A 42 year old lady presents with a 6 month history of oligomenorrhoea and galactorrhoea to her GP. She denies any headache, neurological symptoms, or visual problems. Her mother had a premature menopause. Two separate prolactin readings taken a month apart at the GP surgery have shown prolactin of 590 and 680 (normal range <400 mU/L). Thyroid function tests, FSH and LH are normal. What would be the next most appropriate investigation required?