X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-10-12
2001-10-02
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S901000
Reexamination Certificate
active
06298109
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to x-ray equipment and in particular to x-ray equipment providing real-time imaging with rapid, automatic adjustment of x-ray voltage and current, and with image correction and image rotation.
BACKGROUND OF THE INVENTION
Current x-ray imaging systems may employ an x-ray tube providing a beam of x-rays emanating from a focal spot of the x-ray tube. The x-rays may be received by an image intensifier producing a visible image recorded by a video camera or the like. An object to be imaged is placed within the cone beam of x-rays and the video camera records an image indicating the attenuation of the x-ray beam by the imaged object.
An x-ray tube provides an electrical cathode within an evacuated envelope. Electrons generated at the cathode are accelerated against a target anode to produce x-rays. Controlling the electrical current generally affects the number of x-ray photons per unit time or fluence of the x-ray beam. Controlling the voltage between the cathode and anode affects the energy of each photon or the “hardness” of the x-rays.
In producing an x-ray image, it is often desirable to limit the dose to the extent possible. At the same time, it is desired that the imaging technique, i.e. the voltage across the tube and the current provided to the x-ray tube, be properly adjusted to provide an image with adequate detail. Generally, this adjustment considers the contrast in the image and its signal-to-noise ratio.
The correct technique varies considerably depending on the object being imaged. For medical imaging, it is known to provide certain preset techniques for different body parts. However, use of these presets requires the operator to identify the body part being imaged and will typically be less than optimum as a result of variations in particular patients and even the particular portion of the body part being imaged.
For many imaging situations where real-time imaging is required, it would be desirable to be able to turn the x-ray tube on and off on demand to obtain an instantaneous image and then to stop additional doses. The time required to adjust the proper technique for the particular imaged object is a significant obstacle to this goal.
Automatic exposure control (AEC) of an x-ray tube by varying the current to the x-ray tube based on the flux received by the image intensifier is known. Such AEC systems often work poorly when the imaged object is smaller than the field of view (FOV) of the system and therefore where some unattenuated x-rays are received by the image intensifier. In such cases, the AEC tends to overly decrease the x-ray fluence producing an image of the object that is too dark.
It is known to lower the total dose needed to produce a fluoroscopic image through the use of an image intensifier which uses electrical fields to accelerate photon produced electrons against a phosphorescent target. Such image intensifiers tend to distort the image. Such distortion detracts from the usefulness of the image when instruments are manipulated by an operator viewing the image, especially near the edges of the field of view. Distortion also adversely affects quantitative uses of the image such as morphometric or densiometric analysis.
X-ray imaging systems having movable x-ray tubes and image intensifiers may produce an image on a stationary monitor that appears to rotate depending on the orientation of the machine. Often the operator will desire that the rotational orientation of the image be corrected to provide more intuitive view of the object. This is particularly the case in medical systems where the x-ray image is used to guide medical instruments. Prior art has addressed this problem through the use of a motorized rotating camera or movable deflection yokes on the display screen itself. Both of these approaches provide real-time rotated images.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a robust, automatic, technique-control for an x-ray tube that permits short exposures of an x-ray machine to be made without time consuming technique correction. Computerized analysis of the received image is used to identify the effect of changes in the technique on the image so that the correct technique may be rapidly selected.
Specifically, the present invention provides an x-ray imaging system having an x-ray source positioned on one side of an object to be imaged. The x-ray source is attached to an x-ray source power supply providing electrical energy to the x-ray source. An imaging x-ray detector is positioned on an opposite side of the object from the x-ray source and produces first x-ray reception signals each related to received x-rays passing along a path through the object; and second-x-ray reception signals each related to received x-rays passing along paths outside of the object. An electronic computer receiving the x-ray reception signals operates according to a stored program to identify the first x-ray reception signals and to control the x-ray tube power supply to adjust the exposure of the object based on the identified first x-ray reception signals.
Thus, it is one object of the invention to provide control of the exposure of the imaged object based only on the portion of the image attenuated by the object. By eliminating consideration of background portions of the image, exposure errors resulting from the imaging of small objects that only partially fill the field of view of the imaging system are eliminated.
The electronic computer may identify the first x-ray reception signals by constructing a histogram of signal values versus frequency of occurrence of particular signal values and identifying a peak within the histogram as second x-ray reception signals to be ignored.
Thus, it is another object of the invention to provide an easily automated method determining which portions of the image represent the imaged object when the imaged object may be in arbitrary size and dimension.
The electronic computer may control the x-ray tube power source to take at least two separate exposures of the object with different voltages applied to the x-ray tube to deduce a relationship between voltage and dose that may be used to determine an amperage and voltage to be applied to the X-ray tube.
Thus it is another object of the invention to provide an automatic technique adjustment system that may model the effect of changes in technique on changes in dose and thus more rapidly achieve the correct technique and dose.
The electronic computer may control the electrical power to the x-ray tube to decrease x-ray tube voltage for a given dose.
Thus, it is another object of the invention to identify a unique and consistent value of voltage and amperage among a variety of amperages and voltages that may produce the desired dose. It is further an object of the invention to select one voltage value that may be expected to generally improve tissue contrast.
The present invention employs an electronic computer to process the image data to remove distortion by mapping each point in the received image as stored in memory to a different point in the display according to a transform equation calculated in real-time by the electronic computer. Rotation of the image may be performed by treating the rotation as a form of distortion and adjusting the equation parameters to rotate the image appropriately.
Specifically, the invention provides a fluoroscopic x-ray imaging system having an x-ray tube positioned on one side of an object. Positioned on the opposite side of the object from the x-ray source is an imagining x-ray detector having an imaging surface and producing a plurality of x-ray reception signals, each related to x-rays received at the imaging surface at different spatial locations. An electronic display displays pixels at image locations and communicates with an electronic computer receiving the x-ray reception signals. The electronic computer operates according to a stored program to illuminate a pixel at a particular image location based on the value of a signal received at a particular spatial loca
Dijkstra Marcel J.
Ergun David L.
Kuipers Hendrik
Laanstra Geert Jan
Slump Cornelis H.
Bruce David V.
Lunar Corporation
Quarles & Brady LLP
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