Primary investigation of choice in a patient with suspected subarachnoid haemorrhage should be:

Here the investigation of choice is Non-contrast CT; if it is not successful lumbar puncture can be tried. The most sensitive investigation is Digital subtractions angiography of both carotid and veebral aeries. Ref: Mauricio Castillo (2006), Chapter 8, "Trauma", In the Book, "Neuroradiology Companion: Methods, Guidelines, and Imaging Fundamentals" 3rd Edition, Lippincotts Publications, USA, Page 52 ; Current Medical Diagnosis and Treatment, 2003, Page 965