Abstract:
A viewing apparatus for transparencies or the like masks any faceplate areas not covered by images by generating masks, and adapts the luminance level of the image under study and of other faceplate areas to the optimal viewing conditions required by the observer.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 08/358,603 filed on Dec. 14, 1994, now abandoned which is a continuation of U.S. patent application Ser. No. 07/861,982 filed on Jun. 30, 1992, now U.S. Pat. No. 5,430,964 issued Jul. 11, 1995 which is the national stage of PCT/EP91/00065, filed Dec. 28, 1990 which is a C-I-P of U.S. patent application Ser. No. 07/537,799, filed Dec. 28, 1990, now abandoned. 
     This application is also a continuation-in-part of PCT/IL96/00163 filed Nov. 24, 1996. 
     This application is also a continuation-in-part of PCT/EP95/04693 filed Nov. 27, 1995 (which was filed as U.S. national application Serial No. 08/849,125 filed on Jun. 4, 1997 now U.S. Pat. No. 6,269,565) which claims the benefit of U.S. Provisional application No. 60/007,522 filed Nov. 24, 1995. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a transparency viewing device, more particularly, to apparatus for holding and illuminating X-ray and like transparencies. 
     BACKGROUND OF THE INVENTION 
     Medical X-ray transparencies usually are examined by placing them over the faceplate area of a device commonly referred to as an illuminator (or viewbox). Conventional illuminators normally comprise a box-like structure enclosing fluorescent lighting tubes behind a semi-transparent light diffusing faceplate defining a display area. Commonly, transparencies are retained on a surface of the faceplate by pushing the upper edge of the transparencies under spring-loaded film-holder clips located along the top edge of the faceplate. 
     Standard size illuminators have a faceplate 17 inches high and 14 inches or multiples of 14 inches (i.e. 28 inches or 56 inches) wide. Usually, each 14 inch width of faceplate has its own fluorescent tubes and control switch. Such faceplates enable viewing full size X-ray films which measure 17 inches by 14 inches. In such cases, the sections of the faceplate not covered by transparencies need not be illuminated. This eliminates unnecessary glare from areas outside the transparency. 
     When transparencies smaller than 14 inches by 17 inches are to be examined, they are typically retained on the faceplate area in the same manner as full size transparencies, i.e., suspending them by means of the film-holders along the top of the viewer. This leaves a portion of the faceplate area surrounding the transparencies fully illuminated and the resulting additional glare detracts from the visual perception of the person trying to study the image and assess the information it contains. 
     Often, transparencies contain several very transparent areas, and frequently, radiologists have to examine over-exposed transparencies. In these cases, considerable glare emanates through transparent portions of the transparencies themselves, and from the areas surrounding the transparency. Glare causing portions can especially be found in collimated images, multi-exposure images and in certain anatomical regions, such as for example in a chest image, wherein the abdomen portion of the image is very transparent and may interfere with detecting small lesions in the lower part of the lungs. 
     An important factor in the interpretation of transparencies, is contrast resolution (the ability to discriminate between various levels of light). This ability is determined by Weber&#39;s Law. E. H. Weber found that “the minimum perceptible difference in a stimulus is proportional to the level of the stimulus”. Stated in terms of vision, as formulated by Fechner, δL/L=K (Weber constant); Where δL is the minimal detectable difference in luminance; and L is the luminance, see “Visual Psychophysics”, D. Jameson and L. M. Hurvich (ed.), Berlin, 1972. 
     Accordingly, if the eye is adapted to luminance L, δL is determined. For a radiologist, maximal gray level discrimination is desired. Therefore, the observer&#39;s eye should be adapted to the luminance level of the image under study. In less benign conditions, a person reading an X-ray image will be less able to perceive critical but minor shadings and nuances in the transparency. Moreover, protracted inspection of faceplate areas under less benign conditions involves significant eye strain on the part of the observer. 
     While it is of course feasible for an observer to overlay masking strips on the faceplate area and on the transparency and thus block unwanted and contrast-reducing light passing through the faceplate area, as a practical matter, readers of X-rays rarely resort to such practice. 
     Another important factor in the interpretation of transparencies is the intensity of the backillumination. As is known in radiology psychophysics, lesion delectability is optimized when the Luminance level emerging from the transparency is between 200 and 500 cd/meter 2 , i.e., about 100 nit. 
     Mammograms are among the most challenging x-ray film transparencies to interpret. One difficulty with viewing mammograms is that the densities of the images are relatively high, so that an intense backillumination is required to achieve an optimum acuity of the eye of the observer of the transparency. Further, masking mammograms is difficult and time consuming. For example, the side of the image nearer the chest contains clinically important details even at the edge of the transparency. Masking which overlaps the transparency results in loosing image content, while spacing the masking from the transparency results in glare which might dazzle the reader. Another difficulty with viewing mammograms is that the size of some of the lesions of interest are very small (micro calcifications are 50-150 microns) on one hand and some of the lesions have a low density on the other hand, so that the backillumination must be artifact free. 
     An X-ray viewer is typically suitable for reading a wide variety of transparencies. However, this may be a limitation rather than a benefit for a radiologist who specializes in one particular field. A specializing radiologist may be required to use a viewbox which is not optimal for his particular need and, furthermore, the viewbox may have many complicated features which are not necessary for his work. A mammogram specialist, views films which come only in two sizes: 18×24 cm and 24×30 cm. A nuclear medicine or ultrasound specialist on the other hand views images which are 8×10 inches. 
     This application is a further improvement on a previously filed series of applications for improved display devices for transparencies. These applications, the disclosures, claims, annexes, appendices and drawings of which, if any, are incorporated herein by reference are U.S. Pat. No. 5,430,964 which was filed on Jun. 30, 1992 as U.S. application Ser. No. 07/861,982, a U.S. Provisional application number 60/007,522, entitled “Improved Display Device” filed Nov. 24, 1995 by Inbar, et al., PCT application PCT/EP95/04693 filed Nov. 27, 1995, entitled “Improved Display Device”, published as WO96/17269 and PCT application PCT/EP94/03968, published as WO95/14949. The PCT applications name, inter alia, the United States of America as a designated state. The above referenced applications describe methods of masking, back-illuminating and image processing algorithms generally preferred for the practice of the present invention. Israel patent application 114,911, titled “Backprojection Transparency Viewer”, filed Aug. 11, 1995, and corresponding PCT application PCT/IL96/00026, the disclosures of which are incorporated herein by reference, describe apparatus useful for a backprojection viewbox and which are also preferred for some embodiments of the present invention. U.S. provisional application No. 60/001814 “Transparency Viewing Apparatus”, filed Aug. 1, 1995, its corresponding Israeli application 114,258 of like title filed Jun. 21, 1995, and PCT application PCT/IL96/00023, the disclosures of which are incorporated herein by reference, describe light recycling methods especially suitable for the practice of preferred embodiments of the present invention. 
     SUMMARY OF THE INVENTION 
     It is an object of some aspects of the present invention to provide a viewbox which is dedicated for viewing a specific type of X-ray image, preferably mammograms. 
     It is a further object of some aspect of the present invention to provide a transparency viewer which optimizes viewing conditions for X-ray transparencies. Preferably, the viewer is adapted to viewing mammograms. This adaptation is preferably embodied in at least one of the following ways: 
     (a) providing a backillumination having a high intensity, preferably with variable intensity and/or chromaticity; 
     (b) providing a backillumination having a high uniformity and few artifacts; 
     (c) providing a masking mechanism which provides a high contrast between image carrying portions of the transparency and other parts of the viewing field; 
     (d) providing a masking mechanism which illuminates all of the image carrying portion of the transparency; 
     (e) providing a mechanism for viewing only a slot portion of a single or a pair of Mammograms, preferably, with an even higher intensity level than available when viewing an entire mammogram; and 
     (f) not substantially interfering with the high work load of a radiologist using the viewer by providing automation. 
     Another object of some aspects of the present invention is to provide a high intensity backillumination which is efficient and not overtly power consuming. 
     Yet another object of some aspects of the present invention is to provide simple sensing means which sense the size and placement of transparencies, especially for use in a motorized viewer. 
     There is thus provided, in accordance with a preferred embodiment of the invention, a method of scanning a breast in a mammogram, comprising: 
     determining the location of a nipple in the mammogram; 
     back-illuminating in a rectangular shape a rectangular portion of the mammogram, including the nipple; and 
     rotating the rectangularly shaped backillumination such that the nipple is always back-illuminated. 
     There is further provided, in accordance with a preferred embodiment of the invention a viewbox, comprising: 
     a faceplate adapted for mounting of a transparency thereon; 
     a back-illumination source; 
     a mask-generator which generates an back-illuminated region of interest which scans the transparency; and 
     a control operative to momentarily increase the intensity of back-illumination in the region of interest. 
     Preferably the control comprises a foot-pedal. 
     There is further provided, in accordance with a preferred embodiment of the invention, a viewbox comprising: 
     a light source; 
     a housing defining a first aperture therein, enclosing said light source; 
     a faceplate adapted for holding a transparency thereon; and 
     means for rotating said housing so that light is emitted from the first aperture to scan said transparency. 
     In a preferred embodiment of the invention the first aperture comprises a rectangular slot. 
     Preferably, the housing defines a second aperture, wherein when said housing is rotated such that the second aperture is disposed between the light source and the transparency, substantially the entire transparency is illuminated. 
     Preferably, the viewbox includes means for reducing the intensity of the light source when the transparency is illuminated by light from the first aperture. 
     Preferably the viewbox includes a reflector adjacent to the housing and which reflects light, which is emitted via the aperture not illuminating the transparency, back into the housing. 
     In a preferred embodiment of the invention the viewbox further comprises a masking LCA for masking portions of said emitted light. Preferably the LCA comprises segments having a larger vertical extent than its horizontal extent. 
     Preferably, the viewbox includes a second backillumination source which back-illuminates the transparency. Preferably the viewbox includes a mask generator which modulates light from the second backillumination source for back-illuminating the transparency. 
     There is further provided, in accordance with a preferred embodiment of the invention a motorized viewbox comprising: 
     a faceplate adapted for mounting a transparency thereon; 
     a transparency conveyer, for conveying the transparency from a storage location to the faceplate along a path; 
     only a single sensor located in the path and which generates a first signal at a portion of the conveyer occupied by the transparency and a second signal at a portion of the conveyer not occupied by the transparency; and 
     a patterned back-illuminator which back-illuminates only a portion of the faceplate, responsive the signals generated by the single sensor. 
     Preferably, the sensor is a single sensor. Preferably the single sensor has a rectangular aperture having a significant extent in a direction perpendicular to the path. Preferably the single sensor generates a third signal at a portion of the conveyer occupied by a clear portion of the transparency and wherein the back-illuminator does not back-illuminate a vertical segment of the transparency which corresponds to the clear portion. 
     There is further provided, in accordance with a preferred embodiment of the invention a method of transparency size determination comprising: 
     back-illuminating a faceplate having a transparency mounted thereon at a first intensity; 
     measuring an exit intensity of light exiting the viewbox during the back-illuminating; and 
     determining the transparency size of the transparency responsive to the measured intensity. 
     Preferably the method includes: 
     back-illuminating the faceplate at a second intensity; 
     measuring a second exit intensity of light exiting the viewbox during the back-illuminating at the second intensity; and 
     determining the transparency size of the transparency responsive to the measured intensity and the second measured intensity. 
     There is further provided, in accordance with a preferred embodiment of the invention a transparency holder for holding a conveyed transparency in a motorized viewbox, comprising: 
     means for mounting a transparency thereon; 
     at least one sensor which generates a signal responsive to the size of the transparency; and 
     a guide which guides the insertion of a transparency into a predetermined position in the transparency holder. 
     Preferably the holder is comprised in a belt. 
     There is further provided, in a preferred embodiment of the invention, a viewbox comprising: 
     a faceplate adapted for mounting of a transparency thereon; 
     a back-illumination source; 
     a transparency detector which determines a loci associated with the transparency; and 
     a mask-generator which spatially modulates the back-illumination to illuminate at least a portion of the transparency, responsive to the determined loci, 
     wherein, the transparency detector can only differentiate between two transparency sizes. 
     Preferably the transparency detector detects the distance between the transparency and a second transparency mounted on the faceplate. 
     There is further provided, in accordance with a preferred embodiment of the invention, a viewbox comprising: 
     a faceplate adapted for mounting of a pair of transparencies thereon; 
     a back-illumination source; 
     a transparency detector which determines the size of one of the transparencies and the distance between the pair of transparencies; and 
     a mask-generator which spatially modulates the backillumination to illuminate corresponding portions of each of the transparencies, responsive to the determined size and distance. 
     There is further provided, in accordance with a preferred embodiment of the invention an optical transparency detection clip for a back-illuminated viewbox, comprising: 
     a transparency holder defining a gap therein for grasping a transparency; 
     a light source, on one side of the gap, which transmits light; and 
     an optical sensor, on an opposite side of the gap, which senses light which crosses the gap and generates a signal indicative of the intensity of the sensed light. 
     Preferably, the optical sensor generates a first signal when there is no transparency inserted into the gap and a second signal when there is a transparency inserted into the gap. 
     Additionally or alternatively the optical sensor preferably generates a first signal when there is no transparency inserted into the gap, a second signal when a dark portion of the transparency is inserted into the gap and a third signal when a clear portion of the transparency is inserted into the gap. 
     Preferably the light source conveys light from the backillumination. 
     Preferably the optical sensor is a polarized sensor. 
     There is further provided a clip array comprising a plurality of clips each as described above wherein the clip array generates a signal indicative of a size and a position of a transparency inserted in the clip array. 
     There is further provided, in accordance with a preferred embodiment of the invention a viewbox comprising: 
     a faceplate adapted for mounting of a transparency thereon and having a guide for guiding the transparency into a predetermined mounting position; 
     a back-illumination source; 
     a transparency detector which determines a size associated with the transparency; and 
     a mask-generator which spatially modulates the back-illumination to illuminate at least a portion of the transparency, responsive to the determined size, 
     wherein the faceplate is adapted for mounting a second transparency having a second predetermined mounting position, wherein the guide separates and determines the two predetermined mounting positions. 
     Preferably the mask generator comprises a directly addressed light valve having a first segment corresponding to a first transparency size and a second segment corresponding to a second transparency size. Preferably, the light valve is a segmented LC (liquid crystal). 
     There is further provided, in accordance with a preferred embodiment of the invention a transparency detection clip for a back-illuminated viewbox, comprising: 
     a transparency holder defining a gap therein for grasping a transparency; 
     a first transparency insertion sensor at a first location on the transparency holder; and 
     a second transparency insertion sensor at a second location on the transparency holder, 
     wherein the clip generates three occupancy signals: a first signal when neither of the sensors are occupied, a second signal when both sensors are occupied and a third signal when only one of the sensors is occupied. 
     There is further provide, in accordance with a preferred embodiment of the invention a masking generator for masking backillumination for a transparency, comprising: 
     a plurality of vertical light-valves, comprising at least a first light valve having a first width and at least a second light valve having a second, greater, width, wherein all the light valves have the same length. 
     Preferably the plurality of light valves are vertically segmented along a single line. 
     Preferably the at least a first light valve is associated with mask generation near an edge of the transparency and wherein the second light valve is associated with mask generation near the center of the transparency. 
     In a preferred embodiment of the invention when the transparency has an edge containing clinically important image information, the at least a first light valve is associated with mask generation along the edge containing clinically important image information. 
     Preferably, when the transparency has an underexposed vertical segment, the at least a first light valve is associated with mask generation of the underexposed vertical segment. 
     There is further provided, in accordance with a preferred embodiment of the invention, a motorized viewbox comprising: 
     a faceplate adapted for mounting a transparency thereon; 
     a transparency conveyer, for conveying the transparency from a storage location to the faceplate along a path; 
     at least one sensor located in the conveyer which generates signals indicative of loci of the transparency; and 
     a patterned back-illuminator which back-illuminates only a portion of the faceplate, responsive the signals generated by the at least one sensor. 
     There is further provided, in accordance with a preferred embodiment of the invention, a motorized viewbox comprising: 
     a faceplate adapted for mounting a transparency thereon; 
     a transparency conveyer, for conveying the transparency from a storage location to the faceplate along a path; 
     a sensor which generates a signal indicative of a relative position of the conveyer and the faceplate; and 
     a patterned back-illuminator which back-illuminates only a portion of the faceplate, responsive the signals generated by the sensor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention with regard to the embodiments thereof, reference is made to the accompanying drawings, in which like numerals designate corresponding sections or elements throughout, and in which: 
     FIG. 1 is a front view of a dedicated mammography viewbox according to a preferred embodiment of the invention; 
     FIG. 2 is a partial schematic side view of a viewbox showing preferred locations for an optical sensor in a viewbox according to a preferred embodiment of the invention; 
     FIGS. 3A-B show schematic side views of a position detection clip according to a preferred embodiment of the invention; 
     FIGS. 3C-D show schematic side views of an optical position detection clip according to an alternative preferred embodiment of the invention; 
     FIG. 3E is a schematic bottom view of a position detection clip according to preferred embodiments of the invention; 
     FIG. 3F is an electrical schematic of a resistance type position detector clip according to a preferred embodiment of the invention, such as shown in FIGS. 3A-B; 
     FIGS. 4A and 4B show a preferred embodiment of a guided placement faceplate; 
     FIG. 5A shows, a vertically segmented LCA suitable for masking in a viewbox according to a preferred embodiment of the invention; 
     FIG. 5B shows a variable-resolution LCA suitable for masking in a viewbox according to another preferred embodiment of the invention; 
     FIGS. 6A-C show various preferred regions of interests for viewing mammograms; 
     FIG. 7 is a schematic side view of a projection system for a viewbox according to a preferred embodiment of the invention; 
     FIG. 8A is a schematic side view of a mechanical masking apparatus for a projection viewbox according to a preferred embodiment of the invention; 
     FIG. 8B is a schematic front view of a slotted cylinder, such as shown in FIG. 8A; 
     FIG. 9 is a perspective front view of a motorized viewbox according to a preferred embodiment of the invention; 
     FIG. 10A is a partial front view of two transparencies and an optical sensor for detecting the size and relative placement of the films; 
     FIG. 10B shows an output signal of the sensor of FIG. 10A; 
     FIG. 11A shows a transparency frame having guided placement for a motorized viewbox according to a preferred embodiment of the invention; and 
     FIG. 11B shows a position sensing transparency belt, useful for a motorized viewbox according to a preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As described herein, the present invention employs several sub-systems and encompasses, for some of them, several alternative methods of operation, thus resulting in a large number of permutations. This detailed description illustrates a few such embodiments and variations according to the invention. Other combinations are also useful and fall within the scope of the invention. In particular, although the preferred embodiments may be described in a way which is especially suitable for viewing mammograms, the present invention is also useful for viewing other types of X-ray transparencies. 
     FIG. 1 is a front view of a dedicated mammography viewbox  20  according to a preferred embodiment of the invention. Viewbox  20  is preferably sub-divided into four panels  22 , so that a current and a previous study can be viewed simultaneously. A plurality of transparencies  24  is mounted on faceplate  26 , preferably held by a plurality of clips  28 . Portions of faceplate  26  which are behind image carrying portions of transparencies  24  are preferably back-illuminated so that an operator can interpret images on transparencies  24 . Portions of faceplate  26  which are not covered by transparencies  24  or portions which are covered by transparent portions of transparencies  24  are preferably dark. Preferably, the darkness is achieved by masking backillumination at portions of faceplate  26 , as described below. When so operated, viewbox  20  does not generate glare which might dazzle the operator. 
     Preferably, viewbox  20  controls the ambient lighting and/or the intensity of the backillumination and/or the chromaticity of the backillumination and/or the backillumination of the uncovered portions of faceplate  26 . Thus, viewbox  20  controls and may optimize all of the viewing parameters which may affect the perceived image quality. Preferably, the optimization of the viewing conditions, including at least one of: masking, backillumination level, ambient light level and chromaticity of the backillumination, is in response to the image density of at least a portion of one of transparencies  24 . More preferably, the viewing of a particular region of interest on one or more of transparencies  24  is optimized. 
     The intensity and uniformity of the backillumination of transparencies  24  and the darkness of the non-illuminated portions of faceplate  26  are preferably at least as good as required by the U.S. government, as set forth in the MQSA (mammography quality standards act). 
     In a preferred embodiment of the invention, viewbox  20  automatically detects the transparency size and/or location and controls the backillumination so that faceplate  26  is only back-illuminated at locations covered by transparencies  24 . FIG. 2 shows preferred methods of detecting the location and/or size of transparencies  24 . One preferred method is acquiring an image of faceplate  26  and transparencies  24  using a camera  30 ″ mounted on the outside of viewbox  20 . This image is processed to determine the size, location and, preferably, the morphology of transparencies  24 . An alternative location for camera  30 ″ is at location  30 ′, inside of viewbox  20 . Further alternatively, and more preferably, a camera views the reflection of faceplate  26  in a mirror  30 . 
     Alternatively or additionally, the existence and/or size and/or location of transparencies  24  is detected by a position sensing clip  28 . FIG. 3E is a front view of clip  28 . A plurality of sensors  33  are arranged along clip  28 , wherein a first part of each sensor  33  is a portion  34 , generally in one plane of clip  28  and a second part of each sensor  33  is a portion  36 , generally in a second plane of clip  28 . When transparency  24  is inserted in clip  28 , a plurality of sensor portions  34 ′ are disconnected from their corresponding sensor portions  36 ′. This disconnection can be sensed in many ways, including the following: 
     (a) Sensor  33  may be a capacitance sensor which senses the changes in capacitance caused by the displacement of portion  34 ′ from portion  36 ′ and/or by the imposition of film material having a dielectric constant much higher than that of air (transparency  24 ). 
     (b) Sensor  33  may be a resistance sensor which senses the absence of contact between portions  34 ′ and their corresponding portions  36 ′. A side view of a resistance sensor is shown in FIG. 3A (open position) and in FIG. 3B (closed position). In this embodiment sensor portions  34  and  36  are electrodes. The current source for this resistance sensor preferably has a high impedance output, so that the quality of contact between sensor portion (electrode)  34  and sensor portion (electrode)  36  is not important. FIG. 3F is an electrical schematic of a resistance sensor which incorporates a plurality of sensors  33 . A control unit  37  sequentially connects one of a plurality of current sources  35  to an electrode ( 34 ). A single common electrode ( 36 ) is used for all of the source electrodes. The sensed current is amplified by a current amplifier  39  and passed to a discriminator  41  which decides if there is an electrical contact. A detection logic  43  outputs the state of clip  28 , based on the scanning sequence which logic  43  receives from control unit  37 . Preferably, the back of clip  28  is a PCB board incorporating sensor portions (electrodes)  36 , while sensor portions (electrodes)  34  are mounted on leaf springs, which are connected to the PCB board. 
     (c) Sensor  33  may be an optical sensor. FIG. 3C and 3D show such an embodiment where portion  34  is a light source and portion  36  is a light sensitive element. The light sensitive element is preferably solid state device. The light source is preferably an unmasked portion of the backillumination, which may be guided to behind clip  28 . Alternatively, the light source may be a solid state light source or a light pipe. 
     An optical sensor is preferred since it not only senses the imposition of transparency  24  (even an unexposed portion of X-ray film attenuates light) but also detects the difference between exposed and unexposed portions of transparency  24 . Thus, if a vertical strip of transparency  24  is unexposed (as is often the case in Mammograms), the masking can be configured mask the unexposed vertical strip. Preferably, the light source is a polarized light source, preferably by virtue of a polarizer  38 . Using polarized light enhances the sensitivity of sensor  33  to the imposition of transparency  24 , as transparency  24  typically has a retardent effect on polarized light. The major retardation axis is usually parallel to one of the transparency edges. Thus, the polarization is preferably at 45 degrees to the longitudinal direction of clip  28 . Sensor element  36  preferably has a polarizing input either parallel to or perpendicular to the polarization axis of polarizer  38 . In a case where no transparency is placed between light source  34  and sensor element  36  the amount of light detected by sensor  38  will be low (if the polarizers are perpendicular). If a transparency portion is placed between, the polarization of the light from source  36  will be affected by the bi-refringent properties of the transparency and a substantial amount of light will be detected by sensor element  33 . The precise amount of detected light depends on the density of the transparency. A high density transparency will block most of the light, while a low density transparency will pass most of the light. A further discussion regarding the effect of a transparency on a polarizing sensor (albeit not in direct contact therewith) and various configurations of a polarizing sensor are described in WO96/17269, cited above. 
     (d) Sensor  33  may be an ultrasonic sensor comprising a transmitter and a receiver, wherein the amount of transmission is affected by the imposition of transparency  24 . 
     It should be appreciated that there are only two standard sizes of x-ray mammography transparencies so the signal processing associated with detecting the film sizes and locations is relatively simple. 
     In a further preferred embodiment of the invention, the placement of transparencies  24  is guided into predetermined positions. Thus, a single sensor is sufficient to determine the transparency size of each transparency. A second sensor may be used to determine the existence of said transparencies. FIG. 4A shows a preferred guidance method. A guide  40  vertically bisects faceplate  26  and clip  28 . A left mammogram  24 ′ is inserted in the general direction  42 , so that it is slid along guide  40  and into clip  28 . A right mammogram  24 ″ is inserted in the general direction  44 , so that it is slid along guide  40  and into clip  28 . FIG. 4B shows a top view of the guidance method shown in FIG.  4 A. 
     A first sensor  48  is preferably placed at a distance X from guide  40 , so that, also a smaller sized transparency will be detected by sensor  48 . A second sensor  46  is preferably placed at a distance Y (&gt;X) from guide  40 , so that only a larger sized transparency will be detected by sensor  46 . Thus, both the existence and the size of transparency  24  can be determined. In a preferred embodiment of the invention, sensors  46  and  48  are combined into a single resistive sensor having three states. If there is a transparency at sensor  46  but not at sensor  48 , an electrical resistance between the location of sensor  46  and sensor  48  is measured if there is a transparency also at sensor  48 , the sensor is open, so an infinite resistance is measured and if there is no transparency, the sensor is short circuited, so a zero resistance is measured. 
     Since the backillumination of the mammogram must illuminate transparency  24  up to the edge at its chest side, a soft mask is preferably used at this edge. Soft masking is a mask which does not have a well defined edge, rather, the intensity of the backillumination gradually falls along a direction normal to the edge. As a result, there is a vertical strip along the chest side of transparency  24  which has sufficient backillumination for identifying details, but not so much backillumination through uncovered portions of faceplate  26  that might dazzle the operator. The width of the soft masking is generally the required precision of the masking in general, since there is a wider margin for error on the other side of transparency  24 . Thus, in a preferred embodiment: of the invention using guided placement, soft masking may be dispensed with. It should be appreciated, that the masking, described below, preferably incorporates a diffuser, so that the edges of the mask have a soft transition of, preferably, about 1-2 millimeters. 
     In a preferred embodiment of the invention, the masking of the backillumination is achieved by generating a substantially uniform backillumination and then masking portions thereof using at least one layer of a liquid crystal array (LCA). The incorporated documents referred to above, and especially WO96/17269, describe several preferred ways of generating backillumination and a masking thereof. However, in a dedicated mammogram viewer there are some additional preferred embodiments. 
     FIG. 5A shows a segmented vertical LCA  50 , useful in a preferred embodiment of the invention. LCA  50  may be used instead of one or both of the LCA layers suggested in the above reference applications, depending on the desired contrast ratios, on the necessity of soft masking and on the method of producing the slot, described below. LCA  50  is composed of a plurality of vertical LC elements, each of which are segmented into several, preferably two, segments. A first plurality of segments  52  are the length of the smaller size of mammography film and a second plurality of segments  54  are the length of the difference in lengths between the smaller size of mammography film and the larger size of mammography film. Preferably, segments  52  and segments  54  are independently controlled, so that each of segments  52  and segments  54  is a one dimensional LC array. As can be appreciated such an array can achieve high contrast ratios using direct or active or passive addressing. However, a very high contrast ratio can be achieved by using a passive 1×2 driving scheme as described in PCT/EP95/04693. 
     FIG. 5B shows an LCA  56  which has a non-constant horizontal resolution and which is used in another preferred embodiment of the invention. Two generally low resolution portions  58  are located behind where a pair of transparencies  24  is expected to be placed and a generally high resolution portion  60  is located behind where the chest sides of transparencies  24  are expected to be placed. Thus, a high resolution of masking at the chest side of the transparencies can be achieved, without requiring a high resolution throughout the entire LCA  56 . Preferably, a third portion  62  has a medium resolution behind where the outer edges of transparencies  24  are expected to be. Thus, a transparent vertical strip in transparencies  24  can be easily masked. 
     It should be appreciated that in a viewbox which guides the placement of transparencies  24 , such as described above, the entire masking LCA can be divided into two portions for activation purposes. A first portion includes the portion of the LCA which is behind two smaller mammograms and a second portion which includes the portion of the LCA which is covered by the larger size of mammograms but not by the smaller size. Thus, the LCA is actually a direct addressing two element LC, in which very high contrast ratios can be achieved. 
     It should be appreciated that other masking methods, such as mechanical masking methods, as are well known in the art, can also be used to mask transparencies  24 . 
     In a preferred embodiment of the invention, the operator&#39;s finger or other control means can be used to indicate locations on transparencies  24  to a controller (not shown) in viewbox  20 . This is particularly useful in a computer aided diagnostic station. In such a station, a digitized mammogram is displayed on a monitor and a corresponding transparency is mounted on viewbox  20 . The operator can point to a portion of the transparency, prompting the diagnosing computer to display suggestions, such as suspected lesions and a comparison with a previous study. Alternatively or additionally, a computer analyzes the digitized mammogram (which need not be shown) to detect clinically interesting portions and/or suspected lesions which are then highlighted by viewbox  20 , for example, as described below with respect to ROIs (region of interest). Additionally or alternatively, such reflexive pointing (operator to computer and back) can be used between, preferably registered, images of similar or dissimilar modalities. Finger detection methods and reflexive pointing methods are further set forth in WO96/17269, in WO93/01564 and in PCT/EP94/03791, published as WO95/14950, the disclosures of which are incorporated herein by reference. 
     FIG. 6A shows one preferred method of viewing mammograms, in which a horizontal slot  64  scans the image from top to bottom. Preferably, the intensity of backillumination outside of slot  64  is substantially zero. 
     FIG. 6B shows another preferred method of viewing mammograms. In this method, an ROI  66  is used to highlight a portion of the image. 
     FIG. 6C shows yet another preferred method of viewing mammograms. In this method a slot of lighted area  68  extends through a nipple  70 , and scans the mammogram radially as the slot rotates around the nipple as a pivot. PCT publication WO96/17269 describes methods of image processing which can be used to determine the image features of the mammogram and to generate ROIs which back-illuminate substantially only a portion of transparency  24 . In particular, these methods can be used to determine the location of nipple  70 , since nipple  70  is both the outermost portion of the image and forms the apex for a general triangular shape of the image. In addition, nipple  70  serves as a useful reference point in the image, therefore for reference purposes, the distance and direction from nipple  70  to a detected lesion are preferably measured and noted. 
     The methods of FIGS. 6A and 6C are preferably semi-automatic, with the operator indicating when to start scanning and when to stop. Typically, both left and right mammograms are scanned simultaneously at the same relative positions. 
     In a preferred embodiment of the invention, the intensity of the backillumination can be momentarily intensified by operator command, such as by pressing a foot pedal (See FIG.  9 ). In many cases, the higher intensity level improves the reader&#39;s visual acuity. This intensification is useful to replace the manually positioned (or external) spotlight used by many radiologists today when a high density region is encountered. 
     Generally, it is difficult to achieve high backillumination intensities using florescent lamps. Thus, in a preferred embodiment of the invention, further described in PCT application PCT/IL96/00026, a backprojection illumination system is used. 
     FIG. 7 shows; one preferred embodiment of such a backprojection system. Light from a light source  72 , preferably a metal-halide lamp, is concentrated by a backreflector  74 . The light is projected onto faceplate  26  using a lens  80 . Masking of the light is preferably performed using at least one of the two masking methods: 
     (a) converting the light to a substantially parallel beam using condenser  76 , imposing a masking pattern on the light beam using an LCA  78  and projecting the patterned light beam onto faceplate  26 ; and/or 
     (b) imposing a pattern on the projected light beam using at least one face LCA  82 . 
     Projected backillumination is also preferred due to the ease of mechanically masking and recycling light source  72 . FIG. 8A shows apparatus for generating a scanning slot, as shown in FIG.  6 A. Light source  72  is encased in a slotted cylinder  84 . Cylinder  84  preferably has a reflective interior, so that light which does not exit the cylinder is recycled. A first slot  86  in cylinder  84  is a narrow slot, so that a light cone  92 , exiting though slot  86  creates backillumination similar to that shown in FIG.  6 A. Preferably, a second slot  88  is also formed in cylinder  84  so that it is possible to back-illuminate the entire faceplate  26  by rotating cylinder  84  so that slot  88  faces the faceplate. It should be appreciated that since slot  86  is typically narrower than slot  88 , the intensity of light exiting slot  86  is typically higher than the intensity of light exiting slot  88 , as most of the light is recycled by cylinder  84 , rather than absorbed. As a result, it may be desired to reduce the intensity of light source  72  when using slot  86 , such as by reducing the voltage to source  72  or using other means of reducing light well known in the art. Preferably, cylinder  84  is backed by a reflector  90  which recycles light which escapes through one slot when the other slot is in use. 
     Typically, light cone  92  is wider than necessary for back-illuminating slot  64  (FIG. 6 a ). Typically, a wider light cone is desired if the orientation of cylinder  84  is not precisely known and/or if there are artifacts at the edges of light cone  92 . Thus, in a preferred embodiment of the invention, LCA  82  is activated at least at portions  94  to mask portions of light cone  92 , so that the resulting backillumination of faceplate  26  is limited to a smaller area  96  and there is no illumination “leakage” to undesired locations. Alternatively or additionally, light cone  92  can be masked by LCA  82  to produce highlighted portions shaped differently than slot  86 . Preferably, LCA  82  has vertical segments to mask transparent portions of the transparencies and/or spaces between the transparencies. 
     FIG. 8B shows a front view of cylinder  84 , also showing a preferred method of rotating cylinder  84 . A stepper motor  98  is preferably used to rotate cylinder  84  between rotational positions. A controller  98   a , which controls motor  98 , is preferably operative to: 
     (a) switch between full-field illumination and slot illumination by rotating cylinder  84 ; 
     (b) position cylinder  84 , under viewbox control, so that a particular slot  64  is back-illuminated and/or scanned; and 
     (c) provide an indication of the rotational position of cylinder  84 . 
     In a preferred embodiment of the invention, a slotted cylinder, such as cylinder  84 , is used as a slot scanner or an ROI highlighter in combination with a direct backillumination system, such as florescent lighting. In such an embodiment, cylinder  84  is preferably located between the direct backillumination system and the faceplate or at an upper or lower edge of the direct backillumination system. When cylinder  84  is incorporated into a backprojection system, cylinder  84  can back-illuminate the faceplate either directly or through the backprojection optics, without obstructing the direct backillumination. 
     It should be appreciated that a light recycling/forming element other than a cylinder may be used. For example, a sphere with a square hole formed therein can project a square ROI. 
     Other methods of light recycling can also be used in a backprojection system. For example in a two LCA mask generator, if the masking LCA nearer light source  72  is a PDLC (polymer dispersed LC), light which is not transmitted through the LCA is reflected back towards light source  72  and backreflector  74 . 
     It should be appreciated that light recycling is desirable in most types of viewboxes, including those which are not projection-back-illuminated. For example, a PDLC layer nearer the light source in a florescent-back-illuminated viewbox also yields significant amounts of light recycling. 
     FIG. 9 shows a motorized viewbox  100  according to another preferred embodiment of the invention. There are two main types of motorized viewboxes. One type uses a lateral belt to transport transparencies  24  in lateral direction  102  from a storage location to faceplate  126 . Another type uses a vertical belt to transport transparencies  24  in a vertical direction from the storage location to faceplate  126 . Transparencies  24  may be mounted either on clips or in pockets of a transparent belt. 
     A camera can be used to detect the size, location and/or morphology of transparencies  24  on faceplate  126 , as described WO96/17269. However, in a preferred embodiment of the invention, a single optical sensor is used to determine both transparency size and lateral placement on faceplate  126 . FIG. 10A is a partial schematic view of two transparencies  24  which are laterally transported to faceplate  126  using a belt  106 . A sensor  107  is located so that both of transparencies  24  must pass sensor  127  during their transport. Sensor  107  preferably comprises two portions, one light emitting and one light detecting; transparencies  24  pass between the two portions of sensor  107 . Alternatively, the light source for sensor  107  is the backillumination of viewbox  100 . FIG. 10B shows the output signal of sensor  107  when transparencies  24  are transported past it (in an example where the signal intensity is positively related to the amount of optical obstruction of sensor  107 ). A low output signal  108  corresponds to the times when there is no transparency disposed between the portions of sensor  107  (no obstruction). A medium output signal  110  corresponds to the times when there is a transparent portion of transparency  24  between the portions of sensor  107  (some obstruction) and a high output signal  112  corresponds to dark portions of transparencies  24  (most amount of obstruction). The length  114  of the transparency corresponds to one of the transparency dimensions, thus, the transparency size can be determined from this value. The location of transparencies  24  on faceplate  126  can be determined from a known position of belt  106  relative to faceplate  106 . 
     In a preferred embodiment of the invention, sensor  107  comprises a polarized light source, which can more easily detect unexposed transparency portions, as described above. 
     In a preferred embodiment of the invention, sensor  107  has a rectangular aperture having a significant extent in a direct perpendicular to the movement of belt  106 , to improve the quality of detection. For example, by averaging small changes in the transmission of light through the transparency. 
     Alternatively to using an optical sensor, a resistance sensor, as described above, may be used to determine the extent of the transparency during its passage of sensor  107 . 
     In an alternative preferred embodiment, a vertical transport system is used and the size and locations of transparencies  24  are determined using an imaging bar  116  (shown in FIG.  9 ). 
     It should be appreciated that if the placement of transparencies  24  into the transport belt is guided, the size of the transparency can be determined using a simple binary logic and, typically, only one sensor, such as described above with reference to FIG.  4 A. In addition, the distance between the transparencies can be preset so that the width of the masked portion therebetween is known. FIG. 11A shows a transparency holder  130  having guided placement according to a preferred embodiment of the invention. Holder  130  is adapted to hold two transparency sizes, a transparency  138  and a transparency  139 . Clips  132 ,  134  and  136  are located along holder  130  such that transparency  138  can only be mounted in one stable position, namely, at the left of holder  130 . Transparency  139  is of the same size as holder  130 , so that only one placement option is available for this size transparency. 
     In should be appreciated, that the use of soft masking, as described above, can compensate for imprecise transparency placement, so that the placement guidance does not have to be very precise. 
     FIG. 11B shows transparencies  24  are mounted onto a belt  140  for transport, according to a preferred embodiment of the invention. Belt  140  contains position a plurality of sensing elements  33 , as described above with reference to clip  28 . In addition, the relative positions of belt  140  and faceplate  126  are known, such as by using an optical position encoder. Thus, when belt  140  is brought onto faceplate  126 , the sizes of transparencies  24  and the positions of transparencies  24  (relative to the belt) can be determined as described above with reference to clip  28 . A desired masking pattern for faceplate  126  can then be determined using the determined sizes and position of the transparencies and the known positional relationship between belt  140  and faceplate  126 . It should be appreciated that such a mechanism can be used when the movement of belt  140  is manual, so that an exact position of transparency corners cannot be expected. 
     An alternative method of determining transparency size is to modulate the intensity of the back illumination and sense the amount of emitted light using an light sensor. Since there are only two film sizes it is relatively simple to determine which film size is mounted on faceplate  126  from the effect of the light modulation on the amount of light acquired by the light sensor. 
     Although various embodiments, forms and modifications have been shown, described and illustrated above in some detail in accordance with the invention, it will be understood that the descriptions and illustrations are offered merely by way of examples, and that the invention is not limited thereto but encompasses all variations and alternatives falling within the scope of the appended claims and is to be limited in scope only by the appended claims.