Transcranial Magnetic Stimulation
(TMS)
Research
Involvement
Dr.
Hines Research Group
About
·
Dr. Hines’s research group
caries out research in the field of neuroscience. One of the topics that they
started to do more research in is TMS. Dr. Hines was looking for someone with
expertise to discuss ideas about a TMS circuit, and that is when Dr. Baker came
into the picture. From that point, Daniel got involved in the fascinating
research of transcranial magnetic stimulation.
·
TMS is a non-invasive way to
stimulate neurons in the brain. It is believed that TMS therapy may be able to
help aid illnesses such as Alzheimer’s disease or PTSD.
·
In order to stimulate
neurons, without invasive procedures, high powered electromagnetic waves are
required. The waves are required to have enough power to penetrate through the
skull and into brain to stimulate neurons. The following section goes into more
detail on how to achieve this with a circuit.
TMS Circuit Overview
The main components that comprise a
basic TMS circuit:
·
Switch
·
Capacitor
·
Coil
·
High voltage power supply
Basic operating procedure:
·
Initially, the switch is
open, the capacitor is discharged, and the voltage supply is off.
·
First, the high voltage
supply gets turned on to charge the capacitor. High voltage is used to get a
significant amount of charge stored in the capacitor.
·
Second, the high voltage
supply is turned off to avoid shorting it to ground once the switch closes.
·
Lastly, the switch is
closed, and all the energy stored in the capacitor gets dumped across the coil.
This creates the electromagnetic wave required for TMS.
There are many factors in the
circuit that affect the operation of it. Figuring out those factors and
creating an effective circuit is part of the research that Daniel has been
conducting.
TMS Component Factors
High Voltage Power Supply:
To be able to create powerful electromagnetic waves, high voltages are needed.
This is can simply be traced to the charge formula of a capacitor: 
. The charge stored on a capacitor is proportional to the capacitance of
the capacitor and the voltage applied across the capacitor. The more charge
available, the stronger the electromagnetic wave will be. So, the voltage is
definitely an important factor to consider. In addition, the amount of current
the voltage supply can deliver has an affect on how fast the capacitor gets
charged.
Capacitor:
There are several specifications of a capacitor that need to be considered when
creating a TMS circuit. Probably one of the most important ones is voltage
rating due to the fact that TMS circuits operate at high voltages. Selecting a
capacitor with low voltage ratings can also be a safety issue, so special
attention is required for this specification. Another specification to consider
is capacitance. As seen from the charge formula of a capacitor seen previously,
the capacitance of the capacitor is proportional to the charge stored on it.
Typically, the goal is to have a considerably high capacitance to store more
charge. Again, the more charge available, the stronger the electromagnetic
wave. Also, another factor to consider is creating a capacitor bank by tying
capacitors in parallel and/or in series.
Switch:
The switch may seem simple, but it also has factors to consider. The voltage
rating of the switch is one of the more important specifications to look out
for. If the switch cannot handle the high voltage, the TMS circuit will not
operate correctly. The type of switch is also another factor to consider.
MOSFETs are an option but finding one that can handle high voltages and
currents is challenging and expensive. An IGBT is another option that can
handle high voltages and currents, although these can get expensive. A third
option is an SCR, which can also handle high voltages and currents. The
downside to using an SCR is that once it is triggered, it remains closed until
all the energy that was stored on the capacitor is dissipated. Using MOSFETs
and IGBTs gives the option of opening the switch before all the energy of the
capacitor is dissipated. Having more control over the switching gives rise to
the option of having rTMS (repetitive transcranial magnetic stimulation)
capabilities. The switching system is one of the trickier components to
develop.
Coil:
The coil is also another component that is tricky to figure out. What makes the
coil component so complex is all the parameters that influence the performance.
The first specification to consider inductance. Inductance itself is determined
by the thickness of the wire, the number of turns, the type of coil, etc.
Another factor to consider is the parasitic resistance of the coil, which
affects the maximum amount of current that flow through the coil. And the lastly,
the shape of the coil also has an influence because it determines the shape the
electromagnetic wave will have. The shape and direction of the wave can
influence how a neuron will react. Overall, this is one factor that requires
extensive tests.
The PCBs that have been created and
tested can be found on the following page.
Additional Links