Patent Number: 054815860
Section: summary

FIELD OF THE INVENTION The present invention relates to an automatic position control system for x-ray machines such as mammography machines. In particular, the present invention relates to a position control system which maintains the position of a scanning x-ray beam in coincidence with the center of an x-ray sensor that moves with the scanning x-ray beam. The present invention also relates to the x-ray sensor used in such a position control system. BACKGROUND OF THE INVENTION FIG. 1 is a schematic diagram illustrating a prior art mammography machine (100). The mammography machine (100) has an x-ray source (103), a beam limiting device (104) and a support platform (106) for positioning a part of the patient's body, which in this case is the breast (102). The mammography machine (100) also includes the x-ray sensor (101) and the CCD detector (105). The x-ray sensor (101) is attached to the CCD detector (105) and moves along the track (107). The x-ray sensor (101) is used to determine the quantity of energy passing through the breast (102). The CCD detector (105) is used to create an image for diagnostic purposes. In other mammography machines, the CCD sensor (105) is not utilized and the image is made on a film. The x-ray sensor (101) is also used in automatic exposure controls to control the time of exposure of an x-ray source (103) and, when film is used, to provide the proper optical density on the film. This type of sensor might include solid state devices, ionization chambers or photo-multiplier tubes. In several applications, a narrow x-ray beam (108) is scanned across the breast (102). A slot scanning x-ray procedure is one such application. In slot scanning applications, a beam limiting device (104) is swept to produce the narrow scanning x-ray beam (108). It is desirable to know the location of the scanning x-ray beam (108) as well as its intensity. To obtain an x-ray exposure, the beam limiting device (104), the x-ray sensor (101) and the CCD detector (102) traverse the breast (102) at a constant velocity. Thus the narrow x-ray beam (108) scans across the breast (102) from right to left as indicated by the arrow (121). During exposure of the breast (102), the CCD detector (105) and sensor (101) move along track (107) from right to left as indicated by the arrows (111). FIG. 2 is an enlargement of the lower portion of the x-ray machine shown in FIG. 1, detailing the x-ray sensor (101), the CCD detector (105) and the track (107) used for movement. FIG. 2 also shows arrows (111) which indicate the direction of movement along the track (107). The movement of the sensor (101) and detector (105) along track (107) is aligned with the scanning narrow x-ray beam (108). The x-ray beam (108) is scanned by a servo-positioning motor (110) which moves the beam limiting device (104). FIG. 3 is an enlargement of the upper portion of the x-ray machine shown in FIG 1, detailing the servo-positioning motor (110) and the beam limiting device (104). Slot scanning applications described in the current literature all rely on mechanical interfaces for moving the scanning x-ray beam (108), sensor (101) and the detector (105). These interfaces are complex and costly and are prone to alignment problems which reduce the quality of the produced image. In short, the quality of a slot scanning system is a function of the location of the scanning x-ray beam (108) as well as its intensity. It is a function of the accuracy of synchronously moving the beam limiting device (104), the sensor (101) and the detector (105) in the same direction (111) across the breast (102). In view of the foregoing, it is an object of the invention to provide an automatic position control system for an x-ray machine such as a mammography machine in which the x-ray beam and x-ray sensor synchronously scan across the breast or other part of the patient's body. It is a principal object of the invention to provide an x-ray machine especially for mammography which includes an automatic position control system for maintaining the position of an x-ray beam with respect to an x-ray sensor without cumbersome mechanical interfaces. It is also an object of the invention to provide an x-ray sensor for use in such a position control system, which sensor outputs a signal indicative of the position of a narrow x-ray beam. SUMMARY OF THE INVENTION This and other objects are achieved by the present invention which provides an automatic position control system for x-ray machines for use in mammography or other applications. The control system maintains alignment of a scanning narrow x-ray beam with a sensor. The present invention provides a servo-driven motion control system which automatically aligns the x-ray beam with the x-ray sensor. According to one embodiment of the invention, a narrow x-ray beam is produced. The narrow x-ray beam illuminates the breast (or other body part) of the patient. The x-ray beam, after passing through the breast, is sensed by an x-ray sensor. The narrow x-ray beam and sensor move synchronously to scan across the breast. The x-ray beam is scanned by a servo-positioning motor which controls the position of a beam limiting device. The sensor is moved by a sensor positioning motor. The feedback system operates as follows: The sensor senses the position of the narrow x-ray beam relative to the sensor and sends a signal representing the x-ray beam position to a feedback amplifier. The feedback amplifier compares the signal representing the position of the x-ray beam with a preset reference signal and sends the difference (i.e., error signal) to a servo-positioning motor controller. This motor controller controls the servo-positioning motor which moves the beam limiting device to center the narrow x-ray beam over the sensor. The sensor can be implemented by various technologies, including but not limited to an ionization chamber, a photodiode array, a CCD array or a photocell with phosphor. Illustratively, the sensor receiving the x-ray radiation is an ionization chamber which generates an ionization current. Such an ionization chamber comprises two elements (i.e., electrodes) separated by a gas or air. One of the element comprises a plurality of sensing strips separated by resistors. Furthermore, this element has two terminals each terminated by a terminating resistor. The voltage between the two terminals is a signal representing the position of the x-ray beam. According to another embodiment of the x-ray sensor, a plurality of photodiodes are connected in series and separated by resistors. The sensor has two terminals (at either end of the series connected photodiodes), with each terminal being terminated by a resistor. The voltage between the two terminals is a signal which represents the position of the x-ray beam. The present invention solves the problem of simultaneously correlating and achieving the correct alignment of the sensor and the beam limiting device without cumbersome mechanical interfaces. The x-ray positioning sensor of the present invention allows for an automatic alignment through its ability to sense the position of the scanning narrow x-ray beam. The mechanical mechanisms used in the prior art to move the sensor are replaced by a simple servo-driven motion control system.