This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nervous System, Subject name: Radiology
Which imaging modality is least useful in a case of CNS neoplasm? 
Select and explain. 
A. Radiograph, B. Ultrasound, C. CT scan, D. MRI
B: Ultrasound
Answer: B: Ultrasound 
Explanation: Ans BHere the doubt is between Radiographs and USG. But in CNS tumours radiographs can show many direct and indirect signs like calcification, bone erosion, hyperostosis, widening of various intracranial foramina, features of raised ICT. But ultrasound simply cannot penetrate the skull and is useful only in children when the fontanelles are open.