I had tremendous brain fog a few times - now I use kudzu root and it's helping some.
Brain fog is usually from toxins, and over exertion as Tickbite has said. Any time I push myself too far with mental activity, I suffer from it more than usual though.
I also agree that an MRI likely won't show much. My SPECT scan done many years ago (2009??) showed that I had lowered blood flow to two areas of my brain, which correlated exactly with the areas of my body that I was having the most symptoms in. But, that didn't help with anything, other than to have the information (and, I have to admit I thought it was kind of cool, in an odd, sciencey way).
There is also Dr. Jernigan's anti-tox formulas that have helped some people.
Herb only treatment for Lyme & Bart ended 12/11 - no active symptoms for 2 yrs -Herb only treatment for Babesia ended 12/12www.healingwell.com/community/default.aspx?f=30&m=2977364
Had Lyme, Bart, Babs, RMSF, Ehrlichia, Myco, Anaplasmosis, EBV
New Lyme case 8/2014 - Healed 1/31/15 - unknowingly had Asymptomatic Babs and Asymptomatic Bart, being treated now though (2/2016)