A woman lies still in the MRI machine, the rhythmic pounding oddly reassuring, the images being created inside her skull potentially revealing whether the headaches that brought her here have sinister cause. The scan will show what cannot be seen - the internal structures of her brain in exquisite detail. A child with a broken arm has X-rays that confirm the fracture and guide its treatment. A man with chest pain undergoes CT angiography, the scan revealing whether his coronary arteries are blocked without the need for invasive catheterization.