CT Scanner Geometric Calibration for Large Flat Module Detectors
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Solution Overview
Problem
The geometric calibration of CT scanners with large flat module detectors is challenging due to immeasurable air gaps between adjacent modules and ISO channel inaccuracies, leading to ring artifacts and degraded Modulation Transfer Function (MTF) in reconstructed images.
Innovation Solution
A method and apparatus for geometric calibration using a calibration phantom and nonlinear least square fitting algorithm to determine accurate air gap sizes and ISO channel offset, optimizing the forward projection function to minimize errors and improve image quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a large flat module detector is used to save on detector costs, then manufacturing cost is reduced, but measurement precision of air gap sizes deteriorates because the air gaps become immeasurable due to oblique installation and manufacturing errors
Solution Approach 1:
A calibration phantom is introduced as an intermediary object to indirectly measure the air gap sizes. The phantom includes reference objects with known dimensions that, when scanned, provide projection data used to calculate the actual air gap sizes between detector modules through geometric relationships, thereby enabling measurement without direct physical access to the gaps
Solution Approach 2:
The patent replaces direct mechanical measurement methods with a computational approach. Instead of physically measuring the air gaps with calipers or other mechanical tools, the system uses X-ray projection data, forward projection functions, and nonlinear least square fitting algorithms to computationally determine the air gap sizes based on the detected positions of calibration phantom features
2Reliability
If accurate air gap sizes are required to avoid ring artifacts, then image quality is improved, but device complexity increases because geometric calibration algorithms and calibration phantoms must be implemented
Solution Approach 1:
The geometric calibration is performed as a preliminary action before actual patient scanning. The calibration phantom is scanned once to determine air gap sizes and update the forward projection function, after which the calibrated parameters are used for all subsequent clinical scans, eliminating the need for repeated calibration and minimizing added complexity
Solution Approach 2:
The patent changes the parameters of the forward projection function based on measured air gap sizes. By updating the geometric parameters in the projection model to reflect the actual air gap measurements, the system compensates for manufacturing and installation errors, thereby eliminating ring artifacts and improving image quality without requiring physical modification of the detector
3Ease of operation
If the detector position is fixed after mounting to simplify installation, then ease of operation is improved, but measurement precision of ISO channel deteriorates because position adjustments are no longer possible
Solution Approach 1:
The system implements feedback by using the calibration phantom scan results to determine the actual detector position and ISO channel offset. The measured positions of calibration features are compared with expected positions, and the difference (feedback) is used to update the forward projection function parameters, thereby compensating for installation errors and achieving accurate ISO channel alignment without physical adjustment
Data Source
AI summary
A method for geometric calibration of a CT scanner, including, for each row of at least one row of detector cells, establishing a complete geometric description of the CT scanner, including at least one unknown geometric parameter, establishing a description of a forward projection function using the complete geometric description, acquiring actual projection coordinates of a calibration phantom placed in a scanning field of view (SFOV) on a current row of detector cells and corresponding to a plurality of angles, acquiring calculated projection coordinates of the calibration phantom on the current row of detector cells and corresponding to the plurality of angles using the description of the forward projection function, and acquiring a calibrated value for the at least one unknown geometric parameter by evaluating the at least one unknown geometric parameter based on the acquired actual projection coordinates and calculated projection coordinates via a nonlinear least square fitting algorithm.