Ditto to what the others have said. The weight limit is more a "generalized" limit more geared toward size then the weight. The supine (closed tube) MRI's can only accommodate persons to a certain size.
But you can get an open MRI and ibake&pray is correct, most often, the scan is not as crisp as with the closed MRI.
My spine surgeon that just ordered my MRI stated that this was to be a closed MRI to get the best possible detail/clarity.
Hope this helps. Take care. Tammy