What does frontal and parietal cerebral atrophy indicate? this was the conclusion on my 81 year old mother's ct scan. i'm worried about her changing personality such as irrational behavior?

Some atrophy may occur in the ninth decade. It may be asymptomatic or cause a mixed set of symptoms such as memory and cognition. So by iteslf it is not disease specific.

Focal bilateral cerebral atrophy often is indicative of specific developing dementias, and the findings on your Mother's images should be further evaluated. MRI films will provide more accurate anatomical delineations, and a PET scan may provide the most definitive answer.