The effect on the magnetic field on the image of SEM

"Various reasons of the distortion is based on measurements of the periodic deformations of the images for different electron beam energies and working distances between the microscope final aperture and the specimen. Using the SEM images, a direct influence of alternating magnetic field on the electron beam was distinguished."

The direct electron beam deflection depends on the magnetic field magnitude and the working distance, i.e. distance between the final aperture of the electron gun and the specimen.

Paper that explains the measurement of a B-field in a SEM;

http://www.sciencedirect.com/science/article/pii/S0968432808000188

Expression states to calculate the magnetic field;

Where Bx and By are two orthogonal components of magnetic field vector B, e is electron charge, E is energy of electrons, Me is relativistic electron mass, Vx0 and Vy0 are orthogonal components of initial electron velocity (initial, i.e. at working distance=0) parallel to Lorentz force direction, dx0 and dy0 are orthogonal components of initial electron beam deflection.