Abstract:
An x-ray system that uses an x-ray source to irradiate a subject with x-rays is provided. A collimator is located proximate to the x-ray source and blocks a portion of the x-rays. A detector receives the x-rays and creates subject data indicative of a subject and collimator data indicative of a collimator. A position detector identifies a position of the collimator with respect to a field of view of the x-ray source. A gating module receives the subject and collimator data and passes at least a portion of the subject data. The gating module blocks at least a portion of the collimator data based on the position of the collimator. A display displays an x-ray image based on the portion of the subject data passed by the gating module.

Description:
BACKGROUND OF THE INVENTION 
     Certain embodiments of the present invention relate to medical diagnostic systems, and in particular, to techniques and apparatus for adjusting the contrast of displayed diagnostic images acquired while using a collimator. 
     X-ray systems are well known for creating a series of internal images of a patient, such as cardiology, radiology and fluoroscopy systems. The patient is exposed to x-rays which are then detected after passing through the patient. The radiation is pulsed to produce a continuous sequence of images which are displayed real-time on a monitor. The x-rays are attenuated as they pass through the patient. The amount of attenuation experienced by the x-rays is represented in the image by the grayscale level of the pixels that are displayed. The contrast between grayscale levels is representative of the amount of attenuation. 
     Bones and different types of tissues attenuate the x-rays by different amounts, and thus are detected and displayed on an image monitor with different contrast levels. For example, bone will attenuate x-ray to a larger degree than muscle and may be displayed darker than surrounding anatomy. A region of anatomy containing only soft tissue may have a smaller range of contrast than a region of anatomy containing both soft tissue and bone. In addition, scattered radiation or using an increased kVp level to image a very large patient may also decrease the contrast range. 
     The level of radiation detected by the system is correlated to the contrast of the displayed image by a look-up table, or transfer function. In other words, the system uses the transfer function to assign a specific level of radiation to a specific grayscale level of the display. The system varies the range of the contrast for the displayed image associated with a particular range of grayscale levels by changing the shape (e.g., slope, offset, etc.) of the transfer function. The system may have multiple transfer functions, representing different mathematical models or shapes, from which one transfer function is selected to control the contrast of one or more display images. A particular transfer function is selected for different procedures or when imaging different anatomy. 
     The range of contrast used to display multiple images during a procedure may be set by the system and remain constant throughout the procedure. Therefore, a level of detected radiation, or a particular brightness level, is assigned a particular grayscale level for one or an entire series of scans. This is not advantageous, as during the same procedure, areas of interest within a patient which have various ranges of contrast may be scanned, and thus the contrast may appear to fluctuate. Some images may appear with a high level of contrast, containing areas that are very dark and areas that are very light, while other images may have low contrast and appear light or washed out. An operator may chose to adjust the contrast of the displayed image to correct for the change in contrast in the anatomy, but this is time consuming, error prone, and would need to be repeated as different tissues are examined. 
     To provide a more constant contrast throughout a procedure (fluoroscopic, cardiology, radiology or otherwise), automatic contrast compensation algorithms have been proposed. Automatic contrast compensation, or contrast management, is utilized to present a more pleasing image with better diagnostic utility. The images are examined for maximum and minimum brightness levels as they are acquired. The maximum and minimum brightness levels are then used to determine a new grayscale transfer function to enhance the contrast of the displayed image. Therefore, a radiologist may view images that contain bone and images containing only soft tissue during the same procedure without manually adjusting the contrast. 
     Unfortunately, automatic contrast compensation algorithms are also sensitive to data from regions where radiation has been blocked, such as when a collimator is used. A collimator may be used to block a portion or portions of an x-ray beam to minimize exposure to areas of the body which are not of diagnostic interest. When using a collimator, automatic contrast compensation, when enabled, identifies a minimum brightness level and adjusts the contrast such that the data detected from the collimator is assigned the lower end of the available grayscale, or the minimum brightness levels. Therefore, the grayscale range available to display the anatomic region of interest is decreased, and the displayed anatomic data reflects a sudden decrease in contrast. 
     In an effort to eliminate the effect of the collimator, it has been proposed to determine the maximum and minimum brightness levels from a small area of an image that will never be obstructed by the collimator regardless of where the collimator is actually located. Unfortunately, controlling brightness levels based on a smaller portion of the field of view may compromise contrast enhancement for larger fields of view, such as when no collimator or a less obstructive collimator is used. This method results in anatomic grayscale data being lost when imaging high contrast anatomy. 
     Thus, a need exists in the industry for a method and apparatus for adjusting the contrast of displayed diagnostic images when a collimator is used that addresses the problems noted above and previously experienced. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with at least one embodiment, an x-ray system is provided utilizing an x-ray source that irradiates a subject with x-rays. A collimator is located proximate to the x-ray source and blocks a portion of the x-rays. A detector receives the x-rays from the x-ray source and creates subject data indicative of a subject and collimator data indicative of a collimator. A position detector identifies a position of the collimator with respect to a field of view of the x-ray source. A gating module receives the subject and collimator data and passes at least a portion of the subject data. The gating module blocks at least a portion of the collimator data based on the position of the collimator. A display displays an x-ray image based on at least a portion of the subject data passed by the gating module. 
     In accordance with at least one embodiment, an x-ray system is provided utilizing an x-ray source to irradiate a subject with x-rays. A collimator blocks a portion of the x-rays and is located proximate the x-ray source. A detector receives x-rays from the x-ray source and creates subject data indicative of a subject and collimator data indicative of a collimator. A collimator calculation module identifies at least one of a position, an orientation, a shape, and a boundary of the collimator based on the position of the collimator. A gating module receives the subject and collimator data, and passes at least a portion of the subject data and blocks at least a portion of the collimator data based on the position of the collimator. A display displays an x-ray image based at least on the portion of the subject data passed by the gating module. 
     In accordance with at least one embodiment, a method for enhancing the contrast of an x-ray image is provide. A subject is exposed to an x-ray source which is partially blocked by a collimator. Subject data indicative of a subject and collimator data indicative of a collimator is detected. Position data indicative of a position of the collimator is identified. The subject and collimator data is gated to block the collimator data based on the position data. An x-ray image is displayed based on at least a portion of the subject data. 
    
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 illustrates a fluoroscopic x-ray system that utilizes automatic contrast compensation in accordance with an embodiment of the present invention. 
     FIG. 2 illustrates a diagnostic image acquired without utilizing a collimator in accordance with an embodiment of the present invention. 
     FIG. 3 illustrates a histogram and a transfer function generated from a region of interest in accordance with an embodiment of the present invention. 
     FIG. 4 illustrates a diagnostic image acquired utilizing a collimator in accordance with an embodiment of the present invention. 
     FIG. 5 illustrates two histograms and a transfer function generated from detected radiation in a region of interest including collimator leaves in accordance with an embodiment of the present invention. 
     FIG. 6 illustrates a diagnostic image acquired utilizing a collimator and collimator position data in accordance with an embodiment of the present invention. 
     FIG. 7 illustrates a histogram and transfer function utilizing collimator position data in accordance with an embodiment of the present invention. 
     FIG. 8 illustrates a collimator and x-ray source in accordance with an embodiment of the present invention. 
     FIG. 9 illustrates a method for enhancing the contrast of diagnostic images acquired using automatic contrast compensation and a collimator in accordance with an embodiment of the present invention. 
     The foregoing summary, as well as the following detailed description of the embodiments of the present invention, will be better understood when read in conjunction with the appended drawings. It should be understood, however, that the present invention is not limited to the arrangements and instrumentality shown in the attached drawings. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a fluoroscopic x-ray system that utilizes automatic contrast compensation in accordance with an embodiment of the present invention. The fluoroscopic system  100  includes an x-ray source  102  which directs an x-ray beam  103  toward a subject or patient  104 . The patient  104  may be a human, animal, test phantom, or other object which includes anatomic structure. The image intensifier  106  detects the radiation that passes through the patient  104  and produces an image. The image is transferred via optics  108  to a CCD video camera  110 . The CCD video camera  110  produces an analog video signal which is converted to a digital signal by an analog to digital converter (ADC)  112 . Optionally, the ADC  112  may be included in the CCD video camera  110 . Alternatively, a flat panel detector may replace the image intensifier  106 , the optics  108  and the CCD video camera  110 . The digital signal is then sent to the image processor  114 . A memory  115  may be utilized to store the digital signal data and other data utilized by the fluoroscopic system  100 . 
     The fluoroscopic system  100  includes a region of interest (ROI) gate  116 , a histogram generator  118 , and an auto contrast algorithm  120  that cooperate to perform automatic contrast compensation. The ROI gate  116 , histogram generator  118  and auto contrast algorithm  120  are displayed separately on FIG. 1 but may constitute software, all of which is performed by the image processor  114  or other component of the fluoroscopic system  100 . Alternatively, separate hardware such as individual digital signal processing boards may carry out the functions of the ROI gate  116 , histogram generator  118  and auto contrast algorithm  120 . A control panel  128  may be utilized by an operator to input data, such as to enable automatic contrast compensation or enter patient data. 
     A collimator  130  may be utilized to block a portion or portions of the x-ray beam  103 . The collimator  130  may be comprised of one or more leaves. A detector  131  detects the position of the collimator  130 . The collimator calculation module  132  reads the collimator position data and utilizes the collimator position data to calculate a region of interest. The region of interest may define an area of acquired data upon which a displayed image is based. The region of interest may be used by the ROI gate  116  to identify portions of the acquired data to block. Therefore, the blocked data may not affect the histogram generated by the histogram generator  118 . The auto contrast algorithm  120  identifies maximum and minimum values in the histogram. The output of the auto contrast algorithm  120  is passed to a display look-up table (LUT)  122 . The display LUT  122  uses the parameters identified by the auto contrast algorithm  120  and correlates the levels of detected radiation in the digital image data from the image processor  114  to a selected look-up table, or transfer function. A digital to analog converter (DAC)  124  converts the output of the display LUT  122  from digital to analog, and passes the analog signal to display  126 . The display  126  then displays a diagnostic image of the patient  104 . 
     FIG. 2 illustrates a diagnostic image acquired without utilizing a collimator in accordance with an embodiment of the present invention. A monitor  140  or similar device displays a diagnostic image  142  such as that displayed by display  126 . The region of interest gate  116  utilizes predefined parameters or data from the collimator calculation module  132  to block digital image data outside a region of interest  144 . The region of interest  144  is a part of the diagnostic image  142 , and is typically  60  percent to  100  percent of the diameter of the diagnostic image  142 . For example, the region of interest  144  may be fixed at 90 percent. The diagnostic data within the region of interest  144  will be used by the histogram generator  118  to build a histogram. 
     FIG. 3 illustrates a histogram and a transfer function generated from a region of interest in accordance with an embodiment of the present invention. The histogram generator  118  utilizes the digital image data output from the image processor  114  inside the region of interest  144  over a raw data range  156  to generate a histogram  150 . The digital image data used to construct histogram  150  does not include digital image data located in regions covered by collimator  130 . The auto contrast algorithm  120  determines the detected minimum value (detected MIN)  152  and the maximum value (MAX)  154  within the histogram  150  of the digital image data. The display LUT  122  utilizes the detected MIN  152  and MAX  154  values to generate a transfer function  160  over the displayed contrast range  158 . 
     By way of example only, the displayed contrast range  158  may be divided into 256 discreet grayscale levels, wherein  0  indicates black and  255  indicates white. In a typical fluoroscopic system, the average values of the region of interest  144  may be displayed in the upper half of the displayed contrast range  158 . For example, the brighter elements of the image may be displayed with a value in the range of 180 to 200, while the darkest elements may be displayed in the range of 50 to 100. 
     The histogram generator  118  may be comprised of a table of values stored in the memory  115  to which the digital image data is compared. Alternatively, the histogram generator  118  may use other electronic circuitry to detect the various white, black, and gray levels in the digital image data. The peak white (maximum) and peak black (minimum) signals may be used to generate offset and gain values to drive analog circuitry. The analog circuitry may be combined with an analog gate or clipping circuit, for example, that would reject black values below a certain level identified by the operator through the control panel  128 . 
     The histogram generator  118  plots the histogram  150  on a graph such that the horizontal axis represents the level of detected radiation inside the region of interest  144  for the digital image data over the raw data range  156 , and the vertical axis represents the displayed contrast range  158  available on the display  126 . The displayed contrast range  158  may be a grayscale from black to white. Alternatively, the displayed contrast range  158  may be represented by a color scale, in which each color indicates a level, or band of levels, of signal value or detected radiation. The histogram generator  118  may plot each pixel&#39;s value in the histogram  150 . Optionally, the digital image data may be divided into groups of pixels, such as a square of 10 pixels, and the average value of each group of pixels may be plotted. The histogram generator  118  may plot a histogram  150  for each individual image that is acquired, or for a certain percentage of the images that are acquired. 
     Once the histogram  150  is generated, the auto contrast algorithm  120  determines the detected MIN  152  and MAX  154  values within the raw data range  156 . The detected MIN  152  is the lowest detected value in the region of interest  144 , or the point(s) in the region of interest  144  with the least detected radiation. Pixels with detected radiation at or below the detected MIN  152  may be represented by the darkest element of the grayscale (i.e. black, or approaching black) on the display  126 . The MAX  154  is the highest detected value in the region of interest  144  and indicates the points(s) with the most detected radiation. A pixel with detected radiation at or above the MAX  154  may be represented by the brightest element (i.e. white) on the display  126 . 
     Additionally, the histogram  150  may be further filtered to eliminate discrepant values. For example, in order to identify the detected MIN  152 , the auto contrast algorithm  120  may set a condition which must be met, such as having  10  values in a row plotted on the histogram  150  with greater than a predetermined number of pixels. Similar restrictions may be placed on the upper end to determine the MAX  154 . Therefore, noise present at the minimum and maximum ends of the histogram  150  may not be considered. Alternatively, the auto contrast algorithm  120  may set a condition such that the gradient or slope of the histogram  150  may be below a predefined level as the histogram  150  approaches and combines with the threshold of 1% amplitude. Optionally, a convolving kernel or window may be utilized to smooth the histogram  150  with a spline or cubic function before the detected MIN  152  and MAX  154  are identified. It should be understood that further methods exist to identify the detected MIN  152  and MAX  154  values in the histogram  150 , and any appropriate method may be utilized. 
     After the detected MIN  152  and MAX  154  have been identified, the display LUT  122  draws the transfer function  160  from the detected MIN  152  to the MAX  154  and translates the radiation levels of the histogram  150  to occupy the full contrast range of the display  126 . Therefore, detected MIN  152  is typically assigned a value of zero or near zero on the vertical axis and MAX  154  is assigned a value of 255 on the vertical axis. The transfer function  160  may be stored in memory  115  and is illustrated as non-linear, but it should be understood that any transfer function may be used. For example, the transfer function may be linear (such as the transfer function  184  illustrated in FIG.  5 ), logarithmic, exponential, S-shaped, hyperbolic tangent, or another shape that may not fit any mathematical model. 
     FIG. 4 illustrates a diagnostic image acquired utilizing a collimator in accordance with an embodiment of the present invention. FIG. 4 incorporates similar components as illustrated in FIG. 2, such as a monitor  170 , diagnostic image  172 , and region of interest  174 . The diagnostic image  172  is comprised of an area of subject data  168  and areas of collimator data  176  and  178 . Because the leaves of the collimator  130  are opaque, little or no radiation is transmitted through the leaves. 
     FIG. 5 illustrates two histograms and a transfer function generated from detected radiation in a region of interest including collimator leaves in accordance with an embodiment of the present invention. The histogram generator  118  utilizes the raw data range  194  from the region of interest  174  to build histograms  180  and  182 . Histogram  180  is a first distribution of the collimator data  176  and  178  from the collimator leaves. The detected MIN  186  is the lowest radiation level detected in the raw data range  194 . Histogram  182  is a second distribution of the subject data  168  of the subject or patient  104  similar to histogram  150  of FIG.  3 . 
     When a collimator  130  is used, the auto contrast algorithm  120  identifies the detected MIN  186  as the lowest detectable radiation. The display LUT  122  then generates the transfer function  184  based upon the detected MIN  186  and the MAX  190  with a displayed contrast range  192 . The collimator data  176  and  178  is represented by the lower level of the transfer function  184 , and subject data  168  is pushed into the upper level of the transfer function  184 . Thus, the subject data  168  is displayed with contrast range  196 , which is less than the displayed contrast range  192 . 
     FIG. 6 illustrates a diagnostic image acquired utilizing a collimator and collimator position data in accordance with an embodiment of the present invention. FIG. 6 incorporates similar components as illustrated in FIG. 4, such as a monitor  200 , diagnostic image  202 , subject data  216 , and collimator data  204  and  206 . The region of interest  208 , however, has been calculated utilizing data provided by the detector  131 . Because the leaves of the collimator  130  are symmetrical, the region of interest  208  may be at the center of the diagnostic image  202 , illustrated just inside the collimator data  204  and  206 . Therefore, the region of interest  208  contains only subject data  216 , and may not be impacted by the orientation of the collimator  130 . 
     The region of interest  208  of FIG. 6 has been illustrated as a circle centered within the diagnostic image  202  and inside the collimator data  204  and  206 . However, the region of interest  208  may be any size or shape, such as a rectangle defined by collimator edges  210  and  212 . The region of interest  208  may then contain all or nearly all of the subject data  216 . Alternatively, the operator may define the size and/or shape of the region of interest  208 , then use the control panel  128  to move the region of interest  208  within the area defined by the collimator edges  210  and  212  in order to choose the subject data  216 . Thus, the operator may be able to further control the contrast of the subject data  216  displayed by display  126 . 
     FIG. 7 illustrates a histogram and transfer function utilizing collimator position data in accordance with an embodiment of the present invention. Histogram  220  represents subject data  216  acquired within region of interest  208 , which comprises raw data range  230 . As illustrated in FIG. 6, region of interest  208  includes a portion of subject data  216 , but does not include collimator data  204  and  206 . The auto contrast algorithm  120  determines MIN  222  and MAX  224  corresponding to histogram  220 . The display LUT  122  then draws a transfer function  226  from the MIN  222  to the MAX  224 . By using collimator position data to identify a region of interest  208  that does not include collimator data  204  and  206 , the transfer function  226  maximizes the use of the displayed contrast range  228  by assigning collimator data  204  and  206  (data below MIN  222 ) a black or nearly black value. 
     It may not be desirable to use the maximum grayscale values for black and white when displaying diagnostic data. Therefore, moderation values may be utilized by the auto contrast algorithm  120  to moderate the maximum and minimum values used by the display LUT  122 . As a result, values below MIN  222  may be assigned a black, or nearly black, value, differentiating the data below MIN  222  from the anatomic data. 
     FIG. 8 illustrates a collimator and x-ray source in accordance with an embodiment of the present invention. A collimator  250  is fastened to an x-ray source  252  such that the collimator  250  is between the x-ray source  252  and the patient  104 , as illustrated in FIG.  1 . The collimator  250  may be comprised of one or more collimator leaves  254  and  256  and a frame  258 . The collimator  250  may be placed within an x-y plane, such as the plane defined by x-axis  260  and y-axis  262 . Therefore, the collimator  250  may be rotated relative the x-axis  260  and the y-axis  262 . Alternatively, the collimator  250  may be constructed to permit the collimator leaves  254  and  256  to rotate within the frame  258 . 
     The collimator leaves  254  and  256  may be rectangular in shape, resulting an a substantially rectangular exposure area  264 . Alternatively, the collimator leaves  254  and  256  may be another shape to provide a circular, keyhole, or square exposure area  264 , for example. By way of example only, the collimator leaves  254  and  256  may move toward and away from one another along a linear path  251 , or may be rotated along a circular path  253 . The collimator frame  258  may contain one or more position sensors  275 - 279  which sense the location of the collimator leaves  254  and  256 . The position sensors  275279  may be electromagnetic, mechanical, optical, or any other type of sensor. The collimator leaves  254  and  256  may be fixed to a specified position within frame  258 , or may be adjusted by the operator to increase, decrease, or change the shape of the exposure area  264 . It may be possible to move one collimator leaf  254  or  256 , or the collimator leaves  254  and  256  may be fixed such that each collimator leaf  254  and  256  obstructs a substantially similar sized portion of the exposure area  264 . 
     By way of example only, when the collimator  250  is attached to the x-ray source  252 , one or more position sensors  275 - 279  within the collimator  250  sense the position of one or more collimator leaves  254  and  256 . The collimator calculation module  132  reads position data from the position sensors  275 - 279  and utilizes the position data to calculate the position of the collimator leaves  254  and  256  relative to the x-axis  260  and y-axis  262 . The position of the collimator leaves  254  and  256  may be identified by reading the location of one edge of a collimator leaf  254  or  256 , such as marker  266  or  268 . One marker  266  or  268  may be utilized if the associated position sensor  275 - 279  provides data identifying an x value, a y value, and a rotation value. Alternatively, the position of a collimator leaf  254  or  256  may be identified by sensing a position within the collimator leaf  254  or  256 , such as marker  270 . Marker  270  may be the center of the collimator leaf  256 , for example, and provide a reference as to the location of collimator leaf  256  in relation to the x-axis  260  and y-axis  262 . If more than one marker  266 - 270  is used to identify the location of a collimator leaf  254  or  256 , the collimator calculation module  132  may utilize x and y coordinate data relative to the x-axis  260  and the y-axis  262  provided by the position sensors  275 - 279  to calculate an associated angle. It should be understood that the location and number of markers  266 - 270  and collimator sensors  275 - 279  in FIG. 8 is exemplary. Therefore, one or more markers  266 - 270  may be located elsewhere on collimator leaves  254  and  256 , and one or more collimator sensors  275 - 279  may be located elsewhere on collimator  250 . 
     The collimator calculation module  132  may convert the position data to a numeric number set. The number set may comprise coordinate data, such as in a bit map, configured to store displayed digital image data. The collimator calculation module  132  may use the coordinate data to “draw” the location of each collimator leaf  254  and  256  into the bit map. The orientation of the collimator leaves  254  and  256  may be identified relative to a reference coordinate within the bit map. The shape of the collimator leaves  254  and  256  may also be identified. As discussed above, the shape of the collimator leaves  254  and  256  is not limited to a rectangle. By identifying the orientation, shape, and/or location of the collimator leaves  254  and  256 , the boundaries  272  and  274  of the collimator leaves  254  and  256  may be identified. Therefore, the size, location, and shape of exposure area  264 , which is inside the boundaries  272  and  274 , is known. 
     Once the exposure area  264  is identified, the collimator calculation module  132  may calculate a region of interest  208 , as illustrated in FIG.  6 . The region of interest  208  is defined as a circle within the exposure area  264 . As discussed previously, the region of interest  208  may be any shape and/or size, and may be located anywhere within the exposure area  264 . For example, the region of interest  208  may include all digital image data within the exposure area  264 . Therefore, all of the digital image data within the region of interest  208  may be utilized to adjust the contrast of the displayed diagnostic image  202 . 
     FIG. 9 illustrates a method for enhancing the contrast of diagnostic images acquired using automatic contrast compensation and a collimator in accordance with an embodiment of the present invention. For the method of FIG. 9, the auto contrast option is enabled. For example, the auto contrast option may be enabled through a keyboard or other control panel  128  as previously discussed. In addition, a look-up table used by the display LUT  122  to calculate the transfer function is predefined and may be stored in memory  115 . 
     At step  280 , the collimator calculation module  132  reads the collimator position information from the position sensors  275 - 279 . As discussed previously, the position of the collimator leaves  254  and  256  may be identified by one or more position sensors  275 - 279 . The collimator position information may be in the form of a collimator marker  266 - 270 , where one or more collimator markers  266 - 270  are used to identify the position, orientation, and shape of the collimator leaves  254  and  256 . 
     At step  282 , the collimator calculation module  132  determines whether a collimator  250  is being used. If no collimator position information was read from the position sensors  275 - 279 , the flow passes to step  284 . At step  284 , the collimator calculation module  132  may access a predefined region of interest  174 . Therefore, the region of interest  174  may be assigned a size of 90 percent of the diameter of the diagnostic image  172 , as previously discussed. The predefined region of interest  174  may be stored in memory  115 . If collimator position information is available, the flow passes to step  286 . 
     At step  286 , the collimator calculation module  132  converts the collimator position information to a numeric number set. As previously discussed, the number set may comprise coordinate data in a bit map. The coordinate data identifying the location of the collimator leaves  254  and  256  is then drawn into the bit map. 
     At step  288 , the collimator calculation module  132  utilizes the numeric number set and/or bit map to compute the region of interest  208 . The boundaries  272  and  274  may be identified, thus defining the exposure area  264 . The region of interest  208  is computed to be within the exposure area  264 , based upon size and shape parameters identified by the collimator calculation module  132 . Once the region of interest  208  has been computed, the region of interest  208  is sent to the ROI gate  116 . 
     At step  290 , x-ray source  102  exposes the patient  104  to x-ray radiation. The level of x-ray may be determined by the anatomy being imaged, the type of procedure, or the size of the patient  102 , for example. At step  292 , the image intensifier  106 , the optics  108 , the video camera  110 , the ADC  112 , and the image processor  114  are utilized to acquire digital image data that may be displayed as diagnostic image  202 . The digital image data may be stored in a memory  115  to be further processed later. 
     At step  294 , the histogram generator  188  generates one or more histograms based upon the digital image data within the region of interest  208 , such as the region of interest  208  computed in step  288 . Digital image data outside the region of interest  208  is “gated” or excluded by the ROI gate  116  from the histogram generation. As stated previously, the histogram generator  118  may generate one or more histograms for each acquired image, or combine more than one acquired image to generate a histogram. 
     At step  296 , the auto contrast algorithm  120  determines the minimum and maximum values of the raw data range  230  within the region of interest  208 . For example, MIN  222  and MAX  224  may be determined, such as in FIG.  7 . Next, at step  298 , the display LUT  122  utilizes the predetermined look-up table and the MAX  224  and MIN  222  values of step  296  to calculate transfer function  226 . The transfer function  226  may associate the histogram  220  with a grayscale level, or displayed contrast range  228 . 
     At step  300 , the display  126  displays the diagnostic image  202  by applying the transfer function  226  to the digital image data from the image processor  114 . The MIN  222  of step  296  may be displayed as nearly black, while the MAX  224  may be displayed as white. As discussed previously, a color scale may also be used. The collimator is displayed at black, or approaching black. Therefore, the anatomic data may be displayed with nearly the full displayed contrast range  228 . 
     At step  302 , the method determines whether more diagnostic images  202  are to be acquired. If yes, the flow returns to step  290 . If no, flow passes to step  304  and the method is complete. 
     By using the aforementioned methods and apparatus to acquire diagnostic images  202  while using a collimator  130  to block a portion of the x-ray beam  103 , diagnostic images  202  with improved diagnostic utility may be acquired. Therefore, a fluoroscopic system  100  that utilizes collimator position data together with the auto contrast option will able a radiologist to conduct a procedure comprising multiple x-ray images utilizing the benefits of auto contrast without the disadvantage of displaying anatomic data with a reduced contrast range. 
     While the invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.