Hi! Interesting question indeed. Maybe NODDI, an advanced diffusion technique developed at UCL, can be something to look at? 

http://mig.cs.ucl.ac.uk/index.php?n=Tutorial.NODDImatlab

In comparison to regular DTI measures such as FA and RD, you get measures of orientation dispersion (ODI) as well as isotropic diffusion from NODDI, this might be something to look into. Otherwise, maybe some quantitative T2 relaxometry techniques such as mcDESPOT or Myelin Water Imaging (MWI) could be something to look into. 

You can estimate myelination by mesureing white matter FA.For example, In normal development.

Hi I would use radial diffusivity to differentiate between myelin thickness (FA is thought to be a measure of white matter integrity although there are limits as to what you can conclude based on white matter crossing, averaging the value across the white matter tract etc). As far as number of connections, you may be able to turn these measures into binary data and extract a number of connections that way. 

Hi, Because diffusion anisotropy does not require myelination, DTI parameters are not good markers for myelination status. Myelin Water Imaging (e.g. through multi-exponential T2 relaxation or mcDESPOT) or MTR may indeed give you an additional indication of the presence of myelin water, as mentioned by Emil in the first reply.

There have been attempts to map the g-ratio (outer vs. inner diameter of a myelinated axon), using an estimate of axon density (from NODDI), and an estimate of myelin density (from MTR). Under some assumptions, this could give you a clue about the myelin thickness. Be very careful with interpretation though, as the limitations of both input techniques (NODDI and MTR) are transferred to the g-ratio.

http://www.ncbi.nlm.nih.gov/pubmed/26004502

All answers were useful, although Thibo and Barbara mentioned what I was exactly looking for.

Thanks.