With a diagnosis of depression and ptsd and negativefindings on mri and ct scan, could there be any other organic cause for impaired memory, mild confusion, and slow speech and word finding that does not show up onmri?

Many possibilities: 1st, MRI & ct only demonstrate anatomy & structure; they don't assess function. You need f-mri (functional mri), ct perfusion, pet etc to show abnormal function as described. However this doesn't explain why, like traumatic brain injury eg concussion or knockout. Ask doc (neurologist of geriatrician) for dementia evaluation. Unfortunately, while we look for reversible causes, often doesn't respond.

Not likely: Need to treat your depression and ptsd first so they do not cloud your thinking or your evaluation / testing if remain after feeling back to baseline. Good luck.

Yes, many: Thyroid problems, pernicious anemia, infection with HIV or syphilis, alcohol and nutritional issues, medication reactions, use of recreational drugs, and many others. Sounds like your issues need to be sorted out, and simplest and direct way is to have neuro-psychological testing.