Cycloidal Computed Tomography

Our phase methods use apertured masks to make x-ray systems sensitive to phase effects. One additional advantage of using masks is that the spatial resolution becomes independent from x-ray focal spot and detector pixel size, and is determined purely by the size of the apertures in the masks. However to access such a higher resolution level, the sample or the mask must be laterally translated in steps equal to the aperture size, by a number of steps equal to the ratio between apertures spacing and aperture size (“dithering”). In cycloidal CT, the sample is laterally translated as it is rotated (every point in the object then describes a cycloidal trajectory, hence the name). This allows a resolution very close to that obtained with dithering, but without the extra steps, leading to much faster scans in which the sample can be moved continuously rather than in a “step-and-shoot” fashion. Despite the resolution increase, exposure time and dose are the same as in a “rotation only” scan. Our research has also produced preliminary evidence that the “higher resolution at same dose” argument can be turned on its head to yield “same resolution at reduced dose”, which could provide options to reduce dose in CT in the future.

Image quality obtained with a full lateral scan of the sample (a), rotation only (b), roto-translation (cycloidal) CT (c). The dose and exposure time are the same for (b) and (c), while they are eight times higher for (a).