[Mrtrix-discussion] Response functions at lmax=6 vs lmax=8

[David Grayson]

Hi MRtrixers,

I have HARDI datasets on a large sample of young adults (7-16yrs) taken with 72-dir and b0=3000. I am wondering whether I should proceed with lmax=6 or 8. I am using MRtrix 0.2.12.

I am concerned about using lmax=8 because in the response function there is often a 'nubbin' that appears right in the center of the Z-axis line that doesn't appear at lmax=6. The below link shows an example of the RF's generated for a single subject taken at lmax=6 (on the left) and lmax=8 (on the right). These are generated using default parameters (FA threshold of 0.7 for the single-fiber-orientation mask).
http://imgur.com/a/ODrcf

Hopefully it's obvious what I'm talking about. Sometimes this nubbin is more pronounced than what I'm showing here. I'm not sure how concerned I should be about this, but based on my intuition about what the RF means, the Z-axis itself should represent the global minimum of the DW signal, which is obviously not the case with lmax=8. Should I avoid any appearance of this effect at all costs, or is there some rule of thumb for how big it can be before I should be concerned about it?

Thanks,

David
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