Abstract:
A method for obtaining tomosynthesis images of an object of interest using an imaging system, wherein the imaging system comprises an X-ray source arranged to facing a detector on which the object of interest is positioned. The method comprises acquiring a plurality of projected 2D images of the object of interest in a plurality of orientations identified relative to a perpendicular to the detector, wherein a zero orientation is closest to the perpendicular and applying at least one filter to the acquired projected 2D images to obtain filtered, projection images of the object of interest. The method further comprises determining a reconstruction slice of the object of interest from the backprojection of at least two of the filtered projections, the set of reconstruction slices being the filtered, reconstructed volume of the object of interest, wherein a filter used on the 2D projection images is an adaptive filter.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Embodiments of the present invention related to the general field of tomosynthesis. 
         [0002]    A radiographic image is a projection of an object of interest. It is generally obtained by placing the object between an X-ray emitting source and an X-ray detector, so that the rays reach the detector after passing through the object. The radiographic image is then constructed from the data provided by the detector, and is an image of the object of interest projected onto the detector in the direction of the X-rays. 
         [0003]    In this image, an experienced radiologist is able to distinguish radiological signs indicating a potential problem e.g. microcalcifications or opacities in mammography. 
         [0004]    However, in a 2D projection image, the superimposition of tissues may hide lesions and under no circumstances can their actual position in the object of interest be known, since the radiologist has no information on the position of the radiological sign in the direction of projection. 
         [0005]    Tomosynthesis has recently been developed to meet these problems; it allows a 3D image of an object of interest to be obtained in the form of a series of successive slices. These slices are reconstructed from projections of the object of interest from different angles. To do so, the object of interest is generally placed between an X-ray emitter source and an X-ray detector. The source and/or the detector are mobile so that the direction of the projection of the object onto the detector may vary (in mammography typically over an angle range of the order of 30° or more). 
         [0006]    By looking at the tomosynthesis slices of an object of interest, a practitioner is therefore able not only to detect radiological signs in the object of interest but also to evaluate their 3D position. 
         [0007]    Different techniques for reconstructing 3D images have been proposed allowing good quality reconstructed slices to be obtained with short computing times. 
         [0008]    In particular, techniques using filtered backprojections (FBPs) have recently been proposed in patent US 2011/0150178 for example, or in the article: 
         [0009]    “Generalized Filtered Backprojection Reconstruction in Breast Tomosynthesis”—Bernhard E. H. Claus, Jeffrey W. Eberhard, Andrea Schmitz, Paul Carson, Mitchell Goodsitt &amp; Heang-Ping Chan, International Workshop on Digital Mammography, IWDM 2006, pp 167-174. 
         [0010]    These techniques, in the projection images, particularly make use of high-pass filters which enhance the contrast of the structures. They provide image reconstruction of quality similar to that of the 3D images obtained with reconstruction techniques of iterative type, whilst allowing a limited number of iterations. 
         [0011]    Three objects are illustrated in  FIG. 1 , one GS is of a large size and the two others, PS 1  and PS 2 , correspond to structures of a smaller size, the object PS 1  being more contrasted than the object PS 2 . 
         [0012]    As illustrated in  FIG. 1 , it may happen with these techniques that contrast artefacts can be seen in the reconstructed images obtained, these propagating in the direction corresponding to the scanning of the X-ray beam when the source switches from one acquisition position to another. As can be seen by comparing the artefacts F of the beams generated in the image by the object PS 1  and by the less contrasted object PS 2 , the more an object is contrasted the larger the artefacts and the more they are visible. In addition, the edge of the breast may at times appear to be highlighted which may be confused with an indirect sign of a lesion thereby inducing radiographer error. 
         [0013]    Also, for interventional applications (biopsy, stereotactic surgery, etc.) highly contrasted metallic instruments may cause reconstruction artefacts that it is sought to reduce. 
       SUMMARY OF THE INVENTION 
       [0014]    There is a need for an image reconstruction processing which has particularly short processing times and has further improved image quality. 
         [0015]    Embodiments of the present invention provide a method for processing and displaying tomosynthesis images, and, more particularly, a method which allows both the display of reconstruction slices and the display of two-dimensional images (2D) of the object of interest. 
         [0016]    Embodiments of the present invention can be applied in mammography. 
         [0017]    According to an embodiment of the present invention, there is provided a method for obtaining tomosynthesis images of an object of interest by means of an imaging system. The imaging system comprises an X-ray source arranged facing a detector on which the object of interest is positioned. The method comprises the following steps: acquiring a plurality of 2D projection images of the object of interest in a plurality of orientations identified in relation to the perpendicular to the detector, one so-called zero orientation being the closest to the perpendicular; applying at least one filter to the acquired 2D projection images so as to obtain filtered, projection images of the object of interest; and determining reconstruction slices of the object of interest from the back projection of at least two of the filtered projections, the set of reconstruction slices being the filtered, reconstructed volume of the object of interest. 
         [0018]    In an embodiment, a filter used on the 2D projection images is of an adaptive type. 
         [0019]    In an embodiment, a filter applied to the projection images enhances the amplitude of the high frequencies of an image relative to the low frequencies of this image. 
         [0020]    In an embodiment, a filter applied to the 2D projection images enhances the high frequencies with an amplification factor which varies according to the position of the pixel in the processed projection image. 
         [0021]    In an embodiment, a filter applied to the 2D projection images enhances the high frequencies with an amplification factor which is a function of the local contrast of the processed projection image. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0022]    Other characteristics objectives and advantages of the present invention will become apparent from the following description which is solely illustrative and non-limiting and is to be read with reference to the appended drawings, in which: 
           [0023]      FIG. 1  illustrates the impact of small contrasted structures in an object to be imaged; 
           [0024]      FIGS. 2 and 3  schematically illustrate an imaging system and its acquisition unit according to an embodiment of the present invention; 
           [0025]      FIG. 3  schematically illustrates some operating functions of the imaging system according to an embodiment of the present invention; 
           [0026]      FIG. 4  is a block diagram showing the different steps of processing conforming to an embodiment of the present invention; 
           [0027]      FIGS. 5A and 5B  illustrate an example of an artefact that can be reduced with the proposed method according to an embodiment of the present invention ( FIG. 5A ) and the result obtained with this method ( FIG. 5B ). 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]      FIG. 2  schematically illustrates a medical imaging system  10  for acquiring projected 2D images to reconstruct a 3D image of a region of interest. 
         [0029]    The medical imaging system  10  comprises an image acquisition unit  12 , an image processing unit  13  and a display system  15 . 
         [0030]      FIG. 3  schematically illustrates the acquisition unit  12 . 
         [0031]    The acquisition unit  12  allows a plurality of 2D projections to be obtained of a region of interest O—of an organ—in a patient. The acquisition unit  12  is notably formed by a detector  11  positioned facing an X-ray source  12 . The detector  11  is a digital camera for example. The acquisition unit  12  is an X-ray acquisition system for example, the latter comprising any known means allowing the emitting of X-rays onto the object O and the acquisition of resulting images. 
         [0032]    The display unit  15  can be integrated in the image acquisition unit  12  or the image processing unit  13 , or it can be separate from the acquisition unit  12  and the processing unit  13 . The display unit  15 , for example, is a computer screen, a monitor, flat screen, plasma screen or any type of known commercially available display device. The display unit  15  enables a radiological practitioner to monitor the reconstruction and/or display of the acquired 2D images. 
         [0033]    The processing unit  13  is adapted for implementing a processing method (e.g. the reconstruction of a 3D image from 2D images). The processing unit  13  can be integrated in the acquisition unit  12  or it can be separate from the acquisition unit  12 . The processing unit  13 , for example, is one or more computer(s), processor(s), microcontroller(s), micro-computer(s), programmable logic controller(s), application-specific integrated circuit(s), other programmable circuits or other devices which include a computer such as a work station. 
         [0034]    The processing unit  13  is coupled to memory means  14  which may be integrated in or separate from the processing unit  13 . These memory means may be formed by a hard disk or any other removable storage means (CD-ROM, floppy disc, etc.). These memory means may be used to store a 3D image of the region of the organ visualized as an acquired or processed 2D image. This may be a ROM/RAM memory of the processing unit  13 , a CD-ROM, USB key, a memory of a central server. The processing unit  13  may comprise a read device (not illustrated) e.g. a floppy disc reader or CD-ROM reader, to read the instructions of the processing method (described in the remainder hereof) from an instruction medium (not illustrated) such as a floppy disc or CD-ROM. In an embodiment, the processing unit  13  executes the instructions of the processing method (described in the remainder hereof) that are stored in firmware (not illustrated). 
         [0035]    Acquisition and filtering of the projection images: 
         [0036]      FIG. 4  illustrates different steps to obtain tomosynthesis images. 
         [0037]    At a first step S1, a plurality of projected 2D images  30  are acquired of the object of interest O in a plurality of orientations around the object (scanning between −12° and +12° for example, or between −25° and +25°). 
         [0038]    The method further comprises a step S2 to apply a filter to the acquired, projected 2D images  30  so as to obtain filtered, projection images  40  of the object of interest O. 
         [0039]    To increase the quality of the reconstructed images, the filtering used on the projected pixels is adaptive filtering. 
         [0040]    Different types of adaptive filters can be envisaged, in relation to the types of desired improvements. 
         [0041]    In particular, to reduce blurring which may be generated in projection images by structures of large size (reference can be made to  FIG. 1  in which the object GS of large size generates a large fan-shaped artefact via its back projection), the adaptive filter can be chosen so as to enhance the amplitude of the high frequencies relative to the low frequencies. 
         [0042]    For this purpose, the filtering used may be filtering of “Unsharp masking” type, each pixel p of (x, y) coordinates in the plane of projection being filtered in the following manner: 
         [0000]        p ′( x,y )= LP   σ ( x,y )+α·[ p ( x,y )− LP   σ ( x,y )],
 
         [0000]    where p′(x,y) is the value of this filtered pixel, a being an amplification factor, LP, being a low-pass filter applied to the value of the pixels around the pixel of (x,y) coordinates in the direction corresponding to scanning by the X-ray beams when the source switches from one acquisition position to another, and for example being: 
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         [0000]    where G σ is a Gaussian of standard deviation σ, Win is a parameter defining the size of the neighbourhood it is desired to take into account for the filter around the pixel of (x,y) coordinates. 
         [0043]    With said filter, it is therefore possible to enhance the high frequencies relative to the low frequencies. 
         [0044]    In an embodiment, this low-pass filter can be replaced by a bilateral filter, so as to avoid the effects of under-exposure which may otherwise exist on the edges of highly contrasted structures. 
         [0045]    In addition, the amplification factor may itself be a function of the x, y coordinates of the points in the projection images. It can therefore be adapted to the local contrast of the objects/structures appearing in the processed projection image, in order to attenuate the impact of highly contrasted objects/structures in the reconstructed volume. 
         [0046]    The local contrast in a projected pixel may be given by the difference between the value of the pixel and the value filtered by a low-pass filter for this pixel; in an embodiment, it can be given by the difference between the value of the pixel and the value filtered by a bilateral filter for this pixel. 
         [0047]    In mammography, images may contain some contrast artefacts on the edges of the breast where the skin is thinner. The use of an amplification factor itself varying spatially, allows these effects to be corrected or attenuated. 
         [0048]    For example, in the case of an adaptive low-pass filter, this can be expressed as: 
         [0000]        p′ ( x ,y )= LP   σ ( x, y )+α( x ,y) ·[ p ( x, y )− LP   σ ( x, y )],
 
         [0000]    with 
         [0000]      α( x, y )= f ( p ( x, y )− LP   σ ( x, y )),
 
         [0000]    where f is a function dependent on the local contrast and chosen to optimize the image quality of the reconstructed slices, and where f may depend on local contrast and depend on the distance from the pixel (x,y) to the edge of the breast. 
         [0049]    In addition, the frequency on and after which the amplification is applied may depend on the thickness of the breast, the emission angle of the tube or the target angle thereof 
         [0050]    It will effectively be appreciated that these different parameters also have an effect on reconstruction artefacts. For example, the thicker the breast the greater the impact obtained by backprojection of a structure therein over the entire volume. Lower cut-off frequencies for example are therefore used for large breast thicknesses as well as higher coefficients of amplification. 
         [0051]    The same applies to the angle of emission or target angle of the X-ray tube. For example, the larger these angles are, the greater the increase in the cut-off frequency of the filter used or in the amplification factor. 
         [0052]    Reconstruction of the slices of the 3D image: 
         [0053]    At a step S3, reconstruction slices  50  are determined of the object of interest O. This determination S3 particularly consists of a backprojection of the filtered, projected 2D images  40 . 
         [0054]    This backprojection may in particular be non-linear of the type “Order Statistics Based Backprojection”. In linear backprojection, each voxel of the volume is reconstructed using N data pixels, each pixel being determined by reprojection of the voxel in each of the N projections. In non-linear backprojection, the pixel of maximum intensity among the Ns is not used, which makes it possible to obtain a distinct reduction in replication artefacts caused by the most intense objects. 
         [0055]    It is noted that the reconstruction slices of the object of interest O represent the reconstructed volume of the object of interest O. 
         [0056]    Optionally, at a step S4, the reconstruction slices  50  are re-projected in a given determined direction. 
         [0057]    Examples of such reprojection processing have been described for example in patent application US 2011/0150178. 
         [0058]    This makes it possible to obtain an intermediate 2D image  60  of the object of interest O. 
         [0059]    At a step S5, this then gives a final 2D image of the object of interest by combining the intermediate 2D image  60  with a projection image initially acquired in a given orientation of the source (e.g. the orientation in which the said source emits perpendicular to the plane of the detector). The combination is , in an embodiment, pixel by pixel linear combination. 
         [0060]    The final 2D image thus obtained is then an image similar to a mammography image and can be displayed (step S6). 
         [0061]    In this manner, with one single reconstruction, it is possible to obtain both reconstructed slice images and a 2D image resembling a 2D radiography image of the object of interest. 
         [0062]    As will be appreciated however; steps S4, S5 and S6 are optional. 
         [0063]    It will be noted that the sampling of the images of reconstructed slices can be identical to that of a projection image, the backprojection and reprojection being performed in a single computing step, making it possible substantially to reduce the computing time. 
         [0064]    In an embodiment, the reconstruction of an image of a sectional plane can be made in relation to the ratio between the zoom applied during backprojection and the zoom required for reprojection at one or more given incidences. 
         [0065]    In this manner, a better definition is potentially obtained of some small objects, in particular of microcalcifications for example. 
         [0066]    The different steps just described and in particular the processing steps S2 and S3 (and optionally S4 and S5) are implemented using code instructions of a programme recorded on a medium integrated in the processing unit or associated therewith. 
         [0067]    A two-dimensional image 2D obtained from images of reconstructed slices using a prior art method is illustrated in  FIG. 5A . 
         [0068]    It can be seen that this image,  FIG. 5A , has a sizeable artefact (encircled region in this Figure). 
         [0069]      FIG. 5B  illustrates the same image but obtained using the method which has just been described. As can be seen in this Figure, the artefact has been reduced therein and even eliminated. 
         [0070]    This written description uses examples to disclose the invention, including the preferred embodiments, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural element with insubstantial differences from the literal languages of the claims.