  For any admissible value of the parameters $n$ and $k$ there exist
$[n,k]$-Maximum Rank distance ${\mathbb F}_q$-linear codes. Indeed, it can be
shown that if field extensions large enough are considered, almost all rank
distance codes are MRD. On the other hand, very few families up to equivalence
of such codes are currently known. In the present paper we study some
invariants of MRD codes and evaluate their value for the known families,
providing a new characterization of generalized twisted Gabidulin codes.
