X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1998-08-13
2001-04-10
Church, Craig E. (Department: 2876)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S112000, C378S098700
Reexamination Certificate
active
06215842
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the art of x-ray generation and/or production. It finds particular application in conjunction with CT scanners, and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications where temporally stable x-ray generation is desired.
Generally, CT scanners have a defined examination region or scan circle in which a patient or other subject being imaged is disposed. A beam of radiation is transmitted across the examination region from an x-ray source, such as an x-ray tube, to oppositely disposed radiation detectors. The source, or beam of radiation, is rotated around the examination region while data is collected from the radiation detectors receiving x-ray radiation passing through the examination region.
The sampled data is typically manipulated via appropriate reconstruction processors to generate an image representation of the subject which is displayed in a human-viewable form. Commonly, the x-ray data is transformed into the image representation utilizing filtered backprojection. A family of rays extending from source to detector is assembled into a view. Each view is filtered or convolved with a filter function and backprojected into an image memory. Various view geometries have been utilized in this process. In one example, each view is composed of the data corresponding to rays passing parallel to each other through the examination region, such as from a traverse and rotate-type scanner. In a rotating, fan-beam-type scanner in which both the source and detectors rotate (i.e. a third generation scanner), each view is made up of concurrent samplings of an arc of detectors which span the x-ray beam when the x-ray source is in a given position to produce a source fan view. Alternately, with stationary detectors and a rotating source (i.e. a fourth generation scanner), a detector fan view is formed from the rays received by a single detector as the x-ray source passes behind the examination region opposite the detector.
The demands placed on a x-ray tube by a CT scanner are quite severe. For example, in a rotating anode x-ray tube, a heavy metal or metal/graphite anode, in an evacuated x-ray tube, is spun on its axis at angular velocities of 60 to 180 revolutions per second. The x-ray tube, in turn, is rotated at angular speeds up to 2 revolutions per second on the CT scanner's rotating gantry. The “G” forces are quite high. Moreover, it is generally advantageous that the x-ray tube generate a steady, high-power x-ray flux that is without temporal and spatial fluctuations. However, temporal x-ray variations or x-ray ripple often exist and come from sources such as: anode target surface roughness and density; filament vibration or the resonant frequency of the filament; cathode vibration or the resonance frequency of the cathode mounting structure; and, other effects that cause the beam current to vary.
Fourth generation CT scanners reconstruct temporally varying x-ray beams into images with “tire track” artifacts. The nature of the artifacts vary with the x-ray ripple frequency (typically, very high or very low x-ray ripple frequencies of reasonable magnitudes do not materially contribute to image artifacts), detector sampling rate, and gantry rotational speed.
Methods to compensate for the presence of time varying x-ray CT data have been developed. The methods generally involve the use of reference detectors somewhere on the gantry. The output of the reference detectors is used by the computational systems and/or reconstruction processors to correct for variations in the x-ray data. However, fast, high-quality CT scans employ multiple detectors and high quantities of data. Burdensome corrections and/or data conditioning by software for x-ray ripple artifacts in the data results in slower, more inefficient reconstruction processing.
One method for the correction of temporal variations (ripple) of the x-ray beam has been to utilize data from the radiation detectors that are active, but are out of the imaging field. These detectors “see” the same temporal x-ray variations as the more central imaging detectors. The data from these reference detectors is used to make corrections to the data from the imaging detectors and remove the undesirable effects before the image reconstruction process. The detectors, both imaging and reference, are located opposite the x-ray source, and beyond the object or patient being scanned with the reference detector being at the far left and right sides of the fan beam. An inherent drawback of this system is that on occasion, the patient or appurtenances to the patient (tubes, clothes, sheets, etc.) may interrupt the reference portions of the x-ray beam, invalidating the data from these reference detectors. Therefore, the software is further burdened by having to recognize invalid data and not apply it for corrections.
The present invention contemplates new and improved x-ray generation techniques which overcome the above-referenced problems and others.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, an x-ray radiation stabilization system is provided. It includes an x-ray tube which emits x-ray radiation. The x-ray tube includes an anode, a cathode, and a vacuum envelope housing the anode and the cathode. A high-voltage generator is connected to the x-ray tube which supplies a high-voltage electric potential between the cathode and anode such that an electron beam flows therebetween striking the anode to produce the x-ray radiation. A reference radiation detector samples a representative portion of the x-ray radiation emitted by the x-ray tube and generates a signal in response to an intensity of the sampled x-ray radiation. A feedback circuit is connected between the reference radiation detector and the high-voltage generator. The feedback circuit generates an error signal in response to the detected radiation and directs the high-voltage generator to adjust the high-voltage electric potential supply to the x-ray tube such that in the x-ray radiation ripple having a predetermined frequency range is substantially canceled.
In accordance with a more limited aspect of the present invention, the x-ray radiation stabilization system further includes a radiation filter disposed in front of the reference radiation detector which filters the x-ray radiation before it is sampled by the reference radiation detector.
In accordance with a more limited aspect of the present invention, the radiation filter is selectively tunable.
In accordance with a more limited aspect of the present invention, the radiation filter is tuned to achieve a spectral response to the sampled x-ray radiation which simulates that of a subject being examined with the x-ray radiation.
In accordance with a more limited aspect of the present invention, the feedback circuit includes an amplifier which amplifies the error signal.
In accordance with a more limited aspect of the present invention, the feedback circuit further includes a normalization circuit which normalizes gain from the amplifier.
In accordance with a more limited aspect of the present invention, the feedback circuit further includes a band-pass filter through which the error signal is passed to substantially remove frequency components outside the predetermined frequency range.
In accordance with a more limited aspect of the present invention, the feedback circuit corrects for filtering effects a subject being examined has on an energy spectrum of the x-ray radiation.
In accordance with a more limited aspect of the present invention, the predetermined frequency range is from about 30 Hz to about 700 Hz.
In accordance with a more limited aspect of the present invention, the reference radiation detector samples the x-ray radiation prior to its traversing a subject being examined by the x-ray radiation.
In accordance with another aspect of the present invention, an x-radiation system for the reduction of x-ray ripple is provided in a C
Mattson Rodney A.
Resnick Theodore A.
Church Craig E.
Fay Sharpe Beall Fagan Minnich & McKee
Kocovsky, Jr. Thomas E.
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