Practical Electron Microscopy and Database

The Detective Quantum Efficiency (DQE) in the direct electron detectors (DEDs) relates to the efficiency with which these detectors capture and process image signals relative to the noise they introduce. Defined as the squared ratio of the output to input signal-to-noise ratio (S/N)²_OUT/(S/N)²_IN, DQE effectively quantifies how much noise and signal degradation occurs within the detector. An ideal detector would have a DQE of 1, indicating perfect efficiency with no added noise.

In direct electron detection, DQE is typically plotted against spatial frequency, where values closer to 1 imply optimal performance across frequencies, especially at the Nyquist frequency (determined by pixel spacing). The spatial frequency range that a detector can effectively capture is limited by factors like pixel spacing, which sets the Nyquist limit (the maximum spatial frequency that can be accurately recorded without aliasing). Improvements in DQE, notably in CMOS/MAPS-based detectors compared to traditional CCDs, allow for higher spatial resolution and a clearer signal in electron microscopy applications. Backthinning techniques and counting mode operation enhance DQE further, achieving high values at low spatial frequencies and enabling potential super-resolution imaging by accurately counting individual electron events. Note that backthinning is a process applied to semiconductor detectors, where the detector’s substrate thickness is reduced by removing material from the back. This technique minimizes the amount of backscattered electrons, which can otherwise introduce noise and blur into the images, especially in high-resolution imaging applications like electron microscopy.

Figure 13 shows the DQE values for integrating and counting modes, particularly the comparisons and the range of 40–60% at half Nyquist and around 25% at Nyquist frequency. This figure compares DQE at 300 keV across different detectors, showing variations in DQE as a function of spatial frequency for models such as the DE-20, Falcon II, and K2 Summit.

[1] R.A. Crowther, The Resolution Revolution: Recent Advances In cryoEM, MRC Laboratory of Molecular Biology, Cambridge, United Kingdom, 579, pp.2-445 (2016).