Would you consider trying the MRI without anesthesia, or use with some oral anti-anxiety medication? I know of at least one child here who had a successful unmedicated MRI at around age 8. My daughter had her MRI at age 6 with 0.5 mg of Ativan given orally (under the tongue) 30 minutes before, and she did perfectly. The MRI techs told me later that when they saw us walk in they never thought she could hold still, being so young, but she did. I did quite a bit of preparation with her prior to it, anticipatory guidance about what to expect, and emphasized how she had to hold completely still. I bribed her with a trip to the donut store if she was successful--and happily, she was.
There is a thread here about MRI's in young kids, I will find it and link it here for you.
Gayle, age 50
Oct 2010 fusion T8-sacrum w/ pelvic fixation
Feb 2012 lumbar revision for broken rods @ L2-3-4
Sept 2015 major lumbar A/P revision for broken rods @ L5-S1
mom of Leah, 15 y/o, Diagnosed '08 with 26* T JIS (age 6)
2010 VBS Dr Luhmann Shriners St Louis
2017 curves stable/skeletely mature
also mom of Torrey, 12 y/o son, 16* T, stable