Abstract:
An x-ray densitometry system provides computer assistance to the operator in identifying possible sources of scanning or analysis error through computer review of the acquired data, operator input, and the ultimate diagnostic outputs.

Description:
BACKGROUND OF INVENTION 
     The present invention relates generally to x-ray bone densitometers for measuring bone health and particularly to a bone densitometer providing computer assisted detection of measurement artifacts and operator errors. 
     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. 
     In order to achieve accurate quantitative results from a bone densitometer, the patient must be properly positioned, motionless during the scan, and free from high-density materials such as pins or buttons. For proper analyses of the scanned data, the measurement regions may need to be correctly identified by the operator. 
     if a problem with the scan is not detected promptly, the patient may need to be recalled and scanned again, incurring additional expense and inconvenience. It is also possible that improper scanning may not be recognized at all, producing an erroneous result. 
     SUMMARY OF INVENTION 
     The present invention provides computer-assisted densitometry in which software monitors the steps of acquiring and analyzing the data with the intent of identifying potential positioning and/or analysis errors. This computer assistance provides a backup to the operator or physician review of the measurement, advising them of a possible problem. Computer assistance together with the oversight of the physician or operator may significantly decrease errors in the acquisition and analysis of the data, and decrease errors from any other source that affects the ultimate clinical measurement. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a simplified perspective view of a bone densitometer performing a posteroanterior or lateral scan of a patient with a fan beam under the control of a computer; 
         FIG. 2  is a geometric representation of two successive fan beams in the scanning pattern of  FIG. 1  showing how height of a bone may be determined using shifts in the images produced by the divergent rays of the fan beams; 
         FIG. 3  is a bone image of the lumbar spine such as may be acquired from the apparatus of  FIG. 1  showing its composition from scan lines obtained in the scans of  FIGS. 1 and 2 ; 
         FIG. 4  is a figure similar to that of  FIG. 3  showing a bone image for the proximal femur; 
         FIG. 5  is a plot of attenuation taken along one scan line of  FIG. 3  showing an attenuation peak caused by a metallic foreign object in the proximity of the patient that creates a density artifact; 
         FIG. 6  is a detailed fragmentary view of the bone image of the femur per  FIG. 4  showing a discontinuity caused by patient motion in a lateral direction during the scanning process; 
         FIG. 7  is a plot similar to  FIG. 5  of a column of data from the bone image of the femur taken along line  7 — 7  of  FIG. 4  showing a discontinuity in density such as may indicate patient motion in a superior-inferior direction; 
         FIG. 8  is a schematic representation of the process of correlating a template with a bone image such as that of  FIG. 3  to identify proper patient positioning and proper location of the scan area as well as positioning of various regions of interest used in other measurements of the image; 
         FIG. 9  is a simplified lateral view of a spine showing curvature away from the surface of the table such as creates magnification artifacts that may affect density measurements. This lateral view is positioned over a graph of vertebral height as deduced from the fan beam parallax per  FIG. 2  such as may be used to trigger an operator warning condition; 
         FIG. 10  is a simplified representation of a template per  FIG. 8  having predefined regions of interest at the proximal femur that may be used to analyze the quality of the acquired data; 
         FIG. 11  is a graphical representation of the bone image of lumbar vertebrae such as may be displayed to the operator to allow positioning of intervertebral fiducial points for the segmentation of the vertebral bodies to determine bone density The graphical representation is positioned next to a plot of bone density along the centerline of the vertebrae whose minimums can be used to analyze operator located intervertebral points; 
         FIG. 12  is a histogram of density values used to determine the threshold for defining intervertebral spaces in  FIG. 11 ; 
         FIG. 13  is a sample operator screen showing indications to the operator of possible errors; and 
         FIG. 14  is a flowchart showing various stages of the computer assistance envisioned by the present invention; 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , a bone densitometer,  10 , includes a patient table,  12 , providing a horizontal surface for supporting a patient in 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  as will be described below. 
     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 to  FIG. 3 , a bone image  32  associated with a scan of the lower lumbar vertebrae 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 femur or the forearm) also may be measured. The measurements of each scan line produce a row of 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  32  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  32 , or of scan area  38  of either proximal femur  87  producing bone image  40  shown in FIG.  4 . 
     Referring now to  FIGS. 1 and 14 , the present invention provides a program executable by the computer  26  that assists the operator in ensuring high quality and accurate scans are obtained. At process block  42  and  44  the operator inputs, through a terminal  28 , patient information including patient age, height, weight and gender as well as the particular scan area ( 37  or  38 ) being acquired. 
     The patient  14  is then positioned on the patient table  12  and the C-arm  18  moved to the scan area  37  or  38  as may be appropriate for the particular scan. The operator initiates the scan through the terminal  28  as indicated by process block  46 . 
     The data acquired in the scan provides the first source of error, and therefore at process block  48 , the scan data is checked. This checking process can be concurrent with the scan or performed at the conclusion of the scan. Generally, if the checking is performed during the scan, the particular steps of the check will be conducted repeatedly on all the data of bone images  32  or  40  acquired up to that instant. Otherwise, if the checking is performed after the scan, it is conducted on the entire bone image  32  or  40 . Typically, when the checking is performed during the scan, it is also performed at the conclusion of the scan when a more comprehensive analysis can be performed. 
     The invention contemplates a number of checks of the scan data, not all of which need be performed in the invention. A first step  50  of this checking evaluates the location of the patient  14  on the table  12 . Ideally, for the scan of the lower lumbar spine  89 , the patient  14  is positioned so that the patient&#39;s spine  89  is centered on the table  12  and aligned with the longitudinal axis  16 . 
     This checking of the spine  89 , location can be performed in a variety of ways. In one embodiment, shown in  FIG. 8 , the bone image  32  or  40  is correlated with a template  52  providing a corresponding bone density image standardized to an average patient. The template  52  is mathematically shifted along the bone image  32  or  40  and the two images are correlated by a mathematical correlation process  54  that compares each pixel of the bone image  32  or  40  (B i ) with the aligned pixel of the template (T i ) over the entire image (i pixels). This process is performed by a correlator  54  realized in software on the computer  26  and is continued until the best alignment is obtained. The alignment process may include optionally not only translation laterally and in an inferior/superior direction, but also rotation and scaling to fit the template as accurately as possible to the scan data. 
     When maximum correlation is obtained, indicated by output  57  of the correlator  54 , the location of the patient  14  can be obtained by reviewing the template&#39;s predetermined centerline  58  to determine the location of the patient&#39;s spine  89  or femur  87  with respect to the table  12  and relative angulation of each. Per step  50 , if the angulation of the spine  89  or translation of the spine  89  or femur  87  on the table  12 , as scanned, deviates by more than a predetermined about from the centerline of the table  12 , a warning will be generated. Each such warning is provided to the operator to allow repeat of the acquisition as indicated by process branch  56 . 
     The location of the template may also be used to define certain regions of analysis in the underlying bone image  32  and  40 , and to determine angulation of the bones as may be used in later analysis steps to be described. 
     A second step  60  of checking the scan data as shown in  FIG. 14 , evaluates whether the bone is out of plane, that is, not parallel with the top of the patient table  12  (in the case of the spine  89 ) or the amount of angulation of the bone (in the case of the neck of the femur  87 ). 
     Referring to  FIG. 9 , the spine  89  may arc upward away from the top of the patient table  12 . Vertebrae  62  that are closer to the top of the patient table  12  and thus the x-ray source  20 , will have greater magnification in the image  64  received by the x-ray detector  22  than vertebra  62 ″, whose image  64 ″ at the x-ray detector  22 ″ will be smaller. The smaller image produces an apparent greater areal density, which may affect the integrity of the scan. Accordingly, the present invention may provide a measurement of spine height  66  as a function of longitudinal distance along the spine  89  that may be compared against a desired limit  68  and an operator warning if the spine height  66  exceeds this limit  68 . 
     Referring now also to  FIG. 2 , the spine height  66  (or the height of any bone) may be deduced in a variety of manners including through a lateral scan of the patient. In the preferred embodiment of the present invention, however, the spine height  66  is deduced by analyzing a region of overlap  70  between two successive images obtained by fan beams  24  and  24 ″ in successive transverse scans  33 . Vertebra or other bones  72  that are further off the patient table  12  will produce more widely separated images  74  than bone  72 ′ closer to the table surface, which produce less widely separated images  74 ′. Shifting the images  74  and  74 ′ to obtain alignment in the overlap region of either bone  72  or bone  72 ′ thus provides a triangulation giving a measurement of height of the bones  72 ,  72 ′. 
     Height determinations of this kind need only be made occasionally during the acquisition of the images  32  and  40  because of the slowly varying geometry of the bones and thus the overlap of the fan beams  24  and  24 ″ need not equal the width of the entire fan beam  24 . 
     Referring again to  FIG. 14 , the check of scan data per process block  48  may include the step  73  of evaluating whether the scan area  37  or  38  corresponds with the area of the patient  14  actually scanned. Referring again to  FIG. 8 , this may be done by checking the absolute magnitude of the greatest correlation between the selected template  52  and the particular bone image  32  or  40 . Failure of a threshold correlation to be achieved may indicate that the patient region that was scanned is inappropriate. 
     Step  75  searches for high-density artifacts caused, for example, by pins or metallic items in the patient  14  or on the patient&#39;s clothing or on surface of the table  12 , such as buttons or clips. Referring to  FIG. 5 , these artifacts may be identified by extremely high attenuations  71  in a given scan line  77  of a bone image  32  or globally with respect to all data of a completed bone image  32 . Additional or alternative filters may be applied to these data that evaluate not only the magnitude of the histogram but also its steepness and/or dual energy characteristics, as will be understood to those of ordinary skill in the art. 
     Referring again to  FIG. 14 , an important source of errors in the acquired data, as checked at process block  48 , may be patient motion, which may be evaluated as indicated by step  80 . Referring to  FIG. 6 , lateral motion of the patient will be manifest in a bone image  32  or  40  as a discontinuity  82  in the vertical edges of the imaged bone. Bone edges are readily visible in the bone images  32  and  40  and may be further identified by prelocated analysis zones imprinted on the template  52  aligned with the underlying bone image  32 . A mathematical derivative taken along the edges of the bone near the discontinuity  82  will identify the discontinuity  82  as a value exceeding a predetermined threshold, triggering a warning to the operator as well as a visual marking of the bone image  32  or  40 . Again, this process may be performed upon completion of the scan or on a line-by-line basis. 
     Superior-inferior patient motion, resulting in shifting a vertically oriented bone along a vertical axis, will not reveal pronounced discontinuities per  FIG. 6  but will affect the density taken along the bone as shown in FIG.  7 . Here, a general trend in the density as a function of distance  84  along the bone shows a discontinuity  86  at the moment of patient motion. Again, a simple differentiation process followed by a thresholding will indicate possible patient motion. 
     Referring again to  FIGS. 14 and 4 , a consideration in obtaining good scans of the femur  87  is that the neck  88  of the femur  87  be substantially horizontal so as to render an accurate bone density measurement without overlap with the pelvis or density artifacts caused by foreshortening. Angulation of the neck  88  may be determined through the height measurement technique described with respect to  FIG. 2  or may be deduced by an anisotropic scaling of a template  52  during correlation that shortens its width disproportionately to its height. This checking of bone angle is indicated at step  93 . 
     Referring now to  FIG. 10 , a template  52  for the proximal femur  87  is shown such as may be scaled, as described above, to the collected bone images  32  and  40  and which has embedded analysis zone  90  and two soft tissue measurement zones  92  used to guide analysis of the bone images  32  or  40  after the template  52  is properly aligned. Using the analysis zone  90 , the neck  88  of the femur  87  may be analyzed per step  94  in  FIG. 14  to see if sufficient neck area is available for accurate bone density measurement. If not, an operator warning is provided. In this case, area may be determined by a simple counting of bone in the analysis zone  90 . 
     Similarly, as indicated by step  96 , the availability of soft tissue zones  92  free from bone may be evaluated using the soft tissue measurement zones  92 . Suitable soft tissue is determined by counting soft tissue pixels in the soft tissue measurement zones  92 . A certain amount of soft tissue is necessary to provide an accurate reference to calibrate the bone density measurements, as is understood in the art. 
     Referring now to FIG.  14  and  FIG. 10 , a final step in the analysis of the scan data  100  investigates whether there is sufficient separation (distance  102 ) between the femur  87  and the pelvis  91 . This analysis, again, may use the correlated and scaled template  52 , to review the length of an embedded separation line  95  in the template  52  after scaling. 
     Referring again to  FIG. 4 , these measurements may alternatively be performed by fiducial points marked by the operator on the bone images  32 , as prompted by the computer  26 , or by other image recognition techniques such as, for example, those which identify fiducial points such as the lesser or greater trochanter neck and other landmark features in the particular scanned regions. 
     Returning to  FIG. 14 , once the data have been acquired at process block  46  and confirmed at process block  48 , the program may proceed to process block  107  where operator input is accepted for analysis purposes. If at process block  48  the scan data is not approved, that is it fails one or more of the steps  50 ,  60 ,  73 ,  75 ,  80 ,  93 ,  94 ,  96 , and  100 ), the operator may nevertheless proceed to provide analysis of the data at process block  107 . The data, however, will be marked to indicate possible artifacts. 
     At process block  107 , the operator may provide input to allow the analysis of the data. Referring to  FIGS. 14 and 11 , at step  115  of  FIG. 14 , this operator input data may, for example, be the placement of markers  108  in the intervertebral spaces  110  between vertebrae  62 . The placement of these markers  108  may be done by manipulation of the cursor control device  35  according to techniques known in the art. 
     Such intervertebral markers  106  determine the measurement of vertebral height, which is necessary to compute vertebral area and determine if a particular vertebra  62  has had a crush fracture. The location by the operator of the intervertebral markers  108  may be checked by software review of the underlying data of the bone image  32 . Referring momentarily to  FIG. 9 , the bone density data of the bone image  32 , collected up to the time of the check, may be plotted in a histogram  112 , which may be used to make a determination of a boundary  114  between bone and soft tissue. This boundary  114  may be applied to row-averaged bone density data of the bone image  32  in the area of the spine (aligned generally along line  11 — 11  through  FIG. 3 ) to determine points of minima  116  corresponding with the intervertebral spaces  110 . Referring also to  FIG. 13 , to the extent that the operator places intervertebral markers  108 ″ in locations that deviate significantly from the minima  116 , the operator will be notified in the checking process  118  shown in  FIG. 14  so as to have the opportunity to re-input the data as indicated by the process path  120 . Notification may be by text messages and/or highlighting of the misplaced markers or erroneous operator data. 
     The operator may then proceed to calculation of diagnostic output at process block  122 , in this case measurements of bone area, bone content, bone density, and vertebral height, either after a correction of the operator input or a notation that the input was not corrected (if a correction was suggested by the program). 
     At succeeding block  124 , the diagnostic output (in this case vertebral height) is checked against standard output ranges as a final safety check on the data. Typically, the diagnostic output of a densitometer will be either a bone mineral density reading in grams per square centimeter or a T-score or Z-score, the former being the number of standard deviations of the diagnostic output from a reading expected of a healthy 30-year old standard woman and the latter being the number of standard deviations of the diagnostic output from an age-adjusted standard woman. 
     Specifically, as indicated by step  126 , the computer  26  may store an expected range of clinically experienced BMDs, T-scores, or Z-scores and compare the diagnostic output against these to flag a problem if the diagnostic output is outside of this range. 
     As indicated by step  128 , a similar process may be used to check diagnostic outputs of vertebral height used in assessing possible crush fractures or other morphometric aspects of the vertebra. Here, the diagnostic output may be compared against patient height or against other vertebra of the patient above and below the given vertebra or against an average of the patient&#39;s vertebrae used to define a range within which the diagnostic output reading should fall. Generally, a crush fracture will cause a deviation of vertebra height from its neighbors, but the ranges are established to embrace the normal expected deviation. 
     At step,  132  the report is generated which may include images marked as described above and warnings that were not corrected per branch  56  and  120 . 
     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.