X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2001-11-21
2002-12-10
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S901000
Reexamination Certificate
active
06493416
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to tomographic imaging, and more particularly to methods and apparatus for reducing reconstructed image noise in a computerized tomographic (CT) imaging system.
In at least one known 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 an “imaging plane”. The x-ray beam passes through an 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 an 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.
In some known imaging systems, using sub-millimeter scanning speeds, the demands on the peak and sustained x-ray source power increase substantially resulting in a signal loss to the detector. Additionally, a signal loss can be caused by gaps between individual detector cells resulting in the number of detected x-ray photons decreasing by more than a factor of two thereby facilitating a reduction in image quality. Additionally, an increased x-ray dosage may produce adverse effects in a human body, therefore, a reduced x-ray photon flux facilitates a reduction in the x-ray dosage a patient receives since an x-ray dosage received by a patient should be minimized. This is generally accomplished with a reduced x-ray tube current, which typically results in a decreased number of x-ray photons.
BRIEF SUMMARY OF THE INVENTION
In one aspect, a method for facilitating a reduction in reconstructed image noise in a computed tomography imaging system is provided. The method includes generating projection data, characterizing a noise distribution of the projection data, performing an adaptive noise reduction operation on the projection data using the noise distribution characterization, and reconstructing an image.
In another aspect, a method for facilitating a reduction in reconstructed image noise in a computed tomography imaging system is provided. The method includes generating projection data, pre-processing the projection data using at least one of an offset correction, a primary speed correction, a reference channel correction, and an air-calibration. The method also includes characterizing a noise distribution of the projection data wherein characterizing a noise distribution includes generating an air calibration vector, performing a polynomial fit of the air calibration vector to generate a bowtie shape vector and approximating a standard deviation of the noise distribution using the bowtie shape vector in accordance with
σ
(
γ
)
=
B
(
γ
)
p
(
γ
)
p
(
γ
0
)
.
The method also includes performing an adaptive noise reduction operation on the projection data using the noise distribution characterization, filtering the projection data in accordance with p
r
(&ggr;)=p(&ggr;)−&agr;(&ggr;)└p(&ggr;)−p
f
(&ggr;)┘, and reconstructing an image.
In a further aspect, a computed tomographic (CT) imaging system for facilitating a reduction in reconstructed image noise is provided. The computed tomographic (CT) imaging system includes a detector array, at least one radiation source, and a computer coupled to the detector array and radiation source and configured to generate projection data. The computer is also configured to characterize a noise distribution of the projection data, perform an adaptive noise reduction operation on the projection data using the noise distribution characterization, and reconstruct an image.
In still a further aspect, a computed tomography (CT) imaging system for facilitating a reduction in reconstructed image noise is provided. The computed tomography (CT) imaging system includes a detector array, at least one radiation source, and a computer coupled to the detector array and radiation source and configured to generate projection data, pre-process the projection data using at least one of an offset correction, a primary speed correction, a reference channel correction, and an air-calibration. The computer is also configured to characterize a noise distribution of the projection data, wherein to characterize a noise distribution includes generating an air calibration vector, performing a polynomial fit of the air calibration vector to generate a bowtie shape, approximating a standard deviation of the noise distribution using the bowtie shape vector in accordance with
σ
(
γ
)
=
B
(
γ
)
p
(
γ
)
p
(
γ
0
)
.
The computer is also configured to perform an adaptive noise reduction operation on the projection data using the noise distribution characterization, filter the projection data in accordance with p
r
(&ggr;)=p(&ggr;)−&agr;(&ggr;)└p(&ggr;)−p
f
(&ggr;)┘, and reconstruct an image.
In yet another aspect, a computer readable medium encoded with a program executable by-a computer for facilitating a reduction in reconstructed image noise in a computed tomography imaging system is provided. The program is configured to instruct the computer to generate projection data, characterize a noise distribution of the projection data, perform an adaptive noise reduction operation on the projection data using the noise distribution characterization, and reconstruct an image.
In yet another further aspect, a computer readable medium encoded with a program executable by a computer for facilitating a reduction in reconstructed image noise in a computed tomography imaging system is provided. The program is configured to instruct the computer to generate projection data, pre-process the projection data using at least one of an offset correction, a primary speed correction, a reference channel correction, and an air-calibration. The program is also configured to characterize a noise distribution of the projection data wherein to characterize a noise distribution includes generating an air calibration vector, performing a polynomial fit of the air calibration vector to generate a bowtie shape vector and approximating a standard deviation of the noise distribution using the bowtie shape vector in accordance with
σ
(
γ
)
=
B
(
γ
)
p
(
γ
)
p
(
γ
0
)
.
(The program is also configured to instruct the computer to perform an adaptive noise reduction operation on the projection data using the noise distribution characterization, filter the projection data in accordance with p
r
(&ggr;)=p(&ggr;)−&agr;(&ggr;)└p(&ggr;)−p
f
(&ggr;)┘, and reconstruct an image.
REFERENCES:
patent: 4761819 (1988-08-01), Denison et al.
patent: 5454019 (1995-09
Armstrong Teasdale LLP
Bruce David V.
GE Medical Systems Global Technology Company LLC
Horton Esq. Carl B.
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