Oblique Board Calibration Phantom for Robust Half-Fan CBCT
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Solution Overview
Problem
Existing calibration phantoms for imaging scanners, particularly CBCT systems, are complex and expensive to manufacture due to the need for precise alignment of ball bearings, and suffer from inaccuracies in half-fan imaging modes where not all features are visible, leading to suboptimal calibration results.
Innovation Solution
A calibration phantom with a support structure holding planar boards featuring radiopaque checkerboard patterns, positioned at oblique angles to ensure visibility of calibration patterns in all projection images, allowing determination of nine parameters including flex under gravity, using circuit board manufacturing techniques for accuracy and precision.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If ball bearings are used in the calibration phantom, then calibration accuracy can be achieved, but manufacturing complexity and cost increase due to the need for very high precision alignment
Solution Approach 1:
The patent replaces physical ball bearings with a digital/computational model of ball bearings. The phantom contains markers arranged in a known geometric pattern that simulates the appearance of ball bearings in projection images. The calibration algorithm uses a computational model to identify these markers and determine their positions, eliminating the need for actual spherical objects while maintaining calibration accuracy. This reduces manufacturing complexity as the markers can be fabricated using standard PCB or printing techniques rather than requiring precise mechanical assembly of ball bearings.
Solution Approach 2:
The patent replaces the mechanical alignment system (physical ball bearings requiring precise mechanical positioning) with an image processing and computational geometry system. The calibration process uses algorithms to detect marker positions in 2D projection images and computationally reconstruct 3D positions, replacing the need for mechanical precision in phantom fabrication with computational precision in image analysis.
2Measurement precision
If ball bearings are used in the calibration phantom, then calibration can be performed, but measurement uncertainty increases because each ball bearing occupies only a small number of pixels in the projection image
Solution Approach 1:
The patent transitions from analyzing spherical objects (3D objects projected to 2D) to analyzing planar marker patterns (2D objects in the projection plane). The markers are designed as two-dimensional patterns with high contrast and extended spatial extent, allowing more pixels to contribute to each marker's position determination. This dimensional change from spherical to planar markers increases the number of pixels available for subpixel position measurement while maintaining the known geometric relationships needed for calibration.
3Adaptability or versatility
If the detector is shifted to an offset half-fan position to increase field-of-view, then imaging capability is improved, but calibration robustness deteriorates because not all ball bearings are visible in every projection image
Solution Approach 1:
The patent divides the calibration phantom into multiple sections or views by using multiple markers positioned at different locations and orientations. Instead of relying on a single complete set of ball bearings visible in all images, the system uses multiple partial patterns that can be individually detected. The calibration algorithm integrates information from all visible markers across all projection images, making the calibration robust even when the detector is offset and cannot see all markers simultaneously in half-fan mode.
4Measurement precision
If multiple projection images are taken at different gantry angles, then accurate position determination of kV source and detector is achieved, but calibration time increases
Solution Approach 1:
The patent incorporates preliminary action by pre-designing the marker positions and patterns in the phantom according to an optimized geometric configuration. The markers are arranged in advance with known spatial relationships that are mathematically optimized for calibration efficiency. This preliminary geometric design allows the calibration algorithm to rapidly converge on accurate position and orientation parameters from a limited set of projection images, reducing the number of images needed compared to unoptimized phantom designs.
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 accurate and cost-effective calibration by ensuring visibility of calibration patterns in all projection images, reducing manufacturing complexity and enhancing calibration robustness in both full and half-fan imaging modes.
Implementation Method 1
Each board comprises a planar calibration pattern formed of radiopaque material
Data Source
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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.