Methods and apparatus for calibrating CT x-ray beam tracking loop
First Claim
1. A method for determining tracking control parameters for positioning an x-ray beam of a computed tomography imaging system, the imaging system including a movable collimator positionable in steps and a detector array including a plurality of rows of detector elements, said method comprising the steps of:
- obtaining detector samples at a plurality of collimator step positions while determining a position of a focal spot of the x-ray beam;
determining a beam position for each detector element at each collimator step utilizing the determined focal spot positions, a nominal focal spot length, and geometric parameters of the x-ray beam, collimator, and detector array;
determining a detector element differential error according to ratios of successive collimator step positions; and
selecting a target beam position for an isocenter element in accordance with the determined element differential errors.
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Abstract
The present invention is, in one embodiment, a method for determining tracking control parameters for positioning an x-ray beam of a computed tomography imaging system having a movable collimator positionable in steps and a detector array including a plurality of rows of detector elements. The method includes steps of obtaining detector samples at a series of collimator step positions while determining a position of a focal spot of the x-ray beam; determining a beam position for each detector element at each collimator step utilizing the determined focal spot positions, a nominal focal spot length, and geometric parameters of the x-ray beam, collimator, and detector array; and determining a calibration parameter utilizing information so obtained. For example, in determining a target beam position at which to maintain the x-ray beam, a detector element differential error is determined according to ratios of successive collimator step positions; and a target beam position is selected for an isocenter element in accordance with the determined element differential errors.
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Citations
26 Claims
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1. A method for determining tracking control parameters for positioning an x-ray beam of a computed tomography imaging system, the imaging system including a movable collimator positionable in steps and a detector array including a plurality of rows of detector elements, said method comprising the steps of:
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obtaining detector samples at a plurality of collimator step positions while determining a position of a focal spot of the x-ray beam;
determining a beam position for each detector element at each collimator step utilizing the determined focal spot positions, a nominal focal spot length, and geometric parameters of the x-ray beam, collimator, and detector array;
determining a detector element differential error according to ratios of successive collimator step positions; and
selecting a target beam position for an isocenter element in accordance with the determined element differential errors. - View Dependent Claims (2, 3, 4, 5, 6, 7)
determining a collimator z-axis position offset from the detector array centerline at a point at which outer detector row signals are reduced to a full width at a half maximum; and
determining a focal spot position as a function of the determined collimator z-axis position and the geometric parameters of the x-ray beam, collimator, and detector array.
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3. A method in accordance with claim 1 further comprising the step of offset-correcting and view-averaging the obtained detector samples at a plurality of collimator step positions to obtain a set of detector samples for each collimator step position used in said step of determining a beam position and said step of determining a differential error for selection of the target beam position.
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4. A method in accordance with claim 1 wherein selecting a target beam position for an isocenter detector element in accordance with the determined element differential errors comprises the steps of:
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weighting the detector element differential error by a reconstruction error sensitivity function;
determining a step position at which the weighted detector element differential error exceeds a predetermined limit; and
setting a tracking beam position for the isocenter detector element at a distance from the determined step position preceding a step that exceeds a predetermined artifact limit by an amount that exceeds a tracking loop positioning error.
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5. A method in accordance with claim 4 wherein the reconstruction error sensitivity function is detector element dependent.
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6. A method in accordance with claim 5 wherein the reconstruction error sensitivity varies according to a distance of the detector element from an isocenter element.
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7. A method in accordance with claim 4 wherein the detector rows have at least 214 elements on each side of an isochannel element, and the reconstruction error sensitivity function is:
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8. A method for determining tracking control parameters for positioning an x-ray fan beam of a computed tomography imaging system, the imaging system including a movable collimator positionable in steps and a detector array including a plurality of rows of detector elements including at least an inner row and an outer row, said method comprising the steps of:
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obtaining detector samples at a plurality of collimator step positions while determining a position of a focal spot of the x-ray fan beam;
determining a beam position for each detector element at each collimator step utilizing the determined focal spot positions, a nominal focal spot length, and geometric parameters of the x-ray fan beam, collimator, and detector array; and
determining a beam position transfer function for a ratio of an average of detector outer row signals to detector inner row signals for a set of detector elements at an extreme end of the x-ray fan beam in accordance with a selected approximation over a selected ratio range between a minimum and a maximum ratio for the plurality of collimator step positions. - View Dependent Claims (9, 10, 11, 12, 13)
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14. A computed tomography imaging system comprising an x-ray source, a detector array including a plurality of rows of detector elements, and a movable collimator positionable in steps and configured to collimate and position an x-ray beam produced by said x-ray source on said detector array, said system configured to:
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obtain detector samples at a plurality of collimator step positions while determining a position of a focal spot of the x-ray beam;
determine a beam position for each detector element at each collimator step utilizing the determined focal spot positions, a nominal focal spot length, and geometric parameters of the x-ray beam, collimator, and detector array;
determine a detector element differential error according to ratios of successive collimator step positions; and
select a target beam position for an isocenter element in accordance with the determined element differential errors. - View Dependent Claims (15, 16, 17, 18, 19, 20)
determine a collimator z-axis position offset from the detector array centerline at a point at which outer detector row signals are reduced to a full width at a half maximum; and
determine a focal spot position as a function of the determined collimator z-axis position and the geometric parameters of the x-ray beam, collimator, and detector array.
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16. A system in accordance with claim 14 further configured to offset-correct and view-average the obtained detector samples at a plurality of collimator step positions to obtain a set of detector samples for each collimator step position used in determining said beam position and in determining said differential error for selection of said target beam position.
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17. A system in accordance with claim 14 wherein said system being configured to select a target beam position for an isocenter detector element in accordance with the determined element differential errors comprises said system being configured to:
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weight the detector element differential error by a reconstruction error sensitivity function;
determine a step position at which the weighted detector element differential error exceeds a predetermined limit; and
set a tracking beam position for the isocenter detector element at a distance from the determined step position preceding a step that exceeds a predetermined artifact limit by an amount that exceeds a tracking loop positioning error.
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18. A system in accordance with claim 17 wherein the reconstruction error sensitivity function is detector element dependent.
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19. A system in accordance with claim 18 wherein the reconstruction error sensitivity varies according to a distance of the detector element from an isocenter element.
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20. A system in accordance with claim 17 wherein the detector rows have at least 214 elements on each side of an isochannel element, and the reconstruction error sensitivity function is:
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21. A computed tomography imaging system comprising an x-ray source, a detector array including a plurality of rows of detector elements, and a movable collimator positionable in steps and configured to collimate and position an x-ray beam produced by said x-ray source on said detector array, said system configured to:
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obtain detector samples at a plurality of collimator step positions while determining a position of a focal spot of the x-ray fan beam;
determine a beam position for each detector element at each collimator step utilizing the determined focal spot positions, a nominal focal spot length, and geometric parameters of the x-ray fan beam, collimator, and detector array; and
determine a beam position transfer function for a ratio of an average of detector outer row signals to detector inner row signals for a set of detector elements at an extreme end of the x-ray fan beam in accordance with a selected approximation over a selected ratio range between a minimum and a maximum ratio for the plurality of collimator step positions. - View Dependent Claims (22, 23, 24, 25, 26)
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Specification