X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1998-08-25
2001-12-04
Kim, Robert H. (Department: 2877)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
Reexamination Certificate
active
06327329
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to imaging and, more particularly, to monitoring performance of a detector in an imaging system.
In at least some known medical imaging systems, such as a computed tomograph (CT) imaging system, 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.
Multislice CT systems are used to obtain data for an increased number of slices during a scan. Known multislice systems typically include detectors generally known as 3-D detectors. With such 3-D detectors, a plurality of detector elements form separate channels arranged in columns and rows. Each row of detectors forms a separate slice. For example, a two slice detector has two rows of detector elements, and a four slice detector has four rows of detector elements. During a multislice scan, multiple rows of detector cells are simultaneously impinged by the x-ray beam, and therefore data for several slices is obtained.
The channels of the detector typically are ganged together to form the rows. Channel to channel variation in the z-direction can result in generation of image artifacts. As the detector ages the gain variation changes due to radiation damage. Corrections for such channel to channel variability are know, but the effectiveness of such corrections depend on the magnitude of the variability.
BRIEF SUMMARY OF THE INVENTION
These and other objects may be attained by an algorithm, which may be executed periodically by the imaging system, for detecting cell to cell variation to ensure that the maximum allowable channel to channel variation is not exceeded. More specifically, and in accordance with one aspect of the present invention, an algorithm is periodically executed to measure the relative gains in the channels. The gains are measured, for example, by recording the signal from an air scan and normalizing to a common reference. Part of the normalization process includes accounting for the non uniformity of the x-ray beam, for example, the heel effect. It is assumed that the x-ray flux profile in z is slowly changing in the x-direction and is obtained by low pass filtering in x. The normalized values are then compared to a predetermined specification. If any particular cell is not within the specification parameters, then the module in which such cell resides may be replaced.
In addition to measuring gain variation and comparing it to a specification, a trending analysis also may be performed. The trending algorithm predicts the time at which the detector will fail the specification so that replacement of the detector may take place before failure occurs.
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Bromberg Neil B.
He Hui David
Kulpins Mary Sue
Armstrong Teasdale LLP
Cabou Christian G.
General Electric Company
Kim Robert H.
Rodriguez Armando
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