Justification involves assessing the risk / benefit ratio. The problem with imaging procedures is that no one doctor is in an ideal position to assess both sides of this equation. The imaging specialist can best assess the risks for the individual patient, but the referring doctor - who knows the patient, the clinical context and the question to be addressed by the procedure - is best placed to assess the potential benefit as well as the risk of not performing the test.
Justification is therefore a joint responsibility of imaging specialist and referring doctor. In practical terms, the referring doctor needs to have some knowledge of the risks (if for no other reason than to inform the patient) and the imaging specialist requires adequate clinical information to be able to determine the appropriateness of the examination and the imaging protocol to be undertaken. Close consultation between the two doctors, in situations where there may be cause for discussion, is essential.