The aim of this pilot study is to determine if there are any changes in brain glucose metabolism in the gray matter of patients with PKU and whether Sapropterin (KUVAN) can improve such deficits. This study will also elucidate the relationship between hyperphenylalaninemia, phenylalanine intake in diet, altered brain glucose handling and the neurocognitive profile of the patients with PKU before and after KUVAN therapy.