Biomedical Engineering Reference
In-Depth Information
tube cap. A hole cut on the side of the cap was
fitted with a PVC tube which acts as the
coil handle and wire exit. 24-hour epoxy was used to pot the coil inside the cap. One must
keep in mind that some glues can damage enamel on coil wire.
Coil geometry directly a
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ects the depth and focality (precision of targeting) of the tis-
sue to be stimulated. Standard stimulating coils are circular, with a diameter ranging from
50 to 150 mm. Coils designed for
focal stimulation
, that is, capable of stimulating one
given area without stimulating a nearby area, usually consist of two adjacent coils with
current
ff
gure-
eight coils, yielding twice the stimulus produced at the edges of the individual coils. Four-
leaf coils have been constructed to produce a concentrated dot stimulus useful for
peripheral nerve stimulation. Another design that is now being used is the half-toroid coil
(as in a Slinky), which is wound with the turns in di
fl
flowing in opposite directions. Induced currents add at the center of these
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erent angles while maintaining the
tangency along one edge to drastically reduce the current induced away from the target
stimulation site.
Mouchawar et al.'s paper [1991] provides a thorough analysis of the e
ff
ect of coil geom-
etry on stimulation. Despite this, designing coils for magnetic stimulators is more an art than
an exact science. To play di
ff
nite element
analysis program called FEMM (Finite Element Methods Magnetics) by David Meeker (sup-
plied for your convenience in the topic's ftp site). FEMM is a suite of programs for solving
static and low-frequency problems in magnetics. The programs currently address problems
on two-dimensional planar and axisymmetric domains. FEMM comprises:
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erent “what if ” scenarios, we recommend a free
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•
A preprocessor
: a CAD-like program for laying out the geometry of the problem to
be solved and de
ning material properties and boundary conditions
•
A solver
: a device that takes a set of data
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files that describe a problem and solves the
relevant Maxwell's equations to obtain values for the magnetic
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fi
field through the solu-
tion domain
•
A postprocessor
: a graphical program that displays the resulting
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fields in the form of
contour and density plots
Interestingly, many theoretical studies have demonstrated that today's coils are far from
optimal. These studies also show that power consumption can be reduced drastically, open-
ing many opportunities for individual innovators in coming up with new ideas that can
become platforms for completely unexplored applications.
Commercial Magnetic Stimulators and Their Applications
Commercial magnetic stimulators are not much di
erent than the simple prototype device
discussed above. The three main commercial players in the magnetic stimulation
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field are
Cadwell Laboratories, Inc. (Kennewick, Washington), Magstim Company, Ltd. (Whitland,
UK) and Medtronic Dantec NeuroMuscular (Skovlunde, Denmark). One of the newest
magnetic stimulation modalities is repetitive transcranial magnetic stimulation (rTMS),
capable of delivering trains of stimuli at 1 to 50 Hz. rTMS was
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first produced by Cadwell
Laboratories in 1988. Dantec and Magstim have add-on modules for their single-pulse
devices that can be used to drive one coil with two to four pulses separated by 1 ms to 1 s.
As you can imagine, the duration of sustained operation is limited by coil heating, and
Cadwell solves this problem with continuous water cooling.
Single-stimulus magnetic stimulators are used routinely by neurologists as diagnostic
tools to measure nerve-conduction time. In addition, transcranial magnetic stimulation
(TMS) over the primary motor cortex evokes movement in the contralateral limb and has pro-
vided information on the anatomical organization and functional characteristics of the motor
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