Imaging and Data Evaluation
Cryo-SEM can only be used to image the surface of vitrified samples. Cryo-TEM or cryo-tomography (CET) in contrast allows detailed imaging of the ultrastructure of the native, vitrified specimen. Sub-tomogram averaging and 3D segmentation or rendering are used to visualise the 3D structure. For single particle analysis (SAP) hundreds of thousands of highly purified molecular complexes or viruses are recorded in all axial directions and subjected to an averaging process. Using this mathematical reconstruction, molecular structures can be resolved down to 3 Angstroms. This means that with help of special averaging methods, the signal-to-noise ratio and the resolution of non-contrasting biological phase objects (organic samples are transparent) can be significantly increased. However, these methods require big data processing power. Meanwhile, further developments such as the phase plate help to increase the contrast.