MRIs have made it possible to see many abnormalities in the spine that couldn't be seen before. MRI is best for detecting tumors, infections, and disc herniations. It's used most often when surgery is being considered.
Reduced signal on MRI is used as a sign of disc or bone changes. The MRI shows irregular disc shapes, narrow disc spaces, and tears in the outer covering of the disc. It also shows changes in the joints and narrowing of the spinal canal called stenosis.
All in all, MRIs reveal many things about the spine and its condition. The problem is that many people with changes on MRI don't have any pain or other symptoms. And the opposite is true as well. Patients with back pain may have normal MRI findings. So we can't assume that abnormal findings seen on MRI are the cause of the problem.
Back pain affects many people each year. With or without MRI we don't always have an explanation for the symptoms. There are just too many possible sources of pain in the lumbar spine that don't show up as "abnormal."