Can the brain atrophy caused by b-12 deficiency be reversed?

Sometimes: When B12 deficiency is treated with B12 (also known as cobalamin), all hematologic symptoms (i.e. Anemia) should reverse and neurological symptoms usually start improving after 3 months with maximal improvement at 6-12 months. For unknown reasons though, sometimes the neurological symptoms persist and even progress even after cobalamin treatment.

Not likely: Atrophy is structural change, brain repair is dependent on the age of the individual.