Breaking up the exposure used to record an image into many frames, like a movie, is known as dose-fractionation. This 'movie mode' imaging is possible in transmission electron microscopes (TEMs) equipped with direct electron detectors (DDDs). It allows image drift occurring during the total exposure to be corrected. The individual frames are aligned and then averaged to enhance contrast. The removal of motion-blur is essential to obtain high-resolution.
We are developing advanced, noise-resistant methods for the registration of X-Y drift at lower defocus and dose levels, and corrections for other aberrations such as defocus/Z-drift.
The quality of images recorded in the TEM is not limited by the spatial resolution of the instrument, but by the signal-to-noise ratio due to limited source brightness and exposure time. The TEM as an instrument is highly sensitive to its environment, and long-exposure times lead to excessive image drift, which degrades image quality due to motion-blur. Move-mode imaging, i.e., dose-fractionation allows individual frames to be registered for various drifts, principally X-Y drift via cross-correlation techniques, aligned, and averaged to increase the signal-to-noise ratio.
For cryo-EM of biological particles, dose fractionation is paired with near quantum-efficient DDDs to achieve outstanding signal-to-noise ratios compared to what was historically possible with film emulsions or scintillator fiber-optic coupled CCDs. The DDD consists of a back-thinned CMOS detector with a silicon active layer. The thinness of the electronics limits the interaction volume of an individual electron, improving the point-spread function compared to scintillators, and the direct generation of many secondary electrons (~70) per primary electron.
In order to avoid saturation, a DDD must be read out very quickly (400 Hz for our Gatan K2 detectors). The extremely fast read-out generates approximately 5 GB/s data, so individual electron counting is done in-hardware to reduce the frame rate to a reasonable frequency. The movie is typically broken up into 20-150 frames. Each frame is then registered, aligned, and finally an overall average computed. Generally some minimum contrast levels must be maintained between images to successfully align the series.