Compact transparent sheet digitiser offering angular and positional accuracy of a sheet and uniform sheet motion

Abstract: An apparatus is disclosed for reading out by scanning an image on a transparent sheet (10) such as photographic film in medical applications. The apparatus comprises illumination means (22), scanning means (21) with at least one photosensitive element, and transport means for transporting said sheet between said illumination means and said photosensitive element, wherein said transport means is formed by : means for keeping at least the portion of the sheet which is being illuminated in an upright position ; a planar substantially horizontal sheet support (35) for keeping contact with the bottom edge (19) of said sheet (10), thereby to guide the bottom edge of said sheet in a substantially horizontal plane ; and a carrier (23) arranged for movement in a plane parallel to the sheet support (35) and for abutting against the upstanding rear edge (18) of the sheet, thereby to move the sheet forward over the support (35) by such abutting contact. These arrangements guarantee a uniform slowscanspeed, fixed angular orientations, accurate vertical and transversal positioning during scanning. Vertical pressure means (45), operative on the top edge of the sheet (46) can supplement the force of gravity. Compact design, ease of use and high throughput are improved by a U-shaped guide means.

Description: Field of the invention. The present invention relates to an apparatus for reading out an image on a transparent sheet and is intended in particular for use in medical or graphical applications. Background of the invention. In clinical or private radiology departments, patients are examined by means of non-invasive techniques, such as X-ray imaging, computed tomography, magnetic resonance, ultrasound etc. All these techniques deliver images : directly on transparent photographic film for use in medical imaging ; or computer images, that can be recorded on this type of film too. These films are very useful for the radiologist to examine the pathology in detail. The same film can also be transported to other departments and other physicians. The film can be stored for years and retrieved whenever necessary for comparison with the current situation. Problems arise when the same images must be available at two different locations at the same time for examination, or when the locations are so distant that the transport time becomes critical. The storage organisation and storage area pose also a problem in some hospitals, where by preference the patient carries the films home to keep them. Mostly, these films are even more difficult to retrieve, because the patient has lost them. These problems can be solved by scanning and digitising the films, used for visual examination, directly as they are produced (developed and dry) or before they leave the radiology department, to produce computer images from the films. These computer images can then be stored consistently by electronic means in digital format. Nowadays, a lot of image processing can be done on images in digital format. E.g. in order to compare semi-automatically older medical images on film with currently acquired data, the images on the older films must be made available in digital format as computer images. Reproduction of images from a transparent sheet on other media, is described in the European patent application EP 0 452 570 A1 "Processing and digitizing apparatus for medical radiographic film." by H. A. Primo et al. The conversion of images - on transparent hardcopy media such as medical imaging photographic film - to computer images in digital format, needs the process of digitising the film. Digitising is the process of converting optical densities of an image on a transparent sheet to binary (digital) electrical signals that represent the varying intensities of the transmitted light and thus said densities. The optical densities are measured by transmission of a light beam from a calibrated light source through the transparent sheet and by measuring the amount of light transmitted. The measurement occurs by : one light sensitive element (e.g. a photomultiplier or a similar light detector) or more such elements simultaneously, said elements e.g. organised in a line or regular rectangular matrix, e.g. CCD-arrays (Charge Coupled Devices). Each element senses the light transmitted through a small area of the sheet. The form of said area can be square, rectangular, circular, elliptic or of any other shape. The whole sheet is virtually divided into said small areas, and with every area corresponds an analogue signal that represents the amount of light transmitted through said area. These analogue signals are digitised individually - i.e. each signal is converted by an analogue to digital converter (D/A-converter) - to a limited set of M binary digital signals, representing 0 or 1. These digital signals are called bits. M is mostly in the range of 1 to 16. Each said set of M digital signals represents a digital number in the range of [0, 2 M -1]. The said number represents the amount of light transmitted through the abovementioned small area on the transparent sheet and thus the local optical density of said area. The said number represented by M bits is called pixel or contone pixel, because each pixel has a large range (2 M ) of possible values. Every individually digitised analogue signal results in one individual pixel. These individual pixels are generated sequentially by one or more said D/A-converters. The sequence is dictated among others by the way how the lightsensitive elements are arranged, how the original transparent sheet is illuminated and how the electronics address the said elements. The said sequence of individual pixels is exactly known and can thus be re-arranged in a regular rectangular matrix and as such displayed on a video CRT monitor or recorded by a hardcopy device on a sheet of paper or film. In those forms, the image can be visually inspected by the user. Scanners tend to have problems with the metrical accuracy of the scanned data. This is due to the difficulty to position exactly the transparent sheet to be scanned relative to the scanning system, or - when movement is involved - to keep the slowscan speed constant within narrow tolerances, such that scanning at a fixed rate results in scanning equidistant lines. In most scanning systems known in the art, the relative position can vary due to skew and to uncertainty about the lateral and longitudinal position. Skew is rotation of the sheet around an axis perpendicular to the sheet. The other two factors are due to translations in two orthogonal directions parallel to the plane of the sheet. Another positioning error, namely transversal displacement in a direction perpendicular to the page, can result in defocusing of the material to be scanned, resulting in a degraded spatial resolution. In order to achieve a homogeneous and known distribution of the pixels over the sheet, accurate and stable linear and angular positioning is necessary and every motion, necessary for the digitising process, must be exactly known in time and be stable in time. US-A-4,864,411 discloses an electronic copy board system, including a planar guide member to support a transparent sheet on which image data is written. The guide member houses an elongated light to direct light through the transported sheet towards an array of light detectors. Objects of the invention. It is an object of the invention to provide an apparatus for reading out an image on a transparent sheet, which allows a precise and accurate scanning of an image on such sheet, by keeping the bottom edge of said sheet in contact with a substantially horizontal support, the illuminated portion vertically at a fixed distance from the scanning means, and advancing said sheet forward over said support at a uniform scanspeed. Another object of the present invention is to make the size of a digitiser small, and the output as convenient as the output of a common automatic radiographic film processor. These processors can be coupled to the input of the filmdigitiser of the current invention. Summary of the invention The above mentioned objects are realised by an apparatus having the specific features defined in claim 1. The sheet can be a photographic film e.g. for use in medical imaging, but it can also be any other image bearing transparent sheet e.g. an overlay for use in overhead projection, in which the densities are formed in a non-photographic way. The terms "horizontal" and "vertical" for the purpose of the present disclosure must be understood as substantially horizontal and substantially vertical respectively, such that the ideas of the invention still apply, where verticality is necessary for taking advantage of the gravity and the horizontal motion is perpendicular to the said vertical direction. The purpose of the arrangement is to use the vertically downward directed gravity force and the equal upward reaction caused by the substantially horizontal planar guide means. Said upward force is uniformly distributed over the bottom edge of the sheet, while the gravity force has its origin of force near to the centre - i.e. in the gravity centre - of the sheet. This arrangement causes a stable equilibrium that results in a constant and - within near tolerances - well known orientation of the sheet around an axis perpendicular to the plane of said sheet. Said arrangement also guarantees that the vertical position of the sheet relative to the scanning means is known very accurately, since the horizontal guiding means serves as the linear positional reference in vertical direction. The carrier can be any means abutting to the rear edge of the sheet, pushing said sheet at a known speed over the horizontal guiding means. The carrier itself can be mounted on an endless belt or anything else known in the art to transmit rotational motion of e.g. a stepper motor to a carrier moving linearly during useful operation. The horizontal sheet support can be a planar plate, a rollerbed or anything else known in the art to keep the sheet orientation and vertical displacement fixed. Detailed description of the invention. The invention is described hereinafter by way of example with reference to the accompanying figures wherein : Fig 1 is a perspective view of one embodiment of an apparatus according to the present invention, Fig 2 is a top view of said apparatus, Fig 3 is a top view of the scanning means and illumination means, Fig 4 is a side view of the contact CCD array placement, Fig 5 is a top view of a CCD element and Selfoc in an appropriate housing, Fig 6 is a front view of the digitiser housing, Fig 7 is a back view of the digitiser housing, Fig 8 is a side view of the illumination means. Referring to figure 1, a transparent sheet or photographic film 10 bearing a photographic image is shown during four important phases of the scanning process : during insertion of the film 10 in the digitiser (input rollers 10 and 11 of fig. 6) - as illustrated by the arrow 1, during vertical positioning of the film 10' as shown by arrow 2, during scanning of the film 10" as shown by arrow 3 and at disposal of the film 10"' (arrow 4), e.g. in the output tray 47 (figure 7). The film 10 can be loaded in three manners : manually sheet by sheet, by a rapid-feeder, known in the art such as the film feeding mechanism used in the CURIX CAPACITY PLUS film processor, and automatically as output of a chemical film processor. Film 10 may have been obtained by the processing of an X-ray film, which was exposed to an X-ray image. Suitable processors for carrying out processing are CURIX 460, CURIX HT-530U, CURIX HT-530C, CURIX CAPACITY PLUS, CURIX 260, CURIX 160 and CURIX 60. "CURIX" is a registered trade mark of Agfa-Gevaert N.V. Mortsel Belgium. Figure 6 shows where the film enters the filmdigitiser between rollers 11 and 12. For manual feeding, it's not necessary to take care of the orthogonal orientation or flipping of the film. Mirroring and orthogonal rotation of the scanned images can be done later simply by dedicated hardware means or digital image processing software controlled means respectively. The film is gripped by rollers 11 and 12. One or both are driven by a drive means, not shown in the drawing. Roller 11 is mounted fixed to the apparatus on a rotative axis, not shown, and roller 12 is mounted on an axis parallel to the axis of roller 11, and can move along the plane connecting the axis of roller 12 and roller 11. Pressure means, not shown on the figures, moves the axis of roller 12 towards the axis of roller 11, and causes a pressure force between roller 11 and 12. Once a sheet of film enters between the two rollers 11 and 12, said pressure force is applied from roller 12 to film 10 and from roller 11 to film 10. The friction between roller and film is proportional to said pressure force and moves the film 10 towards the scanning means. Vertical deflecting means, close to the exit of rollers 11 and 12, forces progressively the film to move upward. Once the sheet 10 passed the rollers 11 and 12, the friction force disappears and only gravity acts on the film, thereby causing the film to fall vertically until it reaches position 10' in scanbuffer 24 (Fig. 2). The fall of the film is stopped by a horizontal support means 35. After the film first touches said horizontal support means 35, the gravity and the reaction force of said support means 35 force the bottom edge of the film to be parallel to said support means. Even a film that has been fed with a considerable skew, will have its bottom edge 19 perfectly aligned with the horizontal support means 35. This goal is met thanks to the fact that the input system is totally decoupled from the scanning system after the fall. From this moment on, the film can be seen as a solid body. The film is now in a position in which a carrier 23 can make contact with the rear edge 18 of the film. The arrangement shown in figure 2 forces the film to pass between two glass plates 27 and 28, close to each other, to keep the film in an upright position, parallel to the (X,Y) plane. In figure 1, we define a system with three perpendicular axes X, Y and Z, and rotations around these axes in the directions of arrows w, f, k. Because of the stiffness of the film and the absence of transversal forces with a component parallel to the Z-axis, all forces (i.e. gravity and reaction from support means 35 ; horizontal pushing force from carrier 23 and reaction) act in the vertical (X,Y) plane and won't bend or skew the film, thus behaving as a solid body. A solid body has 6 parameters of positional freedom in the three-dimensional space. Once these parameters are fixed or known, the location of every other point of the solid body in space is known. These parameters are the position (X, Y, Z) and the attitude or three-dimensional orientation (w, f, k). As shown in figure 1, we connect three perpendicular axes to the film such that the film is in the (X,Y) plane. The bottom edge of the film forms the X-axis, the Y-axis is perpendicular to the X-axis and in the plane of the film. The Z axis is perpendicular to the (X,Y) plane, k is the angle of rotation around the Z-axis, f around the Y-axis, w around the X-axis. In the embodiment of this invention, the horizontal guiding means guarantees that the lateral or Y-position of the film is always fixed, also during transport of the film from position 10' to 10'' The horizontal guiding means also guarantees that the orientation k remains fixed during the scanning process, resulting in a correct angular position of the film at any moment. Furthermore, the force of gravity can be supplemented by pressure means 45, that is operative on the top edge 46 of the sheet 10". As illustrated in figure 1, this can be achieved by nondriven rollers, pressed on the top edge by springs, not shown. Pressure means can also be realised by a longitudinal spring, making contact with the top edge 46 of the film. As is illustrated in figure 2, once the film comes to a standstill in position 10' on the horizontal support, it undergoes three motion phases : rapid movement of the front edge 26 towards the illumination means 22 and scanning means 21 ; slow movement passing between illumination means 22 and scanning means 21 while the film 10'' is scanned ; and rapid movement to the output tray 47. The first motion from scanbuffer 24 to scanning means 21 is performed at high speed, so as to improve the total throughput of the digitiser - even for films with a small format - independent from the position where they were deposited by the rollers 11 and 12 in scanbuffer 24. Carrier 23 makes contact with the rear edge 18 of the film 10' and pushes it along the X-axis towards the scanning means 21. The carrier 23 is mounted on an endless belt 17. Carrier 23 on belt 17 is at a height of about 7 inch (ca. 180 mm) above guiding means 35, so as to fit for film sizes between 8'' and 14''. Said belt can occasionally carry two carriers equally distributed over the length of the belt 17, to increase system throughput. Said belt is driven by a stepper motor means, not shown in the drawing. The speed of said motor can be increased or decreased by electronic means. The constant velocity is better guaranteed by use of the described mechanism instead of by using roller pairs, because roller pairs introduce unwanted shocks and thus microbanding effects in the digital image, at least at start of scan and end of scan. At start and end of scanning, the X-position of the film is not a linear function of time, such that the X-position is not known accurately. A detection means 25 senses when the film reaches the reference position, i.e. the position where the front edge of the film reaches the glass plates 27 and 28. The detection means commands the stepper motor to slow down the speed until the film runs at a constant slowscan speed. By the time the front edge 26 of the film 10' reaches the scanning means 21, the film moves at said constant slowscan speed. While the horizontal guiding means keeps Y and k fixed, the carrier 23 guarantees that the position along the X-axis increases uniformly in time. Therefore, the X-position of the film is known very accurately at every moment when a line 14 (figure 1) of densities of the image on the film must be scanned. During the transport of the film 10", scanning and digitising parallel equidistant lines is performed. Therefor, illumination means 22 (figure 2) continuously illuminates the passing sheet. At every point on a vertical line 14 of the film, a light beam incident on the film is partly transmitted through the film and reaches the scanning means 21. The scanning means 21 is further detailed in top view along the Y-axis in figure 3. Scanning means 21 is a sealed module and comprises two contact CCD modules 31 and 32. A side view of these CCD modules along the Z-axis is shown in figure 4. Two CCD modules known in the art, preferably type TCD128AC are butted against each other, to span the maximum filmwidth, being 14'' or 355.6 mm. These are contact CCD's for scanning A4 sized originals (210mm X 297mm). Each CCD module consists of an array of about 3400 CCD elements, each element measuring 1"/400 = 0.0635 mm. This gives a scanresolution of 400 dots per inch in continuous tone. The digitiser must be able to scan images on film having a density range between 0 and 3.2D, with an overall accuracy of at least 0.01D. This quality can be reached with the preferred CCD type. Scanning means 21 (figure 3) further includes Selfoc (Trade Name for selffocusing lens arrays) arrays 33 and 34, to guide and converge the light beams coming from the scanned point on the film 10'' to the CCD modules 31 and 32 respectively. This light guiding means has been preferred over other means known in the art, as to keep the optical path short in a compact design. The sealed module 21 further enables a robust design against shock, vibration and dust. The system can therefore be operated in the field with less care. With CCD array 31, corresponds Selfoc array 33, forming one optical system. The same applies for CCD array 32 and Selfoc array 34. Both optical systems are placed in V-shape, in order to have an overlap of at least one CCD element, at the place where the two CCD arrays are abutted, such that at least one place on the scanline 14 is digitised by two CCD elements. It is necessary to scan one full scanline 14 over the whole width of the sheet at the same moment, and thus from the same optical line 14, for minor deviations in speed of the sheet or internal electronic timing base make the position of the scanline on the sheet unpredictable within the narrow tolerances as the time difference, between two scan operations, grows. The coplanarity and overlap of the two CCD arrays 31 and 32 is done in a controlled environment, before the CCD-module is sealed. The light beam illuminating scanned line 14, is generated by the illumination means 22. This element can have different embodiments. The goal is to produce a planar light beam, perpendicular to the transparent sheet, parallel to the Z-axis. In the preferred embodiment one may start from a point-shaped light source 7 (figure 8), producing a linear light beam 9. This linear light beam is transformed to a planar light beam by optical means 6. In a first preferred embodiment, said optical means 6 consists of a nearly circular glass cylinder, with planar ends 44 and 5, in combination with a light emitting means 7. The glass cylinder 6 has a planar portion 42, parallel to the axis 41 of the cylinder and with edge 43 at end 44. Said planar portion 42 is made knurled for uniform diffraction of the incident light. At one or both ends, a point shaped light source 7 like a halogen lamp e.g. of 400 Watt is installed. The light emitted by this lamp 7 goes partly into the cylinder, is partly reflected by reflection means 13 on the side opposite to the planar end 44, and is partly lost in other directions. Part of the light 9 going into cylinder 6, leaves the cylinder at the other planar end 5. At this end, a reflection means - coated to the planar end 5 - is installed for reflecting leaving light back into the cylinder. In another embodiment (not shown), a second halogen lamp can be installed at end 5, having the same embodiment as the first halogen lamp 7. Inside cylinder 6, lightbeams in different directions incident to the round portions of said cylinder get totally diffracted and further travel into the cylinder until they are incident on planar portion 42 of the cylinder. The light beam is completely dispersed by this portion and part of the light leaves the cylinder as a light beam in a direction 8 perpendicular to the axis of said cylinder ; another part is again totally diffracted against the round portions of cylinder 6. The advantage of a point shaped light source is the ease to filter and eliminate infrared radiation, for which CCD's are sensitive as well. The CCD's can't make selectively the difference between infrared radiation and visible light. Filtering of infrared radiation avoids the transparent optical density to be mixed up with heattransmission of the transparent sheet material. Cooling fins 49 (figure 5) are required in the housing 48 of the sealed module to carry off the heat, since infrared radiation is only partly filtered and because of electrical energy consumption within the individual CCD cells 36. Another embodiment for optical means 6, known in the art, comprises a bundle with optical fibres, having first ends bundled together near a light source, and opposite ends arranged in a linear array. The lightsource can be likewise a halogen lamp or any other type of illumination means that generates electromagnetic waves within the wavelength of visible light (i.e. 300-700 nm). Still further, the illumination means 22 can be formed by a tube-shaped lamp like a TL lamp, producing a uniform light over its length. The light is occasionally reflected by a reflection means placed on the side opposite to the film. The light leaving the cylinder in the preferred embodiment, and going into the direction of the Selfoc lens array 33 and 34, passes the transparent sheet 10'' orthogonally (see Figure 3). By passing said sheet, a very small amount of the light is reflected, and depending on the local optical density of the image on the sheet 10", a certain amount of incident light is absorbed and another part is transmitted. The transmitted part of light is incident to at least one Selfoc array 33 and/or 34. Selfoc arrays 33 and 34 are optical guide means to transport diverging light from one lightsource at a fixed distance D1 from the Selfoc to the opposite side of the lens, converging back again in one point located at 31 (or 32), at a distance D2, which may have a different length from D1, as long as D1+D2 is kept constant for all points. At said distance D2 from said opposite side, the CCD array is placed, capturing all transmitted light that was incident to the Selfoc array. Each individual CCD element 36 (see figure 5) receives the light corresponding to a small portion of the film. The amount of light, incident on the CCD element, gives an analogue electrical signal that can be further processed by electronic means 15 (figure 2), connected to both CCD modules 31 and 32. Said electronic means processes said analogue electrical signals by techniques known in the art of image processing. Some of these techniques are implemented in the HORIZON scanner, marketed by Agfa-Gevaert N.V., Mortsel, Belgium, and can be applied to the abovementioned analogue electrical signals. These signals are converted to a set of digital values, representing a digital image or computer image. Computer images can be electronically stored for a short (seconds) or long (years) period, and if necessary displayed on a video monitor or registered on hardcopy such as another transparent sheet or paper. The accuracy of the distance D1 between the film and the Selfoc array to be equal to the fixed distance D2, can be improved by pressure means 16 (see figure 2), e.g. springs or a longitudinal flexible sheet. As the transparent sheet gets scanned, the detection means 25 detects the rear edge of the film 18. Because the distance between the detection means 25 and the centre line of the scanning means 21 is fixed, moreover the slowscanspeed is also fixed and known and the time T1 needed for the rear edge 18 of the film 10' to reach the centre line of the scanning means 21 is known. After the detection means 25 detects the rear edge 18 of the film 10', the stepper motor causes the sheet 10' to continue the horizontal slowscan motion during a period of time T1. After that time, the image on the film 10' is completely scanned and the third motion can start. In order to keep the size of the digitiser small and the system convenient for combination with film processing apparatus such as the AGFA CURIX system marketed by Agfa-Gevaert N.V., Mortsel, Belgium, the scanned portion of the film is guided through a U-shaped guiding means 29, e.g. formed by two circular cylindrical plates with the same centre. Said guiding means causes the film to be rotated over 180 degrees as it performs the third motion, to reach output tray 47. To start the third motion, two roller pairs - shown in figure 2 - become active. The first roller pair consists of left roller 37 and right roller 38. The second roller pair consists of left roller 39 and right roller 40. Said four rollers are mounted on the U-shaped guiding means 29. The four rollers are activated by a roller drive means not shown, causing a rotation with a linear velocity considerably higher than the slowscanspeed, in order to improve the overall throughput of the system upto five films per minute, films with maximum size of 14"X17". The two rollers of each pair move towards each other, until the film 10" is pressed between them. Once the pressure is built up, the roller pairs cause the film 10" to be transported quickly to film output tray 47, where it can be manually retrieved by the operator. In another embodiment, the direction of the lightbeams is reversed : the scanning means 21 is replaced by a scanning laser lightsource - scanning caused by a rotating polygonal mirror or a galvanometer deflecting a light beam from a steady laser source. The laser light scans vertical line 14 on film 10". The transmitted light is incident on cylinder 6, where planar portion 42 disperses the incident light in all directions, also towards planar ends 5 and 44. Point shaped light source 7 is substituted by a light sensitive element, like a photomultiplier means, that converts an amount of light to an analogue electrical signal, proportional to the amount of light captured by the photomultiplier. The electrical signals generated from said photomultiplier means are converted by electronic means 15 to a digital image as described before. The bottom planar end 5 of the optical means 6 can be reflectively coated, or - in another embodiment - transmit the amount of light incident to the bottom planar end 5 to another photomultiplier means.

Claims: An apparatus for reading out an image on a transparent sheet (10), comprising illumination means (22), at least one photosensitive element (21), and transport means for transporting said sheet between said illumination means and said photosensitive element, wherein said transport means comprises means (27 and 28) for keeping at least the portion of the sheet which is being illuminated in an upright position ; is characterised in that said transport means further comprises : a planar substantially horizontal sheet support (35) for keeping contact with the bottom edge (19) of said sheet (10), thereby to guide the bottom edge of said sheet in a substantially horizontal plane ; and a carrier (23) arranged for movement in a plane parallel to the sheet support (35) and for abutting against the upstanding rear edge (18) of the sheet, thereby to move the sheet forward over the support (35) by such abutting contact. Apparatus according to claim 1, comprising guide means (29) for deflecting the path described by the bottom edge (19) of the sheet. Apparatus according to claim 2, wherein said guide means (29) for deflecting the path is arranged for deflecting the sheet path over 180 degrees. Apparatus according to any of claims 1 to 3, which comprises pressure means (45) which is operative on the top edge (46) of the sheet (10) to supplement the force of gravity. Apparatus according to any of claims 1 to 4, which is arranged for use in medical applications. Apparatus according to claim 5, which is arranged for digitising conventional X-ray films.

Inventors: BOEVE GERARD C O AGFA-GEVAERT, BOEVE, GERARD, BOEVE, GERARD C/O AGFA-GEVAERT N.V.
Applicants: AGFA GEVAERT NV, AGFA-GEVAERT N.V.
Application year: 1993
Main classification: H04N1
IPC codes: H04N1, G06T1