Evaluating image resolution in stimulated emission depletion microscopy

Precise knowledge of the effective spatial resolution in a stimulated emission depletion (STED) microscopy experiment is essential for reliable interpretation of the imaging results. STED microscopy theoretically provides molecular resolution, but practically different factors limit its resolution. Because these factors are related to both the sample and the system, a reliable estimation of the resolution is not straightforward. Here we show a method based on the Fourier ring correlation (FRC), which estimates an absolute resolution value directly from any STED and, more in general, pointscanning microscopy image. The FRC-based resolution metric shows terrific sensitivity to the image signal-to-noise ratio, as well as to all sample and system dependent factors. We validated the method both on commercial and on custom-made microscopes. Since the FRC-based metric can be computed in real time, without any prior information of the system/sample, it can become a fundamental tool for (i) microscopy users to optimize the experimental conditions and (ii) microscopy specialists to optimize the system conditions.