If depression is caused by an inflammation on the brain, why can't we detect it with an mri?

Inflammation: The inflammation hypothesis of depression is just that -- a very plausible, but not completely proven hypothesis. Early stages of inflammation may not show up on an MRI because physical structures like myelin and nerve cells have not been damaged to the point it can be seen anatomically. Change in functional status of these cells and their components precedes anatomical change or damage.