This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

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Only electrons scattered at small angles and without loosing energies will contribute to the information in the HRTEM images while all the others (which have lost energy and have been scattered multiple times or at high angles) also contains important information about the sample, they will for instance give a diffuse background in the TEM images and benefit to EFTEM elemental mapping. High-angle incoherently scattered TDS (thermal diffuse scattering) electrons are subject to significant phase shifts from the rapidly varying portion of the lens transfer function and thus will extremely contribute a constant background to the image [1].

To form Z-contrast images, the STEM objective aperture must be inserted to cut out high-angle electron rays, and an annular detector must be inserted after the projector lens system.

[1] C.B. Boothroyd, Journal of Microscopy 190 (1998) 99.