Extension and flexation mris?

A quick update: It has been awhile since I have been able to be on here, but since then I have went for a second opinion with a NS at Cleveland Clinic and actually he was very nice. He took his time looking at all my scans, and went over everything with me. He explained that although my spinal cord fluid was not "blocked" is was restricted and there was crowding but there still was flow, he did believe that alot my symptoms were most likely from Chiari though. His suggestion though would be to send me for another mri where I would bend my neck up and down to see if there is more compression on the cerrebellum as I do this. I have this scheduled for tomorrow, has anyone ever had this done before? He said that if it shows more compression with my movements then he will know that decompression surgery will be very beneficial in helping my symptoms. If it does not show compression then he wasn't sure if it would help or not. I think I would need a very clear answer myself to go ahead with the surgery, since it is such a major surgery. So, just curious if anyone has had this kind of mri done, or has heard of it?

Thank you