Which investigation, test, imaging for a neuro would make it clear to see and diagnose a dramatic significant loss of brain cells and neurons ?

Neuropsych testing: CT or MRI can show loss of brain volume, which usually means loss of brain cells, but this isn't very helpful. PET-CT can show how much metabolism is going on in areas of the brain. But neuropsychological testing is probably the most helpful, as it tests how well the brain is actually working to solve various tasks. Brain dysfunction may be caused by loss of neurons or impaired neural networks.