Using Brain Scans to Diagnose Mental Disorders

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It's widely recognized that psychiatric conditions like depression and anxiety disorders are based in the brain. Scientists have even started to discover which brain areas are involved in different conditions. For example, post-traumatic stress disorder (PTSD) seems to involve excessive activity in the amygdala, which is involved in processing fear, as well as low activity in certain parts of the frontal lobes.

Much of the evidence for the role of specific brain areas in psychiatry comes from "brain imaging," which involves various ways of looking at the brain. Some technologies like PET imaging and functional MRI can measure the activity of the brain either at rest or while a person does certain tasks. Other technologies, like traditional MRI, measure the brain's structure—its size and shape.

Given how much we've learned about the role of the brain in mental illness, many people are surprised to learn that we can't know what psychiatric diagnosis a person has—or even if the person has any diagnosis—by examining that person's brain.

Several factors make it hard to diagnose a person with a psychiatric condition based on brain imaging.

1. First of all, there's a lot of variation in brain activity among people with the same diagnosis (and without that diagnosis). When we say that brain activity looks a certain way "in people with PTSD," what we really mean is average brain activity in a group of people with PTSD versus average brain activity in a group of people without PTSD. I often use the analogy of average height differences between men and women: All else being equal, men are taller than women on average. But that average difference does not get you very far in predicting if a person who is, say, 5'8" is a man or a woman. It could be a slightly tall woman or a slightly short man because there's a lot of overlap in men's and women's heights. In the same way, there is a lot of overlap in the brains of people with and without PTSD (and other conditions).
2. Psychiatric conditions can look quite different in different individuals. Take depression, for example. There are nine symptoms in the DSM-5, and a person has to have five of them for Major Depressive Disorder. Thus two people can hypothetically have "the same diagnosis" but only share one symptom. Because there's a lot of variation in the symptoms, it's hard to determine with precision what the brain looks like in depression and other conditions.
3. Different psychiatric conditions often share similar symptoms. For example, ADHD and depression can both involve difficulty with concentration, while irritability can be a symptom of both bipolar disorder and generalized anxiety disorder. For this reason, it is likely to be especially hard to tell the difference between two psychiatric disorders based on brain imaging.
4. Finally, similar brain areas are involved in different psychiatric conditions. For example, high activity in the "emotion centers" of the brain and low activity in parts of the frontal lobes is common across anxiety conditions, PTSD, schizophrenia, spouse abusers, and even among healthy individuals who are sleep deprived. It's impossible to say what someone's diagnosis is based on brain imaging when the same areas seem to be involved in various conditions.

The fact that brain imaging can't be used for psychiatric diagnosis doesn't mean there's no place for this technology. It can be useful to understand the areas involved in a psychiatric condition, enabling novel approaches like deep brain stimulation and transcranial magnetic stimulation for the treatment of depression. Brain imaging can also rule out a direct physical cause (like a tumor or a brain bleed) of psychiatric symptoms.

While science hasn't enabled brain-based diagnosis in psychiatry, the "old fashioned" approach—talking with a mental health professional—still works. And good news: It's thousands of dollars cheaper than having a brain scan.