Abstract:
A method and system for producing tomosynthetic images of a patient&#39;s breast. An x-ray source that delivers x-rays through a breast immobilized and compressed between a compression paddle and a breast platform and form an image at a digital x-ray receptor panel. Multiple x-ray images are taken as the x-ray source and the receptor move relative to the immobilized breast. In one preferred embodiment, the x-ray source travels from −15° to +15°. The source can travel in an arc around the breast while the receptor travels linearly while remaining parallel and at the same distance from the breast platform. The sets of x-ray image data taken at different angles are combined to form tomosynthetic images that can be viewed in different formats, alone or as an adjunct to conventional mammograms.

Description:
FIELD 
       [0001]    This patent specification is in the field of x-ray imaging of the breast and, more specifically, obtaining and processing x-ray data for tomosynthesis. 
       BACKGROUND 
       [0002]    Breast cancer remains an important threat to women&#39;s health. X-ray mammography currently is the most widely used breast imaging tool for early detection, and is the modality approved by the Food and Drug Administration to screen for breast cancer in women who do not show symptoms of breast disease. A typical mammography system takes a projection image of the compressed breast, using a collimated x-ray source at one side and a film/screen unit at the other side of the breast. In the United States, typically two views are taken of each breast, one from above (cranial-caudal, or CC) and one from the side (mediolateral-oblique, or MLO). The x-ray source is an x-ray tube typically operating at 25-40 kVp, using a molybdenum or rhodium rotating anode with a focal spot of about 0.3-0.4 mm and, in some cases, 0.1 mm or less. An anti-scatter grid between the breast and the film/screen unit reduces the effects of x-ray scatter. The screen converts x-ray energy to visible light to which the film is exposed to record the image. In each view, the breast is compressed to reduce patient motion and also for reasons such as reducing scatter, separating overlapping structures in the breast, making the thickness of the imaged breast more uniform, and providing more uniform x-ray exposure. Currently, flat panel array receptors are replacing the film/screen units in mammography systems. The Selenia™ digital mammography system with such a flat panel x-ray receptor is offered by Lorad, a subsidiary of the assignee Hologic, Inc. of Bedford, Mass. Digital mammography has significant advantages and in time may fully supplant film/screen systems. Additional information regarding digital mammography systems and processes offered by the common assignee can be found at &lt;www.hologic.com&gt;. 
         [0003]    Mammograms, whether from film/screen units or from digital systems, are particularly difficult to read, and the rate of false negatives and false positives is significant. Many advances have been made in recent years in image acquisition and in image processing, but a need still remains to reduce the rates of false negatives and positives, at least in screening mammography. Additional information can be gained through modalities such as CT and MRI, but examination and interpretation time and cost and other factors have limited their use in screening for breast cancer. Ultrasound breast examination has been proposed as an adjunct to x-ray examination, with synthesized ultrasound images of thick slices of the breast as they would appear in the same projection view as an x-ray view displayed together with the x-ray view, and a unit taking both x-ray and ultrasound images has been proposed. See, e.g., Patent Application Publication No. U.S. 2003/0007598 A1 and U.S. Pat. No. 5,983,123. Digital tomosynthesis has been proposed for x-ray breast imaging, and a laboratory unit is believed to have been installed at the Massachusetts General Hospital (more than a year before the filing date hereof), as reported in Wu, Tao, 2002,  Three - Dimensional Mammography Reconstruction Using Low Dose Projection Images,  PhD thesis, Brandeis University, incorporated here by reference. See, also, Patent Application Publication No. 2001/0038681 A1 and PCT application International Publication No. WO 03/020114 A2 published Mar. 13, 2003, both incorporated herein by reference. Digital tomosynthesis in more general contexts also has been proposed. See, e.g., U.S. Pat. Nos. 6,289,235 and 5,051,904, commonly assigned U.S. Pat. No. 4,496,557, and Digital Clinical Reports, Tomosynthesis. GE Brochure 98-5493, 11/98, all incorporated herein by reference. Reference markers can be used in x-ray imaging for purposes such as checking the rotation angle and unwanted shift of center of rotation of an x-ray source and receptor, and fiducial phantoms can be used in 3D angiography to calibrate for irregular scan geometries. See, e.g., U.S. Pat. Nos. 5,051,904, 5,359,637, and 6,289,235, N. Navab, et al.,  Dynamic geometrical calibration for  3 D cerebral angiography,  SPIE Vol. 2708, pp. 361-370, and said PCT published application WO 03/020114 A2, all incorporated by reference here. 
         [0004]    It is believed that no breast tomosynthesis systems are commercially available currently for clinical use in breast imaging, and that improvements in x-ray imaging and tomosynthesis are a desired goal. Accordingly, it is believed that a need remains for improved and practical tomosynthesis mammography. 
       SUMMARY 
       [0005]    In a typical x-ray imaging according to preferred embodiments disclosed in this patent specification, a patient&#39;s breast is immobilized, at the same or lower compression than in conventional mammography, between a breast platform and a compression paddle. The platform and paddle, and the breast between them, in turn are between an x-ray source and a digital imaging receptor. Unlike conventional x-ray mammography, in which typically a breast is imaged from only one angle at a given compression, here the immobilized breast is imaged with x-rays from the source that pass through the breast and impinge on the receptor, from a greater number of different positions of the source and receptor relative to the breast while maintaining the breast immobilized, to derive image data for the respective positions. To do this, the x-ray source moves around the immobilized breast, typically but not necessarily in an arc, and the receptor also moves relative to the breast, but in a motion that allows it to remain substantially parallel to the same plane. The x-ray data taken at each of a number of positions of the receptor relative to the breast is processed to form images where each of a number of the images is formed from image data acquired from two or more of the different positions, e.g., to form tomosynthetic images. The x-ray dose to the breast can be different for the different imaging positions. One or more of the imaging positions can use an x-ray dose comparable to that used for conventional mammography. These positions may be the same or similar to the source/receptor positions for the typical views used in conventional mammography, e.g., the CC view and the MLO view. Fiducial markers can be used to help assess the positions of the x-ray source and x-ray receptor relative to each other and/or the breast being imaged, and for other calibration purposes. The fiducial markers can be integrated with the compression paddle and/or the breast platform, or can be positioned otherwise to serve the intended purpose. The immobilized breast can be imaged at angular positions extending over a selected range around the immobilized breast, for example ±15°, although other ranges can be used in other examples. The motion can be continuous over some or all of the imaging positions, or can be intermittent such that the x-ray source and/or the x-ray receptor stop for taking and image and then move to the next position for the next image. 
         [0006]    An antiscatter grid can be used, positioned between the breast platform and the x-ray receptor while x-ray image data are taken. One example is the grid available from Lorad under the tradename HTC® grid. Alternatively, the image data at some or all of the imaging positions can be taken without an antiscatter grid that is external to the x-ray receptor. Rather than using the currently conventional materials for the x-ray emitting target in an x-ray tube serving as the x-ray source, in one of the preferred embodiments the target is made essentially of Tungsten, to provide x-rays at energies that are believed more suitable for breast x-ray tomosynthesis. Geometries other than an x-ray source that rotates around the immobilized breast and an x-ray receptor that moves relative to the breast and the source but remains in substantially parallel to the same plane, can be used. 
         [0007]    Image data taken at the different imaging positions can be processed to generate tomosynthetic images of selected slices of the breast. The images can be of thin slices, essentially planar sections through the breast, as in CT slices. Alternatively, they can be of thick slices of the breast, e.g., slices that are about 0.5 cm to about 1.0 cm thick, and simulate projection images of slices of that thickness, projected on one or more selected image planes. In one example, the image plane or planes of the thick slice images are the same as those of the typical conventional mammograms, and can be displayed for viewing in appropriate groupings. For example, projection-like tomosynthetic images of thick slices on a plane parallel to that of a conventional CC image can be displayed together with a conventional CC image of a breast, on the same screen or adjacent screens or other displays. Similarly, projection-like tomosynthetic images of thick slices on plane parallel to that of an MLO image can be displayed on the same display or on adjacent displays with a conventional MLO mammogram of the same breast. In the alternative, the image planes of the tomosynthetic images can be at one or more selected angles to the image planes of the typical images used in screening x-ray mammography. 
         [0008]    Tomosynthetic images can be formed using a form of filtered backprojection modified by using a filter function that is significantly different from a conventional ramp filter used in CT technology backprojection. The novel filter function is believed to be uniquely suited to breast tomosynthesis. In particular, in the frequency domain the novel filter function at low spatial frequencies is much steeper than a CT technology ramp function, it is close to an all-pass filter at intermediate frequencies, and at higher frequencies it falls off to suppress fine detail noise. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0009]      FIG. 1  illustrates schematically a front view of an x-ray source, a breast immobilized between a compression paddle and a breast platform, an antiscatter grid and an image receptor, where the x-ray source and the x-ray receptor move relative to the breast for imaging the breast at different angles according to a preferred embodiment. 
           [0010]      FIG. 2  illustrates a side view of the system illustrated in  FIG. 1  and additional components of a system according to a preferred embodiment. 
           [0011]      FIG. 3  illustrates a display arrangement showing both images that are the same or similar to conventional mammograms and tomosynthetic images according to a preferred embodiment. 
           [0012]      FIG. 4  illustrates in graph form a backprojection filter in the frequency domain used in a preferred embodiment, compared with a conventional ramp filter used in CT technology and an all-pass filter. 
           [0013]      FIG. 5  illustrates coordinate values of a backprojection filter according to a preferred embodiment as control point values in the frequency domain in log scale. 
           [0014]      FIG. 6  illustrates the use of fiducial markers. 
           [0015]      FIGS. 7 and 8  are similar to  FIGS. 1 and 2 , respectively, but illustrate another preferred embodiment. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0016]      FIG. 1  schematically illustrates in front view an x-ray source  100  in three different positions  100   a,    100   b  and  100   c,  between which source  100  moves in an arc centered at the top center of a patient&#39;s breast  102  immobilized between a compression paddle  104  and a breast platform  106 . An antiscatter grid  108  is immediately below breast platform  106 , and an x-ray receptor  110  is below grid  108 , and is shown in three different positions,  110   a,    110   b,  and  110   c,  corresponding to the illustrated positions of source  100 . Receptor  110  moves relative to immobilized breast  102  along a path substantially parallel to breast platform  106  and maintains substantially the same distance from platform  106 , as indicated by the arrows in  FIG. 1 . In operation, source  100  moves from one of its illustrated position to another, and so does receptor  110 . At each position of source  100  and receptor  110 , source  100  is energized to emit a collimated x-ray beam, of which only the center ray is illustrated. The beam irradiates breast  102  and some of the radiation that has passed through the breast also passes through grid  108  and is received by receptor  110 . As in known in the art, receptor  110  and associated electronics generate image data in digital form for each pixel of a rectangular grid of pixels at each of the illustrated angular position of source  100  and translation positions of receptor  110  relative to immobilized breast  102 . While only three positions are illustrated in  FIG. 1 , in practice image data is taken at each of a much greater number of positions, for example, at every 1° of an arc of ±15° of source  100  around immobilized breast  102 . The taking of image data can be while source  100  and receptor  110  are stopped at their respective positions, after moving from one position to the next. In a different preferred embodiment, the motion of one or both of source  100  and receptor  110  can be continuous, with a respective set of image data being accumulated over a small increment of continuous motion, say a 0.1° to 0.5° arc of motion of source  100 . These parameters are only an example. Different preferred embodiments can use different ranges of motion of source  100  and receptor  110 , and can use a motion of source  100  that is arcuate and centered at a different point, such as in immobilized breast  102  or at breast platform  106  or at grid  108  or at receptor  110 , or a motion that is not arcuate but is translational or is a combination of different types of motions, such as partly translational and partly rotational. In the most preferred embodiment, the source  100  motion is arcuate and the receptor  110  motion is translational. As described further below, in practice source  100  can be integrated in a C-arm that is the same or similar to the C-arm used in the commercially available Selenia® or MIV mammography systems available from Lorad, and receptor  110  can be the same receptor as used in such commercially available systems but mounted differently so that it translates in a plane substantially parallel to breast platform  106  while maintaining a substantially constant distance from platform  106  rather that rotating as a unit with source  100 , as in Lorad commercially available systems. 
         [0017]      FIG. 2  schematically illustrates a side view of a mammography system using the arrangement of  FIG. 1 . In  FIG. 2 , x-ray source  100  is at one end  200   a  of a C-arm  200  that is supported for selective rotation about an axis  202 , independently of a support  204  for compression paddle  104  and breast platform  106 . Support  204  also selectively rotates about the same axis  202 . The other end  200   b  of C-arm  200  interacts with x-ray receptor  110  through a motion translator schematically illustrated at  206  that translates the rotational motion of the end  200   b  about axis  202  to a substantially translational motion of receptor  110  that substantially maintains the distance of receptor  110  from breast platform  106  while x-ray data is being taken.  FIG. 2  is not to scale, and receptor  110  can be spaced further from member  200   b  than illustrated, to allow more space for motion translator  206 , and also to allow for receptor  110  to be moved selectively further from or closer to breast platform  104  and thus allow for x-ray image magnification. In operation, C-arm  200  and support  204  are rotated to desired angular positions, either manually or by motor drives, patient breast  102  is positioned on platform  106  and is immobilized by bringing paddle  104  toward platform  106  and compressing breast  102 , with typically the same or less force than for a typical conventional mammogram, such as between one to one-third the conventional force. With breast  102  immobilized, and with C-arm at a selected angle relative to a normal to platform  106  and receptor  110 , such as +15°, imaging starts, and a projection image is taken for each of a number of selected angular positions of source  100  while C-arm  200  rotates, continuously or intermittently, through a selected angle, such as an angle of 30°, is from +15° to −15°. Of course, the motion can be in the opposite direction, from −15° to +15°, or can be over a different angular interval, such as over less than a total of 30°, e.g. 25°, 20°, etc., or more than 30°, such as 35°, 40°, etc. Currently, the preferred range is ±115°. A set of image data can be taken at selected angular positions, such as every degree, or every fraction of a degree, or every several degrees of angle. The angular increments between the different positions for sets of image data need not be the same. For example, the increments around 0° can be less than those at the extremes of the angular positions, or vice versa. Currently, the preferred angular increment is 3°. The sets of image data can be taken after an incremental motion from one angular position of source  100  to another, and from one translational position of receptor  110  to another, such that source  100  and receptor  110  are stationary while a set of image data is being taken. Alternatively, one or both of source  100  and receptor  110  can move continuously while sets of image data are being taken, one set for each increment of continuous motion. In the currently preferred embodiment, in the example of continuous motion while taking image data both source  100  and receptor  110  move while image data are being taken. 
         [0018]      FIG. 2  also illustrates schematically an electrical/electronic system  208  that interacts with the components discussed above. System  208  includes a control  210  for selectively energizing and otherwise controlling x-ray source  100 , an arm rotation control  212  for selectively rotating C-arm  200  and support  204 , a breast compression control  214  for selectively moving compression paddle  104  toward and away from breast platform  106 , data readout electronics  216  coupled with x-ray receptor  110  to read out the sets of image data at the respective positions of source  100  and receptor  110  relative to immobilized breast  102 , and an image reconstruction and display unit  218  coupled with data readout electronics  216  to receive the sets of image data from electronics  216  and to process the image data for reconstruction and other purposes and display images. 
         [0019]    For a given position of breast  102 , such as a position that is the same or similar to the CC position for a conventional mammogram, source.  100  and receptor  110  can be positioned relative to immobilized breast  102  such that at the 0° position a center ray of the x-ray beam from source  100  would be substantially normal to receptor breast platform  106  and receptor  110 . For a first set of image data, source  100  is at +(or −) 15° in a preferred example, and is gradually moved, continuously or intermittently to −(or +) 15°, with a set of image data taken every 3°. The angular range and the increment over which data sets are taken can each be selectively set by the operator, depending of characteristics of the breast being imaged and the screening and diagnostic needs, and can be different for different patients or from one to the other breast of the same patient. For example the source can move through angles that range from a fraction to a degree to several degrees from one imaging position to the next. Each set of image data is supplied by image readout  216  for processing at image reconstruction and display unit  218 . Each set of image data can be taken at the same x-ray dose to the breast, and the dose at any one of the different imaging positions can be substantially less than that for a conventional mammogram. The x-ray dose can be substantially the same for each imaging position, but preferably the dose at one of the position, e.g., at or close to the 0° position, is the same or similar to dose for a conventional mammogram while the dose at the each of the other positions is less, preferably much less. Alternatively, the scan can begin with or end with an exposure close to the 0° position at a dose similar to a conventional mammogram, and the rest of the set of image data can be over the angular range with each exposure at an x-ray dose that is substantially less than that for a conventional mammogram. Thus, two types of images can be produced in accordance with the currently preferred embodiment while breast  102  is immobilized in the same position. One type is the same or is at least similar to a conventional mammogram, which can be read and interpreted in the manner familiar to health professionals. The other type is tomosynthetic images reconstructed from the image data and displayed either separately or as an adjunct to the display of the image that is the same or similar to a conventional mammogram. The process described above for one position of breast  102  can be repeated for another position. For example one process can be for a breast position in a manner that is the same or similar to positioning the breast for a conventional CC view, the breast can then be released, the support  204  and C-arm  200  rotated to other angular positions and the breast repositioned in a manner that is the same and similar to the position for an MLO view, and the procedure repeated. 
         [0020]      FIG. 3  illustrates schematically a display according to a preferred embodiment, where  300  is a display that is the same or similar to a display for a conventional CC mammogram of two breasts and  302  is a similar display for a conventional MLO view. Another display  304  is close to display  300  and can display any one or more of several types of tomosynthetic images reconstructed from the image data taken at the different imaging positions. For example, if display  300  shows views of a right breast taken at or close to the 0° position in one imaging sequence of a total rotation of source  100  over the ±15° range, display  304  can show, at the option of the user, any one or more of the following views of the right breast: (1) one or more of the projection views taken at the different imaging positions; (2) one or more of several tomosynthetic views of thick slices of the right breast simulating projection views of the right breast taken at the same or similar angle as the view at display  300 , with the slices having effective thicknesses selected by the user, such as from several mm to something less than the thickness of the immobilized breast, typically from several mm to about 1 cm; (3) one or more tomosynthetic views of thin slices of the right breast, each simulating a slice through the breast in a respective plane parallel to that of the view at display  300 , where each thin slice has an effective thickness of about a mm or less; (4) thin and/or thick slices corresponding to planes that are not parallel to that of the view at display  300 ; (5) a scrolling image through any of the slices discussed above; and (6) a selected combination of the above displays. A similar display  306  can be associated with display  302 , and show a similar selected views related to the images at display  302 .  FIG. 3  illustrates only one example according to preferred embodiments. Other arrangements also are contemplated, such as different windows on the same display for the different images, differently positioned displays, etc. The user can select the type of images for display, the thickness of a thick slice or a thin slice, the orientation of the slices for display, which slices to display where and in what order, whether to display thick or thin slices as static slides or to scroll through slice images as in movie, etc. Controls  306  can be coupled to each of the displays, and can include manual inputs such as a joystick and/or other known interface devices and a suitably programmed computer to allow a user to control the images that are displayed. For example, the controls can include: (1) selecting one or more areas or volumes of interest at one or more of the displayed images and further processing the image data to form and display additional images of the area or volume of interest (for example, if a lesion is identified that is between tomosynthetic thin slices  32  and  37 , the user can manually designate the corresponding area or volume and command the display of the thin slices in that area or volume, and other parameters such as the orientation of the thin slices); (2) identifying the position of an area or region of interest for use in additional procedures such as needle biopsy (for example, the user can designate with a cursor or in some other way a point in or an outline of a suspected lesion in the breast, and the xyz coordinates of the designated point, area, or volume); (3) the user can manually designate the desired thickness, arrangement, and other parameters of slices whose images are to be displayed, to thereby display images of slices that have user-selected thicknesses and other parameters, such as orientation, so that images of slices of different thicknesses and/or orientations can be displayed concurrently; and (4) the user can point to an area of interest in one of the displayed images and the system can automatically display one or more markers at corresponding or at least related locations in one or more of the other images that are being concurrently displayed. 
         [0021]    At each imaging position, receptor  100  generates respective digital values for the pixels in a two-dimensional array. In one example, receptor  110  has a rectangular array of approximately 4096 by 3328 pixels, with a pixel size of approximately  70  micrometers in each of the column and row directions. The image data of a set (for a respective imaging position) can be processed either at the full spatial resolution of receptor  110 , or at a lower effective spatial resolution, e.g., by binning several original pixel value&#39;s into a single, combined pixel value. For example, each set of 2×2 adjacent pixels can be combined into a single respective pixel value, thus achieving an effective spatial resolution of  140  micrometers in each direction. The binning can be in some other combination of original pixels, such as 2×3, and can be done by data readout electronics  216  or image reconstruction and display unit  218  ( FIG. 2 ). 
         [0022]    Image reconstruction is done using backprojection in the spatial domain or in the frequency domain as in CT technology but with a novel filter that differs from the ramp filter (in the frequency domain) used in CT reconstruction. See, e.g., G. Lauritsch, et al.,  A theoretical framework for filtered backprojection in tomosynthesis,  SPIE Medical Imaging Conference, Vol. 3338, 1998, and U.S. Pat. No. 6,442,288, both incorporated here by reference. Referring to  FIG. 4 , the novel filter is represented by graph  400 , while a graph  402  represents a conventional CT ramp filter, and a graph  404  represents an all-pass filter. As seen in  FIG. 4 , the novel filter according to a preferred embodiment rises sharply in amplitude at lower frequencies, between points a and c (in the range indicated at b), as compared with the conventional ramp filter, then levels off at intermediate frequencies to become the same as or close to an all-pass filter between points c and d, and then gradually drops off at higher frequencies. Using the novel filter produces significantly better breast images compared to using a conventional CT technology ramp filter. 
         [0023]      FIG. 5  gives a sense of the values of the low, intermediate and high frequencies discussed in connection with  FIG. 5 . In  FIG. 5 , the frequency axis is plotted on log scale, and typically is related to spatial resolution of a few mm or more, i.e., it affects mainly to overall impression and flatness of the reconstructed image. With suitable suppression of low frequency content through frequency dependent weights defined by the shape of the novel filter, and gradual suppression of high frequency noise, a tomographic image with good image contrast at all frequency scales can be reconstructed and presented in accordance with a preferred embodiment. Preferably, the filtered backprojection reconstruction of tomographic images is carried out in the frequency domain, using well known processes with the novel filter. However, comparable filtered backprojection can be carried out in the spatial domain, as in known in CT technology, using direct convolution of image data with the spatial domain representation of the filter represented in the frequency domain in  FIGS. 4 and 5 . In either the frequency domain or in the spatial domain the novel filter can be applier to the image data to generate filtered data for backprojection, or backprojection can be carried out first, followed by a filtering operation. The reconstruction process preferably uses the direct fan-beam backprojection method known in CT technology, but with the novel filter, although it may be possible to use the image data to simulate sets of parallel beam paths for backprojection. The reconstructed thin slice images form a three-dimensional image of the breast, comprising voxel values for respective incremental volumes. These voxel values can then be combined as known in CT technology to synthesize any selected thin slice or thick slice in any selected orientation, and in any selected projection plane. 
         [0024]    In a preferred embodiment, while each of all or most of the imaging positions uses lower x-ray dose than that of a conventional mammogram, higher KV can be used as compared with a conventional mammogram in order to boost signal levels from receptor  110  and improve signal-to-noise (SNR) ratios. In addition, preferable an x-ray tube with a 
         [0025]    Tungsten target is used to further exploit the advantage of higher kVp imaging of the breast, such as between 25 and 50 kVp with different x-ray beam filtration. A small focal spot, of the order of 1 mm or less, is preferred, although a larger focal spot of several mm can be used. 
         [0026]    In other preferred embodiments, contrast enhanced breast tomosynthesis can be carried out, by obtaining tomosynthetic images as described above before injecting a contrast agent such as Iodine into the patient, then repeating the process after injection, and subtracting images of the pre-injection and post-injection sets. Another preferred embodiment involves time subtraction tomosynthesis, related to subtracting comparable images obtained at different times. Yet another is dual-energy breast tomosynthesis, whereby two tomosynthetic images at low and high energies are acquired and then subtracted, the two images being acquired through a process such as using successive scans at two different x-ray energy ranges or by alternating x-ray pulses of low and high energy to create the two images. Another other preferred embodiment involves obtaining and displaying both x-ray tomosynthetic images of a breast and ultrasound images of the same breast. Computer aided diagnosis, as known to those skilled in the art and as commercially used currently in the United States, can be applied to selected tomosynthetic images generated as described above. 
         [0027]    Fiducial markers are used in preferred embodiments for off-line (without a breast) mechanical positioning and calibration and/or on-line (while imaging a breast with x-rays) image based positioning encoding of moving components. The fiducial markers can be made of a material such as lead or another metal, and can be in the form of points or lines or arrows or crosses, at locations that would be imaged by the x-ray beam at receptor  110  simultaneously with the imaging of a breast  102  but outside the area of the breast image. The fiducial markers are integrated with compression paddle  104 , or they can be at or near breast platform  106 , or they can be supported separately from paddle  104 , for example on a separate member that can be selectively brought into the path of the imaging x-ray beam or taken out of that path, e.g., to take an image that is the same or similar to a conventional mammogram. Different patterns or types of fiducial markers can be provided, for selection by the user.  FIG. 6  illustrates an example of arranging fiducial markers  601 ,  602 ,  603  and  604  relative to an image of breast  102   a  on receptor  110 , as seen in a top plan view when receptor  110  is horizontal and the fiducial markers are integrated in compression paddle  104 . 
         [0028]    An alternative embodiment, illustrated schematically in a front view in  FIG. 7  and in side view in  FIG. 8 , is similar to the embodiment illustrated in  FIGS. 1 and 2  except that receptor  110  is affixed to the end  200   b  of C-arm  200  that is opposite x-ray source  100 . In this embodiment, receptor  110  moves relative to immobilized breast  102  along an arcuate path from one imaging position to another. Because the change in angle between receptor  110  and breast platform is small, it can be disregarded in processing the sets of x-ray image data. Alternatively, a geometric correction known to those skilled in the art can be applied to each set of image data to convert it to interpolated pixel values that would correspond to those that would have been obtained if receptor  110  had been parallel to and at the same distance from platform  106  at all imaging positions. The so corrected sets of image data can then be used in filtered back projections as described above. 
         [0029]    In each of the embodiments of  FIGS. 1-2  and  FIGS. 7-8 , antiscatter grid  108  may be selectively retractable, so that the user may take any selected set of x-ray image data with or without using grid  108 . As in known in the art, grid  108  can be made to move relative to the x-ray beam during the taking of a set of image data. Similarly, the distance between breast platform  106  and receptor  110  can be selectively changed to effect magnification of the breast image recorded at receptor  110 .