"Psychiatry is the only medical field that rarely looks at the organ it treats. How can you know what’s going on inside your head if nobody ever looks?" Daniel Amen
Most psychiatrists guess (an educated guess but nonetheless, a guess), based on symptoms, what drug will work best for each patient. Our brains are as unique as snowflakes and contain many differences based on our evironments (home, toxins, etc.), development, genetics, etc. This makes it almost impossible to prescribe the correct drug/s and dosage and the one with the least side effects.
Brain Health Questionnaires are the next best thing to having your brain scanned to determine which areas of the brain are working too hard, not working hard enough, or working well.
Additionally, neurocognitive testing provides a baseline score by efficiently and objectively measuring a broad-spectrum of brain function domain performances under challenge. Using proven methods, one can improve neurocognitive outcomes and increase performance baselines as needed.