Hi @Oeslle,
The requirements for minimum number of volumes for a particular maximum spherical harmonic degree applies independently for each shell, as a spherical harmonic representation of the data is calculated for each shell independently. If all DWI volumes were used concurrently, regardless of b-value, then it would no longer be a “multi-shell” approach, but some form of generic q-space approach. You will see this e.g. in the output of response function estimation, where response function coefficients are defined independently for each b-value shell.
Rejecting shells with too few directions is entirely deliberate. If you have just 2 volumes for a given (nonzero) b-value, you have fundamentally not adequately sampled the information within that shell; your measurement and subsequent characterisation will depend very strongly on the directions of diffusion sensitisation of those volumes. Even if you were to choose to ignore cross-terms, we live in a three-dimensional world, and therefore, for an anisotropic signal, you would need to measure diffusion in at the very least three directions in order to obtain something vaguely meaningful. 6 volumes is the better lower limit, since this enables capturing anisotropy that is not aligned with your diffusion directions; this happens to also be the limit both for the tensor model and for an lmax=2 SH fit.
While using the bare minimum number of volumes for a given lmax is possible, we’ve found that there tends to be a large jump in SH fit stability by including even just one extra volume. It’s also worth noting that since introduction of the amp2response command, satisfying this constraint is not as critical as it used to be.