Patent Number: 054815860
Section: description

DETAILED DESCRIPTION OF THE INVENTION A mammography machine (200) in accordance with an illustrative embodiment of the present invention is shown in FIG. 4. The system (200) has an x-ray source (3) and an x-ray sensor (1) which is associated with a support platform (6). The support platform (6) supports an object to be examined, e.g., a patient's breast (2). The foregoing elements are all attached to a column (25). The mammography system (200) also includes a CCD detector (5) for forming an image of the x-rayed breast. Alternatively, a film may be used to form the image instead of the CCD detector. The x-ray source (3) has a beam limiting device (4) which defines a narrow beam of x-ray energy (8) covering the sensor (1). It is desirable in such a system to keep the x-ray sensor (1) centered under the x-ray beam (8) as the beam is scanned across the object (2) to be imaged. Thus, the x-ray beam (8) and sensor (1) travel synchronously in the direction indicated by the arrows (111). The x-ray imaging system (200) has a microprocessor (11) which receives predetermined system requirements, such as a sweep time, from a system operator. Based on these inputs, the microprocessor (11) provides a first control signal (12) to a motor controller (13). In turn, the motor controller (13) provides a second control signal (14) to a servo-positioning motor (10). The servo-positioning motor (10) moves the beam limiting device (4) thus scanning the narrow x-ray beam (8). The programming and operation of the microprocessor to perform this function is well known. The microprocessor (11) also provides third control signal (15) to a sensor positioning motor (16). The sensor positioning motor (16) moves the sensor (1) and detector (5). The sensor (1) senses the position of the x-ray beam (8) and provides a fourth control signal (17) containing position information to a feedback amplifier (18). The fourth control signal (17) is dependent on the position of the x-ray beam (8) relative to the sensor (1). This fourth control signal (17) is part of a feedback loop (21) which controls the position of the x-ray beam (8) to keep it centered over the sensor (1). The feedback amplifier (18) compares the fourth control signal (17) to a reference signal (20) and outputs a fifth control signal (19) which represents the difference between the fourth control signal (17) and the reference signal (20). The fifth control signal (19) is input into the servo-positioning motor controller (13) and completes a feedback loop (21). The closed loop feedback (21) comprises the positioning sensor (1), the amplifier (18), the motor controller (13), and the servo-positioning motor (10). The motor controller (13) controls the servo-positioning motor (10) which in turn moves the beam limiting device (4) so that the x-ray beam (8) is centered over the sensor (1). An x-ray exposure is taken by starting the x-ray imaging system (200) with the narrow x-ray beam (8) at one side of the breast support platform (6). The beam limiting device (4) collimates the beam (8) to expose the CCD detector (5). As the exposure begins, the beam limiting device (4) and the sensor (1) transverse the breast (2) in the direction of arrows (111) at a constant velocity. The sensor (1) and detector (5) move synchronously with the beam (8) as driven by the sensor positioning motor (16). The positioning sensor (1) is used in conjunction with a feedback loop (21) to ensure that the x-ray beam (8) is aligned with a specified location such as the center of the sensor (1). The sensor (1) 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. According to one embodiment of the present invention, the sensor (1) is an ionization chamber (300) shown in FIGS. 7(a), 7(b) and 7 (c). The ionization chamber (300) comprises an element 1 and an element 2 located in a conductive plastic housing (80) as shown in FIG. 7 (c) . The housing (80) is filled with a gas and the elements 1 and 2 are spaced apart by 13 millimeters. As shown in FIG. 7(a), element 1 comprises a series of 15 sensing strips (30) separated by resistors (31) of R ohms. The sensing strips are made of copper and have dimensions of 4mm.times.25mm. There are N resistors (31) of R ohms. Element 1 has two output terminals (32) and (33), each terminated by a terminating resistor (34) of R.sub.2 ohms. Resistor (34) is defined by equation (1) EQU R.sub.2 =N.multidot.R/2 (1) For example, R is 100,000 ohms and R.sub.2 is 700,000 ohms. There are 15 strips and N=14 series resistors. The element 2 of FIG. 7(b) is made of aluminum and has dimensions of 35mm.times.75mm.times.0.1mm thick. One of the fundamental properties of x-rays is that they can ionize gases; that is, remove electrons from atoms to form ions, which can be used for measuring and controlling exposure. Gas-filled detectors, regardless of the shape or size, generally use ionization of the gas by the incoming radiation to produce a signal with a corresponding current and voltage. The important characteristic is that the current is directly proportional to or otherwise represents the intensity of the incoming radiation. The invention uses this principle to generate current in the resistors (31) and (34) and produce a voltage which indicates position as well as intensity. When an x-ray photon ionizes the air or gas separating two sensing strips (30), free electrons are generated. A bias voltage for example 300 volts, is applied between terminal 32 of element 1 and terminal 37 of element 2 so that these free electrons generate a current I. The current I flows through the resistors (31) and generates voltages V.sub.1 and V.sub.2 at each output terminal (32) and (33) of element 1 respectively. The voltage levels V.sub.1 and V.sub.2 are proportional to the number of resistors transversed by the ionization current I. For example, if the x-ray energy was all impinging on the left sensing strip, then the voltage at the left output terminal (32) would be ##EQU1## where I is the ionization current. The voltage at the right output terminal (33) would be EQU V.sub.2 =V/3 (3) The voltages V.sub.1 and V.sub.2 go to a differential amplifier circuit (not shown) whose output is the difference between the two voltages. This difference voltage is given by equation (4). ##EQU2## The difference voltage V.sub.1 -V.sub.2 is proportional to the position of the x-ray energy. When the x-ray beam (8) is centered on the ionization chamber (300), the voltage out of the differential amplifier will be 0 volts. If the beam is off-center to the left or to the right, the voltage out of the differential amplifier will be positive or negative, corresponding to the distance from the center of the chamber (300). It can, therefore, be used in a control feedback loop (21) shown in FIG. 4, to keep the x-ray beam (8) centered over the detector (5). According to another embodiment of the present invention, the sensor (1) is a photodiode (400) shown in FIG. 8. The photodiode (400) is similar to the ionization chamber, but instead of using the ionization of gas, the x-ray photons impinge on junction diodes. The photodiode (400) comprises junction diodes (40), series resistors (31) of R ohms and terminating resistors (34) of R.sub.2 ohms. The value R.sub.2 of the terminating resistors (34) is given by equation (1). These junction diodes (40) generate a current I proportional to the x-ray energy. This current I through the series resistors (31) generates the voltages V.sub.1 and V.sub.2 at the output terminals (42) and (43). The difference between the voltage represents the position of the x-ray beam (8) as described above in the ionization chamber implementation. As in the ionization chamber implementation, voltages V.sub.1 and V.sub.2 are used in a control feedback loop (21) shown in FIG. 4, to keep the x-ray beam (8) aligned to the detector (5). Finally, the above described embodiments of the invention are intended to be illustrative only. Numerous alternative embodiments and equivalent structures may be devised by those skilled in the art without departing from the spirit and scope of the following claims.