Electric lamp and discharge devices: systems – Pulsating or a.c. supply – Transformer in the supply circuit
Reexamination Certificate
2001-01-05
2001-12-11
Riley, Shawn (Department: 2838)
Electric lamp and discharge devices: systems
Pulsating or a.c. supply
Transformer in the supply circuit
C315S246000
Reexamination Certificate
active
06329763
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to pulsed high voltage power supplies and, more particularly, to a pulsed high voltage power supply for use within a radiography system.
2. Description of Related Technology
Generally speaking, radiography and fluroscopy systems include a radiation source that emits high energy photons (e.g., X-rays, gamma rays, etc.) toward a target object and a radiation detector that measures the energy level of photons which have passed through the target object. The radiation detector may, for example, be a charge coupled device (CCD) or a fluoroscope that detects the differential transmission of the high energy photons through the target object to produce images of structures within the target object. These internal images of the target object may be developed and displayed using photographic film and/or may be displayed using a video monitor.
Radiography systems are used in a wide variety of applications and are particularly useful in examining and diagnosing problems with the internal structures of a target object. For instance, in the field of medical diagnostics, medical practitioners use radiography systems to produce radiographic images that reveal the internal conditions of a patient's body. Specifically, radiography systems may be used to assess the condition of damaged or diseased organs, bones, etc. and/or may be used to determine the location of a foreign object within the patient's body. Additionally, radiography systems may be used to determine the internal conditions of machinery and components of a physical plant (e.g., pipes, valves, etc.) to perform preventative maintenance or may be used to perform quality control checks of products being manufactured within a high speed production process.
Of particular concern in using radiography systems for medical applications is that human tissues may be easily damaged by the large doses of radiation which are imparted by conventional radiography systems. Tissue damage is especially critical within the field of pediatrics because children are highly susceptible to tissue damage from exposure to high doses of radiation. In fact, medical guidelines recommend X-ray exposure levels for children that are substantially reduced with respect to the levels acceptable for adult patients. As a result, important developments within the field of radiography have been directed to minimizing the exposure of patients (and medical personnel operating the radiography equipment) to radiation while maintaining or improving radiographic imaging capability.
Additional advances in radiography have been directed to the development of quasi real time imaging capability. With quasi real time imaging, successive radiographic images are acquired at a rate that is perceptible to the human eye (e.g., less than 30 updates or frames per second) and then displayed via a video monitor to a user. Quasi real time radiographic images are particularly useful within the field of medical diagnostics because quasi real time images allow medical practitioners to inspect moving organs, such as the heart, in operation. Additionally, quasi real time radiographic images may be used to view the internal structures of subjects (e.g., patients or any other target objects) that are moving, either deliberately or inadvertently, without blurring of the images. However, because quasi real time video images are updated at rate which is readily perceived by the human eye, the video images “flicker” and, as a result, are generally difficult to view and may be of limited use for diagnostic purposes.
Still other efforts within the field of radiography have been directed to developing portable radiography systems that provide quasi real time imaging capability while addressing the above-noted need to minimize the radiation dosage imparted to a target object. Additionally, these portable radiography systems attempt to provide attributes desirable of equipment designed for field use such as a low cost, lightweight, extended battery powered operation, etc.
Conventional radiography systems typically reduce the radiation dosage imparted to the target object by pulsing the output of the radiation source. In general, these conventional pulsed radiography systems turn the radiation source on and off at a predetermined frequency and duty cycle for a predetermined period of time, which results in an integrated radiation dosage that is at or below desired safe levels. The radiographic images produced by these pulsed systems are acquired during the time intervals when the radiation source is on and are displayed to the user while the radiation source is off and until another image is acquired and ready for display. Typically, these quasi real time medical radiography systems display the images acquired while the radiation source is on using a video monitor that is synchronized with the acquisition of the images.
Traditionally, pulsed radiography systems use an X-ray tube as a radiation source. One common technique of providing a pulsed source of X-rays uses a grid controlled X-ray tube having a constant cathode to anode potential. In a grid controlled configuration, the output of the X-ray tube is gated on and off by applying a series of pulses to the grid terminal, which controls the current flowing between the anode and cathode of the X-ray tube, to generate a corresponding series of X-ray pulses that are directed toward the target object. However, grid controlled X-ray tube configurations are undesirable for many applications because grid controlled configurations result in a radiography system that is heavy, electrically inefficient, and expensive to produce.
More specifically, grid controlled X-ray tubes are significantly more expensive than non-gridded tubes. For example, a grid controlled X-ray tube may cost approximately $10,000, whereas a non-gridded tube having comparable X-ray output characteristics may only cost approximately $200.
Additionally, because grid controlled configurations require a constant high voltage supply to the anode and cathode electrodes of the X-ray tube, the radiography system power supply and the grid controlled X-ray tube continuously dissipate energy and must be capable of operating under high quiescent power levels and high temperatures. These high quiescent energy levels and high operating temperatures increase system material costs, system weight, and reduce overall system performance.
In fact, many commercially available pulsed radiography systems based on grid controlled X-ray tubes, such as those manufactured by Philips Inc., employ oil cooling apparatus and/or must be periodically turned off to prevent overheating and system failure. Further, because grid-controlled X-ray tubes operate at a relatively high temperature, the life expectancy of such tubes is greatly diminished. This reduced life expectancy significantly increases operating costs over the life of the radiography system due to the high costs associated with repeated replacement of a grid controlled X-ray tube. Thus, radiography systems based on grid controlled X-ray tube configurations are undesirable for many radiography applications, particularly for field use applications requiring low cost, reliability, battery powered operation, and ease of portability.
Another common method of providing a pulsed source of X-rays turns the supply voltage (i.e., the anode to cathode voltage) of a non-gridded X-ray tube on and off at a predetermined frequency and duty cycle. Typically, such pulsed supply configurations apply a pulse waveform to the primary winding of a step up transformer and use a conventional diode-based voltage multiplier circuit to further increase the output voltage of the transformer secondary winding to generate a high voltage pulse waveform that is applied across the anode and cathode electrodes of the non-gridded X-ray tube. While these conventional pulsed supply configurations can use relatively inexpensive non-gridded X-ray tubes, they have significant drawbacks. For instance, the diode
Marshall Gerstein & Borun
Riley Shawn
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