X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-11-29
2001-10-23
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S004000
Reexamination Certificate
active
06307912
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to computed tomography (CT) imaging and, more particularly, to methods and apparatus for optimizing image quality in a CT system under low signal conditions.
In at least one known computed tomography (CT) imaging system configuration, 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. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two-dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
When scanning is performed under low signal conditions, electronics noise can cause low signal imaging artifacts. It is known to apply gain to scan signals to reduce low-signal artifacts. Too high a gain, however, can result in over-ranging, which also causes imaging artifacts. Other factors, for example, x-ray tube output current and output voltage and detector slice thickness, also are known to affect image quality and can make it difficult to select a gain appropriate for avoiding image artifacts. It would be desirable to provide a method for optimizing gain relative to input signal to avoid both low-signal artifacts and over-ranging artifacts.
BRIEF SUMMARY OF THE INVENTION
There is therefore provided, in one embodiment, a method for optimizing signal-to-noise performance of an imaging system, including the steps of scout-scanning an object to obtain scout scan data; determining a plurality of normalized x-ray input signal factors using the scout scan data; using the normalized x-ray input signal factors to determine at least one system input signal; selecting at least one gain for the object scan using the system input signal; and applying the selected gain corresponding to the system input signal in the object scan. The above-described method results in lower electronics noise while avoiding over-ranging. Thus both low-signal artifacts and over-ranging artifacts are prevented.
REFERENCES:
patent: 5379333 (1995-01-01), Toth
patent: 5400378 (1995-03-01), Toth
patent: 5416815 (1995-05-01), Hsieh
patent: 5828719 (1998-10-01), He et al.
patent: 5933540 (1999-08-01), Lakshminarayanan et al.
He Hui David
Toth Thomas L.
Armstrong Teasdale LLP
Cabou Christian G.
Church Craig E.
General Electric Company
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