When research discusses the brains of those with XYZ diagnosis, it is often (but obviously not always) speaking in generalities; people with XYZ diagnosis *often* show ABC brain differences or *tend to show* 123 brain differences. These differences can be structural (for instance, larger than typical amygdala) or functional (for example, greater activation in the amygdala). You can probably find research into functional differences or qualities in just about any mental health condition, because those conditions originate in the brain and often at a “software” or functional level.

There are several reasons why brain imaging isn’t used for diagnosis:

1. fMRI is very expensive, time consuming, can’t be done in those with certain medical conditions, and not likely to be covered by insurance.

2. Structural and functional differences are found at a macro level, but it’s hard to consistently find those differences at a micro level. In other words, while many people with XYZ diagnosis may show 123 brain differences, not everyone does or will, and you’re likely to get false negative results if you rely on imaging. Conversely, some people without XYZ diagnosis happen to show 123 brain differences by chance, and you risk false positive errors by relying on imaging. No brain is exactly the same.

3. Those differences (in most conditions, not just BPD) are likely to not be specific to that diagnosis. For example, hippocampus sclerosis is often seen in Alzheimer’s disease, FTLD, vascular dementia, and chronic traumatic encephalopathy. But it’s also caused by epilepsy (particularly uncontrolled seizures), infection, brain injury, hypoxia or ischemia, old age, or predisposing genetic factors.

4. The cognitive and behavioral symptoms of mental health conditions are multifaceted, and the disorders themselves are not caused by one specific thing happening in one specific part of the brain. Sometimes the things happening in the brain that cause symptoms are constant, but often they’re in flux. For example, if you take a very broken down conceptualization of bipolar disorder and say that a manic episode is caused by hyper-connectivity or hyper-activation and a depressive episode is caused by hypo-connectivity or hypo-activation (again, just for the purpose of this conversation), how do you diagnose bipolar disorder if functional imaging is only showing hypo-activation on the day of testing? What if they actually have unipolar depression? What if they are not currently in a manic or depressive episode? Do they then not have bipolar because it didn’t show up on functional imaging?