Abstract:
The present invention provides an apparatus and method permitting one sheet of radiographic film to be exposed multiple times including a control unit for correcting the optical density of the film based upon the spatial variation of x-ray field intensity. In particular, the present invention includes a system and method for determining an optical density of radiographic imaging film utilizing a mask for absorbing soft x-ray radiation thereby creating a reference exposure, and further utilizing template to execute a number of sequential exposures of radiographic imaging film such that the radiographic film is selectively irradiated. The control unit is adapted for deriving a corrective optical density value based upon reference measurements, and correcting the series of template exposures using the corrective optical density value.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/284,682 filed on Apr. 18, 2001. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to the field of radiographic imaging, and in particular, to methods and apparatus for determining the optical density of radiographic imaging film and further enacting corrective adjustments to the optical density of radiographic imaging film used, for example, in mammography.  
           [0004]    2. Description of Prior Art  
           [0005]    Radiographic imaging of body parts is well known and extremely useful as a diagnostic tool in the medical arts. Radiographic imaging involves positioning a part of a patient to be imaged denoted as the “structure of interest” under an x-ray emitter, exposing the structure of interest to an x-ray beam, and recording the interaction between the x-ray beam and the structure of interest on an image receptor. In most cases, the receptor is radiographic film, and the image is amplified by an intensifying screen. After exposing the structure of interest, the film is removed from the cassette and then developed.  
           [0006]    The diagnostic value of radiographic imaging as described is dependent upon the interplay of several factors. One of the most important of these factors is the optical density of the radiographic film. In order to ensure optimal film exposure, typical imaging devices implement an automatic exposure control system (AEC). A typical AEC utilizes a radiation detector for detecting the amount of radiation. The radiation detector is generally disposed adjacent to the radiographic film such that the radiation impinges upon the detector prior to irradiating the radiographic film. In general, the typical energy per photon of the x-ray is sufficient to render the detector transparent or invisible on the radiographic film. Thereafter, the AEC utilizes algorithms and lookup tables to account for the varying properties of the intensifying screen, the radiographic film, and the processor. The AEC then makes the necessary adjustments to the intensity and duration of the radiation. Thus, the AEC is essentially a closed-loop feedback mechanism for optimizing the quality of radiographic images.  
           [0007]    Although an AEC is generally sufficient for a standard imaging device, mammography machines present additional problems. First and foremost among these problems is that the typical energy per photon of the x-radiation emitted by a mammography machine is far less than that of other types of imaging devices. Mammography machines emit so-called “soft” x-rays that are optimal for detecting subtle differences in the soft tissue found in the human breast. However, soft x-rays present a limitation in that they cannot be transmitted through the radiation detector without imaging parts of the detector on the radiographic film. Thus, the detector must be disposed such that the radiographic film is irradiated prior to detection of the amount of radiation impinging upon the film. As a result, the detector will detect amounts of radiation that can be significantly less than what has irradiated the film. Consequently, the reliability of an AEC is slightly compromised when used in a mammography machine.  
           [0008]    Consequently, each mammography machine must have the AEC system frequently calibrated and adjusted on site by a service engineer. The process of calibration is tedious and expensive for many reasons. First, each cassette is slightly different, and thus, the same cassette must be used for multiple exposures. Secondly, radiographic films must be used from the same box to maximize the homogeneity of the film. Thirdly, significant variations can arise in the processor including chemical variation and temperature variation.  
           [0009]    Thus, there is a need in the art for an apparatus and method that permit one sheet of radiographic film to be exposed multiple times including a simple and reliable method for determining the optical density of the film. However, such a solution must account for the decrease in the intensity of the x-ray field with an increase in distance from the source, known as the inverse square law. Optical density is also dependent upon the intensity of the x-ray field, such that there is additionally a need for an apparatus and method that compensates optical density measurements for changes due to the spatial variation.  
         SUMMARY OF THE PRESENT INVENTION  
         [0010]    The present invention provides an apparatus and method permitting one sheet of radiographic film to be exposed multiple times including a control unit for correcting the optical density of the film based upon the spatial variation of x-ray field intensity. In particular, the present invention includes a system for determining an optical density of radiographic imaging film comprising an emitter of soft x-ray radiation, a sheet of radiographic imaging film disposed a distance from the emitter, an attenuator simulating the radiological properties of a human breast, a mask for absorbing soft x-ray radiation disposed between the attenuator and the radiographic imaging film thereby creating a reference exposure, and a template for selectively irradiating the radiographic film.  
           [0011]    The combination of the foregoing elements allows for multiple exposures on a single sheet of radiographic film. The mask defines a divide along its central axis that is coincident with the center portion of the film, thereby allowing the center portion of the film to be exposed during a reference exposure. The mask prohibits the exposure of two portions of the film that are laterally adjacent to the divide. The template is then aligned such that it permits selective irradiation of the portion of the film that was previously unexposed, and prohibits exposure of film that is yet to be exposed. The template is then moved such that previously unexposed film may be exposed, and again, the template prohibits exposure of film that is yet to be exposed. The foregoing process is repeated until there are enough sequential exposures to warrant processing the film. The template defines  
           [0012]    The method of the present invention includes a step of irradiating a single sheet of radiographic film while simultaneously absorbing portions of radiation with a mask thereby preventing exposure of portions of the film. The mask defines a divide along its central portion, which allows the center of the film to be irradiated while preventing other portions from being irradiated. The unexposed portions of the film is selectively exposed multiple times utilizing a template having apertures for transmitting x-radiation such that film exposure occurs at various distances from the center of the film. The multiply and selectively exposed film is interpreted by a densitometer that measures the optical density of the film. The optical density data is corrected by the controller, which corrects the optical density measurements based upon the relationship between the distance between the point of interest and the center of the film.  
           [0013]    The above and further objects of the present invention will more fully be apparent from the following detailed description with accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is an exploded perspective view of the present invention in schematic representation depicting the orientation of the elements.  
         [0015]    [0015]FIG. 2 is a schematic depiction of the components of the present invention including the relevant dimensions of the present invention.  
         [0016]    [0016]FIG. 3 depicts a masking element of the type used in the present invention.  
         [0017]    [0017]FIG. 4 depicts a template of the type used in the present invention.  
         [0018]    [0018]FIG. 5 is a schematic view of a sheet of radiological film that has been shielded by a mask and been exposed multiple times while employing a template to form a grid of exposures along two portions of film adjacent to the center of the film.  
         [0019]    [0019]FIG. 6 is a flow chart illustrating the method of the present invention.  
         [0020]    [0020]FIG. 7 is a flow chart illustrating the method by which a controller determines a corrective optical density. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]    In accordance with the preferred embodiment of the present invention, FIG. 1 is an exploded schematic diagram of the elements of the present invention. An emitter of soft x-ray radiation  10  of the type commonly used in mammography applications is utilized as a source for x-ray beams (not shown). Emitter  10  is disposed a distance from a surface of a sheet of radiographic film  28  that is normal to axis  8 . Radiographic film  28  is of the type that is sensitive to x-ray radiation and, as such, will absorb incident x-ray beams thereby forming an image indicative of the optical density of radiographic film  28 , as discussed further herein.  
         [0022]    An attenuator  18  is disposed between the emitter  10  and the radiographic film  28  such that x-ray beams are transmitted through attenuator  18  prior to irradiating radiographic film  28 . Attenuator  18  is a homogeneous material, such as BR-12 or acrylic plastic, which simulates the density and radiological properties of the human breast.  
         [0023]    A mask  22  and a template  21  are sequentially disposed between attenuator  18  and radiographic film  28 . Mask  22  and template absorb x-ray beams thereby preventing x-ray beams from irradiating portions of radiographic film  28  and permitting multiple exposures of the radiographic film. The specific designs of mask  22  and template  21  are discussed further herein.  
         [0024]    A table  30  is commonly used as a support structure for a mammography machine. In a typical mammography machine, the table  30  will house a cassette (not shown) for housing radiographic film  28  and will be utilized by medical staff to ensure the proper placement and alignment of the structure of interest. In FIG. 1, the table  30  is shown as supporting radiographic film  28 .  
         [0025]    Multiply exposed radiographic film  28  is generally processed in a processor  32 , which is adapted for processing the radiographic film  28  such that an image of the structure of interest is shown in gray scale.  
         [0026]    A densitometer  24  of the type commonly used in mammography application is utilized for detecting levels of a gray scale formed upon radiographic film  28 , and is further adapted for determining an optical density of radiographic film  28 .  
         [0027]    A controller  26  is utilized for processing optical density data determined by the densitometer  24 . The controller  26  is adapted for correcting the optical density data and determining a corrective optical density as discussed further herein.  
         [0028]    [0028]FIG. 2 depicts the relevant geometrical dimensions of the present invention. The distance from the emitter  10  to the radiographic film  28  is r o . An x-ray beam  12  emitted along axis  8  will travel distance r o  and orthogonally impinge upon radiographic film  28  at a point x o . X-ray beam  16  is emitted from emitter  10  at an angle θ from axis  8 , and will impinge upon radiographic film  28  at an angle equal to the difference between ninety degrees and θ. The point at which x-ray beam  16  impinges upon radiographic film  28  is a distance x from the point x o . In determining a corrected optical density, the controller will arbitrarily assign the point x o  a value of zero. Additionally, FIG. 2 depicts attenuator  18  as having a thickness t o  through which x-ray beams  12 ,  16  must pass before impinging upon radiographic film  28 .  
         [0029]    [0029]FIG. 3 is a preferred profile of the mask  22  that is disposed between the attenuator  18  and the radiographic film  28  for absorbing x-ray radiation. The mask  22  defines a center portion  51 , two lateral portions  50 , and a divide  52 . Center portion  51  and lateral portions  50  prohibit the transmission of x-ray radiation, thereby preventing exposure of portions of radiographic film coincident therewith. Divide  52  permits the transmission of x-ray radiation, and radiographic film coincident therewith may be exposed.  
         [0030]    [0030]FIG. 4 is a preferred profile of the template  21  that is selectively disposed between attenuator  18  and radiographic film  28  in order to selectively permit irradiation of radiological film  28 . Template  21  defines body portion  55  that prohibits the transmission of x-ray radiation thereby preventing exposure of portions of radiographic film coincident therewith. Template  21  further defines a pair of apertures  56  disposed within body portion  55  that permit the transmission of x-ray radiation. Template  21  further defines an open aperture  54  that is square in shape and that has one side coincident with an edge of the template  21 , thereby forming an open aperture.  
         [0031]    [0031]FIG. 5 is a schematic view of a sheet of radiographic film  28  having an image  20  irradiated on it by x-rays and having been exposed multiple times utilizing the mask  22  and template  21  of the present invention. An axis  6  is shown perpendicular to an axis  7 . Axes  6  and  7  intersect at a point where axis  8  intersects radiographic film. In a typical mammography machine, the shoulders of a patient are perpendicular to vertical axis  6  and parallel to horizontal axis  7 .  
         [0032]    The fringe area  40  of the radiographic film  28  has been exposed multiple times, and is generally not considered in the calibration procedure or in measuring optical density.  
         [0033]    Reference areas include a central portion  42   a , a lateral portion  42   b , and a lateral portion  42   c . Reference areas  42   a ,  42   b , and  42   c  are created in a reference exposure when the mask depicted in FIG. 3 is disposed between attenuator  18  and radiographic film  28  and x-rays are emitted from emitter  10 .  
         [0034]    Template areas  44   a ,  44   b  are adjacent to mask area  42   a  and are generally rectangular in nature such that axis  7  bisects both template areas  44   a ,  44   b . Template areas  44   a ,  44   b  can be further described by referring to a grid pattern. In the preferred embodiment, template areas  44   a ,  44   b  include columns C 1 , C 2 , C 3 , C 4 , C 5 , and C 6  as well as rows R 1 , R 2 , R 3 , and R 4 . Thus, any grid point in template areas  44   a ,  44   b  can be referred to as a combination of a row coordinate and a column coordinate.  
         [0035]    Referring now to FIG. 6, a flow chart of the describing the system and method of the present invention. In step S 1 , the mask  22  is centered on radiographic film  28  such that divide  52  is bisected by axis  6  and such that center portion  51  is parallel to axis  7 . Step S 1  also includes placing the attenuator  18  over mask  22  such that the x-rays are attenuated before irradiating the mask  22  and radiographic film  28 .  
         [0036]    In step S 2 , a reference exposure is executed wherein fringe area  40 , and reference portions  42   a ,  42   b ,  43   c  are initially exposed to x-radiation. Only template areas  44   a ,  44   b  remain unexposed. The reference exposure executed in step S 3  is for calibrating controller  26 . The optical density of reference points located in reference areas  42   a ,  42   b , and  42   c  is utilized by controller  26  to interpolate a corrective optical density.  
         [0037]    In step S 3 , the mask  22  and attenuator  18  are removed.  
         [0038]    In step S 4 , the template  21  is placed over a template area  44   a ,  44   b  and the attenuator  18  is placed thereupon. Step S 5  consists of irradiating radiographic film  28  such that template  21  selectively irradiates the film through apertures  54 ,  56 . In step S 6 , the position of the template  21  is changed relative to template areas  44   a ,  44   b . Step S 6  may then proceed to step S 7  or loop back to step S 5  depending on the outcome of logic gate L 1 . In the preferred embodiment, step S 6  loops back to step S 5  until template areas  44   a ,  44   b  have been repeatedly exposed to provide sufficient data for an optical density test.  
         [0039]    The process of alternating between steps S 6  and S 5  has many embodiments, and the following description is merely illustrative of one of such embodiments requiring eight iterations through logic gate L 1 . In a first S 5  exposure, the template  21  is aligned such that open aperture  54  coincides with the area designated by C 6 , R 1 . In such an alignment, apertures  56  will coincide with the areas designated by C 4 , R 1  and C 2 , R 1  respectively. The exposure of step S 5  is then executed, and step S 6  is performed.  
         [0040]    A representative performance of step S 6  is to move template  21  such that open aperture coincides with area C 6 , R 2 , and further such that apertures  56  coincide with areas C 4 , R 2 , and C 2 , R 2  respectively. The exposure of step S 5  is then executed, and step S 6  is performed.  
         [0041]    Two further iterations of the preceding pattern are performed such that the areas designated by columns C 6 , C 4 , and C 2  and rows R 1 , R 2 , R 3 , and R 4  have all been exposed in a series of four cycles of steps S 5  and S 6 .  
         [0042]    A fifth iteration of the procedure includes rotating template  21  about axis  7  such that open aperture  54  is coincident with area C 1 , R 4 , and further such that apertures  56  are coincident with areas C 3 , R 4  and C 5 , R 4  respectively. Iterations six, seven, and eight of steps S 5  and S 6  are performed by repeatedly moving template  21  parallel to axis  7  such that the areas designated by columns C 1 , C 3 , and C 5  and rows R 1 , R 2 , R 3 , and R 4  are exposed sequentially.  
         [0043]    Upon sufficient exposure of template areas  44   a ,  44   b , logic gate L 1  directs that the radiographic film  28  is processed by film processor  30  as given in step S 7 . Radiographic film  28  has been exposed nine times and requires processing only once. As noted, this is a principal advantage of the present invention.  
         [0044]    In step S 8 , a densitometer  24  is utilized to measure the optical density of the processed film  20 .  
         [0045]    In step S 9 , the optical density data is inputted into controller  26  for correcting. Although many methods of inputting the optical density data are available, in the preferred embodiment, the data is entered manually by a medical physicist or other technician.  
         [0046]    In step S 10 , the controller determines a corrective optical density to correct any aberrations in the optical density data incurred due to the spatial dependence of the x-ray field intensity. In doing so, the controller employs a corrective algorithm corresponding to certain physical and mathematical properties of the mammography process. The controller is adapted to account for the following properties. A distance x is the distance from the point x o  to an exposed point on radiographic film  28 ; r o  is the distance between the emitter  10  and the radiographic film  10 , θ is the angle between the axis  8  and the path of x-ray beam  16 , and t o  is the thickness of attenuator  18 . The attenuation coefficient of attenuator  18  is denoted α. In general, the attenuation coefficient α varies inversely with the emitter potential of the emitter  10  in kilovolts (kV), such that if the kilovoltage of the emitter  10  is increased, then the attenuation coefficient decreases.  
         [0047]    The corrective optical density ΔOD follows mathematically from the relationships between the foregoing properties. In general, the exposure of the radiographic film is related to the energy deposited by the x-ray beams at the radiographic film  28 . The magnitude of the x-ray field strength can be described as a function of the parameter x in the following equation:  
               E        (   x   )       =     A          e       -   α                     t        (   x   )               x   2     +     r   2                   (   1   )                               
 
         [0048]    where t(x)=t o secθ, and A is a constant proportional to the net charge of the emitter  10  measured in milliAmp seconds (mAs).  
         [0049]    The ratio of the x-ray field strength at a point x relative to the point x o  is given by the following equation:  
                 E        (   x   )         E        (   0   )         =       e     -     α   [                  t        (   x   )       -     t   0       ]           1   +       (     x   /     r   o       )     2                 (   2   )                               
 
         [0050]    Assuming that x is much less than r o , equation 2 can be simplified to the relationship shown in the following equation:  
                 E        (   x   )         E        (   0   )         =       e       -       α                   t   o       2              (     x   /     r   o       )     2           1   +       (     x   /     r   o       )     2                 (   3   )                               
 
         [0051]    Note that the foregoing approximation eliminated any need to measure θ, which was the angle between the path of x-ray beam  16  and axis  8 . Further approximation using a Taylor expansion yields the change in electric field relative to the electric field at the point x o , or rather  
                 Δ                 E       E        (   0   )         ≈       [     1   +       α                   t   o       2       ]            (     x     r   o       )     2               (   4   )                               
 
         [0052]    From empirical analysis, it is realized that  
               ∂              OD     =       1.72        (       ∂   mAs     mAs     )       =     1.72        (       ∂   E     E     )                 (   5   )                               
 
         [0053]    Therefore, combining equations (4) and (5) yields the following relationship between ΔOD and the system constraints:  
               Δ                 OD     ≈       -     1.72        [     1   +       α                   t   0       2       ]                (     x     r   0       )     2               (   6   )                               
 
         [0054]    Equation 6 is a quadratic equation in x, or conversely, if x is known, then equation 6 is a quadratic equation for ΔOD. A previously complex formula dependent upon a number of variables is simplified to a quadratic equation that controller  26  may utilize to compute a corrective optical density.  
         [0055]    [0055]FIG. 7 illustrates the method by which controller  26  computes the corrective optical density. Controller  26  fits a minimum of three data points to a parabola by interpolating optical density data measured by the densitometer  24 . In the preferred embodiment, controller  26  will select three data points each from reference areas  42   a ,  42   b , and  42   c  as shown in step S 101 . The data points will vary in space relative to axes  6  and  7  such that at least three columns of columns C 1 , C 2 , C 3 , C 4 , C 5  and C 6  will contain at least three data points.  
         [0056]    Given equation 6, it is understood that the optical density should decrease by a certain amount as the distance from the intersection of axes  6 ,  7 , and  8  increases along axis  7 . Thus, controller sorts data points relative to their spatial orientation and their relative optical density as shown in step S 102 . Controller  26  presumes the quadratic form of equation 6, and interpolates reference curves that fit a set of parabolas as shown in step S 103 . In step S 104 , the controller  26  determines an actual decrease in optical density. The parabolas obey functions that represent the actual ΔOD of the system, which implicitly contains the properties of the attenuation constant, the attenuator thickness, and the distance from the radiographic film  28  to the emitter  10 .  
         [0057]    Once the foregoing calibration is completed, then optical density data from a selection of points within template areas  44   a ,  44   b  may be selected in accordance with step S 105 . In step S 106 , the controller  26  corrects the optical density data from the template areas  44   a ,  44   b  by adding a corrective ΔOD as determined from the calibration procedure. Thereafter, the corrected optical density, having been gathered over a series of exposures as determined in FIG. 6, should ideally be representative of the actual optical density of the radiographic film  28 .  
         [0058]    In calculating a corrective ΔOD, the controller  26  permits a medical physicist or other technician to eliminate the variation of optical density as a function of space. Assuming that the other factors determinative of optical density are constant, any discrepancy between the corrective ΔOD and the reference value determined during calibration may be indicative of machine error. Therefore, the present invention provides an improved system and method for assessing the optical consistency of the mammography machine.  
         [0059]    While specific illustrative embodiments of the system and method of the present invention have been disclosed in the foregoing specification, it is understood that various modifications within the scope of the invention may occur to those skilled in the art. For example, alternative profiles of mask  22  and template  21  may be utilized to execute a greater or lesser number of exposures exposing different reference and template areas of radiographic film  28 . It is intended, therefore, that all such adaptations and modifications should be comprehended as falling within the scope and meaning of the following claims.