Dose effects are an important topic in electron microscopy (EM) due to the close connection to radiation damage and hence to quantitative image analysis . Two main problems restricting the application of exit wave reconstruction to radiation-sensitive materials are the high noise level in low-dose images and the contrast reversals close to zero defocus. Both problems cause difficulty in correctly registering a focal series of images and a well-aligned image series is a prerequisite for valid wave restoration .
In this work, a simulation-assisted cross-correlation function (SA-XCF) registration scheme is proposed and tested with five focal series of the same cerium oxide (CeO2) nanoparticle taken under identical imaging conditions except for varying electron dose between different focal series. The registration results demonstrate the superiority of the new registration scheme over a simple neighboring-reference cross-correlation function (NR-XCF) registration (Figure 1). The impact of registration quality on exit wave reconstruction is explored by the comparing the IQ factor, an image quality measurement calculated from the power spectrum of an image of the phase restored from the same focal series aligned by two different registration methods. Better phase restoration results are obtained with the focal series registered by SA-XCF in comparison to NR-XCF.
With improved image alignment exit waves reconstructed from focal series data at variable dose were compared. The comparison result confirms the natural hypothesis that higher electron dose series give improved restored exit waves than lower dose series. However, this improvement with increased dose appears to plateau beyond a certain dose threshold. Exit wave reconstruction from focal series at very low dose tends to preserve more noise from the images and leads to worse IQ factor values (Figure 2).
It has also been observed that the restored exit waves from focal series at very low dose can be unreliable for quantitative interpretation (Figure 3) where the phase shift is noticeably attenuated. This leads to the question of how a critical dose can be determined in order to obtain a quantitatively interpretable exit wave subject to the most efficient use of an allowed electron dose budget.
To cite this abstract:Chen Huang, Hidetaka Sawada, Angus Kirkland; Effects of dose and image registration on exit wave reconstruction of low-dose focal series. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/effects-of-dose-and-image-registration-on-exit-wave-reconstruction-of-low-dose-focal-series/. Accessed: December 1, 2022
EMC Abstracts - https://emc-proceedings.com/abstract/effects-of-dose-and-image-registration-on-exit-wave-reconstruction-of-low-dose-focal-series/