In a conventional radiographic system, an x-ray source is actuated to direct a divergent area beam of x-rays through a patient. A cassette containing an x-ray sensitive screen and light and x-ray sensitive film is positioned in the x-ray path on a side of the patient opposite the source. Radiation passing through the patient's body is attenuated to varying degrees in accordance with the various types of tissue through which the x-rays pass. The attenuated x-rays emerge from the patient in a pattern and strike the phosphor screen which in turn exposes the film. The x-ray film is processed to yield a visible image which can be interpreted by a radiologist as defining internal body structure and/or condition of the patient.
In conventional systems of the type described above the x-ray source is mounted to a support structure. Such structure is commonly a ceiling supported, telescoping carriage which permits selection of various source to film distances. The weight of the source and associated componentry are counterbalanced against gravity via a spring motor and a cable/pulley arrangement. A support cable take-up drum or cam is provided to compensate for the variance in the spring tension force over the operative range of spring extension. The take-up drum is provided with a helical groove which receives the support cable. By decreasing the support cable drum take-up radius as the counterbalance springs are extended, a substantially constant counterbalance force is applied to the support cable. Further details of the above described counterbalance system can be found in U.S. Pat. No. 3,902,070 to Amor Jr. et al. which is owned by the present assignee.
It is to be noted that the above described counterbalance system is useful where the center of gravity of the moving component moves in a substantially vertical, straight line.
More recently, digital radiography techniques have been developed. In digital radiography, the source directs radiation through a patient's body to a detector in the beam path beyond the patient. The detector, by use of appropriate sensor means, responds to the incident radiation image to produce analog signals representing the sensed radiation, which signals are converted to digital information and fed to a digital data processing unit. The data processing unit records, and/or processes and enhances the digital data. A display unit responds to the appropriate digital data representing the image to convert the digital information back into analog form and produce a visual display of the patient's internal body structure.
Digital radiography includes radiographic techniques in which a thin spread beam of x-rays is used. In this technique, often called "scan" (or slit) projection radiography (SPR), a spread beam of x-rays are scanned across the patient's body, or the patient is movably interposed between the source and an array of individual cellular detector segments. In such an embodiment, relative movement is effected between the source/detector arrangement and the patient's body, keeping the detector aligned with the beam, such that a large area of the patient's body is scanned by the beam of x-rays.
One such SPR system is described in more detail in U.S. Pat. 4,626,688 to Barnes entitled Split Energy Level Radiation Detection and in the following publication:
Tesic, M. M. et al.; "Digital Radiography of the Chest: Design Features and Considerations For a Prototype Unit", Radiology, Vol. 148 No. 1, pp 259-64, July 1983.
The above described SPR Systems are configured such that the scanning motion is about a substantially vertical axis, i.e., the detector moves along a path defining an arc lying substantially in a horizontal plane.
It has also been proposed to provide an SPR system wherein the scanning motion is about a substantially horizontal axis thereby causing the detector to move along a path defining an arc lying substantially in a vertical plane.
In both configurations the scanning motion is provided by means of electromechanical servo-systems driven by controllable electric motors. An encoder is utilized to provide a closed loop feedback system wherein motor performance is adjusted in accordance with the sensed location of the detector.
In the second system wherein the scanning motion is about a horizontal axis, a difficulty is encountered in that the center of gravity of the rotating body rotates about the pivot point. The torque required to effect scanning motion about the pivot axis varies sinusoidally. A electromechanical servo system designed to compensate for the torque variations would be unduly complex. Further, a motor able to provide sufficient torque to overcome the system torque at the extremes of the scan motion would be large and prohibitively expensive.
It is therefore an object of this invention to provide a lightweight, reliable, simple and inexpensive counterbalance system which compensates for gravitation torques experienced in a system which rotates about a substantially horizontal axis thereby permitting the use of less costly and complex drive components.