Olivier Reynaud1
1Human Neuroscience Platform, Foundation Campus Biotech Geneva, Switzerland
Synopsis
This lecture will highlight the very different
physics of MRI and TMS, and will give the audience a brief introduction to the
characteristics of TMS and TMS experiments outside any MRI environment. The challenges
of performing TMS in a magnetic environment will then be laid out, and
appropriate solutions to the various challenges to overcome explained. The
audience will be presented with the current state of applications in concurrent
TMS-MRI. Hopefully the interested listener will leave the talk with all the
tools necessary to setup his own multimodal study.
Introduction
This course on Complementing MRI with Other
Modalities: Hardware & Method Development will
focus on the concurrent (and not simultaneous) application of TMS and MRI.
This lecture will highlight the very different
physics of MRI and TMS, and will give the audience a brief introduction to the
characteristics of TMS and TMS experiments outside any MRI environment. The challenges
of performing TMS in a magnetic environment will then be laid out, and
appropriate solutions to the various challenges to overcome explained. The
audience will be presented with the current state of applications in concurrent
TMS-MRI. Hopefully the interested listener will leave the talk with all the
tools necessary to setup his own multimodal study.Target audience
Researchers,
engineers and physicians willing to set-up a multimodal study involving – at least
– TMS and MRI in the same session.Objective
This lecture aims at:
1. Understanding the fundamental physical principles of TMS
2. Knowing the main challenges and providing guidelines to follow to setup a successful concurrent TMS-MRI experiment
3. Providing a brief overview of current research using concurrent TMS-MRI1. The Physics of TMS
TMS and MRI rely on very different physical
principles. While MRI relies on the quantum properties of the hydrogen atoms
within a magnetic field, TMS relies instead of the application of Faraday’s
induction law following a massive change in current through a loop or coil positioned
above the subject’s head.
Before stimulation, a strong charge is
accumulated in an open circuit. When the circuit closed, electrical current
rapidly flows through the coil made of multiple loops generating a magnetic
flux below. Magnetic fields penetrate the skull and brain. Its rapid change in magnitude
in turns generates an electrical current strong enough to depolarize the
membrane of neurons, triggering an action potential at interneuron level. Different TMS orientations activate
different populations of cortical neurones/axons in the motor cortical
circuitry, or the same populations but at different sites.2. The characteristics of TMS
TMS can be used as a readout: The white matter
and corticospinal tract can be probed via the magnitude / conduction time of
motor evoked potentials. Neurotransmission can be studied using short / long
term cortical facilitation / inhibition.
But TMS can also be used as a perturbation. Brain
connectivity can be assessed using causality / virtual lesions approaches. Repeated
TMS can also induce long term potentiation / depression, which is the basis of
FDA-approved treatment for depression.
TMS is versatile. It can be performed offline
[Session 1 – TMS – Session 2] or online [repetitions of Block with TMS / Block
without TMS].3. The challenges of concurrent TMS-MRI and potential solutions
The main challenges when performing TMS inside
the MRI include Neuronavigation in the MRI scanner (are we targeting the proper
area?), characteristics of the TMS coil [1-11] (strength, depletion,
cooling, weight, holder, space), sham [8], artefacts [9,10],
characteristics of the MRI coil (compatibility [7]), TMS control
(synchronization), simultaneous vs. concurrent use of TMS and MRI and many
more. Doing it wrong can lead to the destruction of the TMS coil and/or MR signal.
We’ll review those challenges and propose proven solutions to each particular issue.4. Applications of concurrent TMS-MRI
When used properly, concurrent TMS-MRI has the
potential to: bridge the gap between clinical and biomedical research; better
understand cognitive processes by stimulating specific neuronal populations;
and manipulate the brain and/or create reversible virtual lesions; all of this
while measuring the brain neurovascular response. A review of past and current
approaches[1-11] will be done.Acknowledgements
No acknowledgement found.References
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