X-ray or gamma ray systems or devices – Accessory – Testing or calibration
Reexamination Certificate
1999-07-19
2001-12-04
Dunn, Drew (Department: 2882)
X-ray or gamma ray systems or devices
Accessory
Testing or calibration
C378S004000, C378S019000
Reexamination Certificate
active
06325539
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to computed tomograph (CT) imaging and, more particularly, to a calibration of a CT system.
In at least some computed tomograph (CT) imaging system configurations, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal spot. X-ray detectors typically include a post patient collimator for collimating scattered x-ray beams received at the detector. A scintillator is located adjacent the post patient collimator, and photodiodes are positioned adjacent the scintillator.
Uniformity between individual detector elements is important for securing good image quality of CT images. Otherwise, anomalies may occur in the collected data. A consequence of data anomalies are image distortions, commonly referred to as artifacts. Detector uniformity may be impacted by many factors including radiation damage and sensitivity of the scintillator materials. To correct for this uniformity, periodic calibrations of the detector are required.
A common method for calibration employs devices known as a phantoms. Different size phantoms, providing known attenuation paths for x-ray beams passing therethrough, are utilized to generate calibration data for different size objects. Using any non-uniformity demonstrated during the calibration, an error value is generated for the specific size object. The generated error value is then used to correct the measured values of the detector so that detector response is uniform for subsequent use when imaging a patient.
The calibration method, described above, has the major drawback of requiring separate calibrations for different modes of operation, or scan types, of the imaging system. These scans include different size objects and different types of filtration of the x-ray beam, using a filtration device. Such a filtration device, in one known system, is a bow-tie filter having a head portion and a body portion. As a consequence, a separate calibration must be performed for each scan type. Therefore, the calibration of the imaging system is very time consuming and is likely to cause inconvenience to the CT system operator.
Accordingly, it would be desirable to provide a CT system that reduces the number of calibration scans which must be performed. It is also desirable to provide a method for utilizing calibration data from a first scan type to determine the error value for other scan types.
BRIEF SUMMARY OF THE INVENTION
These and other objects may be attained by a CT system which, in one embodiment, utilizes calibration values from a first type of calibration scan to determine the calibration values for at least a second type of calibration scan of the system. The CT system, in one embodiment, includes an x-ray source, a detector array, and a filtration device. The CT system detector, in one embodiment, includes a multislice detector.
In one embodiment, prior to scanning a patient, first calibration scan attenuation data is collected by radiating x-ray beams through a phantom. The collected attenuation data is then compared to ideal attenuation data for the phantom and a difference value is generated for each detector element. The difference values are then used to determine the first type of calibration scan values. Values from the first type of calibration scan are then used to determine calibration values for at least the second type of scan. After selecting a scan type, the patient scan is completed. The attenuation data collected during the patient scan is then adjusted by the selected calibration scan values.
By using the above described CT system the number of calibration scans is reduced. In addition, such calibration improves CT system efficiency and improves system availability.
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Bromberg Neil B.
Pan Tin-Su
Armstrong Teasdale LLP
Cabou Christian G.
Dunn Drew
General Electric Company
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