Methods and apparatus for calibrating CT x-ray beam tracking loop

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:

• 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.

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:

• 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.

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:

• 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.