Hi and welcome to the chiari forum.
I may be wrong, but a dedicated MRI seems to be the type of machine that is calibrated to read into the tiniest of areas...and dedicated to certain areas.
Again, in looking at this, it appears there r dedicated MRI's for Breast MRI's...and some for extremities....
I would imagine it is just to take a closer look...depending on the slices ordered for ur last one it was not focused on the area where ur herniation is.The drs need to determine if u have a CSF blockage and overcrowding...and by knowing ur DX they can dedicate the MRI for that condition so everything is looked at.I hope that makes sense?
As for the upright or standing/sit MRI's...there isn't one around here either...and common sense says if u lie down the blockage won't be as great as u do not have gravity pulling the tonsils down...but many chiari drs are able to determine the severity of ur condition with a MRI set for chiari or the chiari area.
Most important is to find a true chiari specialist. We do have a thread with names, but u must research to find the best one for u.
Glad to have u here, sorry for the reasons u sought us out.
"selma"