Abstract:
A variable self-compensating detent control system for improved positioning accuracy and repeatability is provided. The detent control system provides a system for reducing positioning errors in the positioning of an X-Ray tube in an X-Ray imaging system, such as accurate and repeatable positioning of the X-Ray tube at detents. The control system preferably includes a sensor unit generating positional or velocity signals indicative of the position or velocity of the X-Ray tube and a microprocessor receiving the positional signals and determining an overshoot correction. The overshoot correction is used by the X-Ray system to control a locking system controlling the position of the X-Ray tube. The sensor unit may employ a potentiometer, a digital encoder, or preferably both in combination to determine the positional or velocity signals.

Description:
BACKGROUND OF THE INVENTION 
     The preferred embodiments of the present invention generally relates to improvements in a medical X-Ray imaging system, and more particularly relates to an improved positioning control for positioning an imaging X-Ray tube. 
     FIG. 1 illustrates an exemplary medical X-Ray imaging system  100 . The imaging system  100  includes a X-Ray tube  110 , a collimator  120 , a table detector  130 , an X-Ray table  140 , a patient  150 , and a clinical operator  160 . In operation, a patient  150  to be imaged is placed upon the X-Ray table  140  as shown. A clinical operator  160 , such as a radiologist or technologist, then positions the X-Ray tube  110  and collimator  120  at one of several pre-determined positions relative to the patient. Once the clinical operator has positioned the collimator  120  at the desired position, the X-ray tube  110  is energized and emits X-Rays. The X-Rays pass through the collimator  120  which directs the X-Rays through the patient to the table detector  130 . The energy of the X-Rays passing through the patient is attenuated by the anatomical features of the patient  150 . The table detector  130  detects the energy of the X-Rays and develops an image of the anatomical features of the patient  150 . 
     The X-Ray tube  110  and collimator  120  are typically fixed together to form an X-Ray assembly and are typically able to move in three dimensions relative to the X-Ray table  140 . That is, the collimator  120  may be moved upward or downward along the patient&#39;s  150  body, right to left across the patient&#39;s  150  body, and closer to or farther from the patient&#39;s  150  body in any of several fixed positions called detents. Each of the several fixed positions or detents may correspond to different X-Ray exposure and imaging parameters that have been predetermined in order to produce the clearest possible images of the patient  150 . For example, placing the collimator  120  farther from the patient may result in a different parameter for dynamic range of energy of the X-Rays received by the detector  130 . 
     Typically, imaging parameters are calibrated only for the several predetermined fixed positions, and not continually throughout the path of movement of the collimator  120 . That is, the imaging parameters are typically configured for only a single, specific position, and may change rapidly as the collimator is moved. Thus, precise positioning of the collimator  120  helps provide clearer, more clinically relevant images of the patient  150 . 
     Referring to FIG. 1, typically, an medical X-Ray imaging system may employ and configure detents to identify the several fixed imaging positions for radiographic examinations. As the collimator  120  is moved to one of several fixed imaging positions, a detent is engaged which holds the collimator  120  in the desired position while imaging takes place. Detents may be mechanical or electrical, however, detents employing electromagnetic locks and a position reference triggering device may preferably be employed because of, for example, better wear properties. 
     Positioning errors as small as a millimeter may significantly reduce the quality of the resulting image. For example, anatomical cutoff may occur due to misalignment or misregistration of the beam with respect to the detector. Improving positioning control of the X-Ray tube also aids in the repeatability of X-Ray images which may be of great importance in comparing X-Ray images taken at time intervals during a patient&#39;s treatment. Thus, a need exists for an improved X-Ray tube and collimator positioning system for a medical imaging system. 
     BRIEF SUMMARY OF THE INVENTION 
     The preferred embodiments of the present invention provide a system for reducing positioning errors of an X-Ray tube in an X-Ray imaging device. The system facilitates the accurate and repeatable positioning of the X-Ray tube at detents. A preferred embodiment of the present invention preferably includes a sensor unit generating positional or velocity signals indicative of the position or velocity of the X-Ray tube and a microprocessor receiving the positional signals and determining an overshoot correction. The overshoot correction is then used by the X-Ray system to control a locking system controlling the position of the X-Ray tube. The sensor unit may employ a potentiometer, a digital encoder, or preferably both in combination to determine the positional or velocity signals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a conventional exemplary medical X-Ray imaging system. 
     FIG. 2 illustrates an exemplary detent positioning system for a medical X-Ray imaging system according to a preferred embodiment of the present invention. 
     FIG. 3 illustrates a locking system of the medical X-Ray imaging system according to a preferred embodiment of the present invention. 
     FIG. 4 illustrates a top view of the electromagnetic locks of FIG. 3 according to a preferred embodiment of the present invention. 
     FIG. 5 illustrates a calibration sequence according to a preferred embodiment of the present invention. 
     FIG. 6 illustrates a flowchart of the calibration system according to a preferred embodiment of the present invention. 
     FIG. 7 illustrates a sensor unit with a self-tensioning belt assembly according to a preferred embodiment of the present invention. 
     FIG. 8 illustrates a sensor unit according to a preferred embodiment of the present invention. 
     FIG. 9 illustrates a top view of the sensor unit  800  of FIG. 8 according to a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 illustrates an exemplary detent positioning system  200  for a medical X-Ray imaging system according to a preferred embodiment of the present invention. The detent positioning system  200  includes an X-Ray tube  210 , a X-Ray assembly  205 , a pair of vertical rails  230 , a pair of horizontal rails  240 , and a sensor unit  275 . The X-Ray tube  210  and collimator  220  are collectively known as an X-Ray assembly  205 . Both the horizontal rails  240  and the vertical rails  230  include a number of detents  250 . In operation, the X-Ray assembly  205  is moved in two dimensions along the vertical rails  230  and horizontal rails  240 , first by sliding the X-Ray assembly  205  and vertical rails  230  within the horizontal rails  240  to a detent  250  position on the horizontal rails  240 . Then the X-Ray assembly  205  is slid within the vertical rails  230  to a detent  250  position on the vertical rails  230 . Preferably, at each detent  250 , electromagnetic locks are employed to lock the collimator in the desired detent position. The sensor unit  275  will be discussed below in detail. 
     FIG. 3 illustrates a locking system  300  of the medical X-Ray imaging system according to a preferred embodiment of the present invention. The locking system  300  includes electromagnetic locks  310 , a bridge rail  320 , and a power supply  330 . In operation, the locking system  300  is mounted inside the vertical rails  230  and horizontal rails  240  of the detent positioning system  200  of FIG.  2 . Once a given detent  250  position is reached, the electromagnetic locks  310  are activated and the position is locked in place. The electromagnetic locks  310  are activated by a voltage supplied by the power supply  330 . 
     FIG. 4 illustrates a top view  400  of the electromagnetic locks of FIG. 3 according to a preferred embodiment of the present invention. The view  400  includes electromagnetic lock coils  410 , a lock strip  420 , and bearings  430 . In operation, as discussed above, the electromagnetic lock coils  410  may be slid inside a rail until they are activated by an externally supplied voltage. The externally supplied voltage generates a magnetic force between the electromagnetic lock coils  410  and the lock strip  420  sufficient to maintain and secure the collimator in a fixed position. 
     In operation, an electromagnetic lock requires a certain, finite time to develop sufficient magnetic force to begin decelerating the collimator  120 . In addition, some time is required before the electromagnetic lock develops sufficient force to hold the collimator  120  in place. Referring to FIG. 2, because the X-Ray assembly  205  (and their support/positioning apparatus) have significant mass, and consequently significant momentum while being positioned by a clinical operator, the magnetic force generated by the electromagnetic locks may not be sufficient to overcome the momentum of the X-Ray assembly  205  within a desired time and, consequently, the X-Ray assembly  205  may not be stopped precisely at the desired detent. Thus, the activation and stopping time of the electromagnetic locks may introduce a positioning error in the positioning of the collimator. As mentioned above, this positioning error may adversely affect the quality and repeatability of the X-Ray images. 
     To put it another way, the speed at which the X-Ray assembly  205  is being positioned by an operator along with the electromagnetic lag or time delay of the electromagnetic lock may contribute to a final positioning error if the initial speed of the X-Ray assembly  205  is below a critical value (V c ). This positioning error is approximately proportional to the approach speed of the X-Ray assembly  205  to the detent position. However, if the speed of the X-Ray assembly  205  is sufficiently high, the electromagnetic lock may not react completely to engage and hold the device. If the electromagnetic lock does not engage completely, the X-Ray assembly  205  may simply pass through the intended detent location. Because the lock may not fully engage and hold the collimator at higher speeds, the operators must begin to slow down as they approach the detent position so that the X-Ray assembly  205  may be positioned and locked at the preset, pre-configured detent position. Additionally, unless the incoming speed is quite slow, the final offset positioning error may be significant, that is, from five to ten millimeters. Consequently, because the X-Ray assembly  205  must be moved slowly, additional time may be required. When additional time is required, customer productivity may be adversely affected because of the additional time per image. 
     In order to counter these effects, the preferred embodiment of the present invention calibrates a positional control system by measuring the detent positional overshoot at various approach speeds. The positional overshoot may be determined by using electronic feedback as further described below. Next, a transfer function between speed and overshoot is developed in order to determine the overshoot correction. Finally, the overshoot correction is applied to the collimator positioning during clinical use. Detent positional overshoot is preferably measured by using a microprocessor-based positioner control wherein both position and velocity feedback is available as described below with reference to FIGS. 8-10. 
     FIG. 8 illustrates a sensor unit  800  according to a preferred embodiment of the present invention. The sensor unit  800  includes an encoder sprocket  810 , a potentiometer sprocket  820  having an alignment mark  830 , a position sensor belt  840 , a belt tensioner screw  850 , a drive belt assembly  860 , and a belt displacement sprocket  870 . The position sensor belt  840  passes over the encoder sprocket  810  and the potentiometer sprocket  820 . The tension on the position sensor belt  840  may be adjusted to a desired tension by use of the belt tensioner screw  850 . 
     The X-Ray assembly, and thus the attached sensor unit  800  is typically manually positioned. Preferably, however, the sensor unit  800  is motor driven and positioned. For example, the sensor unit may be motor driven with a closed loop servo motor using the drive belt assembly  860 . Positioning the sensor unit  800  using a motor, instead of manually, may help ensure consistent placement of the X-Ray assembly at the detent positions. 
     FIG. 9 illustrates a top view  900  of the sensor unit  800  of FIG. 8 according to a preferred embodiment of the present invention. The encoder sprocket  810 , potentiometer sprocket  820  and belt displacement sprocket  870  are shown. The sensor unit  800  also includes a drive belt assembly  910 , a microprocessor interface  920 , and securing points  930 . The sensor unit  800  is preferably mounted on the X-Ray assembly as shown in FIG.  2  through the use of securing points  930 . 
     In operation, the sensor unit  800  is associated with motion of the X-Ray assembly  205  along each of the rails. That is, one sensor unit  800  provides data concerning motion of the X-Ray assembly  205  along the pair of vertical rails  230  and one sensor unit provides data concerning motion along the pair of horizontal rails  240 . A notched drive belt (not shown) is preferably mounted inside at least one of the pair of vertical rails  230  and in at least one of the pair of horizontal rails  240  of FIG.  2 . The drive belt is preferably secured at each end of the rail and passes through the drive belt assembly  910  of the sensor unit  800  of FIG.  9 . As the X-Ray assembly  205  is displaced, the fixed drive belt passing through the drive belt assembly  910  induces motion of the position sensor belt  840 . The motion of the position sensor belt  840  induces revolution of the encoder sprocket  810  and the potentiometer sprocket  820 . 
     The potentiometer sprocket  820  preferably includes an analog potentiometer. Preferably, a voltage is induced across the potentiometer so that the voltage changes with the rotation of the potentiometer sprocket  820 , and thus with the position of the X-Ray assembly  205 . The encoder sprocket  810  preferably includes a digital encoder. Preferably, the digital encoder provides data regarding the position and velocity of rotation of the encoder sprocket  810 , and thus the position and velocity of the collimator. Preferably, the potentiometer sprocket  820  is used to establish an initial position for the X-Ray assembly  205  when the collimator is initially powered-up. The encoder sprocket  810  may be unable to provide this initial information because of data loss at the previous system shut-down. However, the initial position for the X-Ray assembly  205  is recoverable from the potentiometer sprocket  820  because the rotation of the potentiometer sprocket  820  alters its included potentiometer mechanically and thus avoids loss-of-power difficulties. 
     Once the initial position of the X-Ray assembly  205  has been established by the potentiometer sprocket  820 , the encoder sprocket  810  may be employed to provide highly accurate position and velocity information. The digital encoder of the encoder sprocket  810  preferably provides a clean, digital signal indicating the position of the X-Ray assembly  205  which may be easily analyzed to determine velocity information. The potentiometer sprocket  820  may be utilized to provide positional information regarding the X-Ray assembly  205  throughout operation, but the digitally encoded signals from the encoder sprocket  810  may be easier and simpler to use. 
     The initial positional information determined by the potentiometer sprocket  820  and the positional and velocity information determined by the encoder sprocket  810  are passed to an external microprocessor (not shown) by means of the microprocessor interface  920 . As further described below, the microprocessor may analyze the positional and velocity information of the X-Ray assembly  205  to control the activation of the electromagnetic locking system  300  of FIG. 3, above. The microprocessor is typically housed within an external system cabinet. 
     Before use, the sensor unit  800  is calibrated to the specific rail for which it is to provide positional and velocity information. The potentiometer insider the potentiometer sprocket  820  is preferably a multiple-turn potentiometer (most preferably a 10-turn potentiometer) with hard stops at each end of its travel To calibrate the system, the potentiometer may be first rotated to a hard stop and then rotated to the middle of the potentiometer&#39;s range (in the case of a 10-turn potentiometer, 5 turns). The sensor unit  800  including the potentiometer may then be positioned at the center of its path of movement along the rail and the drive belt assembly  910  and position sensor belt  840  engaged. Additionally, the sensor unit  800  may be calibrated by adjusting the tension of the position sensor belt  840  using the belt tensioner screw  850 . 
     FIG. 7 illustrates a sensor unit with a self-tensioning belt assembly  700  according to a preferred embodiment of the present invention. The self-tensioning belt assembly  700  includes an encoder sprocket  710 , a potentiometer sprocket  720 , an alignment mark  730 , a position sensor belt  740 , and a drive belt assembly  760 , similar to the sensor unit  800  of FIG.  8 . The self-tensioning sensor unit  700  also includes a tensioner arm  750 , instead of the belt tensioner screw  850  of the sensor unit  800  of FIG.  8 ,which automatically applies a desired tension to the position sensor belt  740 . Either the sensor unit  800  of FIG. 8 or the self-tensioning sensor unit  700  of FIG. 7 may be employed in the preferred embodiment of the present invention. 
     Once sensor unit has been selected and installed, the potentiometer sprocket of the sensor unit is calibrated and position sensor belts are engaged as described above. Then the assembly positioning system is calibrated. In order to calibrate the assembly positioning system, the collimator assembly is set into motion and information concerning the position and velocity of the collimator are sent to the microprocessor. A detent latch is then simulated. That is, power is applied to an electromagnetic lock on the X-Ray assembly and the assembly is brought to a halt. The position at which the assembly comes to rest may be different from the desired, predetermined, pre-configured, detent position. The difference in position between the detent position and the actual position of the assembly is then analyzed and an overshoot correction is determined. 
     FIG. 5 illustrates a calibration sequence  500  according to a preferred embodiment of the present invention. First, at location  510 , the X-Ray tube assembly is in motion at some initial velocity, V o , which is greater than zero and is located at an initial position, X o , also greater then zero. Then, the electromagnetic lock is engaged. The electromagnetic lock applies a braking force in the opposite direction of the motion of the assembly. The tube assembly then comes to rest at location  520 , that is, the final velocity V f  is equal to zero and at the assembly is located at a final position X f . Then the overshoot, ΔX, the change in position between the initial position X o  where the electromagnetic lock was activated and the final position X f  where the assembly came to rest is determined at  530 . Once the initial and final velocities and positions have been determined, the braking force may be determined at  540 . The mass of the assembly is known and does not change during the calibration process. The calibration sequence is then repeated at several different initial velocities and an empirical relationship between the initial speed V o  and the overshoot ΔX is determined to determine an overshoot correction. 
     The overshoot correction may, for example, be expressed as a linear relationship based on a least-squares regression fit of several speed-overshoot calibration tests. This linear relationship may be expressed as 
     
       
         Δ X=B   0   +B   1   V   
       
     
     Alternatively, the overshoot correction may, for example, be expressed as a more genera non-linear polynomial form such as: 
     
       
         Δ X=A   0   +A   1   V   0   +A   2   V   0   2   +A   3   V   0   3   +A   4   V   4   4 + . . .  
       
     
     where the order of the polynomial depends upon the number of discrete speeds incorporated in the calibration process. 
     Once the overshoot correction has been determined, the overshoot correction is used to determine the position at which the electromagnetic brake should be enabled by the system so that the assembly comes to rest at the desired detent position. That is, the calibration sequence determines the position at which the brake should be enabled by the system controller in order to minimize the position overshoot with respect to the detent position target, as a function of the initial velocity of the tube assembly. 
     A second embodiment of the present invention includes providing continuous positional error monitoring. That is, instead of only using the velocity and position references from an initial calibration process, continuous positional sensing is provided. If the detent positional error exceeds a certain maximum, the operator may be notified, the electromagnetic lock may disengage, and the operator may re-position the assembly. 
     A third embodiment of the present invention includes adaptively calibrating the offshoot correction by continuously updating the offshoot correction after each positioning of the tube assembly. That is, each time the assembly is positioned at a detent, the initial velocity and positional error are measured. The velocity and positional error measurements may then be used to generate a corrected offshoot correction for the assembly. This embodiment also allows the positioning system to compensate for system degradations that occur with use. For example, sustained use of the assembly may result in increased friction in the rails, which may cause the assembly to stop more quickly. By adaptively calibrating the offshoot correction, the effect of increased friction may be minimized and the assembly continuously positioned with minimal positional error. 
     By employing any of the embodiments of the present invention to generate an overshoot correction, the alignment between the X-Ray tube and detector assembly is made more accurate and repeatable than with existing implementations that employ only detents and that do not incorporate the velocity feedback and predictive algorithms of the preferred embodiments of the present invention. 
     The improvements in accuracy and repeatability of positioning provided by the present invention may also minimize radiographic re-takes associated with a variety of factors such as patient anatomical cutoff. Patient anatomical cutoff occurs when an X-Ray image does not contain the desired anatomical information and must be re-taken. Because one of the significant causes of patient anatomical cutoff is positioning error of the assembly, by minimizing positioning error of the assembly, patient anatomical cutoff may also be reduced. Additionally, the present invention may also improve customer productivity in a number of ways. For example, the operator may position the X-Ray assembly rapidly without fear of positional error. Thus, the speed of positioning the assembly is increased and the additional time associated with radiographic re-takes is minimized. 
     FIG. 6 illustrates a flowchart  600  of the calibration system according to a preferred embodiment of the present invention. First, at step  610 , the X-Ray tube assembly is in motion. At step  620 , the electromagnetic lock is activated and the initial velocity V o  and position X o  are determined. Next, at step  630 , the X-Ray tube assembly comes to a halt and the final velocity V f  and position X f  are determined. Then, at step  640 , the initial X o  and final positions X f  are used to determine the overshoot ΔX. Then, at step  650 , steps  610  to  640  are repeated a predetermined number of times at differing initial velocities to generate an empirical relationship between the initial speed V o  and the offset, ΔX. Next, at step  660 , the results of the repeated measurements at different initial velocities are used to determine an overshoot correction. Finally, at step  670 , the overshoot correction is applied to the motion of the X-Ray tube assembly during clinical use. As mentioned above, to implement the third embodiment of the present invention, steps  610  to  640  may be repeated for each clinical positioning of the assembly. 
     While the invention has been described with reference to a preferred embodiment, 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.