Hildur Olafsdottir, Presenter: Research funded, The Phantom Laboratory Research funded, Image Owl, Inc

Jesper Fredriksson, Abstract Co-Author: Employee, Raforninn EHF

Maria Marteinsdottir, Abstract Co-Author: Intern, Image Owl, Inc

Atli Andresson, Abstract Co-Author: Research funded, The Phantom Laboratory Research funded, Image Owl, Inc

Paul Austin Healy, Abstract Co-Author: Research funded, The Phantom Laboratory Research funded, Image Owl, Inc

Joshua Levy, Abstract Co-Author: Research funded, The Phantom Laboratory Research funded, Image Owl, Inc President, The Phantom Laboratory

Smari Kristinsson, Abstract Co-Author: Research funded, The Phantom Laboratory Research funded, Image Owl, Inc

David John Goodenough PhD, Abstract Co-Author: Consultant, Phantom Laboratory Consultant, Image Owl, Inc

For almost four decades bead and wire ramps have been used in the evaluation of slice width as part of QA testing for CT scanners. While each of these approaches have been recognized and accepted as reliable, in this paper we investigate the differences, advantages and limitations of these tools and study the effect of varying the field of view (FOV), filter, mAs and scan type (sequential vs spiral).

The Catphan® 700 phantom (The Phantom Laboratory, Salem NY) includes two pairs of bead ramps (coarse and fine) and a pair of wire ramps in the same module providing an ideal setting for comparing bead ramps and wire ramps. The phantom was scanned repeatedly on the same scanner with varying slice thickness, FOV, mAs and filter. This resulted in 934 CT slices. The images were analyzed using Image Owl Catphan® QA (which provides web-based QA measurements such as slice thickness, MTF, low-contrast and more).

For thin slices (0.5mm), slice thickness estimates from wire ramps were significantly different between the four tested FOVs, where smaller FOVs gave results closer to nominal slice width. For bead ramps, the FOV had no effect. Switching from small to large focal spot resulted in overestimated slice widths for both bead and wire ramps, for thin slices. The smoother the reconstruction filter, the higher the estimated slice thicknesses for wire ramps. The effect of filter type was insignificant for bead ramps possibly due to minimal partial volume effects. The effect of mAs was insignificant for both types of measurement. For spiral slices, higher estimates were obtained than for sequential when using wire ramps while the bead ramps showed equal means but higher variance for sequential than spiral acquisitions.

It is essential to use the small focal spot when measuring thin slices. Bead ramps are more robust to changes in scan parameters while wire ramps give less variability but are sensitive to changes in FOV and filter. 

An accurate measurement of the CT scanners slice thickness is of high importance to the physician who needs to quantify clinical data in 3D.

Olafsdottir, H, Fredriksson, J, Marteinsdottir, M, Andresson, A, Healy, P, Levy, J, Kristinsson, S, Goodenough, D, Optimizing Scan Protocols for CT Slice Width Measurements.  Radiological Society of North America 2012 Scientific Assembly and Annual Meeting, November 25 - November 30, 2012 ,Chicago IL. http://archive.rsna.org/2012/12043897.html