Electrons and X-rays for diffraction and imaging

A comparison is provided between the use of electrons and X-rays for collecting diffraction data from small protein crystals and imaging data from cells and tissues. The paper contains a review element written to enable an understanding of the relevant properties of electrons by those (including one of the authors) more used to X-ray imaging and diffraction. Radiation-damage mechanisms, sample thickness-dependent dose efficiency and energy-dependent scattering cross sections are discussed, together with contrast mechanisms in electron and X-ray imaging. Crossover points are calculated for diffraction from crystals where electrons and X-rays could yield equivalent data quality in the presence of radiation damage. Increasing the electron energy from 300 to 1000 keV results in a ∼43% rise in the electron/X-ray crossover point. However, the maximum information coefficient (useful signal/absorbed dose) for electrons alone occurs for a 250 nm crystal examined at around 800 keV. The impact of inelastic scattering on electron imaging and diffraction is examined, including its role in coherence loss, Bragg spot broadening and background elevation. The possibilities are investigated for locating regions of interest using X-rays for subsequent higher resolution imaging using electrons. For locating a 30 nm diameter protein or virus, the required X-ray dose would be much less than the tolerable dose for electron imaging at 0.5 or 0.25 nm. Overall, these findings are relevant for imaging at different length scales while minimizing dose-induced structural damage.