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`Folder Name URL
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`01
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`https://web.archive.org/web/20120127225340/http://www.icts.uci.edu/neuroim
`aging/GuidetoMagneticStimulation2008.pdf
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`DATE: ________________________
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`________________________
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`11/17/2020
`
`Allergan EX1015
`
`

`

` JURAT
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`FLORIDA
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`Orange
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`Ranisha Chapman
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`Duncan D Hall
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`driver_license
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`Ranisha Chapman
`HH21278
`07/20/2024
`
`Affidavit of Duncan D Hall
`
`11/17/2020
`
`3
`
`Allergan EX1015
`
`

`

`•
`magst1m·
`
`Pioneers in nerve stimulation and monitoring
`
`THE GUIDE TO
`MAGNETIC
`STIMULATION
`
`by
`Chris Hovey BSc
`and
`Reza Jalinous PhD
`
`© The Magstim Company Limited
`
`Allergan EX1015
`
`

`

`A Guide to Magnetic Stimulation
`
`The New Guide to Magnetic
`Stimulation
`
`by Chris Hovey BSc and Reza Jalinous PhD
`
`The ability of a transient magnetic field to induce
`an electrical current within body tissue permits the
`researcher and clinician to influence or monitor
`functions of the neuromuscular system and to
`affect sensory neurons in the brain. It is able to
`reach deep neural structures such as the motor
`cortex and spinal nerve roots, non-invasively and
`without pain.
`
`The effects of magnetic fields have been a source
`of research and interest, and indeed of fear, since
`Faraday's experiments showing magnetic flux
`coupling in the early part of the nineteenth century.
`Magnetic fields are able to pass unhindered
`through skin, muscle and bone and would
`therefore potentially be useful in the examination
`of human tissue if the magnetic field were to have
`any effect on the tissue through which it passed.
`
`Fortunately, a time-varying magnetic field will
`induce an electrical current in any tissue through
`which it passes, and therefore magnetic fields can
`be used to stimulate muscles, peripheral nerves
`and cortical neurons without surgical access or
`anaesthetic agents.
`
`This guide provides an overview of the techniques
`involved in magnetic stimulation, from first
`principles through to some of the clinical
`applications now feasible. Also included are
`details about different stimulator types and the
`effects of different waveforms, and a look at more
`recent developments. A list of reference papers
`organised by discipline is available separately as a
`supplement to this guide. We thank our readers
`who have contributed helpful information or
`suggestions towards this edition.
`
`Please note that this guide describes the state
`of the art in magnetic stimulation and is
`intended for a world-wide readership. Some
`techniques and magnetic stimulator devices
`described represent uses that are considered
`as investigational in the USA. In particular this
`applies to the use of cortical magnetic
`stimulation. Further details on the regulations
`governing the use of investigational devices
`can be obtained from the FDA (www.fda.gov).
`
`Contents
`
`The New Guide to Magnetic Stimulation
`Part 1: Fundamental and Technical Aspects
`Brief History
`Principles of Magnetic Stimulation
`Stimulating Coils
`Single Coils
`Double Coils (Butterfly/Figure of Eight)
`Special Coils
`The Double Cone Coil (Type 9902)
`Double 70mm Air-Cooled Coil (Type 1600-00)
`Small Double Coils
`Sham (Placebo) Coils
`MRI coil
`Custom Coils
`Stimulating Coil Construction
`Magnetic Field Strength vs. Stimulus Strength
`Part 2: Types of Magnetic Stimulators and their Related Area of Function
`Magnetic Single-Pulse Systems
`Types of output waveform
`Magnetic Pulse Pair Systems
`BiStim Set-up
`Silent Period
`Repetitive Magnetic Stimulation
`BrainsightÔ Frameless
`References
`Part 3: Clinical Aspects
`Motor Evoked Potentials (MEPs)
`Facilitation
`MEP Variability
`Central Motor Conduction Time (CMCT)
`Corticomotor Threshold
`Response amplitude
`Limitations
`Demyelinating Neuropathies
`Magnetic Pulse Pairs
`Brain Mapping
`Sensory Evoked Potentials (SEPs)
`Sample Applications
`Coma
`Drug Monitoring
`Epilepsy
`Facial Nerve
`Spinal nerve roots
`Motor Neurone Disease
`Movement Disorders
`Dystonia
`Huntington's Disease
`Myoclonus
`Parkinson's Disease
`Tremor
`Tourette Syndrome
`Multiple Sclerosis
`Neuroscience
`Operating Room Monitoring
`Pain
`Peripheral Nerves
`Plasticity
`Psychiatry
`Depression
`Safety Papers which are Essential Reading:
`Mania
`Schizophrenia
`Psychology
`Rehabilitation
`Muscle injury
`Relief of Spasticity
`Simulation of a Cough
`Urology
`Spinal Injuries
`Cervical Spondylosis
`Sports Medicine
`Stroke
`Thoracic Medicine
`Phrenic Nerve Stimulation
`Urology
`Safety Precautions & Issues
`Low Frequency Stimulation
`High Frequency Stimulation Guidelines
`Related Web Sites
`
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`The Magstim Company Ltd
`
`1
`
`21 July 2006
`
`Allergan EX1015
`
`

`

`A Guide to Magnetic Stimulation
`
`Figures
`
`Figure 1: Silvanus P. Thompson with his experimental equipment; the two coils can be clearly seen
`Table 1: the physical characteristics and maximum calculated outputs of the coils used with the Magstim 2002
`Figure 2: Block diagram of the Magstim 2002 monophasic stimulator
`
`Figure 3: a circular coil showing the lines of force generated when current flows through the winding
`
`Figure 4: the magnetic field plot of a 90mm circular coil
`
`Figure 5: the magnetic field plot of the double 70mm coil
`
`Figure 6: the double cone coil, designed to fit overhead near the vertex and stimulate the lower limbs
`
`Figure 7: diagram showing the concentration of the lines of force appertaining to the double cone coil
`
`Figure 8: the air cooled coil; air is sucked across the surface of the coil, with the heated air removed through the tubing
`
`Figure 9: a double 25mm coil used for phrenic nerve stimulation or in animal studies
`
`Figure 10: a small selection of special coils built for individual customers
`
`Figure 11: the magnetic field plot against time for the 90mm coil, measured at the coil surface
`
`Figure 12: the electric field plot against time for the 90mm coil, measured at the coil surface
`Figure 13: The Magstim 2002 single pulse magnetic stimulator and remote control double 70mm coil
`
`Figure 14: the output waveforms of monophasic, biphasic and polyphasic stimulators
`
`Figure 15: The BiStim system, showing the master unit (top), the slave unit (bottom), and the BiStim module (top left)
`
`Figure 16: the effect of varying the interpulse spacing - cortical stimulation of ADM
`
`Figure 17: cortical evoked potential to ADM showing silent period; 161ms - 21.6ms @ 140ms
`
`Figure 18: an example showing the effects of facilitation using a BiStim;
`
`Figure 19: the Standard Rapid package with optional trolley, showing touch sensitive screen, MEP Pod and double 70mm coil
`
`Figure 20: the Super Rapid showing trolley, coil stand, double 70mm coil and MEP Pod
`
`Figure 21: the Session software screen showing the parameters which can be controlled by the program
`
`Figure 22: Curvilinear reconstructions
`
`Figure 23: Targets can be defined based on anatomical and functional information
`
`Figure 24: Enables coil location and trajectory to be recorded for archiving and correlation with stimulus results.
`
`Figure 25: Co-registering a subject with anatomical structures allows BrainsightÔ Frameless to implement 3D coil targeting
`
`Figure 26: responses from ADM when stimulated at motor cortex; C7 cervical root; Erb’s point; the elbow; and the wrist, using a double 70mm coil
`
`Figure 27: diagrammatic view of the brachial plexus and associated cervical nerve roots
`
`Figure 28: view of the upper limb showing major nerves and their general muscle innervation
`
`Figure 29: Cortical magnetic stimulation of the upper limbs;
`
`Table 2: summary of measurements made on a single subject to illustrate CMCT calculation
`
`Figure 30: Responses from sensory nerves can also be recorded using magnetic stimulation
`
`Figure 31: 50mm double coil positioned to stimulate the facial nerve in the labyrinthine segment of the facial canal
`
`Figure 32: cortical stimulation of the facial motor cortex with the facial muscles relaxed
`
`Figure 33: peripheral facial nerve stimulation with a double 50mm coil and electrodes located on mentalis
`
`Figure 34: cortical stimulation of the facial motor cortex with slight facilitation - note the decreased latency
`
`Figure 35: cortical stimulation of the facial motor cortex,
`
`Figure 36: the double 70mm coil placed over the sacrum
`
`Figure 37: stimulation of S1 nerve root and resultant EMG from medial gastrocmenius muscle
`
`Table 3: some suggested muscle sites for specific vertebral levels
`
`Figure 38: the 90mm coil placed over the sacrum, showing that a number of nerve roots are likely to be stimulated
`
`Figure 39: The example waveforms shown here have all been recorded over the left first dorsal interosseous (FDI).
`Figure 40: The 2002 can be used for the stimulation of lumbosacral nerve roots.
`
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`The Magstim Company Ltd
`
`2
`
`21 July 2006
`
`Allergan EX1015
`
`

`

`Part 1: Fundamental and Technical
`Aspects
`
`repeated and confirmed this experiment, as shown
`in Figure 1.
`
`A Guide to Magnetic Stimulation
`
`Brief History
`
`Electromagnetic induction was first described by
`Michael Faraday in 1831 at the Royal Institution of
`Great Britain. It is probably the most relevant
`experimental observation for magnetic stimulation.
`Faraday wound two coils on an iron ring and
`showed that whenever the coil on one side was
`connected or disconnected from a battery, an
`electrical current passed through the coil on the
`other side. The iron ring helped to position the
`coils in relation to each other and an experiment a
`few weeks later produced the same effect from two
`coils closely positioned in air. With non-invasive
`magnetic stimulation the stimulating coil acts as
`the first coil, air as the medium for the flow of the
`magnetic field, and the electrically conductive
`living body tissue as the second coil.
`
`In 1896 d'Arsonval [C R Soc Biol; 1896, 3: 450-51]
`reported phosphenes (flickering lights in the visual
`field) when placing his head between two coils
`driven from an alternating 110 volt supply at 30
`amperes. It is now known that this was due to the
`direct stimulation of the retina. Silvanus Thompson
`
`Figure 1: Silvanus P. Thompson with his experimental
`equipment; the two coils can be clearly seen
`
`Bickford and Fremming in 1965 [Digest 6th Int
`Conf Med Elec Biol Eng, 1965, p112]
`demonstrated the non-invasive magnetic
`stimulation of facial nerves. In 1982 Polson et al.
`produced a magnetic stimulator capable of
`peripheral stimulation and recorded the first
`muscle evoked potential [Med Biol Eng Comput,
`20: 243-4]. The technique of magnetic stimulation
`came of age in 1985 when Barker et al. in
`Sheffield [Lancet, 1985, 1106-1107] achieved
`magnetic stimulation of the human motor cortex.
`For a more detailed historical review the reader is
`referred to a publication by Geddes LA [J Clin
`Neurophysiol, 1991, 8:1-9].
`
`Progress has been rapid since 1985 with several
`new areas of research using new developments.
`Equipment reliability has been improved, and
`stimulators with differing output waveforms
`developed. Coil design has been as important as
`stimulator development, the most important
`advance being the development of coils with
`multiple windings for precise stimulation of nerves
`or cortical neurons. Other developments have
`included trains of pulses for therapy in
`rehabilitation, sports medicine and in the treatment
`of psychiatric disorders; the use of fast repetitive
`stimuli to determine the laterality of speech
`centres; and high energy focal stimuli as an
`adjunct to ECT to relieve drug resistant
`depression.
`
`Principles of Magnetic Stimulation
`
`The first commercial magnetic stimulators were
`produced in Sheffield in 1985 and the Magstim
`Model 200, based on the original Sheffield design,
`was launched in 1986. Stimulator information
`given in this document is based on the current
`Magstim 2002, which has replaced the original
`machine using the latest technology.
`
`The Magstim range of magnetic nerve stimulators
`are developed under an exclusive licence with the
`University of Sheffield. Magnetic nerve stimulators
`typically consist of two distinct parts: a high current
`pulse generator producing discharge currents of
`5,000 amps or more; and a stimulating coil
`producing magnetic pulses with field strengths up
`to 4 tesla, and with a pulse duration from 100μs to
`1ms, dependent on stimulator type. A block
`
`The Magstim Company Ltd
`
`3
`
`21 July 2006
`
`Allergan EX1015
`
`

`

`diagram of a typical stimulator is shown in Figure
`2; a transformer charges a capacitor under the
`
`120 - 230V
`
`Transformer
`
`Chaf\jing
`Circuitry
`
`Capacitor
`
`Eleclronic
`Sw1tc.h
`
`Control
`Cil!CUitry
`
`Power
`Selling
`
`Safety
`ln!ellocks
`
`footswitch
`Coil swi cties,
`Software
`
`Coil
`
`Figure 2: Block diagram of the Magstim 2002
`monophasic stimulator
`
`control of a microprocessor, which accepts
`information such as the capacitor voltage, power
`set by the user, and various safety interlocks within
`the equipment to ensure proper operation, and the
`capacitor is then connected to the coil via an
`electronic switch when the user wishes to apply
`the stimulus.
`
`A Guide to Magnetic Stimulation
`
`The discharge current of c.5,000 Amps flows
`through the coil and lasts for up to 1ms, but 90%
`of the discharge occurs within the first 100μs. This
`is important because it is the rate of change of the
`magnetic field which causes the electrical current
`within tissue to be generated, and therefore a fast
`discharge time is crucial to stimulator efficiency
`(see figure 12). The discharge current flowing
`through the stimulating coil generates the
`necessary magnetic lines of force as shown in
`Figure 3. As the lines of force cut through tissue, a
`current is generated in that tissue, whether skin,
`bone, muscle or neural; if the induced current is of
`sufficient amplitude and duration such that the cell
`membrane is depolarised, neuromuscular tissue
`will be stimulated in the same manner as
`conventional electrical stimulation.
`
`It is therefore important to understand that a
`magnetic field is simply the means by which an
`electrical current is generated within the tissue,
`and that it is the electrical current, and not the
`magnetic field, which causes the depolarisation of
`the cell membrane and thus the stimulation of the
`target muscle/nerve.
`
`Since the magnetic field strength falls off with the
`square of the distance from the stimulating coil,
`the stimulus strength is at its highest close to the
`coil surface. The stimulation characteristics of the
`magnetic pulse, such as depth of penetration,
`strength and accuracy, depend on the rise time,
`peak electrical energy transferred to the coil and
`the spatial distribution of the field. The rise time
`and peak coil energy are governed by the
`
`Circular
`70mm
`P/N 9762
`
`Circular
`90mm
`P/N 3192/3
`
`Double
`25mm
`P/N 1165
`
`Double
`50mm
`Prototype
`
`Double
`70mm
`P/N 3190/1
`
`Double
`Cone
`P/N 9902
`
`Cooled
`Coil
`P/N 1640
`
`Placebo
`Coil
`
`Circular
`50mm
`P/N
`9999
`
`25
`
`77
`
`18
`
`Inside diameter (mm)
`
`Outside diameter
`(mm)
`
`Number of turns
`
`Inductance (μH)
`
`13.5
`
`40
`
`94
`
`15
`
`16
`
`66
`
`18 (x2)
`
`34 (x2)
`
`56 (x2)
`
`96 (x2)
`
`56 (x2)
`
`N/A
`
`123
`
`42 (x2)
`
`74 (x2)
`
`87 (x2)
`
`125 (x2)
`
`87 (x2)
`
`3190
`casing
`
`14
`
`23.5
`
`2.0
`
`14 (x2)
`
`11 (x2)
`
`9 (x2)
`
`7 (x2)
`
`9 (x2)
`
`N/A
`
`10
`
`4.0
`
`23
`
`N/A
`
`15.5
`
`17.8
`
`16.4
`
`2.55
`
`2.2
`
`1.4
`
`0.93
`
`0.2
`
`Peak Magnetic Field
`Strength (Tesla)
`
`Peak Electric Field
`Strength (V/m)
`
`Number of
`discharges @ 100%
`
`3.6
`
`2.6
`
`600
`
`530
`
`530
`
`660
`
`N/A
`
`660
`
`N/A
`
`N/A
`
`N/A
`
`65
`
`63
`
`145
`
`40
`
`78
`
`60
`
`584
`
`>>3600
`
`984
`
`Table 1: the physical characteristics and maximum calculated outputs of the coils used with the Magstim 2002
`
`The Magstim Company Ltd
`
`4
`
`21 July 2006
`
`Allergan EX1015
`
`

`

`electrical characteristics of the magnetic stimulator
`and stimulating coil, whereas the spatial
`distribution of the induced electric field depends on
`the coil geometry and the anatomy of the region of
`induced current flow.
`
`--- - -
`
`/'
`
`I
`I
`I
`\
`\
`
`" '-
`
`;(
`'-
`I
`
`\
`
`\
`I
`I
`
`I
`
`I \
`
`I
`
`\
`\
`
`I
`
`~
`
`/
`
`Coil Winding
`
`/
`
`Figure 3: a circular coil showing the lines of force
`generated when current flows through the winding
`
`Stimulating Coils
`
`The stimulating coil consists of one or more tightly
`wound and well-insulated copper windings,
`together with temperature sensors and safety
`switches. The physical description of some of the
`coils used with the Magstim 2002, with their
`estimated magnetic and electrical fields, are
`shown in Table 1.
`
`Single Coils
`
`A circular 90mm mean diameter coil is supplied as
`standard with single pulse systems. This coil is
`most effective in stimulating the human motor
`cortex and spinal nerve roots. A more recent
`development is the remote control coil which
`allows the user to operate the stimulator from
`control buttons situated on the coil handle. To
`date, circular coils with a mean diameter of
`80-100mm have remained the most widely used in
`magnetic stimulation. A 3D representation of the
`magnetic field produced on the surface of a 90mm
`circular coil is shown in Figure (Type 9784 in
`Table 1). In the case of circular coils it is important
`to note that the induced tissue current is near zero
`on the central axis of the coil and increases to a
`maximum in a ring under the mean diameter of the
`coil. Stimulation occurs under the winding and not
`under the coil centre.
`
`During the stimulating phase, when the magnetic
`field is increasing from zero to its maximum, the
`induced tissue current flows in the opposite
`direction to the coil current. In the case of the
`
`A Guide to Magnetic Stimulation
`
`Magstim 2002, all single circular coils are marked
`with Side A and Side B. With the coil placed on the
`body and Side A visible, the induced tissue current
`flows in the clockwise direction, indicated by an
`arrow pointing in a clockwise direction on the coil
`surface. With Side B visible, induced tissue current
`flows in the anti-clockwise direction, again
`indicated by an arrow. The use of the correct coil
`side is particularly important in cortical stimulation
`as the human motor cortex is more sensitive when
`the induced current is flowing from posterior to
`anterior. With the coil placed centrally on the
`vertex and Side A visible, the induced current
`predominantly stimulates the left motor cortex and
`
`Magnetic
`Field
`Strength
`
`D ista
`c e
`Figure 4: the magnetic field plot of a 90mm circular coil
`
`m
`
`m
`
`m
`
`n c e fr o
`ntr e in
`
`hence the right side of the body; with Side B
`visible the right motor cortex is stimulated and the
`response will occur on the left side of the body.
`
`Double Coils (Butterfly/Figure of Eight)
`
`The most notable improvement in coil design has
`been that of the double coil (also termed butterfly
`or figure of eight coil). Double coils utilise two
`windings, normally placed side by side. A 3D
`representation of the magnetic field produced on
`the surface of a 70mm double coil (Type 9925 in
`Table 1) is shown in Figure 5. Typically double
`coils range from very small flat coils for brain
`mapping work to large contoured versions
`designed to stimulate deeper neural structures in
`the brain. The main advantage of double coils over
`circular coils is that the induced tissue current is at
`its maximum directly under its centre, where the
`two windings meet, giving a more accurately
`defined area of stimulation. A remote control
`
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`

`

`double 70mm coil is supplied as standard with the
`new Magstim 2002.
`
`areas controlling the muscles of the lower torso
`and limbs.
`
`A Guide to Magnetic Stimulation
`
`Magnetic
`Field
`Strength
`
`m
`
`m
`
`m
`
`D ista n c e fr o
`c e ntr e in
`
`Figure 5: the magnetic field plot of the double 70mm coil
`
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`Figure 7: diagram showing the concentration of the
`lines of force appertaining to the double cone coil
`
`Special Coils
`
`Double 70mm Air-Cooled Coil (Type 1600-00)
`
`The Double Cone Coil (Type 9902)
`
`This coil uses a forced air flow to cool the coil
`surface so that it can be used for long trains of
`pulses. It is usually used in conjunction with the
`
`Figure 6: the double cone coil, designed to fit overhead
`near the vertex and stimulate the lower limbs
`
`The Double Cone Coil (Figure 6) has two large cup
`shaped windings positioned side by side, with a
`flat central section and angled sides closely fitting
`the patient's head. The coil geometry allows for
`better magnetic coupling, giving significantly higher
`induced current in the central fissure (70% higher
`than with the 90mm circular coil). Figure 7
`illustrates the effect of angling the coil windings,
`showing that the lines of force are concentrated in
`the area underneath the central area of the coil.
`This coil is useful in stimulating the motor cortex
`
`Figure 8: the air cooled coil; air is sucked across the
`surface of the coil, with the heated air removed through
`the tubing
`
`Magstim Rapid stimulators, enabling long
`protocols to be completed, or to allow several
`subjects to be treated in a morning or afternoon
`session.
`
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`

`Small Double Coils
`
`MRI coil
`
`A Guide to Magnetic Stimulation
`
`15-25mm for research applications. (1-2T
`output)
`
`A coil suitable for use in a MRI scanner has been
`developed. The stimulator is located outside the
`MRI room, with the coil cable, via an extension,
`being fed through the wall so as to maintain the
`Faraday cage; the coil itself is designed to
`minimise the artifacts generated by placing a
`metallic object within the scanner. At present, the
`coil is suitable for use in 3T scanners; more
`powerful scanners may lead to the coil being
`damaged which could damage the scanner.
`
`Custom Coils
`
`Figure 9: a double 25mm coil used for phrenic nerve
`stimulation or in animal studies
`
`These small diameter coils are built to enable
`precise stimulation of superficial structures, since
`the depth of penetration of the magnetic field is
`related to the diameter of the coil windings. These
`coils are also used in animal research, often to
`stimulate rats.
`
`Because they are of small diameter, and use
`lighter gauge copper for the windings, they will be
`able to stimulate fewer times before overheating
`when compared to the larger diameter coils.
`
`Sham (Placebo) Coils
`
`These coils are designed to be used by
`researchers who wish to complete blind or double
`blind trials. Sham coils consist of 2 coils, one
`active, one passive, but which look and feel
`identical. It is only possible to determine which is
`the active coil by checking the serial number of the
`coil.
`
`The active coil is a standard coil which is
`connected via a box to the stimulator; the sham
`coil looks identical, and is also routed via a box.
`Inside the box, in the case of the sham coil, most
`of the energy is discharged; a small portion of the
`current is taken to the coil head and discharged
`into a small coil so that the characteristic ‘click’ is
`heard when the stimulator is fired. In addition, this
`small coil, which is built into the housing of the
`standard coil, is sufficient to stimulate the skin and
`muscle overlaying the scalp, thereby giving the
`patient the sensation of magnetic stimulation, but
`without the penetrating stimulus of the larger coils.
`
`Figure 10: a small selection of special coils built for
`individual customers
`
`The Magstim Company is unique in manufacturing
`special coils for individual customers. These coils
`are built either to a customer’s own specification,
`or as a variant of a standard coil to enable
`particular research to be undertaken. Custom coils
`usually have a polyurethane coating rather than
`plastic covers, but the safety of the coils is
`maintained through advanced insulation
`techniques. The only limits are your imagination
`and practical safety!
`
`Stimulating Coil Construction
`
`The stimulating coil is the only part of a magnetic
`nerve stimulator which needs to come close to, or
`into contact with, the patient. During the discharge
`of the magnetic pulse the coil winding is subjected
`to high voltages and currents. Although the pulse
`generally lasts for less than 1ms, the forces acting
`on the coil winding are substantial and depend on
`the coil size, peak energy and construction.
`Careful coil design is, therefore, a very important
`aspect in the safe construction of a magnetic
`stimulator. Large coils utilise more copper mass
`than small coils and generally have a lower
`
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`A Guide to Magnetic Stimulation
`
`of the phrenic nerve roots, the double 70mm is
`used for mono-hemispheric transcranial
`
`1J
`
`500
`
`400
`
`I 600
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`300
`
`200
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`100
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`-100
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`-200
`
`0
`
`0.2
`
`0.4
`
`0.6
`
`0.8
`
`Time (ms)
`
`Figure 12: the electric field plot against time for the
`90mm coil, measured at the coil surface
`
`stimulation, the circular 50mm coil is well suited to
`stimulation at Erbs point, and the double cone is
`most powerful in cortical stimulation of the lower
`extremities.
`
`electrical resistance. As a result, less heat is
`dissipated in their windings and because of their
`higher heat capacity they remain usable for much
`longer periods of time without becoming warm.
`
`Magnetic Field Strength vs. Stimulus Strength
`
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`2.6
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`0
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`0.2
`
`0.4
`
`0.6
`
`0.8
`
`Time (ms)
`
`Figure 11: the magnetic field plot against time for the
`90mm coil, measured at the coil surface
`
`Magnetic field strength (as shown in Figure 11)
`alone is a poor measure of magnetic stimulator
`performance. The suitability of a coil for its
`intended application must be taken into account.
`Magnetic field strength is defined as the magnetic
`flux density and does not reflect the total magnetic
`flux produced by the stimulating coil over its total
`area. In a small coil where the magnetic flux is
`concentrated in a small area, the magnetic field
`intensity will be higher than in a larger coil, but the
`field reduces much more rapidly with distance.
`Hence a small coil is somewhat more powerful in
`the stimulation of superficial nerves and a large
`coil is more suitable for structures at depth.
`
`The electric field, as shown in Figure 12, is a better
`measure of the efficiency of the coil, as it shows
`that the greater part of the stimulus occurs within
`the first 100μs, whereas the magnetic field rises to
`a maximum at 100μs and then slowly decays for
`up to 1ms. However, it is the rate of change of the
`magnetic field, rather than simply the magnitude of
`the field, which has the more important role to play
`in the efficiency of the coil design. This is more
`clearly demonstrated by the electric field plot.
`
`The amplitude, waveform and spatial
`characteristics of the induced current all play a role
`in magnetic nerve stimulation. As examples, the
`90mm coil is very effective in bilateral stimulation
`
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`

`A Guide to Magnetic Stimulation
`
`Part 2: Types of Magnetic
`Stimulators and their Related Area of
`Function
`
`The Magstim 200, 220 and Rapid systems have
`now been replaced by the Magstim 2002, 2202 and
`Rapid 2002.
`
`Magnetic Single-Pulse Systems
`
`Single Pulse Systems may be used for cortical or
`peripheral stimulation with either single circular or
`double figure of eight coils. A single pulse is of
`
`attention and plasticity. It is also an aid in the brain
`mapping of motor sites and in evoking motor
`responses to aid in neurological diagnosis.
`
`Figure 13 shows the Magstim 2002 and a double
`70mm stimulating coil. This machine, and its
`predecessor, the Magstim 200, are widely used in
`neurology departments throughout the world to
`evoke motor responses from patients undergoing
`a clinical neurological examination. Its ability to
`stimulate without pain makes it useful to both
`patient and clinician, and its property of being able
`to stimulate the motor cortex makes it