  Since the seminal paper by Mitra et al., diffusion MR has been widely used in
order to estimate surface-to-volume ratios. In the present work we generalize
Mitra's formula for arbitrary diffusion encoding waveforms, including recently
developed q-space trajectory encoding sequences. We show that surface-to-volume
ratio can be significantly misestimated using the original Mitra's formula
without taking into account the applied gradient profile. In order to obtain
more accurate estimation in anisotropic samples we propose an efficient and
robust optimization algorithm to design diffusion gradient waveforms with
prescribed features. Our results are supported by Monte Carlo simulations.
