Imagine that for the last three months, you have had a very bad headache – the worst in your life – that won’t go away, even when you take aspirin. In addition to the headache you have also been feeling dizzy. Your doctor tells you that you need to get a brain scan to test whether the headache is being caused by something serious. There are two possible scans you can get: a Magnetic Resonance Imaging (MRI) or a Computerized Axial Tomography (CT) scan. The MRI provides a slightly more detailed picture and might find something that the CT misses, such as an extremely uncommon blood vessel problem, but nearly all problems serious enough to need treatment would be seen on either the MRI or the CT.