Hello and thanks for the query
ALS is a motor neuron disease
and usually diagnosed by clinical symptoms. On other investigations is a definite test for diagnosis. MRI scan came back normal in patient with ALS. The problem you are facing is most likely to be PIVD of the cervical spine
as your doctor has in his mind. Weakness, numbness
are present in this disease. So don't worry and go through the investigation and than get to a decision. Their might be some technical problem that result in repeat scan.