X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1998-11-24
2001-07-03
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S019000
Reexamination Certificate
active
06256364
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to computed tomography (CT) imaging and more particularly, to correction of z-axis x-ray beam movement in an imaging system.
In at least one known CT 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 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 that 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.
To reduce the total scan time, a “helical” scan may be performed. To perform a “helical” scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a one fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.
At least one known CT system uses a real-time z-axis beam sensing detector to measure the position of the x-ray beam for each view. From the measured position, an error signal representative of the difference between the measured and desired position is determined. Using the error signal, the position of a collimator may be adjusted to reduce the z-axis error between the measured and desired positions. However, the measured position signal at each view contains noise which may have a standard deviation approaching the z-axis error. Although the noise may be filtered, the filtering causes a phase lag and a position error in following the dynamic movement during the scan. As a result, a compromise must be made between loop response time and beam position measurement noise resulting in significant tracking errors.
Accordingly, it would be desirable to provide a system to facilitate correction of z-axis x-ray beam movement. It would also be desirable to provide such a system which improves image quality without increasing patient dosage.
BRIEF SUMMARY OF THE INVENTION
These and other objects may be attained by a system which, in one embodiment, corrects for position errors, or movements, of an x-ray beam caused by dynamic movement and thermal drift of an imaging system. More specifically and in one embodiment, signals from a detector array are utilized to determine a dynamic movement error and a thermal drift error. Utilizing the dynamic movement error and the thermal drift error, the position of the x-ray beam is corrected for the position errors.
Specifically, the dynamic movement error profile is determined by measuring the position error caused by the rotation of the components within a gantry of the imaging system. Particularly, as a result of the dynamic movement error being consistent from rotation to rotation of the gantry, the determined dynamic movement error for an initial rotation of the gantry may be utilized to correct for dynamic movement errors in subsequent rotations of the gantry. In one embodiment, the dynamic movement error is determined by generating a difference between a measured, or actual, position of the x-ray beam and a desired position of the x-ray beam. By characterizing the difference for a plurality of locations of the gantry, the dynamic movement error is determined.
In addition to the dynamic movement error, the position of the x-ray beam is altered by the thermal drift of the imaging system. In one embodiment, the thermal drift error includes an actual thermal drift from a previous rotation of the gantry and a predicted thermal drift. The predicted thermal drift is determined by generating a difference between a measured x-ray beam position and a desired x-ray beam position for the operating range of an x-ray source as a function of time. Prior to starting a scan, the thermal drift error is utilized to adjust the position of the x-ray beam to correct for the thermal drift of the system.
In one embodiment, the dynamic movement error and the thermal drift error are combined to alter the position of a pre-patient collimator to correct for the dynamic movement and thermal drift errors of the imaging system. More particularly, as the gantry is rotated during a scan, the position of pre-patient collimator cams are adjusted, or moved, for each location of the gantry as determined by the combined dynamic movement error and thermal drift error.
The above described system adjusts the position of the x-ray beam to facilitate correction of z-axis x-ray beam movement of the imaging system. In addition, the described system reduces patient dosage without reducing image quality.
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Nilsen Roy A.
Toth Thomas L.
Armstrong Teasdale LLP
Cabou Christian G.
Church Craig E.
General Electric Company
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