This dilemma is faced by lot many people, as besides individual fears, it also increases the duration of scans. But for total information about the disease or even screening, it is advisable that both are done. At times, the tumors specially, vascular ones, may not be picked up on a plain scan. In such a scenario, the information received is incomplete and may not even give a clue about the impending problem.. So explain about it to your mother and she would surely agree.