SINGLE-HEAD PHOSPHOR SCREEN SCANNING SYSTEMS

Abstract: A system for scanning an imaging plate (10), including a continuous belt drive(36), a drive assembly (40) connected to the continuous belt drive (36), a scanning head (35) connected to the drive assembly (40), and a laser (50) positioned to direct a laser beam (51) in a beam path across the surface of the imaging plate (10), wherein the drive assembly (40) is adapted to move the scanning head (35) back and forth in a path which is collinear with the beam path as the continuous belt drive (36) is rotated in one direction.

Description: SINGLE-HEAD PHOSPHOR SCREEN SCANNING SYSTEMS CROSS-REFERENCE TO RELATED APPLICATIONS The present application is a regular application which claims the benefit of U.S. Provisional Patent Application No. 60/110,151 filed November 25, 1998 (Atty. File No. 18482-000400). TECHNICAL FIELD The present invention relates to methods and systems for reading images stored on photostimulable media, and in particular to reading images stored on phosphor radiation screens. BACKGROUND OF THE INVENTION The use of photostimulable phosphor image storage screens as a replacement for an x-ray film and other sensors is well known. Phosphor image screens work by trapping individual x-ray photons in a storage layer. The latent image trapped in the screen can then be read by scanning the storage layer using a suitable wavelength excitation beam, preferably from a focussed laser. The laser excitation beam causes the screen to release the latent image in the form of emitted stimulable phosphor light that is proportional to the x-ray energy applied to the screen during exposure. The emitted light is collected by an optical system and is converted into an electronic signal proportional to the emitted light. The electrical signal is then converted into a digital value and passed to a computer which generates and stores an image file. The image file can then be displayed as a representation of the original radiograph, with image enhancement software applied to augment the radiographic information. Various known systems for moving a scanning head or directing a scanning beam across image or data storage screens are known. In one family of systems, an X-Y raster scan is taken as follows. The scanning head or beam first scans in a straight line across the screen in an X direction. The screen is then moved a short incremental distance in the Y direction. (Alternatively, the scanning head or the optics directing the beam can be moved incrementally in the Y direction). Thereafter, an X directional scan is repeated. Accordingly, by scanning back and forth in one direction, while intermittently advancing the screen, (or re-directing the scanning beam), in a perpendicular direction, an X-Y raster scan is generated. In a second family of systems, the image or data storage screen is rotated in the plane of the screen about a center point in the screen while a scanning head is moved radially across the screen. A problem common to both families of scanning systems is the problem of precisely controlling the movement of the scanning head, (or the movement of the optical system directing the scanning beam, which may comprise a galvanometric mirror). Moreover, problems exist when attempting to accurately position such a moving scanning head or beam direction system to direct an incident beam at a desired location on the phosphor screen. A second problem common to existing imaging systems is that such systems are configured such that the response radiation emitted by the screen is not directed back to a light detector through the same optical train that was used to direct incident laser light at the screen. Accordingly, a first optical train is required to direct and focus the incident light on the screen, and a second optical train is required to detect and measure the response radiation emitted by the screen. It would instead be desirable to provide a system for high speed scanning of a phosphor screen, (or any other photostimulable media), which moves a scanning beam head in a path across the surface of the phosphor screen to generate a raster scan, yet avoids the problems of controlling the back and forth movement of the scanning head across the screen. It would also be desirable to avoid potential inaccuracies, control and wear and tear problems caused by acceleration forces moving such a scanning head back and forth in one or two directions, at the same time achieving near 100% duty cycle read efficiency. Moreover, it would be desirable to create a high speed scanning system which has minimal dead time during its operation such that a near continuous data stream can be generated as the phosphor screen is scanned. Additionally, it would be desirable to create a high speed scanning system which does not require a transport mechanism which either moves the phosphor screen in two perpendicular directions (such as would be accomplished with an X-Y transport mechanism), or rotates the phosphor screen. Additionally, it would be desirable to create a high speed scanning system which uses the same optical train for phosphor screen stimulation and data collection. SUMMARY OF THE INVENTION The present invention provides systems and methods for scanning a photostimulable imaging plate, (which may comprise a phosphor storage screen), with a single-head scanning system comprising a continuous belt drive which moves the scanning head back and forth across the surface of the imaging plate as the continuous belt drive is rotated in one direction. As such, an advantage of the present invention is that the scanning head can be moved across the surface of the imaging plate in two directions, without having to change the direction of the continuous belt drive to which the scanning head is attached. An additional advantage of the present invention is that the scanning head passes over the surface of the imaging plate at the same speed when moving in either direction. In a preferred aspect, the present system comprises a continuous belt drive, having a drive assembly attached thereto. The scanning head is connected to the drive assembly and a laser positioned to direct a laser beam in a beam path across the surface of the imaging plate, wherein the drive assembly is adapted to move the scanning head back and forth in a path which is collinear with the beam path as the continuous belt drive is rotated in one direction. In preferred aspects of the present invention, the drive assembly may comprise a drive member attached to the continuous belt drive, and a drive member guide supported by the drive member such that the drive member is adapted to slide back and forth along the drive member guide, wherein the drive member guide moves back and forth across the imaging plate as the drive member moves around the continuous belt drive. An advantage of the present invention is that scanning of the phosphor screen is achieved without changing the direction or speed of rotation of the belt which moves the scanning head across the screen. Therefore, high speed scanning can be achieved. In a preferred aspect of the present invention, the scanning head is moved by a rotating belt drive which is wrapped around two spaced apart pulleys. The single scanning head is preferably attached to a reciprocating drive assembly which is connected directly to the rotating belt. Preferably, the scanning head is mounted to a drive assembly which comprises a drive pin which is attached directly to the rotating belt. As the drive pin moves back and forth between pulleys and moves around the dual pulleys, the drive pin causes the drive assembly to move back and forth in a straight path such that the single scanning head oscillates back and forth over the phosphor screen. The scanning head is subject to acceleration forces as it oscillates back and forth. Accordingly, in certain preferred aspects of the present invention, systems for counteracting the effects of acceleration on the scanning head are included. In particular, a spring system or voice coils positioned near the pulleys can be used to assist in reversing the direction of the scanning head and its associated drive assembly. A counterweight system can also be provided such that vibration in the system can be substantially eliminated. Concurrently with the rotation of the belt (which moves the scanning head back and forth in an X direction), the phosphor screen is preferably advanced in a perpendicular Y direction relative to the rotating scanner. In one approach, the scanner (i.e., continuous belt drive, drive assembly and laser), are held at a fixed position above the phosphor screen while a transport mechanism, (which may comprise a series of rollers and a guide), moves the phosphor screen under the rotating scanner. In an alternate approach, the scanner is mounted to a transport mechanism to move the rotating scanner across the surface of the stationary phosphor screen. In either case, a raster scan of the phosphor screen is generated by moving the scanning head over the phosphor screen as the scanning device is moved in a perpendicular direction across the surface of the phosphor screen. Motion in the Y direction can be motion in incremental steps. Incremental motion in the Y direction requires only a simple decoding algorithm for generating the image. Alternatively, motion in the Y direction can be continuous, producing a zigzag raster scan, which can also be easily decoded for generating the image. It may be desirable that the pair of pulleys which drive the continuous belt drive are positioned a distance apart such that the scanning head moves at the same speed across the phosphor screen, and slows down to reverse direction at locations off the sides of the phosphor screen. Specifically, having the pulleys positioned at a distance beyond the edges of the phosphor screen assures that the speed of the scanning head will be constant as it passes back and forth over the surface of the screen. When the scanning head travels a distance greater than the width of the phosphor screen, a "dead time" data gap occurs. Although it may be desirable to minimize this dead time gap, (so as to increase duty cycle), it may also be desirable to have a small dead time gap present. Specifically, this data gap may be used to distinguish between successive scans across the screen such that a raster scan image can be generated of the phosphor screen. Dimensioning the scanner such that there is minimal, (or preferably no), difference between the maximum linear distance of travel of the scanning head and the width of the phosphor screen ensures near continuous scanning is achieved. In a accordance with the present invention, a single head scanning system is used in conjunction with a single laser light source and a single photodetector. Light from the single laser source is directed towards the scanning head in a beam path which is parallel to a straight portion of the rotating belt drive spanning between the two pulleys. The scanning head comprises an optical system which intercepts the laser beam and reflects and focuses the beam downwardly onto the phosphor screen as the scanning head moves across the phosphor screen. Response radiation emitted by the phosphor screen is directed back through the same optical system as the incident laser beam such that separate optical lenses to scan the laser beam across the phosphor screen and to collect the response radiation emitted by the phosphor screen are not required. Response radiation emitted by the phosphor screen is received by the scanning head and is directed towards a light detector which may preferably comprise a photomultiplier tube, but may also comprise a photodiode. In various preferred aspects, the optical system comprises a collimated laser which directs a laser beam in a path across the surface of the phosphor screen. The path of the laser beam is disposed parallel to the straight portion of the continuous belt drive passing over the surface of the phosphor screen. As such, the scanning head, (which is attached to the continuos belt drive by the drive assembly), moves back and forth across the phosphor screen in a path which is collinear with the laser beam. A dichroic mirror is preferably used to separate incident laser light from the collimated response radiation emitted by the phosphor screen such that only the response radiation is directed to a photomultiplier tube. In one aspect, the dichroic mirror is mounted onto the scanning head. In an alternate aspect, the dichroic mirror is positioned at a stationary location in the path of the laser beam between the laser and the scanning head. In this alternate aspect, the dichroic mirror is preferably mounted near the photomultiplier tube. A reflecting mirror is located in each scanning head such that incident laser light is directed towards the phosphor screen and the phosphor emitted light is directed back through the same optical path as the incident laser beam. A focussing lens is also located in each scanning head for focussing the collimated laser beam to a point spot of about 50 microns on the phosphor screen. An advantage of the present invention is that it is not necessary to alter the direction or speed of movement of the rotating belt drive as the scanning head passes over the phosphor screen. This substantially reduces wear on the system, and provides a system which is balanced and has a slim aerodynamic profile for high speed rotation. In the present system, the only necessary moving parts are a system to rotate the continuous belt drive around the two pulleys, (such that the scanning head is moved back and forth), and a system to advance the relative motion of the phosphor screen to the scanner in a direction perpendicular to the rotating belt. By moving the phosphor screen perpendicular to the direction of scanning head movement, high resolution scanning is achieved as the phosphor screen can be advanced in very small increments relative to the path of the scanning head passing thereover. Accordingly, a pixel by pixel resolution of the image can be derived. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a top plan view of the present single head scanning system in a first position. Fig. 2 is a top plan view of the present single head scanning system in a second position. Fig. 3 is a top elevation view of the present single head scanning system in a third position. Fig. 4 is a perspective view of an end of the single head scanning system of Figs. 1 to 3 showing the details of the drive assembly and drive pin. Fig. 5 is a top plan view of an alternate scanning head drive assembly for use with the single head scanner. Fig. 6 is a side elevation view of an optical system. Fig. 7 is a side elevation view of an alternate optical system. Fig. 8 is a side elevation view of another alternate optical system. BEST MODES OF CARRYING OUT THE INVENTION Figs. 1 to 4 illustrate a first embodiment of the present single head scanner system. Fig. 5 illustrates a second embodiment of the present single head scanner system. Figs. 6 to 8 illustrate various optical systems for use with either of the embodiments of the present single head scanner system. Figs. 1 to 3 illustrate the sequential movement of a scanning head moving back and forth across the surface of a phosphor screen in a path collinear with a laser beam, as follows. Referring to Fig. 1, a scanner 30, comprising a continuous belt drive 36; a drive assembly 40 connected to the continuous belt drive; a scanning head 35 connected to the drive assembly; and a laser 50 is provided. Continuous belt drive 36 is wrapped around two pulleys 32 and 34, which are rotated at a constant speed in direction R. Laser 50 is positioned to direct a laser beam 51 in a path which is parallel to portion 36a of continuous belt drive 36, such that scanning head 35 moved in a path collinear with laser beam 51, as shown. As pulleys 32 and 34 are rotated in direction R, scanning head 35 will move to the position shown in Fig. 2, (at which time its movement stops and direction of motion changes), and then return back along the same path to the position shown in Fig. 3. Further details of a first embodiment of drive assembly 40 of Figs. 1 to 3 are shown in Fig. 4. As can be seen in Fig. 4, scanning head 35 is attached to drive assembly 40 which comprises a guide rail 42, a guide pin 46, a transverse beam 44, and sleeves 47 and 49. Guide pin 46 is attached to belt 36 and slides freely back and forth along transverse beam 44 as scanning head 35 moves back and forth in a path collinear with laser beam 51. Guide pin 46 may be slidably received over transverse beam 44 (as shown), or alternatively, may be dimensioned to slide along a groove in transverse beam 44. Specifically, (as seen in Fig. 1), as belt 36 is rotated about pulleys 32 and 34, guide pin 46, (being attached to belt 36), will first pull scanning head 35 across the surface of phosphor screen 10 in direction Dl. As shown in Fig. 2, further rotation of belt 36 will cause guide pin 46 to slide to a position mid-way along transverse beam 44 when scanning head 35 reaches its maximum travel displacement in direction Dl. Still further rotation of belt 36, (to the position shown in Fig. 3), will cause guide pin 46 to slide completely along transverse member 44 such that guide pin 42 will pull scanning head 35 in direction D2. (Direction D2 being exactly opposite that of direction Dl). Accordingly, scanning head 35 will move back and forth across the same linear path as guide pin 46 moves completely around pulleys 32 and 34. An optional rebound system 43 comprising springs or voice coils positioned at opposite ends of guide rail 42, adjacent pulleys 32 and 34, can assist drive assembly 40 in "bouncing back" when it reaches the opposite ends of guide rail 49. In the case of springs or voice coils, each of the springs or voice coils are preferably designed to provide a near perfect elastic response over a distance equal to the radius of pulleys 32 and 34. To avoid vibration as scanning head 35 oscillates back and forth, a counter weight 60 can be attached to belt 36 at a location opposite drive assembly 40 such that drive assembly 40 and counter weight 60 simultaneously reach opposite ends of scanner 30 with drive assembly 40 reaching pulley 32 as counter weight 60 reaches pulley 34, and vice versa. As can be appreciated, scanning head 35 remains in the path of laser beam 51 at all times, regardless of the position or movement of belt 36. Accordingly, a near continuous data scan can be made of phosphor screen 10. By advancing phosphor screen 10 in a Y direction perpendicular to the direction of travel of scanning head 35, a raster scan is generated when phosphor screen 10 is advanced incrementally between passes of scanning head 35. Specifically, by advancing phosphor screen 10 in a Y direction perpendicular to the direction of travel of scanning head 35, a zigzag scan is generated when phosphor screen 10 is advanced continuously as scanning head 35 passes thereover. Using appropriate software algorithms, the zigzag scan is converted to a standard X-Y raster scan. An alternate drive assembly 100 is shown in Fig. 5, (in which only one end of the scanner is shown for clarity). Pulley 102 has a belt 104 wrapped there around. A guide pin 106 is attached to belt 104. Guide pin 106 moves freely in channel 108 on traverse member 110. Wheels 112 and 114 position transverse member 110 such that it moves back and forth in direction Dl and D2. Guide assembly 100 further comprises a collar 115 which is received over guide rail 120. Guide rail 120 extends both to the opposite pulley and to the center of pulley 102. Scanning head 130 is mounted below transverse member 110. As belt 104 is moved by rotation of pulley 102 in direction R, guide pin 106 will move to the opposite end of shaft 108 as the drive assembly 100 passes over the center of pulley 102. Specifically, scanning head 130 moves back and forth in a straight path along guide rail 120 as pin 106 reciprocates back and forth along the length of channel 108 as pin 106 moves around pulley 102. As is shown in Figs. 1 to 3, laser 50 generates a beam 51 which is intercepted by scanning head 35. Light detector 55 received the emission radiation emitted from screen 10, and reflected by scanning head 35, thereby generating a signal representative of the image stored on screen 10 as it is scanned. A variety of optical systems which may be used in conjunction with the present invention are set forth in Figs. 6 to 8. It is to be understood, however, that these optical systems are exemplary, and that other optical systems may also be used, all keeping within the present scope of the invention. As is shown in Fig. 6, scanning head 212 may comprise a reflective mirror 222 which directs the laser beam through a dichroic mirror 224 and a focusing lens 226 such that the laser beam is focused as a point on the surface of phosphor screen 10. A dichroic mirror 224 can be used to separate response radiation emitted by phosphor screen 10 and divert such response radiation towards photomultiplier tube 225. Photomultiplier tube 225 thus provides a signal which can be used in generating a pixel by pixel image of phosphor screen 10. As is shown in Fig. 7, an alternate optical system positions a stationary dichroic mirror 224 to reflect emission radiation into photomultiplier tube 225. by using a single stationary dichroic mirror, (preferably placed adjacent to both laser 220 and photomultiplier tube 225), the weight of scanning head 212 can be minimized. As is shown in Fig. 8, the positions of laser 220 and photomultiplier tube 225 can be reversed, with a light tight tube 227 preventing stray photons outside the wavelength of interest from entering between dichroic mirror 224 and photomultiplier tube 225. In any of the above preferred optical systems, a filter 241, which may comprise a red light blocking filter, may be included, and is preferably positioned between scanning head 212 and photodetector 225, as shown. Filter 241 will preferably permit blue wavelength emitted response radiation beam 221 to pass therethrough, yet prohibit the passage of reflected or scattered red wavelength incident laser therethrough. Optionally as well, a collimating lens 236 can be positioned adjacent laser 220 for producing a collimated laser beam. Using any of the various above described embodiments of the optical train, the laser light beam 221 emitted from laser 220 may preferably have a wavelength of about 635 to 680 nM and a power in the range of 0 to 50 mW. The beam of response radiation will typically have a wavelength centered at about 390 nM. Focussing/collimating lens 236 may comprise a 5 to 15 mm diameter lens with a focal length of 4 to 10mm which will focus the collimated beam of laser light into a beam width of about 25 to 250 microns, and most preferably 30 to 80 microns on the surface of phosphor screen 10. Minimizing the diameter of the incident laser light beam upon the phosphor screen will minimize destructive pre-reading of the image data caused by forward overlap of the focused beam and reflected and scattered laser light. It is to be understood that the foregoing wavelengths, powers and sizes are merely exemplary and that other wavelengths, powers and sizes may also be used.

Claims: WHAT IS CLAIMED IS: 1. A system for scanning an imaging plate, comprising: a continuous belt drive; a drive assembly connected to the continuous belt drive; a scanning head connected to the drive assembly; and a laser positioned to direct a laser beam in a beam path across the surface of the imaging plate, wherein the drive assembly is adapted to move the scanning head back and forth in a path which is collinear with the beam path as the continuous belt drive is rotated in one direction. 2. The system of claim 1, wherein the imaging plate is a phosphor screen. 3. The system of claim 1, wherein the drive assembly comprises: a drive member attached to the continuous belt drive; a drive member guide, wherein the drive member is adapted to slide back and forth along the drive member guide, such that the drive member guide moves back and forth across the imaging plate as the drive member moves around the continuous belt drive. 4. The system of claim 3, wherein the scanning head is mounted to the drive member guide. 5. The system of claim 3, wherein the drive member is slidably received over the drive member guide. 6. The system of claim 3, wherein the drive member slides along a groove in the drive member guide. 7. The system of claim 3, further comprising: a guide rail, wherein the scanning head is adapted to slide back and forth along the guide rail such that the scanning head moves back and forth across the imaging plate in a path collmear with the laser beam path. 8. The system of claim 7, further comprising: a pair of springs disposed at opposite ends of the guide rail. 9. The system of claim 7, further comprising: a pair of voice coils disposed at opposite ends of the guide rail. 10. The system of claim 1, further comprising: a pair of pulleys adapted to rotate the continuous belt drive. 11. The system of claim 1 , wherein, the continuous belt drive is dimensioned such that a straight portion of the belt drive spans fully across the imaging plate. 12. The system of claim 1, wherein the scanning head comprises: a mirror for directing a beam of incident laser light towards the imaging plate. 13. The system of claim 1, wherein the scanning head comprises: a dichroic mirror for separating incident laser light from response radiation emitted by the imaging plate. 14. The system of claim 13, further comprising: a photodetector, wherein the dichroic mirror directs the incident laser light towards the imaging plate, and directs the response radiation towards the photodetector. 15. The system of claim 14, further comprising: at least one red wavelength blocking filter positioned between the photodetector and the dichroic mirror. 16. The system of claim 1, wherein the scanning head is adapted to direct the laser beam to the surface of the imaging plate and direct response radiation emitted by the imaging plate toward the photodetector. 17. The system of claim 1, wherein the scanning head comprises: a focusing lens for focusing incident laser light on the imaging plate. 18. The system of claim 17, wherein the focusing lens focuses a laser beam to a diameter of 25 to 250 microns. 19 . The system of claim 17, wherein the focusing lens focuses a laser beam to a diameter of 50 to 80 microns. 20. The system of claim 1, further comprising: a transport mechanism adapted to move the imaging plate in a direction perpendicular to the straight path across the imaging plate. 21. A method of scanning an imaging plate, comprising: directing a laser beam in a beam path across the surface of the imaging plate; rotating a continuous belt drive, wherein the continuous belt drive is connected to a drive assembly which is adapted to move a scanning head back and forth across the surface of the imaging plate in a path which is collinear with the beam path as the continuous belt drive is rotated in one direction. 22. The method of claim 21 , wherein the imaging plate is a phosphor screen. 23. The method of claim 21 , wherein the drive assembly comprises: a drive member attached to the continuous belt drive; a drive member guide, wherein the drive member is adapted to slide along the drive member guide, such that the drive member guide moves back and forth across the imaging plate as the drive member moves around the continuous belt drive. 24. The method of claim 23, wherein the scanning head is mounted to the drive member guide. 25. The method of claim 21 , wherein the continuous belt drive is wrapped around at least two pulleys. 26. The method of claim 21 , wherein, the scanning head is moved back and forth along a guide rail such that the scanning head moves back and forth across the imaging plate in a path collinear with the laser beam path. 27 . The method of claim 21, further comprising: reflecting the laser beam onto the surface of the imaging plate with a minor disposed in the scanning head. 28. The method of claim 21, further comprising: directing response radiation emitted by the imaging plate towards a photodetector. 29. The method of claim 28, wherein directing response radiation emitted by the imaging plate towards a photodetector comprises: separating the incident laser light from the response radiation with a dichroic minor. 30. The method of claim 29, wherein the dichroic mirror is disposed in the scanning head. 31. The method of claim 29, wherein the dichroic minor is disposed adjacent the laser. 32. The method of claim 28, further comprising: blocking red wavelength light with a blocking filter positioned between the photodetector and the dichroic mirror. 33. The method of claim 26, further comprising: reversing the direction of movement of the scanning head at opposite ends of the guide rail with a pair of springs disposed at the opposite ends of the guide rail. 34. The method of claim 26, further comprising: reversing the direction of movement of the scanning head at opposite ends of the guide rail with a pair of voice coils disposed at the opposite ends of the guide rail. 35. The method of claim 21, further comprising: advancing the imaging plate in a direction perpendicular to the beam path.

Inventors: CANTU GARY, EVANS WAYNE, LEWIS TODD, CANTU, GARY, EVANS, WAYNE, LEWIS, TODD
Applicants: PHORMAX CORP, PHORMAX CORPORATION
Application year: 1999
Main classification: G03B42
IPC codes: G01T1, G03B42, G06T1, G21K4, H04N1