Abstract:
Medical images from x-rays or the like are enhanced by characterizing pixels of the image as to the type of underlying tissue and selectively applying image enhancement techniques only to particular tissue types.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
     BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to medical imaging systems and in particular to an imaging system that sharpens portions of an image based on a determination of the underlying tissue type.  
         [0002]     Diagnostic images of the lateral spine, for example, using dual energy x-ray, may be used to assess the presence of spinal fractures incident to osteoporosis and other bone diseases. A vertebra with upper and lower surfaces that are wedge shaped, concave, or compressed together may have experienced a fracture.  
         [0003]     Often the edges of the vertebra are indistinct in the image. Sharpening filters, operating on the data underlying the image, may be used to highlight the edges of the vertebrae but will also highlight features in soft tissue around the bone such as the diaphragm, organs, ribs, abdominal gas, and other distracting tissue structures.  
       SUMMARY OF THE INVENTION  
       [0004]     The present invention provides a method of sharpening only selected tissue types in a medical diagnostic image. In this way, for example, the bone image may be sharpened without generating distracting artifacts in the surrounding soft tissue. Alternatively, features in soft tissue may be sharpened without accentuating surrounding bone.  
         [0005]     The invention is particularly suited to dual energy x-ray images which may automatically characterize image data based on tissue types, but the invention may also be applied to other imaging modalities where tissue type may be approximately identified. The user may manually adjust the regions automatically identified. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0006]      FIG. 1  is a simplified perspective view of a bone densitometer such as may collect dual energy x-ray attenuation measurements over a region of a patient supported on a patient table;  
         [0007]      FIG. 2  is an example lateral bone scan taken by the densitometer of  FIG. 1  showing the vertebral column surrounded by soft tissue and further showing a paintbrush cursor and a virtual slider control;  
         [0008]      FIG. 3  is a histogram sorting the data underlying the image of  FIG. 2  showing a bi-modality such as may be used to identify tissue types; and  
         [0009]      FIG. 4  is a block diagram showing the steps of a computer program implementing the present invention in the densitometer of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0010]     Referring now to  FIG. 1 , a dual energy x-ray densitometer  10  may include a patient table  12  for supporting a patient (not shown) near a C-arm  14  having two horizontally extending arms, one positioned above and one positioned below the surface of the patient table  12 . The lower arm of the C-arm  14  supports an x-ray source  16  providing for two energies of x-rays in an upwardly directed beam passing through the patient table  12  and patient to be received by an x-ray detector  18  mounted on the upper arm of the C-arm  14 . Both the x-ray detector  18  and x-ray source  16  are mounted for scanning across the patient to obtain attenuation information through the patient at two energy levels.  
         [0011]     Control of the scanning motion and operation of the x-ray detector  18  and x-ray source  16  of the x-ray densitometer  10  is provided by a computer system  20  executing a stored control program stored in computer memory. The computer system  20  includes generally a screen  22 , a cursor control device  24  (such as a mouse), and a keyboard  26  as are well understood in the art. An x-ray densitometer  10  as described above, and suitable for use with the present invention, is commercially available from General Electric Company of the United States under the trade name Prodigy.  
         [0012]     Referring now also to  FIG. 2 , the x-ray densitometer of  FIG. 1  may be used to produce an image  30  such as may be displayed on the screen  22 . As shown, an example image  30  may be that of laterally viewed lumbar vertebrae  34  such as form a portion of the spinal column taken with the patient lying on his or her side on the patient table  12 . Per conventional practice, this image  30  is developed by conducting a scan of the x-ray detector  18  and x-ray source  16  to collect attenuation measurements along a number of vertical rays through a volume of the patient. The attenuation measurements along with the location of the rays is stored in the memory of the computer system  20  where this data is mapped to pixels  32  together forming the image  30 . The location of each pixel  32  within the image  30  corresponds to the location of the underlying ray and the brightness of each pixel  32  is a function of the attenuation measurements at that location. When two attenuation measurements are associated with each pixel  32 , as is the case in an x-ray densitometer  10 , the brightness of the pixels  32  may be determined from a simple average of the attenuation values or other mathematical combination, or from either the high energy attenuation or the low energy attenuation value.  
         [0013]     Referring to  FIG. 3 , the attenuation measurements underlying each of the pixels  32  may be sorted by a program executed on computer system  20  to develop a histogram  36  indicating the number of pixels  32  at each “pixel value”. For the case of the dual energy x-ray densitometer  10  of  FIG. 1 , two attenuation measurements (one for high energy and one for low energy) are associated with each pixel  32  and the pixel value forming the horizontal axis of the histogram  36  may be, for example, a ratio of attenuation attributable to bone to attenuation attributable to soft tissue.  
         [0014]     The histogram  36  in this case will show multiple modes  38  and  38 ′ corresponding to different tissue types (e.g. bone and soft tissue). A threshold value  40  may be established separating the modes  38 ′ and  38 , for example, by finding a local minima within an empirically established range and used to sort each pixel  32  of  FIG. 2  into one of two tissue types of bone and soft tissue depending on whether it is above or below the threshold value  40 . The threshold value  40  may be adjustable by the user and the empirically established range may be determined for each particular x-ray densitometer  10  based on its calibration and studies of patients.  
         [0015]     According to the sorting with the threshold value  40 , each pixel  32  of the image  30  is tagged in the memory of the computer system  20  with its tissue type to generate a bone pixel set  44  having attenuation caused principally by bone and a soft tissue pixel set  46  having attenuation caused principally by soft tissue. These sets may be optionally filtered using a spatial filtering system based on pixel location to provide that each of the bone pixel set  44  and soft tissue pixel set  46  define locally continuous regions uninterrupted by single pixels of the other tissue type. (?)  
         [0016]     Generally, an x-ray densitometer  10  may only distinguish between two types of tissues, however more complex algorithms, for example, those which look at spatial locations of the pixels  32  in addition to attenuation values, may approximate divisions into greater numbers of tissue types as may be used by the present invention or may be used to refine the two tissue characterizations described. The present invention may also find use with other tissue identification techniques and may be used with single energy x-ray systems in which tissue types are deduced from single energy attenuation, for example, in a CT machine or standard x-ray system. While the tissue types of bone and soft tissue are used in this example, clearly other tissue types such as fat and non-fat tissue may be used.  
         [0017]     Referring again to  FIGS. 2 and 3 , the pixels of one of the pixel sets  44  and  46  (in this case soft tissue pixel set  46 ) may be tinted to produce a semi-transparent colored masked area  51  (depicted in  FIG. 2  by cross-hatching) overlaid on otherwise gray-scale pixels  32  to distinguish one pixel set from the other in the image  30 . The particular tissue associated with the masked area  51  may be selected by the user and the masked area  51  may be altered by the user to change the characterization of the underlying pixels  32  irrespective of their pixel values. Specifically, using a menu command, the user may invoke a paintbrush tool  50  movable over the screen  22  by use of the cursor control device  24 . The paintbrush tool  50  may be used to “paint” on additional masked area  51  or to erase masked area  51  to produce additional unmasked area  52  per standard computer graphics techniques. In this way, the user may correct or alter the selection of or sorting of the pixels  32  into the two tissue types particularly if the automatic tissue identification does not pick the area of interest.  
         [0018]     In a preferred embodiment, this masked area  51  may be low-pass filtered to create a “soft mask” eliminating abrupt visual transitions in the final filtered image. For example, in a mask that provides a binary state of 1 for areas included by the mask and 0 for areas excluded by the mask, where the mask is applied by a simple multiplication of pixels of the underlying image times corresponding mask pixels, the mask is filtered to create a transition region at the interface between mask regions of 0 and 1, the transition region having fractional values, the lower the fractional value the less the contribution of the underlying image in the final masked image.  
         [0019]     Referring now to  FIG. 4 , the bone pixel set  44  may be provided to a high-pass filter  48  which accentuates spatially high frequency components of the image  30 , for example, image edges to produce high-pass filtered data  56 . The high-pass filter  48  affects only the image formed from the bone pixel set  44  and thus can be considered as being restricted to the unmasked area  52  as possibly modified by the user. In the example of  FIG. 2 , therefore, the edges of the vertebral column  33  would be emphasized but no emphasis would occur in the soft tissue of masked area  51 . To the extent that the invention is used to thus sharpen the bone soft tissue interface, it can aid in analyzing bone morphology.  
         [0020]     The high-pass filter  48  may be implemented in a number of ways well known to those of ordinary skill in the art including, for example, by taking a derivative of the unmasked area  52  of the image  30  or use of the Fourier transform, a truncation of low frequency data and a reverse Fourier transform of operating on the unmasked area  52  of the image  30 . In a simple embodiment, a low-pass filtered image may be obtained using averaging techniques or the like and subtracted from the unmasked area  52  of the image  30  leaving high-pass filtered data  56 .  
         [0021]     The high-pass filtered data  56  is provided to a multiplier  58  which receives a weighting value x as will be described below. The product of the high-pass filtered data  56  and the weighting value x is provided to an adder  54 .  
         [0022]     The soft tissue pixel set  46  is provided directly to the adder  54 .  
         [0023]     The bone pixel set  44 , prior to high-pass filtering, is also provided to multiplier  62  which receives a weighting value 1−x. The product of the high-pass filtered data  56  and the weighting value 1−x is provided to an adder  54 .  
         [0024]     The adder  54  provides an output  66  which provides new brightness values for pixels  32  to be displayed as an enhanced image on the screen  22  providing improved bone edge enhancement without enhancing features of the soft tissue.  
         [0025]     Referring now also to  FIG. 2 , the amount of edge enhancement in the unmasked area  52  will be a function of the weighting variable x. In the preferred embodiment, this value of weighting variable x is determined by the user through a virtual slider  64  that may be displayed on the screen  22  and manipulated by the cursor control device  24  according to techniques well known in the art. The user in real time may adjust the slider  64  varying x between zero, and 1 where x=0 provides for no edge emphasis and x=1 provides full emphasis to the selected tissue of the unmasked area  52 . Note that the image of the masked area  51 , for example soft tissue, is unaffected by this process. The invention may in this way allow enhancement of selected tissue types without creating distracting artifacts in adjacent different tissue types. The slider  64  allows the user to simply adjust the enhancement amount without complex controls requiring a detailed knowledge of image processing.  
         [0026]     By changing the particular tissue selected in the unmasked area  52 , other areas of the image can be accentuated or de-emphasized including regions including air or artifacts such as metal or the like. In addition, other filter strategies can be applied to the masked area  51 , for example, the soft tissue may be further processed by low pass filtering to decrease its presence in the image or a high-pass filter to control or accentuate its presence in the image. The high-pass filtering may be applied to fat tissue when fat and non-fat tissue are analyzed.  
         [0027]     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.