mesh DICOM trop haute resolution

I need suggestions regarding how to lower the resolution of a DICOM mesh, that I imported in stl?
It contains 318 754 faces and tris. I did try to apply tris to quad but it seems that I cannot go further…
I used the decimate in collapse mode and in unsubdivide mode, each time on a new version, but I still have lots of tris and a high numer of faces 120.000
Is there a way to get a better results?, I am looking for around 40000 faces maxi…

W / remove doubles, change merge distance
X / limited dissolve
Decimate modifier
Retopologise you mesh
Tris to Quads

Use in combination to give the result you want

Thank you Richard I did not think about remove doubles or merge distance … will do that thanks again