TMS and Brain Mapping with PET
Positron emission tomography (PET), when applied to the brain, usually measures changes in regional cerebral blood flow (rCBF) or regional cerebral metabolic rate of glucose (rCMRglc) – depending on the radioactive tracer used. The combined use of PET and TMS was established around 1997 (Paus et al. 1997). There are two main methods of delivering PET and TMS: 1. On-line PET imaging – where PET mapping is conducted at the same time as TMS stimulation 2. Off-line PET imaging - where TMS precedes or follows PET mapping.
The combination of on-line PET and TMS was first used in 1997 (Paus et al. 1997) to stimulate the frontal eye fields (FEF) demonstrating the potential to map functional cortico-cortical connectivity.
Ongoing research
Off-line PET imaging demonstrates rTMS-induced regional plasticity and several rTMS protocols have been developed to induce lasting changes in the stimulated cortex and connected areas (Oxford Handbook):
- Continuous rTMS (Hallett 2000)
- Intermittent (theta) burst rTMS (Huang et al. 2005)
- Paired-associative stimulation (Stefan et al. 2000)
Future research
Several PET studies have shown that rTMS produces substantial changes in synaptic activity without any visible change in behaviour (Lee at al. 2003, Rounis et al. 2005) – If not behavioural, future studies may involve finding out on what level this is affecting brain function. Exploring the site, interval and strength of TMS using PET will enable a better understanding of how TMS affects brain function when measures of behaviour or movement are not available.
Combining TMS with ligand-PET provides a means of assessing regional changes in neurotransmitter binding following TMS, eg, to show dopamine concentrations (Strafella et al. 2001) with potential applications in therapy.
References
- Hallett et al., Supplements to Clinical Neurophysiology, 2000.
- Huang et al., Neuron, 2005.
- Lee at al., The Journal of Neuroscience, 2003.
- The Oxford Handbook of Transcranial Stimulation.
- Paus et al., The Journal of Neuroscience, 1997.
- Rounis et al., Neuroimage, 2005.
- Strafella and Paus. Journal of Neurophysiology, 2001.
- Stefan et al. 2000.
