Patent Publication Number: US-6702459-B2

Title: Mobile radiography system and process

Description:
FIELD OF THE INVENTION 
     This application claims priority to U.S. patent application Ser. No. 60/282,978, which was filed on Apr. 11, 2001 and which is incorporated herein by reference. The present disclosure relates to radiography and, more particularly, to mobile radiography with improved x-ray scatter rejection. 
    
    
     BACKGROUND OF THE INVENTION 
     In the hospital setting, mobile radiographic exams are performed on patients that are incapable of being moved, or are difficult to move. In tertiary care medical centers, mobile radiographic exams represent a significant percentage of the radiographic exams performed. X-rays passing through an object, such as a human body, experience some degree of scatter associated with interactions with atoms or electrons. The primary x-rays transmitted through an object travel on a straight line path from the x-ray source (also referred to herein as the x-ray focal spot) to the image receptor and carry object density information. Scattered x-rays form a diffuse image that degrades primary x-ray image contrast. In thick patients, scattered x-ray intensity exceeds the intensity of primary x-rays. Scattering phenomena is well known and routinely compensated for in general radiography, fluoroscopy and mammography through the use of anti-scatter grids. 
     An anti-scatter grid includes a laminate of lead foil strips interspersed with strips of radiolucent material (FIG.  1 ). The grid is positioned between the object of interest and the x-ray image receptor plate and oriented such that the image forming primary x-rays are incident only with the edges of the lead foil strips. Thus, the majority of primary x-rays pass through the radiolucent spacer strips. In contrast, scattered x-rays are emitted in all directions after interaction with the object and as such, scattered x-rays are incident on a larger area of the lead strips and only a small percentage of scattered x-rays are transmitted by the grid, as compared to primary x-rays. The degree of scatter control for a given grid depends upon the grid ratio, which is defined as the ratio of the radiopaque strip thickness in the direction of the x-ray path to the width of the radiolucent spacer material as measured orthogonal to the x-ray beam path. Thus, the higher the grid ratio, the greater the scatter control. A high grid ratio, while more effective, is also more difficult to align relative to a focal spot. In order to compensate for x-ray beam divergence in a focused grid, the radiopaque strips are tilted to a greater extent with increasing distance from the center of the grid. The planes of the grid vanes all converge along a line known as the focal line. The distance from the focal line to the surface of the grid is referred to as the focal length of the grid. The focal line coincides with the straight line path to the focal spot (illustrated in FIG.  2 ). Thus, when the focal spot is coincident with the focal line of the grid, the primary x-rays have minimal interaction with the radiopaque lead strips and maximal primary transmission is obtained. Misalignment of the focal line of the anti-scatter grid with the focal spot diminishes primary x-ray transmission while scattered x-ray transmission remains unchanged. Thus, optimal primary x-ray transmission requires alignment (positional and orientational) of the focal spot with the focal line of the anti-scatter grid. 
     In general radiography, fluoroscopy and mammography, the image receptor and x-ray tube are rigidly mounted and in a fixed position relative to one another, thereby making focal spot and grid alignment a simple process. In mobile radiography, an image receptor is placed under a bedridden patient and the x-ray source is positioned above the patient. Since the relative separation of the focal spot and the image receptor is variable, determining the proper position and orientation of an anti-scatter grid between a patient and the image receptor becomes a difficult alignment problem. If a grid is not used, only a small fraction of the possible contrast is obtained in the x-ray image. As a result, scatter to primary x-ray ratios of 10:1 or more are common in chest and abdominal bedside radiography resulting in less than 10% of the possible image contrast being obtained in mobile radiographic films ([1,2]Barnes, G T,  RadioGraphics  11:307-323, 1991; Niklason et al.,  Med. Phys . 8:677-681, 1981). Contrast limitations are exacerbated if digital storage phosphor image receptors are utilized in place of the more conventional screen-film systems ([3]Tucker et al.,  Radiology  188:271-274, 1993). 
     When grids are utilized in conjunction with mobile radiography, the grid is typically not aligned. Misalignment problems are diminished by utilizing a grid having a low ratio of 8:1 or less. Although x-ray image contrast is improved with the use of a low ratio grid, the contrast remains significantly lower than otherwise could be obtained with a properly aligned, high ratio grid having a grid ratio of 10:1 or greater. 
     Thus while mobile radiography is in many ways more convenient than fixed installation radiography, its clinical utility is diminished due to the inferior image quality caused by scattered radiation which is a greater problem in mobile radiography due to the difficulty in producing the proper alignment of the focal spot with the anti-scattering grids. A means to produce proper alignment that is easy for the operator to use would significantly improve mobile radiographic image contrast and image quality, and thus increase the clinical utility of mobile radiography. 
     A system is disclosed in U.S. Pat. No. 4,752,948 which includes a rigid arm mounted on the grid tunnel, with a coupling on the other end that connected to the x-ray source housing. A hinge on the grid end of the arm allowed it to be folded for transportation. A radiography technologist unfolds the arm, locks the hinge, slides the grid tunnel and film cassette under the patient, and attaches the x-ray source housing to the other end of the arm. The arm then holds the x-ray source rigidly in alignment with the grid tunnel. This system demonstrated the image quality and clinical advantages of employing properly aligned high ratio grids in bedside radiography. However, difficulty in using the system limited the application thereof in mobile radiography. 
     Loren Niklason et al. disclosed a mobile radiography system utilizing a telescoping arm ([4]Niklason et al.,  Radiology  173(P):452, 1989). One end of the arm was permanently attached to the mobile x-ray unit column and the other end was attached by the radiography technologist to the grid assembly after the grid and cassette were positioned under the patient and the mobile unit was centered right-to-left to the grid assembly. Dials indicated to the technologist the transverse direction the tube needed to be moved and the angle the tube had to be rotated to align it with the grid. The time consuming and complex steps to align the x-ray tube using this system limited the application thereof in mobile radiography. 
     U.S. Pat. Nos. 5,241,578 and 5,388,143 disclose a laser alignment device that required a user to align a laser and a mark in the alignment light field with a reflector device that mounted on a corner of a grid tunnel. As with Niklason&#39;s system, this system required the user to manually align the x-ray source by trial and error. Further, it required that part of the grid tunnel extend past the patient, which limited the application thereof in portable radiography. 
     Peter O&#39;Donovan et al. disclosed a system involving electronic levels on the grid tunnel and source housing, an alignment target attached to the source, and crosshairs in the alignment light field ([5]O&#39;Donovan et al.,  Radiology  184:284-285, 1992). A tape measure was used to ensure that the source was the proper distance from the grid tunnel. The user rotated the source housing until the two levels indicated that the central axis of the source was normal to the grid tunnel in one direction; turn on the collimator light; and move the tube housing until the shadow of one of the cross-hairs fell on a mark on the grid tunnel. The complexity of this procedure limited the application thereof in mobile radiography. 
     The prior art systems have been limited in their utility in clinical acceptability owing to the considerable additional effort required on the part of a radiography technologist to align the x-ray source. Thus, there exists a need for a mobile radiography system having a simple means to place the focal spot and the central x-ray beam in correct alignment (position and orientation) with regard to the anti-scattering grid. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of an anti-scattering grid common in the field; 
     FIG. 2 is a schematic view of a focused anti-scattering grid common in the field; 
     FIG. 3 is a side view of a mobile radiography system according to the present disclosure; 
     FIG. 4 is an illustration of the optimal and acceptable state of alignment for the mobile radiographic system of the present disclosure; 
     FIG. 5 is a perspective detail view of a target arm and fiducial markers according to the present disclosure; 
     FIGS. 6A and 6B are perspective views of a grid tunnel of the present disclosure; 
     FIG. 7 is a flowchart illustrating the steps involved in the calibration procedure; 
     FIG. 8 is a flowchart illustrating the steps involved in the alignment procedure; 
     FIG. 9 is a flowchart illustrating the steps involved in measuring the position of the x-ray source assembly; 
     FIG. 10 is a flowchart illustrating the steps involved in acquiring an image of the target array; 
     FIG. 11 is a flowchart illustrating the steps involved in the procedure of localizing individual fiducial markers on the target array (illustrated in this embodiment as LEDs); 
     FIG. 12 is a flowchart illustrating the steps involved in the calculating the position of the x-ray source assembly; and 
     FIG. 13 illustrates one embodiment of the x-ray source assembly showing the different degrees of rotation in each component; 
     FIG. 14 is a flowchart illustrating the interaction between the automatic measuring system and the motion control system whereby the position of the x-ray source assembly relative to the console is determined. The arrowed lines indicate the flow of signals of controls. The dotted arrows indicate that the control function is optional. 
    
    
     DETAILED DESCRIPTION 
     The present disclosure provides a device and method to increase x-ray scatter control of mobile radiography equipment through optimal alignment of a focal spot with the focal line of an anti-scatter grid. A mobile radiography device and method according to the present disclosure affords a rapid and accurate alignment between a mobile radiographic device focal spot and the focal line of an anti-scattering grid. In a preferred embodiment, the present disclosure describes a system comprising an x-ray source assembly, an automatic measuring means, a motion control means and a processing means. The automatic measuring means utilizes a detecting means attached to a mobile radiographic system to determine the position and orientation of the grid tunnel relative to the radiographic system by reference to a target array or other external object, a processing means to determine the position and orientation of the anti-scatter grid relative to the a fixed point on the system, as well as the alignment of the focal spot and central x-ray beam relative to the anti-scattering grid for production of an optimal image, and driving means in communication with the processing means to position the x-ray focal spot to a state of alignment relative to the focal line of an anti-scattering grid. It is preferred that a high ratio anti-scattering grid be employed. For the purpose of this specification, a high ratio anti-scattering grid is defined as a grid having a grid ratio of 10:1 or greater. Through the device and method of the present disclosure, the process of positioning the components of a mobile radiographic system to a state of alignment is automated with minimal operator involvement. 
     Referring now to FIG. 3, a mobile x-ray generator system  10  includes a wheeled base  12 , an operator&#39;s console  14 , an x-ray source assembly and a tube housing mounting. The x-ray source assembly preferably has at least one degree of freedom of motion and comprises an x-ray tube housing  22  containing an x-ray source, the tube housing  22  having an x-ray emission aperture (not shown), and a collimator  24  attached to the tube housing  22  and aligned with the x-ray emission aperture. The tube housing mounting has a plurality of degrees of freedom of motion to allow the x-ray source assembly to be positioned at a desired position and orientation. In one embodiment, the tube house mounting comprises an adjustable, vertical column  16 , an adjustable, horizontal arm  20  mounted to the column  16  and an adjustable gimbal  23  for coupling the tube housing  22  to the arm  20 . 
     The mobile system  10  further comprises a processing means, a detecting means (described above as a camera  26 ) in communication with the processing means and the grid tunnel  30  with rigid arm  36  equipped with a target array  28  comprising a plurality of fiducial markers  50 . A detecting means, illustrated in FIG. 3 as an optical detector, specifically as a digital camera  26 , is attached to the system  10 , preferably on collimator  24 . The camera  26  is positioned to produce an image of a target array  28  and its fiducial markers  50  attached to the grid tunnel  30 . The image produced by the detecting means may be any information that allows the processing means to determine the position of the fiducial markers  50 . The grid tunnel  30  incorporates an anti-scattering grid  32  and contains a cavity to receive an image receptor  34  (illustrated in FIGS.  6 A and  6 B). An object to be imaged  1  is interspersed between the collimator  24  and the grid tunnel  30 . An x-ray image receptor  34  is placed proximal to anti-scattering grid  32  and distal from an object, such as a patient  1 . 
     The processing means analyzes images of the target array  28  acquired by the detecting means to determine the position and orientation of the target array  28  (which is equivalent to the position and orientation of the anti-scatter grid  32 ) relative to the detecting means, illustrated as camera  26  (which is equivalent to the position and orientation of the focal spot when the detecting means is positioned on the collimator  24 ). The processing means then calculates the optimal position and orientation of the x-ray tube housing  22  such that the focal spot and central ray are in a state of alignment with regard to the anti-scattering grid  32 . The driving means (not shown) located within the mobile x-ray system  10  are directed by the processing means to position the system to the state of alignment. 
     Referring to FIG. 4, the preferred location  42  of the focal spot is the intersection of the focal line  44  of the grid and a line  46  normal to the surface of the grid that passes through the center of the grid. This location is defined as the optimal focal spot position, and when the focal spot is in this location the transmission of x-rays through the anti-scatter grid is at its maximum value. The x-ray source assembly is in its preferred orientation when the central ray of the x-ray beam passes through the center of the grid, and the long and short axes of the x-ray beam are parallel to the long and short axes of the grid tunnel. When the x-ray focal spot is in its preferred location  42  and the x-ray source assembly is in its preferred orientation, then the system is defined to have optimal alignment. 
     The focal spot is in an acceptable position if the transmission of primary x-rays through the grid is at least 90% of the its maximum value over the entire grid, and if the focal spot is within 5 cm of its ideal location in a direction parallel to the focal line  44 . For example, for a standard size 12:1 grid with a focal length of 100 centimeters, the focal spot position will be acceptable if it is on the focal line  44  and within 5 centimeters from the optimal location  42 , on the normal line  46  and within 2 centimeters from the optimal location  42 , or on a line  45  normal to both grid focal line  44  and the grid normal line  46  and within 0.8 centimeters of the optimal location  42 . Similarly, the x-ray source assembly is in an acceptable orientation if the central ray of the collimated x-ray beam passes substantially close to the center of the grid, and the long and short axes of the collimated x-ray beam are substantially parallel to the long and short axes of the grid tunnel. When the x-ray focal spot is an acceptable position, and the x-ray source assembly is in an acceptable orientation, the system is defined to have acceptable alignment. 
     While it is preferred that the detecting means be affixed to the collimator housing  24 , it is appreciated that the detecting means according to the present disclosure can be mounted in a variety of positions on a mobile x-ray system  10  to provide position and orientation data for control of the x-ray tube housing  22  position adjustment. It is further recognized that other detecting means in addition to a digital camera are operative herein. These additional detecting means may be optical in nature, or be based on other principles such as magnetic interactions, ultrasound, or inertial navigation. Some of these means mat not require the target arms  28 , but may directly detect the grid tunnel  30 , or fiducial markers attached directly to the grid tunnel. 
     In operation according to the present disclosure, grid tunnel  30  is placed under an imaging object  1 , such as a hospital patient. A radiological technician thereafter attaches the rigid arm  36  to the grid tunnel  30 . The arm  36  fits into a socket  40  on grid tunnel  30  and extends past the lateral dimensions of the object  1 . Thus, the end of the arm  36  is visible to the detecting means, in this case camera  26 . The operator places the detecting means in rough alignment with the target array  28 . The rough alignment process may be aided by the use of a positioning means on the detecting means, such as a light, that will assist the operator in aligning the system properly. After the rough alignment, the automatic measuring system (AMS) is activated by the operator. The detecting means collects an image of the target array  28  and delivers the data to the processing means. The processing means calculates the position and orientation of the target array  28 , and therefore the anti-scatter grid  32 , relative to the detecting means, and therefore the focal spot when the detecting means is located on the collimator. Once the AMS calculates the relative position and orientation information, the operator activates the motion control system (MCS). On activation of the MCS, the processing means then directs the drive means to move the system to a state of alignment as determined by the AMS. The detecting means may collect a confirmatory image of the target array  28  to assure proper alignment of the system. 
     The mobile system  10  may be equipped an indicating means to alert the operator of the condition of the system  10 . For instance, the indicating means could be a plurality of indicator lights, such as LED lights. If three indicator lights are used, one light could indicate the detecting means is unable to “see” all the fiducial markers  50  of the target array  28 , two lights could indicate the detecting means “sees” all the fiducial markers  50 , but is not yet aligned, and a third light could indicate the system is ready for use. The indicating means could also be a display panel to graphically display information regarding the condition of the system to the operator. In addition, the system  10  may have at least one control means, such as a button, toggle switch or similar device, on the system  10 , preferably on the collimator handles. One control means will release the drive means and allow the operator to roughly align the tube housing  22  with the target array  28 . Another control means will activate the MCS. The operator will be required to continually depress the control means for the MCS to remain active (referred to as a dead-man switch). If the control means is released during any point at which the tube housing  22  is under control of the MCS, the MCS stops immediately. This is a safety precaution designed to prevent the tube housing  22  or other parts of the system  10  from hitting nearby objects, such as intravenous stands or sensitive medical equipment. 
     The grid tunnel  30  and the rigid arm  36  are shown with greater clarity in FIGS. 5 and 6. The grid tunnel  30  is manufactured from material selected from the group including, but not limited to, rigid sheet metal, carbon fiber composites and impact resistant plastics, such as LEXAN (GE), polycarbonate, ABS and the like, or a combination of any of the above. It is preferred that the grid tunnel  30  is manufactured from carbon fiber composites. The grid tunnel  30  has sufficient strength to support the patient  1 , and is typically designed to support more than 200 kilos. Preferably, the grid tunnel  30  has rounded edge surfaces  38  to facilitate insertion under the hospital patient  1 . The arm  36  supports the target array  28 , and may be constructed of the same materials as the grid tunnel  30 . The arm  36  is adapted to insert within a channel  40  within the grid tunnel  30 . Preferably, a channel  40  is provided along opposing edges of the grid tunnel  30  to accommodate transverse (parallel to the short axis of the grid tunnel) and longitudinal (parallel to the long axis of the grid tunnel  30 ) orientations of the grid tunnel under the patient  1 . The arm  26  is preferably hollow to provide space for the electronic circuitry employed in the mobile system  10 . The electronic circuitry allows the processing means to control the target array  28  (such as activating the fiducial markers  50  in a specific sequence) and allows the target arm to communicate with the processing means so the processing means can determine whether the target arm is in the transverse or longitudinal configuration. This communication allows the processing means to adjust certain parameters of the system  10  (such as the collimator 24 settings). The communication can occur via wireless communications or through wires, however, wireless communication is preferred. This configuration of the target array (which is a proxy for the configuration of the anti-scattering grid  32  and the image receptor  40 ) is important in obtaining optimal image quality. The processing means will determine the configuration of the target arm and orientate the collimator  24  along the long axis of the grid tunnel  30  and adjust the collimator blade settings to adjust the width and length of the x-ray beam to the size and orientation of the image receptor. In addition, the channel  40  will have electrical contacts to determine when the arm  36  is fully inserted into channel  40 . Optionally, a hand grip  42  is included in the grid tunnel  30  to facilitate crude alignment of the grid tunnel  30  beneath patient  1 . Preferably, the target array  28  extends at least five inches beyond the track  40  to ensure visibility when a large patient  1  covers the grid tunnel  30 . 
     FIG. 5 shows a target array according to the present disclosure having a plurality of fiducial markers  50 , the position of the fiducial markers  50  being fixed relative to the grid tunnel  30 , and therefore, to the anti-scatter grid  32 . In the embodiment illustrated in FIG. 6, three markers  52  are provided in the plane of the anti-scattering grid  32  to provide a measure of the distance from the target array  28  to the detecting means (illustrated as camera  26 ), and therefore, the x-ray tube housing  22 . A fourth marker  53  out of plane relative to the markers  52  provides a measure of transverse misalignment. In one embodiment, the fiducial markers are light emitting diodes (LEDs). When LEDs are used as the fiducial markers  50  of the target array  28 , the detecting means collects images of the target array  28  with all the LEDs energized, all of the LEDs non-energized, and each of the four LEDs energized in succession. These images are analyzed by the automatic measuring system (as described below) and the images are converted to position and orientation information of the target array  28  relative to the detecting means. Alternatively, the fiducial markers  50  may be four differently colored LEDs and the detecting means may be a color digital camera. In this embodiment, the system can uniquely identify each of the LEDs with only the collection of two images corresponding to the energized and non-energized states. 
     In the embodiment described, a position and orientation measurement comprises the following steps. First, the detecting means, in this embodiment camera  26 , acquires one or more images of the target array  28 . The processing means analyzes these images to determine 6 parameters that describe the position and orientation of the target array  28  relative to the camera  26 . This process is described diagrammatically in FIG.  9 . 
     Before the system is used clinically, the mobile system  10  undergoes calibration. This calibration step need be performed just once for a given mobile system  10  and grid tunnel  30 , as the calibration information is stored in a calibration file. The first step in the calibration is to generate a correction for the spatial non-linearity of the camera  26 . This is accomplished by acquiring an image of a matrix of black dots, and fitting the measured position of the dots to a mathematical function. Next, the camera  26  is mounted on the collimator  24 , the target arm  28  is mounted on the grid tunnel  30 , and the tube housing  22  is positioned optimally so the x-ray focal spot falls on the focal line of the anti-scattering grid  32 . The techniques involved in centering the focal spot are common the field and are within the ordinary skill of one in the art. The AMS then measures the position and orientation of the fiducial markers  50  on the target array  28  relative to the detecting means. The results of this measurement are stored in the processing means. The process is depicted diagrammatically in FIG.  7 . 
     The first step in the clinical alignment procedure is to determine the position and orientation of the anti-scattering grid  32  relative to the tube housing  22  through the measurement of the position and orientation of the fiducial markers  50  on the target array  28 . The processing means takes this position data and calculates the position of the tube housing  22  relative to the console  14  so that the tube housing  22  will be in a state of alignment relative to the anti-scattering grid  32 , that is, the relative position stored during the calibration of the tube housing (described above). The processing means then directs the drive means of the MCS to move the tube housing  22  to this position so that the focal spot is in a state of alignment with the focal line of the anti-scattering grid  32 . In summary, the AMS determines the position and orientation of the fiducial markers  50  of the target array  28  in relation to the x-ray tube housing  22 , and uses this information to calculate a state of alignment for the system, and the MCS (under the control of the AMS) moves the system to the state of alignment. 
     The position and orientation of the x-ray source assembly has 6 degrees of freedom. Three degrees of freedom allow the x-ray source to move to the central position on the focal line of the anti-scatter grid  32 , two degrees of freedom allow the x-ray source assembly to direct the central ray of the x-ray beam to the center of the anti-scatter grid  32 , and one degree of freedom allows the collimator  24  to align with the long axis of the cassette (discussed in more detail below). The optimal alignment is achieved by the AMS and the MCS. The AMS measures  6  parameters that describe the Cartesian coordinate system of the target arm  28  (or grid tunnel  30 ) in relation to the Cartesian coordinate system of the camera  26  (or other detecting means). Encoders in the MCS measure 6 parameters that describe the Cartesian coordinate system of the collimator  24  in relation to the Cartesian coordinate system of the console  14 . Comparing the MCS parameters to the AMS parameters, it is possible to determine the 6 parameters that describe the Cartesian coordinate system of the grid tunnel  32  in relation to the Cartesian coordinate system of the console  14 , and therefore the 6 parameters that describe the optimal position and orientation of the tube housing  22  relative to the console  14 . This flow of signals and/or controls is illustrated schematically in FIG.  14 . 
     It is appreciated that an acceptable degree of alignment can be accomplished with fewer degrees of freedom in the MCS. For example, with three degrees of freedom the MCS could automatically move the focal spot to the center point on the focal line of the grid, aligning the focal spot with the grid. The user could then manually adjust the collimator orientation, achieving a result that is nearly as good as that obtainable with a six degree of freedom system. Similarly, the source assembly rotation adjustments which are generally small and less important can be done manually to further improve the alignment. In principle, the focal spot could be moved onto the focal line with as few as two degrees of freedom in the MCS, although with no guarantee that it would fall close to the line normal to the center of the grid. Such approaches align the source assembly and grid at the expense of more effort on the part of the user. 
     FIG. 9 diagrammatically illustrates one embodiment of the steps involved in measuring the tube position. An image is acquired from the camera  26  or other detecting means. FIG. 10 shows an example of the image acquisition process. As the acquisition process is initiated, all or some of the LEDs are turned on and a foreground image is obtained (in FIG. 10, assume all LEDs are illuminated). The LEDs are then turned off and a background image is acquired. The LEDs are controlled by the processing means as discussed above. The process may be repeated with less than all of the LEDS illuminated, and less than all of the LEDs turned off. In addition, the background image may be obtained before the foreground image, as the order of acquisition of the images is arbitrary. 
     The acquired image is compressed in order to more efficiently locate the fiducial markers (in FIG. 9, the fiducial markers are LEDs). The LEDs are then located in the compressed image, and the neighborhood (i.e., general area) of the LED is identified. This neighborhood is scanned in the uncompressed image to identify the exact position of the LEDs. One embodiment of a sequence for locating LEDs is shown in FIG.  11 . The foreground and background images are gamma corrected so that the pixel values are proportional to the light intensity of the LED. The gains of the foreground and background are then matched. The background image is then subtracted from the foreground image. A threshold is then applied to the difference image and pixels with intensities above the thresholds are marked as possible candidates for the location of a LED. The LED candidates are traced and analyzed, and candidates that do not meet certain criteria (for example, size, shape, color, intensity, etc.) are discarded. Finally, a list of candidate LED positions is returned to the calling process. 
     By locating the general position of the LEDs in a compressed image, the speed of the process is greatly increased. Any LEDs in the neighborhood are identified and added to a final list of LED locations. The identification steps are repeated until all four LEDs are located and added to the final list of LED locations. If the final list does not contain exactly four LEDs, the tube measurement process is terminated and an error light displayed. If there are exactly four LEDs in the final list, the tube position is calculated (as shown in FIGS.  9  and  12 ). 
     FIGS. 9 and 12 describe how the LED information is analyzed to determine the position of the camera relative to the LED array (i.e., the fiducial markers  50 ), and by inference the position and orientation of the tube housing  22  relative to the grid tunnel  30 /anti-scatter grid  32 . First, the camera linearity calibration is used to convert the centroid of each LED (in pixel units) to a physical position (in cm) projected onto a fiducial image plane. The Marquardt algorithm is used to calculate the position of the LED array relative to the tube housing  22  from these measurements. The Marquardt algorithm is a general iterative algorithm for fitting a non-linear function to a set of data, starting from an initial estimate of the function parameters. The implementation generates the initial estimate by assuming that the distance to the camera is infinite, and that the magnification of the camera image is unknown. The iterations continue until a convergence criterion is reached. The final estimated parameters are considered good if the measured and estimated LED positions match to within some limit (e.g. 0.05 cm). The mathematics involved in the calculation of the algorithm to convert the position measurements of the fiducial markers to a desired position and orientation of the tube housing  22  involve Cartesian coordinate transforms. The details of this field of mathematics are well known to those of ordinary skill in the art. 
     In response to calculation of the optical position and orientation, the drive means, such as servo motors, located within mobile system  10 , position the tube housing  22  to align the x-ray focal spot to an optimal position and orientation for use with the anti-scattering grid  32 . In order to be able to exactly match the position and orientation of the anti-scattering grid  32 , the x-ray tube housing  22  should have six degrees of freedom, as discussed above. Three degrees of freedom correspond to the three spatial dimensions of the focal spot location, two degrees of freedom correspond to the direction (altitude and azimuth) of the central ray of the X-ray beam, and one degree of freedom corresponds to a rotation of the collimator around the X-ray beam. The MCS should have associated with each degree of freedom of motion of the drive means to drive this motion, and an encoder  29  (either relative or absolute) in communication with the processing means to determine the current position of the components. 
     In one embodiment (FIG.  13 ), the X-ray tube housing  22  is mounted in a gimbal  23 . The gimbal  23  is mounted on a horizontal extensible arm  20 , which in turn is mounted to a vertical column  16 . The X-ray collimator housing  24  is mounted on the x-ray tube housing  22 . The arm  20  can be extended or retracted (motion R), moved up and down the column  16  (motion H), and the column can be rotated about a vertical axis (motion Φ). The three motions R, H, and Φ together provide the three degrees of freedom necessary to locate the center of the gimbal  23  at a given spatial location. 
     Once the gimbal  23  is located, the two bearings of the gimbal can be rotated (motions Θ and Ψ) defining two additional directional degrees of freedom. If the focal spot is located at the intersection of the axes of motions Θ and Ψ, then its position is determined uniquely by motions H, R, and Φ. Otherwise, the position of the focal spot is determined also by motions Θ and Ψ as well. The last degree of freedom lies in the rotation Ω of the collimator housing  24  around the central ray of the X-ray beam. 
     When the AMS is activated, an image is acquired of the target arm assembly and the desired position and orientation of the X-ray tube assembly is calculated as described above. The AMS then activates the MCS and the drive means direct motions H, Φ, and R to place the gimbal in its desired location. Once the gimbal is in place, the drive means directs the remaining three motions to orient the x-ray beam and collimator properly. These motions could be activated sequentially or in parallel. Sequential activation would have the advantage of reduced alignment time, but the disadvantage of increased cost and possible distraction of the technologist by a relatively complex motion. 
     Optionally, the operating console  14  is equipped with an inner lock disabling the x-ray exposure until the x-ray focal spot and grid have been aligned according to the present invention. Further, it is appreciated that an increase in tube voltage is expected to provide improved images as compared to imaging done absent an anti-scattering grid. The increase in tube voltage is intended to increase x-ray transmission through the patient  1  and thereby allow a shorter exposure time. Optionally, a mobile x-ray system according to the present disclosure is provided with an alarm system which is activated upon movement of the system  10  absent grid tunnel  30  to prevent accidental loss of the grid tunnel  30  and the target arm  28 . 
     It is appreciated that localization techniques can be performed not only by the optical methods detailed herein, but also through the use of magnetic dipole technology, ultrasound technology, direct mechanical sensing, and internal navigation technology. Magnetic dipole arrays and sensors operating with the benefit of current loops or electromagnets are detailed in U.S. Pat. No. 4,054,881. 
     Inertial navigation technology would differ from other technologies described in that it would independently measure motion of the grid tunnel and either the console or tube housing, producing measurements of the absolute positions of these devices rather than the position of one relative to another. As the process of determining a relative position from two absolute positions is well established, the use of this technology would be essentially the same as for the others. It is possible to manufacture inertial navigation units (INU)  31  (FIG. 13) small enough for this application using MEMS (Micro-electromechanical system) technology. Such units can incorporate acceleration sensors, rotation sensors, or both. 
     The motion of the grid tunnel could be tracked by a single attached INU with 3 acceleration sensors and 3 rotation sensors, or by 3 INUs with two acceleration sensors per INU attached to 3 corners of the grid tunnel, or by other similar combinations. The position of the grid tunnel would then be calculated by tracking its motion from the moment it leaves a mount fixed to the side of the mobile radiographic system console. To track the position of the source assembly one could mount INUs on it. Alternately, one could track the position of the console by mounting INUs in it and then calculate the position of the source assembly using the readings from the MCS. Finally, one could lock the wheels of the console after the grid tunnel is removed from its mount, and assume that the console remains stationary. 
     It is also appreciated that while the invention as described here provides closed loop control of the x-ray source assembly position and orientation, it is also possible to use open loop control. In this approach, the measurement means is equipped with a display that provides the user with information that directly or indirectly describes the position and orientation of the grid tunnel relative to the x-ray source assembly. The measurement means could be digital, such as the camera/target array system described here, or analog, such as the system described by Niklason ([2]Niklason et al.,  Med. Phys . 8:677-681, 1981). Such analog techniques are known in the prior art, and are described in the references. The user then uses this information to control an automatic motion control system to move the x-ray source assembly to an aligned position. For example, the unit could be equipped with a control module, a mechanical linkage between the source assembly and the grid tunnel, and a series of dials mechanically attached to the linkage which indicates the degree of misalignment. The user controls the MCS through the control module, moving the x-ray focal spot until all the dials indicate a value of zero, which indicates alignment. Alternately, the unit could be equipped with an automatic measurement system, a digital display that indicates the degree of misalignment, and a keypad connected to a computer that controls the MCS. The user would enter the displayed numbers on the keypad, and the computer would then calculate the position required to achieve alignment and direct the MCS to move the x-ray focal spot to this position. These examples are mean to illustrative, and are not an exhaustive catalog of open-loop control approaches to achieving alignment. Such approaches are inferior to the closed-loop control approach and require more effort on the part of the user to align the source assembly and grid. 
     Patents and publications mentioned in this specification are indicative of the levels of those skilled in the art to which the disclosure pertains. These patents and publications are incorporated herein by reference to the same extent as if each individual patent or publication was specifically and individually incorporated herein by reference. 
     The foregoing description is illustrative of particular embodiments of the invention, but is not meant to be a limitation upon the practice thereof. The following claims, including all equivalents thereof, are intended to define the scope of the invention. 
     REFERENCES 
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     2. Niklason L T, Sorenson J A, Nelson J A: Scattered Radiation in Chest Radiography.  Med. Phys . 8:677-681, 1981. 
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     4. Niklason L T, Barnes G T, Carson P: Accurate Alignment Device for Portable Radiography.  Radiology  173(P):452, 1989. 
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