--make sure that when the MRI is done, they visualize not only the lumbar spine, but the sacrum and the nerve roots that come through that area as well. A number of the larger nerves in the lower part of the body and legs are contributed to by a number of nerve root trunks and while it is certainly more common to have problems with lumbar spine compression, compression iof the sacral nerves through movement in the sacrum does happen (and you report of vibration in the rectal area does imply some possible S1/S2/S3 involvement).