  Electron and x-ray microscopes allow one to image the entire, unlabeled
structure of hydrated materials at a resolution well beyond what visible light
microscopes can achieve. However, both approaches involve ionizing radiation,
so that radiation damage must be considered as one of the limits to imaging.
Drawing upon earlier work, we describe here a unified approach to estimating
the image contrast (and thus the required exposure and corresponding radiation
dose) in both x-ray and electron microscopy. This approach accounts for factors
such as plural and inelastic scattering, and (in electron microscopy) the use
of energy filters to obtain so-called "zero loss" images. As expected, it shows
that electron microscopy offers lower dose for specimens thinner than about 1
micron (such as for studies of macromolecules, viruses, bacteria and
archaebacteria, and thin sectioned material), while x-ray microscopy offers
superior characteristics for imaging thicker specimen such as whole eukaryotic
cells, thick-sectioned tissues, and organs. The required radiation dose scales
strongly as a function of the desired spatial resolution, allowing one to
understand the limits of live and frozen hydrated specimen imaging. Finally, we
consider the factors limiting x-ray microscopy of thicker materials, suggesting
that specimens as thick as a whole mouse brain can be imaged with x-ray
microscopes without significant image degradation should appropriate image
reconstruction methods be identified. The as-published article [Ultramicroscopy
184, 293--309 (2018); doi:10.1016/j.ultramic.2017.10.003] had some minor
mistakes that we correct here, with all changes from the as-published article
shown in blue.
