CHAPTER 4:3D Imaging Using an X-ray Free Electron Laser

The prospect of high-resolution three-dimensional (3D) imaging of biological objects was a major part of the science case for constructing X-ray free-electron lasers (FELs). Rather early after the construction of the FLASH facility in Hamburg, the first X-ray FEL, two-dimensional (2D) images of cells were presented. It did, however, take another 9 years for the first non-trivial sample to be imaged in 3D with similar methods. Why is this? And why is it that even now the resolution of this 3D image is almost an order of magnitude worse than the highest resolution achieved in 2D? This chapter gives a detailed explanation of the data analysis required to reconstruct a sample in 3D. These analysis methods took a long time to develop and did not exist at the time when the first FEL data was available. Understanding this set of algorithms also makes it possible to understand if current resolution limits are really a limit to the method or a limit imposed by experimental parameters that are likely to change in the near future. Last, this chapter includes some speculation on the future of the method.