Variable focus detector distance in an X-ray CT system
In this article:
- Enhanced Spatial Resolution via Geometric Magnification: Adjusting the Focus-Detector Distance (FDD) allows for higher geometric magnification, improving voxel resolution and enabling more detailed 3D imaging in industrial CT scans.
- Optimized Signal Intensity and Scan Efficiency: Reducing FDD increases X-ray intensity on the detector due to the inverse square law, enabling faster scans or improved contrast-to-noise ratio (CNR) without increasing tube power.
- Minimized Reconstruction Artifacts: A larger FDD reduces the cone beam angle, which helps minimize Feldkamp-Davis-Kress (FDK) reconstruction artifacts, especially in non-central slices of the CT volume.
- Expanded Field of View for Larger Samples: Increasing FDD decreases the cone beam opening angle, allowing for a wider effective field of view and accommodating larger or more complex parts.
- Application-Specific Flexibility: Variable FDD empowers operators to balance resolution, scan time, and image quality based on specific inspection needs, making CT systems more adaptable across industries.
The benefits of a variable focus detector distance in an X-ray CT system
Some CT systems feature an axis to change the distance of the detector from the tube, i.e., the Focus-Detector Distance, FDD.
Together with the Focus-Object Distance FOD, FDD determines the geometric magnification M: M= FDD/FOD
The basic spatial resolution in the image is limited by the voxel size Δv which results from the pixel size Δp of the detector by:
Δv=Δp/M
So, it is clear: longer FDD gives higher magnification M for a given Region of Interest (ROI).
What other benefits come with a variable Focus-Detector-Distance?
In practice the operator will balance the benefit with the downsides for each application.
FDD and the Inverse Square Law
As the drawing illustrates, doubling the FDD quarters the intensity on the detector since the flux at distance 2L to the source distributes over a four times bigger area at distance L. This is the inverse square law.
Conversely, the intensity can be increased considerably without increasing the power of the tube (and the size of the focal spot): if we reduce the FDD by a factor of x, the intensity rises by a factor of x² .
Thus, in the simple example of halving the FDD (and FOD to maintain magnification), the scan time would therefore only be a quarter at the same image quality. Or, if we stick with the initial scan time, the CNR would double, as the absolute contrast is linear with the signal and would be four times larger, but the (quantum) noise is the square root of the signal and would be only two times larger.
FDK reconstruction artifacts and cone beam opening angle
The Feldkamp-Davis-Algorithm (FDK) only works exactly in the central slice. Above and below this slice, some approximations and assumptions are used, which result in slight characteristic artifacts in the non-central slices. The larger the angle of incidence at the position of the slice on the detector, the stronger these artifacts are.
Nevertheless, the angle of incidence is smaller for large FDD than the small FDD, so that the FDK artifacts can be reduced by choosing a larger FDD.
Cone beam opening angle and field of view
At larger FDD the cone beam angle is smaller. So, the cone beam is wider close to the detector and bigger samples can fit in. In other words, the field of view is larger in effect for larger FDD.
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