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
[{'content': 'Investigation of choice for SAH is \nSelect and explain. \nA. Non enhanced CT, B. Enhanced CT, C. Non enhanced MRI, D. Gd enhanced MRI', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans: a) Non enhanced CT SAH is caused by damage to blood vessels on the pia- arachnoid.Non contrast CT scanning of the brain is the first step in the evolution of SAH. If the scan is performed within 12- 24 hrs after hemorrhage, 92 - 100 % cases can be detected. After 1 day and upto 1 week CT scans performed will appear normal.On CT scans, SAH present as hyperdensity representing acute hemorrhage visualized in the sulci overlying the cerebral convexities, within the sylvian fissurs, basal cisterns and interhemispheric fissure.Non enhanced CT is the modality of choice in the evaluation of SAH because of the hyperdensity clotted blood.It is difficult to visualize SAH with MRI because the high O, concentration in CSF prevents the degradation of oxyHb to deoxyHb.', 'role': 'assistant'}]