Multi-Board Calibration Phantom for Half-Fan CBCT Accuracy
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Solution Overview
Problem
Existing calibration phantoms for imaging scanners, such as CBCT systems, are complex and expensive to manufacture due to the need for precise alignment of radio-opaque ball bearings, and suffer from uncertainty in determining sub-pixel positions and sensitivity to deviations when used in half-fan imaging modes, leading to suboptimal calibration accuracy.
Innovation Solution
A calibration phantom with a support structure and multiple boards featuring planar calibration patterns of radiopaque material, positioned at oblique angles to ensure visibility of at least two patterns in each projection image, allowing determination of nine calibration parameters despite flexing under gravity, using intersecting lines like checkerboard patterns for accurate geometric calibration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a traditional calibration phantom with ball bearings is used, then calibration can be performed, but the positions of ball bearings must be aligned with very high precision making manufacturing complex and expensive
Solution Approach 1:
The calibration phantom is divided into multiple independent boards, each containing a subset of calibration features. This segmentation allows each board to be manufactured separately with standard PCB techniques, eliminating the need for complex overall alignment while maintaining calibration accuracy through the distributed geometry of multiple boards
Solution Approach 2:
The mechanical alignment system using precisely positioned ball bearings is replaced with a PCB-based system where calibration patterns are fabricated using standard electronic manufacturing techniques. This substitution eliminates complex mechanical alignment requirements while providing sufficient calibration precision through the known geometry of PCB features
2Area of stationary object
If the detector is shifted to an offset half-fan position to increase field-of-view, then field-of-view is increased, but the projection images may not capture the entire phantom leading to reduced calibration robustness
Solution Approach 1:
The calibration phantom is segmented into multiple boards positioned at different orientations. This ensures that when the detector is in half-fan mode with limited field-of-view, at least two different boards are visible in every projection image, maintaining calibration robustness even when the complete phantom cannot be captured
Solution Approach 2:
Boards are positioned at different angular orientations around the central axis, adding an angular dimension to the calibration feature distribution. This ensures that projection images taken at any gantry angle in half-fan mode will intersect with multiple boards, providing sufficient calibration data even with reduced field-of-view
3Measurement precision
If ball bearings are used as calibration features, then calibration can be performed, but each ball bearing occupies a small number of pixels adding uncertainty in determining sub-pixel position
Solution Approach 1:
The calibration patterns use high-contrast radiopaque designs on PCB boards, creating distinct visual features in projection images. The alternating copper and non-copper regions produce strong contrast similar to color changes, enabling precise edge detection and sub-pixel position determination with more pixels per feature than small ball bearings
Solution Approach 2:
The calibration features are changed from three-dimensional ball bearings to two-dimensional planar patterns on PCB boards. This parameter change increases the number of pixels representing each calibration feature while maintaining precise known geometry, improving the accuracy of sub-pixel position determination through enhanced signal-to-noise ratio in the projection images
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution provides robust and accurate calibration by ensuring visibility of calibration patterns in all projection images, reducing manufacturing complexity and cost, and enabling precise determination of imaging scanner parameters regardless of gantry rotation angle, even in half-fan modes.
Implementation Method 1
each board comprises a planar calibration pattern formed of radiopaque material
Data Source
AI summary
Disclosed herein is a calibration phantom for calibrating an imaging scanner. The imaging scanner comprises a rotatable gantry, and a source of imaging radiation and a detector both configured to rotate with the gantry. The calibration phantom comprises a support structure and a plurality of boards, wherein each board comprises a planar calibration pattern formed of radiopaque material. The boards are positioned along a first axis and each board is positioned, by the support structure, at an angle with respect to each of the other boards such that each planar calibration pattern faces a different direction. The directions faced by each planar calibration pattern are substantially perpendicular to the first axis.