Prefrontal cortex transcranial magnetic stimulation does not change local diffusion: A magnetic resonance imaging study in patients with depression

Objective: To determine whether transcranial magnetic stimulation over the left dorsolateral prefrontal cortex produces pathologic changes or leakage of the blood-brain barrier in patients with depression by using apparent diffusion coefficient magnetic resonance imaging. Background: Transcranial magnetic stimulation is a new technology for noninvasively stimulating the brain. It appears to be a relatively safe technique, with some important exceptions. Its neurobiologic mechanisms of action are poorly understood. One theory to explain its apparent antidepressant effects involves a potential change in local blood-brain barrier settings, allowing passage of peripheral substances directly into brain parenchyma. Knowing whether transcranial magnetic stimulation changes local brain diffusion is important as well from a safety perspective. To test whether transcranial magnetic stimulation changes local brain diffusion, we used apparent diffusion coefficient magnetic resonance imaging in depressed patients undergoing interleaved transcranial magnetic stimulation/functional magnetic resonance imaging over the left prefrontal cortex. Methods: Within a 1.5 Tesla magnetic resonance imaging scanner, 14 depressed patients were stimulated with a figure-eight transcranial magnetic stimulation coil over the left prefrontal cortex. Apparent diffusion coefficient magnetic resonance imaging was acquired before, and immediately after, 1 Hertz transcranial magnetic stimulation (147 stimuli) intermittently delivered at a motor threshold of more than 7.35 minutes. Phase maps of the transcranial magnetic stimulation magnetic fields were used to guide region-of-interest placement. Results: No significant qualitative apparent diffusion coefficient differences were observed before and after 1 Hertz transcranial magnetic stimulation underneath the coil. Conclusions: One Hertz transcranial magnetic stimulation over the left dorsolateral prefrontal cortex as applied in this study did not result in pathologic changes or leakage of the blood-brain barrier in patients with depression. If prefrontal transcranial magnetic stimulation at these usage parameters changes local diffusion, it is not an obvious or large effect.