TITLE OF THE INVENTION
SYSTEM FOR READING A PHOSPHOR
SCREEN USED IN FILMLESS RADIOGRAPHY
BACKGROUND OF THE INVENTION The present invention relates generally to a method and apparatus for reading a storage phosphor screen which has been previously exposed to radiation. It is well known to use photostimulable storage phosphor sheets or screens (hereinafter referred to as a "phosphor screen") for performing filmless radiography. Some such phosphor screens are created by applying a coating of a phosphor layer onto a thin, flexible, rugged substrate, generally formed of a polymeric material. Such substrates are generally rect.angular]y shaped in top plan view and have a thickness in the range of 0.1 mm to 30 mm. The substrates are typically made of acrylic or MYLAR® although other polymeric materials may be employed. Although the material used to make the substrate, the size of the substrate and even the shape of the substrate may vary from application to application, rectangularly shaped polymeric substrates are generally preferred and such substrates are generally available in several different sizes, including 14 inches x 17 inches, 8 inches by 10 inches, and 7 inches x 17 inches, or any size in between.
The phosphor coating layer may be applied to the substrate using a variety of processes including creating a fine powder of the mixed phosphor elements or components and, thereafter, applying the powder generally evenly over one principal surface of the substrate and securing the powdered phosphor components to the substrate using a suitable binder, adhesive, or the like.
In the preferred embodiment of the present invention, as described in greater detail hereinafter, the components of the phosphor powder include strontium sulfide doped with cerium and samarium (SrS:Ce,Sm). Other components, such as CaS:Ce,Sm; SrS:Eu,Sm; CaS:Eu,Sm; or components having the general composition CaxSr|_x:R,Sm, where R is Ce or Eu, could be used if desired. Phosphor screens of
-ι - this type are commercially available from Liberty Technologies, Inc., of Conshohocken, Pennsylvania. Still other components, such as alkali-earth halides having the composition BaFX:Eu+2 (X=CI,Br,J) may be used, if desired. Although a single type of phosphor screen having the above-described phosphor components will be discussed throughout the present application, it should clearly be understood that the principles involved with the present invention are not limited to a particular size, shape, or type of phosphor screen nor are they limited to particular phosphor components of the phosphor screen.
The processes employed for creating and, thereafter, "reading" radiographic images using a phosphor screen of the type described above are also generally well known in the art. In general, a phosphor screen (having negligible stored energy) is positioned adjacent to a product, device, person or item (hereinafter referred to as an "item") for which an image is desired, and the item and the phosphor screen are exposed to radiation from a radiation source positioned in such a manner that at least some of the radiation passes through the item before being exposed to the phosphor screen. The phosphor screen absorbs energy from the received radiation at varying levels and, depending upon the structure, material, and other aspects of the item, a latent image of the item is created on the phosphor screen through a well known process known as "electron trapping". Typically, the phosphor screen is first placed in a special cassette or other packaging device which prevents the phosphor screen from being exposed to ambient light that could detrimentally affect the latent image of the item stored on the phosphor screen. An intensifier, comprised of a thin sheet of lead, copper or some other metal, may be positioned between the item and the phosphor screen to enhance the quality of the latent image created on the phosphor screen when high radiation energies are employed.
After creation of the latent image on the phosphor screen, the phosphor screen is "read", typically by a laser scanner and digitizer using a photostimulated luminescence process which is generally well known in the ait. In the reading process, the entire phosphor screen is scanned, in accordance with a predetermined scanning pattern, by a high resolution near-infrared laser having wavelengths between
750 nanometers to 1,600 nanometers, with a peak at about 1,000 nanometers and preferably at a wavelength of about 1,000 nanometers. Examples of scanners include a scanner having a 980 nanometer laser diode, a scanner having a 1,064 nanometer YAG:Nd laser, and a scanner having a pulsed diode laser operating in the 900-1 ,000 nanometers range. A typical scanner setup is shown in prior art Fig. 1. The laser scan has the effect of stimulating or releasing trapped electrons. The stimulation and release of the trapped electrons causes visible luminescence to be emitted from the phosphor screen in proportion to the energy level stored at specific locations on the phosphor screen (i.e., pixels). The intensity of the emitted luminescence for each area or pixel of the phosphor screen is electronically measured, utilizing a light sensitive device such as a photomultiplier tube, digitized and stored in a computer memory as a function of the laser position on the phosphor screen, thereby creating a gray scale image. Once stored within the computer memory, the digitized data representative of the latent image read from the phosphor screen may be recalled and displayed, typically on a high resolution monitor, for analysis or may be printed for later review and analysis, including trend analysis.
The above-described filmless radiography process is generally well known and equipment for performing the process is generally available from manufacturers including Liberty Technologies, Inc., of Conshohocken, Pennsylvania. In general, the laser used to stimulate the trapped electrons for release of the latent image from the phosphor screen is scanned very fast so that the image data is acquired fast. Typically, the intensity of the laser is between 1 and 500 milliwatts and the size of the laser spot is 25 - 250 μm with the scan speed being between 1 and 500 μ seconds per pixel. Thus, only a fraction of the total of the energy stored within the phosphor screen to create the latent image is released, depending upon the amount of laser energy employed. As a result, a substantial amount of energy, including the latent image, remains stored in the phosphor screen once the image reading process has been completed and the digital image data of the radiographed item has been stored in the computer memory.
Furthermore, during scanning, up to 80% or more of the incident near- infrared energy can be reflected from the SrS:Ce,Sm phosphor screen. Thus, only a small fraction of the incident energy is absorbed by the phosphor layer, which then causes emission of visible luminescence. During such scanning, the internal conversion efficiency of near-infrared energy is relatively high because the amount of luminescence produced is a high percentage of the amount of energy absorbed by the phosphor layer. However, the practical efficiency is relatively low because of the large amount of reflected energy. That is, the amount of luminescence produced is a low percentage of the total amount of incident energy. The result of a low practical efficiency is that only a small amount of the information on the phosphor layer is read. In one example, a near-infrared laser having a wavelength of 980 nanometers, an intensity of about 50 milliwatts, a laser spot of 50 μm in diameter, and a scan speed of 5 μseconds per pixel, reads only about 10% of the information on the phosphor layer. Accordingly, the signal output of the reader is relatively low.
Phosphor screens of the type described above are relatively expensive to produce. It is thus desirable to reuse such phosphor screens multiple times. A phosphor screen which is still storing a substantial amount of energy, including a latent image of a previously radiographed item, cannot be reused to obtain a latent image of a second or subsequent item without creating double or multiple latent images or distortion on the phosphor screen.
As a result, to reuse the phosphor screen, a process is employed for removing energy and reducing the overall energy level stored in the phosphor screen. sometimes called erasing, at least to a level where the previously stored latent image is no longer readable and no longer detrimentally affects any latent image which is thereafter created on the phosphor screen. In general, the erasing process which is currently employed involves exposing the phosphor screen to infrared radiation at a predetermined intensity and within a prescribed wavelength range (about 1 ,000 nanometers) for a predetermined period of time. Typically, erasing a phosphor screen in this manner involves exposing the phosphor screen to an infrared radiation source for an extended period, typically between 30 minutes and one hour or longer than one hour, the period being determined by several factors, including the amount or intensity of the energy stored within the phosphor screen. While the above-described erasing process is effective in eliminating or substantially reducing the energy level within a phosphor screen, at least to a level low enough so that the previously stored latent image can no longer detrimentally affect any later created latent image, the erasing process takes an inordinately long time to complete, particularly in view of the much shorter time necessary for creating and reading the latent image. In view of the known deficiencies in present phosphor screen image readout and erasing techniques, there is a need for techniques which improve the percentage of information read from the phosphor layer and which also speed up erasing time. The present invention fulfills these needs by providing methods .and corresponding apparatus for reading a phosphor screen in a manner which reads out a significantly greater percentage of the information stored thereon, while simultaneously substantially reducing and/or eliminating energy stored within the phosphor screen. The present invention thus significantly increases signal level output from the phosphor screen reader, while also erasing or releasing a significant amount of stored energy from the phosphor screen.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a radiation image readout apparatus for reading a stimulable phosphor screen storing a radiation image by exposing the phosphor screen to stimulating rays. The readout apparatus causes the phosphor screen to emit light in proportion to the amount of stored energy. The emitted light is detected by a light detector to obtain an image signal of the radiation image. The apparatus comprises at least one stimulating ray source for producing the stimulating rays. The at least one stimulating ray source comprises a light source which outputs light at a wavelength or wavelengths of from about 570 nanometers to about 620 nanometers, such as a light source which outputs a wavelength of about 590 nanometers.
Another embodiment of the present invention provides a method for reading a stimulable phosphor screen storing a radiation image. During the reading process, the phosphor screen emits light in proportion to the amount of stored energy. The emitted light is detected by a light detector to obtain an image signal of the radiation image. The method comprises the steps of producing stimulating rays from at least one stimulating ray source, and exposing the phosphor screen to stimulating rays from the light source for a sufficient period of time to read the image. The at least one stimulating ray source comprises a light source which outputs a wavelength or wavelengths of about 570 nanometers to about 620 nanometers, such as a wavelength of about 590 nanometers. The energy level of the phosphor screen after the image is read is significantly reduced from an initial energy level which exists before the image is read.
BRIEF DESCRIPTION OF THE DRAWINGS The following detailed description of preferred embodiments of the invention will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings an embodiment which is presently preferred as well as data resulting from testing of such embodiment. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings: Fig. 1 is a schematic diagram of a phosphor screen readout setup in accordance with the prior art;
Fig. 2 is a schematic diagram of a phosphor screen readout setup in accordance with a preferred embodiment of the present invention; and
Fig. 3 is a more detailed schematic diagram of a phosphor screen readout setup using a preferred embodiment of a readout apparatus in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION The present invention is based on the discovery that the phosphors used in connection with phosphor screens of the type described above, in addition to releasing energy when scanned with a high resolution laser having a wavelength in the near-infrared range of the electromagnetic spectrum, also release stored energy at one or more other wavelengths. More particularly, in the case of a phosphor screen made from strontium sulfide doped with cerium and samarium (SrS:Ce,Sm), it was discovered that energy is also released from the phosphor screen when the phosphor screen is exposed to light having a wavelength of approximately 590 nanometers which is in the yellow band of the visible spectrum.
It was further discovered that light having a wavelength of approximately 590 nanometers is also suitable for use in the readout process, in place of, or in conjunction with, the prior art near-infrared lasers. More particularly, it was discovered that when light having a wavelength of approximately 590 nanometers is used in the readout process, a significantly greater amount of energy is released from the phosphor screen than when using the near-infrared light in the prior art wavelengths. The greater energy release significantly increases signal level output from the phosphor screen reader in comparison to a level typically obtained with prior art readers. Not only does the increased signal level output result in improved images, but the phosphor screen becomes erased to a greater extent at the same time. Total erasure time is thus reduced in comparison to prior art.
In accordance with the present invention, Fig. 2 is a schematic block diagram of a readout apparatus 10. The apparatus 10 reads a phosphor screen 12
(which has previously been exposed to radiation, such as x-rays,) with light having a wavelength from about 570 nanometers to about 620 nanometers, and preferably about 590 nanometers. The apparatus 10 is defined by the elements within the dashed lines of Fig. 2. The apparatus 10 includes at least one stimulating ray source or light source 16 for producing stimulating rays having a wavelength from about 570 nanometers to about 620 nanometers, and preferably about 590 nanometers. One light source 16 suitable for use with the present invention is a frequency shifted He-Ne laser having an energy peak at approximately 590 nanometers (yellow light). One such laser is manufactured by Research Electro-Optics, Inc., Boulder, Colorado and is identified as Model No. LHYR-0200. This laser emits a 2.0 milliwatt yellow laser beam 18 with a peak wavelength of 594 nanometers and a 0.58 mm beam diameter. However, lasers having greater or less power and/or different beam diameters and different types of light sources are also suitable for use with the present invention. When using this particular laser, it is preferable to add a 590 nanometer bandpass filter 20 to the laser output to block any visible glow at wavelengths other than about 590 nanometers emitted therefrom. More generally, the bandpass filter 20 filters out light from about 570 nanometers to about 620 nanometers, wherein the filtering wavelength depends upon the desired wavelength of the light source 16. The incident laser beam 18, after passing through the filter 20, impinges on the phosphor screen 12 at a prescribed angle, releasing energy therefrom in the usual well known manner as visible luminescence in proportion to the energy level stored at specific locations or pixels on the phosphor screen. A phosphor screen 12 of SrS:Ce,Sm emits visible luminescence at about 485 nanometers. The released energy includes a visible light beam 22. Preferably, the visible beam 22 passes through a short-pass filter 24 associated with the apparatus 10 which blocks any 594 nanometer light reflected from the phosphor screen 12 and transmits the photostimulated luminescence at about 485 nanometers. That is, the short-pass filter 24 filters out light reflected from the phosphor screen 12 which has wavelengths similar to the light source 16 and transmits light having wavelengths emitted by the phosphor sheet 12 as a result of stimulation. After exiting the short-pass filter 24, the light beam 22 is detected by a visible light detector 26 which may be the human eye, a photodetector such as a photomultiplier tube, or the like. If the light detector 26 is an electronic device, its output is connected to subsequent processing circuitry (not shown), as is well-known in the prior art.
The apparatus 10 may be a separate device used in conjunction with apparatus for detecting and processing the visible light beam, or the apparatus 10 may be incorporated into a reader/scanner.
The light source 16 may also be a low pressure, generally elongated sodium vapor (SOX) lamp which emits energy primarily at a wavelength of 589.8 nanometers within the yellow band of the visible spectrum, as well as some infrared energy. The light source 16 may also comprise other sources of light at or near a wavelength of 590 nanometers, such as a yellow light emitting diode. The light source 16 may also be a He-Ne laser having an energy peak at wavelengths in the vicinity of 590 nanometers, such as a yellow laser having an energy peak of 594 nanometers, or an orange laser having an energy peak of 612 nanometers. These lasers also generate photostimulated luminescence even though the phosphor sensitivity does not peak at these wavelengths. Fig. 3 shows a preferred embodiment of a readout apparatus 10'. The apparatus 10' includes a yellow laser light source 16', an optional yellow interference filter 28 for performing the function of the bandpass filter 20 of Fig. 2, optical focusing and directing subcomponents 30, and an optional gel filter 32 for performing the function of the short-pass filter 24 of Fig. 2. One suitable light source 16' is the laser manufactured by Research Electro-Optics, Inc., discussed above. The yellow interference filter 28 effectively filters out any unwanted blue light from the plasma inside the laser tube of the light source 16'. The light detector in Fig. 3 is a photomultiplier (PMT) tube 34. The gel filter 32 is placed over the PMT tube 34. One suitable gel filter 32 is a Roscolux #74 Night Blue gel filter, available from Rosco
Laboratories, Inc., Stamford, CT, which has a peak transmittance of 50% at about 480 nm.
In operation, the PMT tube 34 collects the photostimulated luminescence from the phosphor screen 12, generates a signal therefrom, conditions and digitizes the signal, and displays it on a high resolution monitor (not shown). The phosphor screen 12 preferably uses a conventional phosphor which has known sensitivity peaks at 590 nanometers and 970 nanometers.
Based on the imaging results, the density value, the raw laser power, the PMT filter attenuation ratio, the laser filter attenuation factor and the PMT voltage settings, one can estimate the sensitivity of the phosphor screen at peaks of 590 nanometers and 970 nanometers. It is estimated that the phosphor screen is about twice as sensitive at the yellow peak than at the 970 nm. infrared peak.
The optical subcomponents 30 include a front surface folding mirror 36, a plano-convex focusing lens 38 and a galvanometer mirror 40. The light exiting from the light source 16' is reflected by the mirror 40 and focused by the lens 38. In one suitable embodiment of the invention, the initial laser spot diameter is 0.58 mm. (1/e2) and is about 1.36 mm. of the surface of the image plane. A 17X beam expander (not shown) may be used to focus the laser spot down to about 85 μm. for better image resolution. The galvanometer mirror 40 provides horizontal scanning of the laser beam at 50 Hz. Vertical scanning of the laser beam is provided by physically moving the phosphor screen 12 in the vertical direction. For example, the phosphor screen 12 may be passed by the PMT tube 34 on a conveyor (not shown). Together, the galvanometer mirror 40 and the movement of the phosphor screen 12 provide two- dimensional scanning of the phosphor screen 12. Alternatively, the phosphor screen 12 may be stationary and the galvanometer mirror 40 may be a two-dimensional scanning mechanism.
A single apparatus 10 or 10' may also be constructed to process two different types of phosphor screens 12, especially if both screens 12 have peak responses at approximately the same wavelength. For example, a phosphor screen 12 made of BFBr:Eu+2 has a stimulation spectrum with a peak wavelength at about 590 nanometers, and an emission spectrum that peaks at a wavelength of about 390 nanometers. It was discovered that under specific test conditions, this material exhibits a signal strength which is about one order of magnitude greater than a phosphor screen made of strontium sulfide phosphor, discussed above. Since both of these subst,ances (BFBr:Eu .and strontium sulfide) have peak responses near the 594 nanometer wavelength of a yellow laser, a single apparatus may be used to simultaneously read, and significantly reduce the energy level of, phosphor screens 12 of either type. In such an apparatus, the short-pass filter 24 is selected to block reflections of the yellow laser, and to pass the stimulated light at 390 nanometers or 485 nanometers from either of the phosphor screens 12. For ex.ample, the cutoff frequency may be selected at the midpoint between the 594 nanometer yellow laser wavelength and 485 nanometers, which is the higher emission wavelength of the two types of phosphor screens. The apparatus may be calibrated so that both types of phosphor screens 12 display the same equivalent optical density for the same x-ray exposure. The apparatus 10 or 10' can be used to obtain improved images from an exposed phosphor screen 12 while reducing the time and cost of processing the phosphor screens and preparing them for reuse. In particular, erase times are reduced because the energy level of the phosphor screen, after being read using the light source 16, is significantly lower than the energy level when read by a conventional near-infrared laser. (Erase times are generally proportional to the amount of stored energy that must be removed.) While exposure of the phosphor screen 12 to the light source 16 will not completely erase the stored energy in the screen 12, less time will be required to erase the remaining energy than is required when the screen 12 is read by a conventional near-infrared laser. Qualitative observations have been made to show that when a phosphor screen 12 is read by being exposed to comparable energies from a conventional near-infrared laser and a light source 16 in the form of a sodium lamp, erase times for the sodium lamp are from ten to fifty times faster.
It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.