Doing ultrasound treatments over the center of the spine is tricky. In rare cases, a patient has a small spinal defect in the lower thoracic or lumbar vertebrae. These people were born with a spinal vertebra that was not completely formed. The incomplete formation could allow sound waves from the ultrasound to reach the nerves in the spinal canal. Too many sound waves could damage these nerves.
Sometimes people with this condition have a darker patch of skin or a tuft of hair on the center part of the spine. But this isn't always the case. It is better to be cautious and apply ultrasound treatments along the edges of the spine rather than over the center.