The difficulty of electron tomography

The difficulties limit the direct application to electron tomography

1. The tilt range has to be from -90 degree to +90 degree
2. The number of projections in a tilt series needs to be 2N for an N*N object
3. The grid points past the resolution circle cannot be experimentally determined

These limitations are overcome by combining the PPFFT with an iterative process.

Ref: http://www.physics.ucla.edu/research/imaging/EST/index.html