1. Field of the Invention.
The present invention relates to the field of X-ray equipment and more specifically to mobile C-arm X-ray imaging systems.
2. Related Application.
The present application is related to a copending U.S. application Serial No. 083,626, filed Aug. 7, 1987, entitled "Dynamic Calibration for an X-Ray Machine", and assigned to the assignee of the present application.
3. Prior Art.
The use of X-ray equipment for medical diagnostics is well known in the prior art. The earlier X-ray devices were fixed devices requiring the patient to be brought to the unit for medical diagnosis. The earlier techniques involved a simple concept of shooting an X-ray beam through the patient and having the beam impinge on a photographic film which resulted in a exposure of the film termed a "negative". As technology advanced, more sophisticated X-ray equipment became available. Instead of just providing a single exposure onto a photographic film, imaging systems were developed which permitted the diagnostician to view an image on a video monitoring screen. Further, with the advent of computer systems, it became possible to store image information for future use. In some instances, the operation of the X-ray equipment was under a control of a computer system.
However, the basic concept of taking an X-ray exposure has not changed since the earliest devices. That is, an X-ray emitter, such as an X-ray tube, transmits an X-ray beam and a receptor is disposed to receive this X-ray beam. When a patient (or an object to be viewed) is to be imaged, the patient is placed in the path of the beam between the emitter and the receptor. A typical X-ray tube is comprised of a filament, which also operates as a cathode, and an anode which includes a target. When electrons emitted from the filament strike the anode, X-ray photons are generated. The energy of the resultant X-ray beam is determined by the voltage potential (KVP) between the filament and the anode and the quantity of X-ray photons generated is determined by the rate of electron emission of the filament. Therefore, the filament current, which determines the mount of heating of the filament element, is a key factor in determining the characteristics of the X-ray beam. For each X-ray device manufactured, the characteristics of the primary beam are substantially dependent on the combination of filament current and anode voltage of the tube. An objective of an X-ray control circuitry is to set the proper value of filament current and voltage potential that will "give rise to" the desired energy (KVP) and intensity of the X-ray beam as specified by an operator.
Because diagnostic instrumentation devices subject live patients under conditions which may prove harmful when improperly used, such equipment must meet stringent safety requirements. Typically, the equipment is calibrated at the factory prior to initial use. Then field calibration is required prior to regular operation of the equipment, as well as continuing calibration maintenance to keep the equipment within required tolerances. Prior art calibration techniques involve the process of coupling supplemental calibrating equipment to the X-ray device and fine tuning the x-ray device for meeting calibration tolerances. The most common prior art method for sensing beam current and anode voltage requires the use of dynalyzers. In most instances this method is adequate, as prior art practices have shown, but present difficulties.
Calibration equipment in most instances are supplemental devices which are coupled to the X-ray device for obtaining calibration measurements. That is, the calibration equipment does not provide a true representation of the actual in-circuit parameters when the X-ray device is operating normally, because the calibration equipment is not part of the X-ray device itself.
Further, anode voltage and tube current are not independent parameters. Typically it is difficult to vary one parameter without affecting the other. During calibration it is difficult to keep one parameter constant while attempting to calibrate the equipment for the other parameter. An anode voltage measurement obtained by the use of non-invasive measurement devices still will affect the tube current parameter, such that calibration limitations will restrict the sensitivity of the X-ray device. In the measurement of tube current with external equipment, such as the dynalyzer, the X-ray device is extremely sensitive to current offset problems, due to noise, ground loops and misadjustments. Due to the errors encountered when external devices are connected to the X-ray equipment it is difficult to derive a reliable method to measure tube current externally on prior art X-ray systems. This is especially true in the calibration of a system having low milliampere tube current values. In this instance more error is introduced than is removed during a calibration procedure. Although non-invasive methods may reduce this error somewhat, the potential for calibration induced errors are still present. Also, calibration procedures performed by a calibration technician may require repetitive adjustments of the anode voltage and tube current values to derive at an acceptable tolerance zone.
It is to be appreciated that what is needed is a means to adjust dynamically the X-ray control parameters, KVP and tube current, in response to the internal sensors in a manner that reduces or eliminates the errors that occur due to the use of intrusive test equipment.
It is appreciated that what is needed are permanent sensors which are part of the actual device such that non-invasive calibration measurements can be obtained without altering the operative circuit parameters. Such internal sensors, can then be used for self calibration while the device is functioning.
Also, what is needed is a means to perform a calibration that minimizes the calibration error in an optimum manner. Such a method would utilize data acquired from internal sensors to perform the initial calibration and maintain that calibration over time.
A further concern of modern day X-ray equipment deals with the portability of such equipment. In most instances it is more cost efficient to bring the equipment to the patient instead of bringing the patient to the equipment. The advantage is not only in the cost but concern for the patient in not having to be transported. Therefore, a trend has been to develop a class of mobile X-ray equipment which can be transported to the patient for providing X-ray imaging.
In most instances, these mobile X-ray units are self contained except for the power unit. With the prior art X-ray units a specialized power source is needed to operate the unit. For example, when these units are used in a hospital, a special power source, such as 220 VAC are required to operate the unit. These special power requirements, such as the 220 VAC outlet, severely restrict the mobility of these units because the units must be in proximity to the specialized outlet. Attempts to develop mobile X-ray units which can be plugged into ordinary house current, 110 VAC, could not meet the power requirements necessary for high-intensity beam generation.
It is appreciated that what is required is an X-ray machine which is capable of providing the high voltage and current power source requirement to its X-ray tube such that high precision operation of the device can be achieved, yet having such a device operate from ordinary 110 VAC.