Why is alcoholism thought to be a disease not just a behavior problem?

Because it is consistent with the disease model...just like other chronic diseases, addiction usually involves cycles of relapse and remission. Without treatment or engagement in recovery activities, addiction is progressive and can result in disability or premature death.
Long Definition of Addiction:
Addiction is a primary, chronic disease of brain reward, motivation, memory and

Difference between normal and abnormal tissue is much clearer on mri than on ct and mri much suitable to view soft tissues