# Low-signal limit of X-ray single particle imaging

## TL;DR

This study demonstrates that X-ray single particle imaging can successfully determine virus structures at extremely low photon counts, approaching the limits of current experimental capabilities, with potential improvements from upcoming high-repetition sources.

## Contribution

The paper provides experimental validation that orientation and phase retrieval are feasible at very low signal levels, extending the applicability of Bayesian methods to realistic, low-signal conditions.

## Key findings

- Successful imaging at photon counts as low as 1/256 of typical levels
- High-quality electron density reconstructions from low-signal data
- Potential for future improvements with high-repetition-rate X-ray sources

## Abstract

An outstanding question in X-ray single particle imaging experiments has been the feasibility of imaging sub 10-nm-sized biomolecules under realistic experimental conditions where very few photons are expected to be measured in a single snapshot and instrument background may be significant relative to particle scattering. While analyses of simulated data have shown that the determination of an average image should be feasible using Bayesian methods such as the EMC algorithm, this has yet to be demonstrated using experimental data containing realistic non-isotropic instrument background, sample variability and other experimental factors. In this work, we show that the orientation and phase retrieval steps work at photon counts diluted to the signal levels one expects from smaller molecules or with weaker pulses, using data from experimental measurements of 60-nm PR772 viruses. Even when the signal is reduced to a fraction as little as 1/256, the virus electron density determined using ab initio phasing is of almost the same quality as the high-signal data. However, we are still limited by the total number of patterns collected, which may soon be mitigated by the advent of high repetition-rate sources like the European XFEL and LCLS-II.