Abstract:
An x-ray densitometry system provides improved analysis of bone images taken of a patient over a course of time by comparing the images to deduce positioning errors and/or to correct positioning errors for improved quantitative assessment of the bone.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. application Ser. No. 10/065,109 filed Sep. 18, 2002, now U.S. Pat. No. 6,892,088, and hereby incorporated by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to x-ray bone densitometers for measuring bone health and particularly a densitometer adapted to monitor changes in bone health of a patient over a period of time. 
     X-ray bone densitometers make measurements at two x-ray energies to provide separate attenuation images of two basis materials, typically bone and soft tissue. The bone attenuation image is substantially free from attenuation caused by soft tissue allowing areal bone density (g/cm 2 ) to be accurately determined in vivo for assessments of bone strength and health. The bone attenuation image also provides improved definition of bone outlines, allowing measurements, for example, of bone morphology (e.g., vertebral height) such as may be useful for detecting crush fractures associated with osteoporosis. 
     Normally, such measurements evaluate a bone density within a region of interest (ROI) located within a bone (typically the neck of the femur or body of lower vertebrae) as referenced to one or more landmarks on the bone. 
     Often it is desired to detect changes in particular bones over time or over the course of a treatment. Positioning errors caused by changes in the position of the patient with respect to the densitometer can affect measurements of bone density in an ROI, by changing the apparent location of the landmarks used to locate the ROI and/or by changing the apparent density of the bone within the ROI by foreshortening caused by bone rotation. 
     It may be desired to evaluate localized changes in bone density, for example, in subregions distributed about the bone to detect subtle changes obscured when average bone density in a large area is examined. Detailed comparison of subregions of bone are also hampered by positioning errors which prevent direct comparison of bone images taken at different times. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a computer-assisted bone densitometer in which software assesses changes in positioning of a patient between image acquisitions, alerting the operator to reposition the patient and/or correcting the acquired images for errors caused by mispositioning. Detection and correction of positioning errors allows more diagnostic information to be obtained from the images including not only improved measurements of regions of interest, but also novel measurements that investigate many subregions within the bone. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified perspective view of a bone densitometer performing a posterior-anterior or lateral scan of a patient with a fan beam under the control of a computer; 
         FIG. 2  is a depiction of foreshortening of a femur caused by rotation of the femur about a femur axis; 
         FIG. 3  is histograms of the femur and foreshortened femur of  FIG. 2  showing determination of mispositioning of the patient by histogram area comparison; 
         FIG. 4  is a block diagram showing the processing steps of comparing two bone images taken at different times to determine mispositioning of the patient and to correct distortions in the images for accurate inter-image comparison; 
         FIG. 5  is a schematic representation of a density artifact caused by bone foreshortening; and 
         FIG. 6  is a geometric representation of a bone image pixel having an area caused by warping of the images such as may be compensated for mathematically. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , a bone densitometer  10  includes a patient table  12  providing a horizontal surface for supporting a patient in a supine or lateral position along a longitudinal axis  16 . 
     A C-arm  18  has a lower end positioned beneath the patient table  12  to support an x-ray source  20  and an upper end positioned above the patient table  12  supporting an x-ray detector  22 . The x-ray source  20  and x-ray detector  22  may be moved in a raster pattern  25  so as to trace a series of transverse scans  33  of the patient during which dual energy x-ray data are collected by the x-ray detector  22 . This raster motion is produced by actuators under control of a translation controller  19  according to methods well understood in the art. 
     In the preferred embodiment, the x-ray source  20  provides two x-ray energies and the x-ray detector  22  is a multi-element CZT detector providing for energy discrimination. However, other methods of dual energy measurement including those providing for rotating filter wheels or variations in x-ray tube voltage may also be used. 
     The x-ray source  20  produces a fan beam  24  whose plane is parallel to the longitudinal axis  16 . The raster pattern  25  is adjusted so that there is a slight overlap between successive scan lines of the fan beam  24 . 
     The x-ray source  20 , x-ray detector  22 , and translation controller  19  communicate with, and are under the control of, computer  26  which may include both dedicated circuitry and one or more processors having the ability to execute a stored program, portions of which will be described in detail below. The computer  26  communicates with a terminal  28  including a display  30  and a keyboard  31  and a cursor control device such as a mouse  35  allowing for operator input and the output of text and images to the operator as is well understood in the art. 
     In operating the bone densitometer  10 , the computer  26  will communicate with the translation controller  19  to scan a region of the patient in one or more transverse scans  33  during which a number of scan lines  34  of data will be collected, each with a different ray of the fan beam  24 . These data will include attenuation measurements at two distinct energy levels. At each data point, the two measurements may be combined to produce separate bone and soft tissue images. 
     Referring now also to  FIG. 2 , a bone image  40  associated with a scan of the femur that may be composed of data of a variety of scan lines  34  associated with each of the rays detected by the x-ray detector  22 . Bone density of other skeletal sites (for example, the lower lumbar vertebrae or the forearm) also may be measured. The measurements of each scan line produce measurements at a set of discrete pixels  36  representing an areal bone density along the ray line of that measurement. The bone density may be mapped to a gray scale to present the bone image  40  on the terminal  28  to the operator. 
     In a typical study, images of one or both of two areas are obtained of a scan area  37  of the lower lumbar spine  89  producing bone image  40 , or of scan area  38  of either proximal femur  87  producing bone image  40  shown in  FIG. 4 . 
     Referring still to  FIG. 2 , the densitometer  10  may provide a first bone image  40  taken at a first time and showing the femur  87  in a first position with the neck of the femur  87  generally parallel to the image plane. A second image  40 ′ (indicated by dotted lines) may be obtained at a later time with the femur  87 ′ in a second position rotated as indicated by arrow  50  with the neck of the femur extending out of the image plane and causing a foreshortening  52  in the image  40 ′. 
     A region of interest  54  may be positioned on one or both of these images  40  and  40 ′ for a bone density assessment according to techniques well known in the art. The region of interest  54  may be manually or automatically positioned with respect to landmarks on the femur  87 . 
     Referring now to  FIGS. 2 and 3 , pixels  36  of each of the images  40  and  40 ′ are associated with a bone density value derived from dual energy measurements. These pixels  36  may be collected in corresponding histograms  60  for image  40  and histogram  60 ′ for image  40 ′, each histogram  60  and  60 ′ sorting pixels  36  into vertical bars by density values indicated along the horizontal axis. 
     Such histograms,  60  and  60 ′, allow the separation of pixels  36  into bone pixels within a range  62  and nonbone pixels  36  (soft tissue) within a range  64  typically based on analyses of peaks in the histograms  60  and  60 ′. 
     Detection of Patient Positioning Errors 
     In a first embodiment of the present invention, the areas of the histograms  60  and  60 ′ within the range  62  are compared to give an estimate of the amount of bone in each of the images. The areas may be computed simply by counting the number of pixels in the range  62  for each histogram  60  and  60 ′. Significant difference between these areas indicates a mispositioning of the patient and may be signaled to the operator, for example, in the form of a message stating that the patient may need to be repositioned. 
     Referring now to  FIG. 4  in a second embodiment, a more sophisticated comparison process provides first for an iterative translation  66  of image  40 ′ with respect to image  40  and a rotation  68  of image  40 ′ with respect to image  40  to maximize the correlation between bone pixels  36  in range  62  in the images  40  and  40 ′ as performed by correlation engine  70 . The correlation engine  70 , which may be realized in software, generally tallies the number of bone pixels in range  62  that overlap with bone pixels in image  40  at each iteration. This and the previous tally is used to generate a translation/rotation signal  76  that guides the translation and rotation process, for example, in a hill-climbing algorithm to approach the best alignment. As will be understood to those of ordinary skill in the art, the maximum allowed amount of translation  66  and rotation  68  may be constrained according to predefined limits and the correlation only considers an area of overlap  72  of the images  40  and  40 ′. 
     The correlation engine  70  thus aligns the images within the image plane as closely as possible. When the best alignment is found, the correlation engine may send the amount of translation  66  and rotation  68  as indicated by line  80  to a superposition circuit  82  which receives the two images  40  and  40 ′ and displays them superimposed as offset per display  84 . This display  84  may distinguish areas of overlap by color or brightness with, for example, a first color or brightness  86  applied to areas where there is no overlap and a second color or brightness  88  applied to areas where there is overlap, in this way providing a clear visual indication to an operator of the misalignment of the bones in the images  40  and  40 ′ so as to allow repositioning by the operator of the patient for rescanning image  40 ′. 
     Correction of Patient Positioning Errors 
     Referring still to  FIG. 4 , the shifted image  40 ′ may alternatively be provided to a warping engine  90  as indicated by line  81  which also receives image  40  and warps image  40 ′ without additional translation or rotation to better fit image  40  with respect to the pixels  36  identified as bone. This warping may use standard mesh warping techniques and may be conducted automatically or manually to produce a warp-corrected image  92  transforming image  40 ′ to matching image  40  after translation and rotation have been completed. 
     Referring now to  FIG. 5 , to the extent that the warping serves to correct for rotational foreshortening of the bone in the images  40  and  40 ′, a density correction may be optionally performed. As shown in  FIG. 5 , an object  100  lying generally within the image plane provides an elongate image  102  having a given areal density value. A rotated object  100 ′ of identical dimensions and material produces a shorter image  102 ′ having a greater areal density value. This apparent change in areal density is caused by an increase in traversal length  104  of the x-rays passing through the object  100 ′ as compared to object  100 . 
     Referring now to  FIG. 6 , generally this apparent increase in areal density may be corrected by dividing the density values of image  102 ′ by the change of area effected by the warping engine  90 . Thus, for example, if corner points  108  of a pixel  36  of original image  40 ′ and having area A 1  are expanded to expanded corner points  110  after warping, a new area A 2  may be produced. The density value associated with the pixel  56  may be multiplied by A 1 /A 2  to make this density correction. This correction may be performed by the warping engine  90 . 
     The warp-corrected image  92  from the warping engine  90  may be subtracted from the original image  40  at block  93  to reveal a bone loss/gain display  120  providing not simply average bone density values in a region of interest, but a spatially resolved indication of bone loss or bone gain at different locations in the bone. This imaging requires extremely accurate registration of the images  40  and  40 ′ provided by the present invention. This bone loss/gain display  120  may distinguish areas of bone loss or gain by color or brightness with, for example, a first color or brightness  86  applied to areas where bone has been gained and a second color or brightness  88  applied to areas where bone has been lost, in this way providing a clear visual indication to an operator of changes in the bone over time. 
     In an alternative embodiment, the warp-corrected image  92  having been corrected for rotation and density errors may be forwarded for subsequent processing including measurement of bone density within a predefined region of interest  54  in a display  55  of the corrected image  40 ′. Even without the correction for density errors, the correction of the geometric outlines of the bone can allow more accurate region of interest placement and thus, more accurate longitudinal studies of the patient. 
     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.