A research, using techniques that imaged the brain, has shown that adolescents and adults with bipolar disorder, have structural changes in brain areas that govern emotions.
Brain emotional areas, known as amygdala and hippocampus, are found to be smaller in bipolar patients.
People with bipolar disorder can have abnormalities in areas toward the front of the brain, the orbitofrontal cortex and the anterior cingulate, that process emotions.
Using MRI of the brain, researchers have found enlarged ventricle spaces (spaces which carry cerebrospinal fluid through and around the brain), that indicates less brain tissue.
On MRI picture, bipolar patients, have an abnormal, increased amount of small, white areas in the brain, involved in transmitting information from one part of the brain to the other.
Also, there is a reduction in glial cells, making up to 90% of brain cells, providing physical and nutritional support for neurons, making them communicate more efficiently.
Less glial cells, means impaired brain communications, leading to mood fluctuations.

My neurologist just told me that my brain looks really good. I definitely have bipolar disorder, so apparently you can have it without any physical abnormality of the brain.