I beg to differ Ricky- Alzheimers is the commonest form of dementia- in fact one of its other names is Dementia Alzheimers Type (DAT).
Alzheimers can only be diagnosed with certainty at autopsy.
Ct scans and MRI scans are used to rule out tumours, brain injuries and other possible causes of the symptoms. They can be used to image atrophy of the brain which happens in Alzheimers but also in many other conditions (incl. eg. alcoholism!) but the scan can not be cause specific.
There has been a study done into trying to diagnose the condition early by using SPECT-Single Photon Emmission Computerised Tomography. They looked at people with memory loss and correctly identified 80% of those who went on to develop Alzheimers.