation image recording and reproducing system , the stimulable phosphor sheet once used for recording a radiation image can be reused for further radiation image recording after radiation energy stored i nthe sheet is released, i.e. after the stored radiation image is erased. Therefore, after the stored radiation image is read out by exposing the stimulable phosphor sheet to stimulating rays which cause the sheet to emit light in proportion to the stored radiation energy, radiation energy (radiation image) remaining in the sheet is released and erased by exposing the sheet to erasing light having at least light of a wavelength within the wavelength range of the stimulating rays obtained by a light source such as a fluorescent lamp, a tungsten-filament lamp, a sodium lamp, a xenon lamp or an iodine lamp. After erasing, the sheet is reused for radiation image recording. However, in order to erase the radiation image remaining in the stimulable phosphor sheet after image read-out so that the remaining radiation image is not detected at the step of reading out a radiation image recorded next, it is necessary to expose the sheet to a large amount of erasing light be keeping a large-capacity erasing light source turned on for long periods. US-A 4 350 998 discloses a medical examination device in which a fluoroscopic image is converted via an image intensifier and image pick-up chain into a video-signal which is then digitized. A memory is provided for storing a digitized image. Furthermore, a subtraction circuit, a multiplication circuit and an addition circuit are provided. The addition circuit has a first input for receiving elements of image information from an image produced and a second input for receiving previously determined image information which is stored in the memory. Each one of the image informations applied to the first and second input of the addition circuit is multiplied by a constant factor which is bigger than or equal to zero and smaller than or equal to one. The image information output from the addition circuit is set to one input of a subtraction circuit to the other input of which the digitized image information of the current image is fed. The output of the subtraction circuit may be formed to a visible image on a viewing screen. This known device allows noise integration and differential image determination between consecutive images. SUMMARY OF THE INVENTION The primary object of the present invention is to provide a method of reproducing a radiation image in a radiation image recording and reproducing system wherein the amount of erasing light used for erasing radiation energy remaining in a stimulable phosphor sheet after image read-out is decreased. Another object of the present invention is to provide a method of reproducing a radiation image, which is suitable particularly for a built-in type radiation image recording and reproducing system. The specific object of the present invention is to provide an apparatus for carrying out the method. These objects are achieved by the features comprised in claims 1 and 5 respectively. Advantageous embodiments are claimed in the dependant claims. In the present invention, n denotes an arbitrary natural number. In short, the present invention is characterized by reproducing a visible image of a radiation image recorded at the n+1'th image recording step on the basis of an electric image signal obtained by subtracting the electric image signal corresponding to a radiation image remaining in the stimulable phosphor sheet obtained after previous (n'th) image recording from the electric image signal obtained by image read-out after the n+1'th image recording when the same stimulable phosphor sheet is repeatedly used for radiation image recording. In the present invention, when the radiation image recorded at a previous image recording step is not erased and is involved in the electric image signal detected at the current image recording and read-out step, the electric image signal corresponding to the previous radiation image remaining in the stimulable phosphor sheet is subtracted from the electric image signal detected at the current image recording and read-out step. A visible image is then reproduced by use of the electric image signal obtained by the subtraction. Therefore, it is possible to obtain a visible image free from noise caused by the previous radiation image remaining in the stimulable phosphor sheet. Also, since the presence of the previous radiation image remaining in the stimulable phosphor sheet is allowable, erasing of the radiation image after image read-out may not be conducted completely, and it is possible to decrease the amount of erasing light. Further, since the amount of erasing light can be decreased, it is possible to shorten the erasing time for which the erasing light source is turned on. Accordingly, when the present invention is applied to a built-in type radiation image recording and reproducing system proposed in EP-A- 125800 wherein an image recording section, an image read-out section and an erasing section are connected by a path for conveying and circulating the stimulable phosphor sheet and a plurality of the stimulable phosphor sheets are circulated and reused for image recording and read-out, it becomes possible to shorten the image recording and reproducing cycle and to realize high-speed image recording. Also, in order to store all of the previous image signals detected from many stimulable phosphor sheets, an expensive storage unit of a large capacity is necessary. However, in the aforesaid built-in type radiation image recording and reproducing system, since it is not necessary to use so many stimulable phosphor sheet, a storage unit having a limited storage capacity may be used, and no problem arises with regard to the cost. Therefore, the method and apparatus of the present invention are applicable particularly advantageously to the built-in type radiation image recording and reproducing system. BRIEF DESCRIPTION OF THE DRAWING The single drawing is a schematic view showing an embodiment of the method and apparatus for reproducing a radiation image in accordance with the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the present invention will hereinbelow be described in further detail with reference to the accompanying drawing. In the drawing, reference numerals 1a, 1b and 1c denote an image recording step, a read-out step and an erasing step in the first image recording and reproducing process. Reference numerals 2a, 2b and 2c denote an image recording step, a read-out step and an erasing step in the second image recording and reproducing process. At the image recording step 1a in the first image recording and reproducing process, a stimulable phosphor sheet 5 is exposed to a radiation emitted by a radiation source 3 and passing through an object 4 to have a radiation image of the object 4 stored in the sheet 5. The stimulable phosphor sheet 5 is then sent to the read-out step 1b at which the sheet 5 is scanned by stimulating rays such as a liner beam emitted by a stimulating ray source 6. Light emitted by the stimulable phosphor sheet 5 in proportion to the stored radiation energy when it is exposed to stimulating rays is photoelectrically detected by a photoelectric conversion means 7 to obtain an image signal. The image signal is stored in a storage means 9 and used for reproducing a visible image on a recording medium much as a photographic film or on a CRT by a reproduction means 8. At the erasing step 1c, the stimulable phosphor sheet 5 is exposed to erasing light emitted by an erasing means 10 such as a fluorescent lamp, and the radiation image remaining in the sheet 5 is erased to a predetermined ratio. Then, the stimulable phosphor sheet 5 is sent to the second image recording step 2a at which a radiation image of a new object is stored in the sheet 5 in the same manner as the first image recording step 1a. At the second read-out step 2b, image read-out is conducted in the same manner as in the first read-out step 1b, and an electric image signal obtained is stored in the storage means 9. The second reproduction of a visible image is conducted by the reproduction means 8 by use of an electric image signal obtained by subtracting the stored first image signal multiplied by a factor α indicating the ratio of the residual image signal after the first erasure step 1c and read out in step 2b with respect to the formerly stored image signal read out in step 1b (hereinafter referred to as "remaining image ratio") determined by the first erasing conditions and characteristics of the stimulable phosphor constituting the stimulable phosphor sheet 5, from the electric image signal obtained by the second read-out step 2b. At this time, the multiplication is conducted by a multiplication means 11 constituted by a multiplier or the like, and the subtraction is conducted by a subtraction means 12 constituted by a subtracter or the like. After the second read-out step 2b, the radiation image remaining in the stimulable phosphor sheet 5 is erased at the second erasing step 2c in the same manner as in the first erasing step 1c. Subsequently, image recording, read-out, reproduction and erasing are repeated in the same manner as in the second image recording and reproducing step. In the subtraction between the image signals for the second and subsequent visible image reproductions, a signal of each picture element obtained by multiplying the previous stored image signal by the predetermined constant may be subtracted from the image signal of the corresponding picture element obtained by each of the second and subsequent read-out steps. The remaining image ratio in each erasing step may be adjusted to any value. However, when a radiation image recorded prior to the previous image remains in the stimulable phosphor sheet, the images are sequentially accumulated in the sheet. Therefore, the remaining image ratio should preferably be adjusted so that only the radiation image recorded just before the current step remains in the stimulable phosphor sheet, i.e. so that an image is erased completely when the sheet is subject to the erasing steps twice. Also, the image recording conditions such as the intensity of radiation at each image recording step and the read-out conditions such as the intensity of stimulating rays and the read-out gain at each read-out step should preferably be maintained the same. When these conditions change, the image signal to be subtracted, i.e the constant α by which the stored image signal is multiplied, must be adjusted in accordance with the degree of the change. In the case where the stimulable phosphor sheet 5 is exposed to the radiation without making the radiation pass through the object 4 at the first image recording and reproducing step, the signal detected by the read-out from the sheet 5 represents nonuniformity caused by nonuniformity of sensitivity of the sheet 5 or of the read-out means. Therefore, it is possible to reproduce a visible image free from nonuniformity caused by the apparatus or the like by subtracting the signal representing nonuniformity caused by the apparatus or the like multiplied by a predetermined constant from the read-out electric image signal at the second image recording and reproducing step (i.e. the first image recording with the radiation passing through the object and the first reproduction of the object image), or by subtracting the signal representing the unerased and remaining radiation image and the signal representing the nonuniformity from the read-out electric image signal at the third and subsequent image recording and reproducing steps. In the drawing, the image raid-out means, the storage means, the reproduction mains, the subtraction means and the erasing means are shown for each image recording and reproducing step. However, normally, since the stimulable phosphor sheets are circulated and reused, it is sufficient that a single combination of these means are installed.

Claims: A method of reproducing a radiation image in a radiation image recording and reproducing system in which object image recording is conducted by exposing a stimulable phosphor sheet (5) to a radiation passing through an object (4) to have a n'th radiation image of an object stored in the stimulable phosphor sheet (5), image read-out is conducted by scanning the stimulable phosphor sheet (5) carrying the radiation image stored therein by stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy and by photo-electrically detecting (7) the emitted light to obtain an electric image signal, a visible image is reproduced by use of the electric image signal, and which further comprises exposing the phosphor sheet to an erasing radiation and re-using the stimulable phosphor sheet. characterized in that the method comprises the steps of: i) storing the electric image signal obtained by said image read-out of the n'th radiation image in a storage means ii) exposing the stimulable phosphor sheet after the read-out of the n'th radiation image such that still a predetermined ratio of the n'th radiation image remains stored in the phosphor sheet , and which remaining image is erased by a predetermined number (v) of subsequent later steps (n + v) in which the phosphor is exposed to the erasing radiation iii) reading-out an n+1'th radiation image stored in said stimulable phosphor sheet (5) after n+1'th object image recording on the same stimulable phosphor sheet (5), thereby obtaining an electric image signal after the n+1'th image recording, iv) multiplying the electric image signal obtained by said storing the image read-out of the n'th radiation image in a storage means by a predetermined constant α and substracting this multiplied electric image signal from said electric image signal after the n+1'th image recording, and v) reproducing a visible image of a radiation image recorded by the n+1'th image recording only on the basis of the electric image signal obtained by said subtraction in step iv) (wherein n denotes an arbitrary natural number). The method as defined in claim 1 wherein said predetermined constant α is a remaining image ratio determined on the basis of the erasing conditions at n'th erasing step and characteristics of the stimulable phosphor constituting said stimulable phosphor sheet. The method as defined in claim 1 or 2 wherein said erasing is conducted so that only the radiation image recorded by said n'th image recording remains in said stimulable phosphor sheet when said n+1'th image recording is conducted. The method as defined in claim 1 further comprising the steps of conducting an image recording without an object before the first object recording step; obtaining an electric nonuniformity signal corresponding to the nonuniformity of the sensitivity of the phosphor sheet; either subtracting said non uniformity signal multiplied by a predetermined constant from said electric image signal obtained after n'th object image recording or subtracting said nonuniformity signal and said n'th object image signal from said n+1'th object image signal whereby said electric image signal obtained by said subtraction is free from any nonuniformity. An apparatus for reproducing a radiation image in a radiation image recording and reproducing system in which object image recording is conducted by exposing a stimulable phosphor sheet (5) to a radiation passing through an object (4) to have a radiation image of the object stored in the stimulable phosphor sheet (5), image read-out is conducted by scanning the stimulable phosphor sheet carrying the radiation image stored therein by stimulating rays which cause the stimulable phosphor sheet (5) to emit light in proportion to the stored radiation energy and by photo-electrically detecting (7) the emitted light to obtain an electric image signal, a visible image is reproduced by use of the electric image signal, the stimulable phosphor sheet (5) after the image read-out is exposed to an erasing radiation and is re-used for the radiation image recording, characterized in that the apparatus comprises: a) a storage means (9) for storing the electric image signal obtained by said image read-out, b) a multiplication means (11) for multiplying said electric image signal stored in said storage means by a predetermined constant α, c) a subtraction means (12) for subtracting the electric image signal obtained by multiplying an electric image signal stored in said storage means and corresponding to the image read out of the n'th radiation image by said predetermined constant α by said multiplication means, from an electric image signal obtained by an image read-out conducted after n+1'th object image recording on the same stimulable phosphor sheet (5), and d) a reproduction means (8) for reproducing said visible image on the basis of an output image signal of said subtraction means (12). The apparatus as defined in claim 5 wherein said predetermined constant α is a remaining image ratio determined on the basis of the erasing conditions at n'th erasing and characteristics of the stimulable phosphor constituting said stimulable phosphor sheet (5). The apparatus as defined in claim 5 or 6 wherein said erasing is conducted so that only the radiation image recorded by said n'th image recording remains in said stimulable phosphor sheet (5) when said n+1'th image recording is conducted.

Inventors: NAKAJIMA, NOBUYOSHI C/O FUJI PHOTO FILM CO., LTD., WATANABE, HIDEO C/O FUJI PHOTO FILM CO., LTD.
Applicants: FUJI PHOTO FILM CO., LTD.
Application year: 1985
Main classification: G01T1
IPC codes: G01T1, A61B6, H04N7, H04N1, G03B42