All entries for Tuesday 19 April 2016

April 19, 2016

Modelling the Penumbra in Computed Tomography

Kueh, A., Warnett, J. W., Gibbons, G. J., Brettschneider, J., Nichols, T. E., Williams, M. A., & Kendall, W. S. (2016). Modelling the Penumbra in Computed Tomography. Journal of X-Ray Science and Technology, to appear, 14pp.

BACKGROUND: In computed tomography (CT), spot geometry is one of the main sources of error in CT images. Since X-ray tubes do not arise from a point source, artefacts are produced. In particular there is a penumbra effect, leading to poorly defined edges within a reconstructed volume. Penumbra models can be simulated given a fixed spot geometry and the known experimental setup.

OBJECTIVE: This paper proposes to use a penumbra model, derived from Beer’s law, to confirm spot geometry from penumbra data, as well as quantify blurring in the image.

METHODS: Two models for the spot geometry are considered; one consists of a single Gaussian spot, the other is a mixture model consisting of a Gaussian spot together with a larger uniform spot.

RESULTS: The model consisting of a single Gaussian spot has fits which are poor. The mixture model (which adds a larger uniform spot) exhibits much improved fits. The parameters corresponding to the uniform spot are similar across all powers, and further experiments suggest that the uniform spot produces only soft X-rays of relatively low-energy.

CONCLUSIONS: Thus, the precision of radiographs can be estimated from the penumbra effect in the image. The use of a thin copper filter reduces the size of the effective penumbra.


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  • The paper describing this webapp is now available (open access!) at: Brettschneider, Julia A, Oscar … by Wilfrid Kendall on this entry

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