## 4 Summary and Future Prospects  
Since the start of the SPI initiative, eight experiments have been completed to try to overcome the technical challenges in achieving atomic resolution in single particles imaging with XFELs at the  
Figure 10: Three-dimensional diffraction pattern and reconstructed structure obtained without conformational analysis. (**a**) Diffraction pattern extracted from 37,550 2D single-particle diffraction snapshots of the PR772 virus, obtained at LCLS; (**b**) Corresponding 3D reconstructed image with a resolution of 9 nm, corresponding to scattering to the edge of the detector. (reproduced with permission from [206]. Copyright Springer Nature, 2017). Figure 9: Diffusion map analysis of all hits. Large data sets were reduced to single-particle hit patterns. (**a**) Single-particle hit, (**b**) multiple-particle hit, (**c**) water diffraction that may contain contaminant residue from samples and buffer solutions; (**d**) background diffraction from detector; (**e**) detector defect pattern. (reproduced with permission from [197]. Copyright Springer Nature, 2017). Linac Coherent Light Source. Two data papers have been published to describing experimental details and progress to date. Progress was also made by the SPI initiative members developing conformation change detection and new phase retrieval algorithm. The goals of past SPI experiments mainly focused on light source and instruments optimization, sample preparations and delivery development, background reduction and data analysis methods. The team achieved measurement signals at \(\sim\)6 A resolution, to the corner of the CSPAD detector used with the 100 nm-focusing system of the CXI instrument. At the moment, the data is insufficient to reconstruct the 3D real-space image. On the other hand, hundreds of thousands of diffraction patterns were recorded by a far-field pnCCD at the AMO instrument using soft X-rays. Different team members have successfully reconstructed 3D real-space images using different methods at a full-period resolution of \(\sim\)10 nm. Some of the SPI beamtime have been dedicated to the improvement of sample injection. Different injection methods, such as GDVN and electrospray for aerosolization, have been tested to reduce the background. This is still a challenge for further experiments. For the data analysis, classification has made rapid progress via the use of diffusion maps and machine learning. Powerful programs, such as _psana_, _Condor_, _Huimmingbird_, _Ondata_, _Dragonfly_ have made great contributions to SPI. For higher resolution in the near future, such as 5 A or better, more challenges lay ahead. The reproducibility of samples and homogeneity of internal structures are hard to guarantee. The hit rate and sample contaminations will have a great influence on the validity of patterns and numbers of valid frames. Higher resolution means more frames, this also requires more semi- and automatic data analysis methods. Not only are enough frames needed for 3D reconstructions but they also need enough photons in each frame for interpretation above noise. To totally outrun the radiation damage, the pulse duration should be reduced from 40 fs to below 10 fs without too much loss in flux. High QE, high dynamic range and large array pixel area detector will also be a bottleneck for higher resolution approach using higher repetition rate machines. Fortunately, the European XFEL and LCLS-II are both developing such detectors. A new tendency in scientific research is to share the data, sources and codes to the public [209; 210; 211]. With data sharing, the validity and repeatability of the analysis and interpretation can be tested by different institutes and personnel [212; 213; 214; 215; 216]. \"Open science, data sharing, software sharing is the future of science\", [PERSON] says [209]. For SPI, this concept has also been accepted and will be extended to a wider range [217; 218; 219; 220]. At the moment, most published data has been uploaded to the Coherent X-ray Imaging Data Bank (CXIDB) [221; 222].