Brain mri tmrw. i think the pain may be neck posture (leaning head back). will this show up on the scan if it is the case?

Possibly: Without knowing your exact symptoms, it is difficult to determine whether a brain MRI will find the source of pain. Since your doc has likely done a thorough history and physical, he/she is likely to have some ideas what might be going on with you. A brain MRI should be able to find many things in your head and upper neck that might be causing your pain and should at least r/o serious problems.