FEASIBILITY STUDY AND PROTOTYPING OF AN ELECTROMAGNETIC CORTICAL STIMULATOR FOR BRAIN

MAPPING IN OPEN SKULL NEUROSURGERY

Thesis of: Anna Mafrica

Supervisor: Giancarlo Ferrigno

Co-supervisors: Elena De Momi, Riccardo Bertacco, Christian Rinaldi

Anna Mafrica

INTRODUCTION

BRAIN TISSUE REMOVAL

BRAIN MAPPING

To localize principal functional areas of the brain,such as:- Motor cortex- Areas related to memory- Areas related to speech- …

Preserving brain functionalities during resection of thepathological area:

- Brain tumors ⟶ new cases each year: 21 peopleover 100.000 (http://www.cbtrus.org)

- Epilepsy ⟶ 300 operations in Italy each year(http://www.ospedaleniguarda.it/in-evidenza/leggi/chirurgia-dellepilessia)

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EXISTING TECHNIQUES FOR BRAIN MAPPING

Functional Magnetic Resonance Imaging

PREOPERATIVE

Transcranial Magnetic Stimulation

INTRAOPERATIVE

Direct Cortical Stimulation

Electromagnetic Cortical Stimulation

Safe, non-invasive

Not good for language mappingLow resolution

Simple

InvasiveSeizuresLow penetration depth

Safe, non-invasive

Coil dimensions

Electromagnetic coil

Pulsed magnetic field

Stimulated cortical region

(Ilmoniemi, Ruohonen, and Karhu 1999; Rossi et al. 2009)

(Matz, Cobbs, and Berger 1999; Hervey-Jumper et al. 2015)

(Buzzi et al. 2015; Developed by NearLab, DEIB)

(Gore 2003)

Safe, non-invasive

Big coils (not suitable for intraoperative mapping)

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AIM OF THE WORK

Pulse generator

Stimulation coil

Magnetic wire

B

E −𝜕𝜙 𝐵

𝜕𝑡= ∆𝑉 → 𝑖𝑓 ∆𝑉 > 𝑉𝑠𝑝𝑖𝑘𝑒 → 𝑠𝑝𝑖𝑘𝑒

FEASIBILITY STUDY of an alternative toolfor the intraoperative cortical mapping.

Pulse generator

B

𝑖 → 𝐵 →

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REQUIREMENTS

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BPulse generator

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DIRECT CORTICAL STIMULATION

PULSE SHAPE

AWAKE PATIENT

Penfield protocol:- Current: 2-6 mA- Pulse width: 1 ms- Frequency: 50–60 Hz

ANESTHETIZED PATIENT

Train-of-five protocol:- Current: 4.9-8 mA- Pulse width: 200–500 µs- Frequency: 250–500 Hz

(Matz, Cobbs, and Berger 1999; Hervey-Jumper et al. 2015)

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TRANSCRANIAL MAGNETIC STIMULATION

BIPHASIC

Electric field: 70÷140 V/m

Current: 2-8 kAPulse width: 200–600 µs

PULSE SHAPE

𝑒𝑚𝑓 = −𝜕𝜙 𝐵

𝜕𝑡

MONOPHASIC

𝑒𝑚𝑓 = −𝜕𝜙 𝐵

𝜕𝑡

(Ilmoniemi, Ruohonen, and Karhu 1999; Rossi et al. 2009)

Electromagnetic coil

Pulsed magnetic field

Stimulated cortical region

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ELECTROMAGNETIC CORTICAL STIMULATOR

Current: 32 APulse width: 80 µs

Electric field: 1 V/m

PULSE SHAPE

80 µs 80 µs 80 µs

25 ms 25 ms 90 ms 140 ms

Coil current

Electric field

NOT ENOUGH TO STIMULATE

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REQUIREMENTS

KANTHAL A-1 (FeCrAl alloy)

wires of ϕ = 0.4 mm

- FLEXIBLE- HIGH PERMEABILITY- HIGH SATURATION

MAGNETIC MATERIAL

BPulse

generator

STIMULATION COIL

𝑙 = 3 𝑐𝑚𝜙𝑖𝑛𝑡 = 0.9 𝑐𝑚𝜙𝑒𝑥𝑡 = 2.7 𝑐𝑚𝑁 = 292𝜙𝑤𝑖𝑟𝑒 = 0.82 𝑚𝑚𝜙𝑖𝑛𝑡

𝜙𝑒𝑥𝑡

𝑙

𝑅 =𝜌𝑙

𝑆= 0.5 Ω

𝐿 = 509 𝜇𝐻

CIRCUIT PARAMETERS 𝑑 = 3 𝑚𝑚

𝐿𝑇𝑂𝑇 = 20 𝑐𝑚𝜙𝑐𝑜𝑟𝑒 = 7 𝑚𝑚

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PERFORMANCE EVALUATION

1) Stimulation coil current.

2) Core material: static properties.

3) Core material: dynamic properties

⟶ Comparison with MATLAB simulations

4) Test of the magnetic circuit

⟶ Comparison with COMSOL simulations

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BPulse generator

EXPERIMENTAL PROTOCOL

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1) STIMULATION COIL CURRENT

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2) KANTHAL A-1 – STATIC PROPERTIES

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2) KANTHAL A-1 – STATIC PROPERTIES

Field along the wire

Coercive field

Virgin 𝐻𝑐 = 5.2 𝑂𝑒Positive 𝐻𝑐1 = −4.43 𝑂𝑒Negative 𝐻𝑐2 = 5.95 𝑂𝑒

Retentivity 50 mT

Saturation 𝑀𝑠𝑎𝑡 = 971 𝑘𝐴/𝑚

𝐻𝑠𝑎𝑡 = 100 𝑂𝑒

Loop squareness𝑚𝑟/𝑚𝑠𝑎𝑡

3.74%

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2) KANTHAL A-1 – STATIC PROPERTIES

𝑩 = 𝜇0 𝑯+𝑴(𝑯) = 𝜇0 𝜇𝑟𝑯Pulse

generatorB

Coil Magnetic material

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2) KANTHAL A-1 – STATIC PROPERTIES

Maximum relative permeability ⟶ 80

Saturation ⟶ 100 Oe

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3) KANTHAL A-1 – DYNAMIC PROPERTIES

STIMULATION CURRENT

𝑓 = 100 𝐻𝑧𝐼0 = 3.2 𝐴

Pick-up coil

PICK-UP COIL

𝑒𝑚𝑓 = −𝜕𝜙 𝐵

𝜕𝑡

Up to saturationStimulation coil

Kanthal A-1

𝐵

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3) KANTHAL A-1 – DYNAMIC PROPERTIES

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EXPERIMENTAL DATA SIMULATION DATA (MATLAB)

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3) KANTHAL A-1 – DYNAMIC PROPERTIES

𝑩 = 𝜇0 𝑯+𝑴

𝑒𝑚𝑓 = −𝜕𝜙 𝐵

𝜕𝑡

𝑒𝑚𝑓 = −𝜕𝜙 𝜇0𝐻

𝜕𝑡+𝜕𝜙 𝜇0𝑀

𝜕𝑡

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4) MAGNETIC CIRCUIT

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Kanthal A-1Stimulation coil

Pick-up coil

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4) MAGNETIC CIRCUIT – FREQUENCY

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Kanthal A-1Stimulation coil

Pick-up coil

𝑓 = 37 𝐻𝑧 , 𝐼0 = 3.2 𝐴

COMSOL SIMULATION

EXPERIMENTAL DATA

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4) MAGNETIC CIRCUIT – STIMULATOR

𝐸𝑚𝑎𝑥 = 0.06 𝑉/𝑚

NOT ENOUGH TO STIMULATE

OPTIMIZATIONS:

- MAGNETIC MATERIAL- GEOMETRY

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Kanthal A-1Stimulation coil

Pick-up coil

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OPTIMIZATION OF THE MATERIAL

MATERIAL 𝝁𝒓 𝑯𝒔𝒂𝒕 [𝑨/𝒎]

Metglas nano finemet 50 hz nofieldannealed 100000 10

Metglas nano nanocrystalline viproterm 50 Hz 30000 50

Nickel steel 4750 70000 10

Nickel steel permalloy NGO 50000 10

Nickel steel molypermalloy 70000 20

Stainless steel 430 annealed 800 1000

Stainless steel annealed sus 403 300 2000

Stainless steel 455 annealed 300 5000

Stainless steel chrome 35% steel 80 9000

Kanthal A-1 80 7960

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OPTIMIZATION OF THE GEOMETRY

𝐸ma𝑥 = 30 𝑉/𝑚 ~ 𝐸𝑠𝑡𝑖𝑚𝑢𝑙𝑎𝑡𝑖𝑜𝑛 = 70 − 140 𝑉/𝑚

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CONCLUSIONS

• Agreement among experimental set-up, simulations and theory

⟶ EXPERIMENTAL PROTOCOL VALIDATION

• Identification of critical parameters:

Saturation, magnetic core material, stimulation circuit

• FEASIBILITY of a magnetic stimulator through magnetic circuit

THANK YOU FOR THE ATTENTION!

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FURTHER DEVELOPMENT

- Perform new simulations

- Core material

- Geometry

- Realize and test a new prototype

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DIRECT CORTICAL STIMULATION

PULSE SHAPE

AWAKE PATIENT

- Current: 2-6 mA- Pulse width: 1 ms

ANESTHETIZED PATIENT

- Current: 4.9-8 mA- Pulse width: 200–500 µs

SPATIAL RESOLUTION ANDPENETRATION DEPTH

- Spatial resolution: up to 0.5 cm- Penetration depth: up to 0.8 cm

Amplitude

Distance

Amplitude

Distance

MONOPOLAR

BIPOLAR

(Matz, Cobbs, and Berger 1999; Hervey-Jumper et al. 2015)

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TRANSCRANIAL MAGNETIC STIMULATION

BIPHASIC

Electric field: 70÷140 V/m

Current: 2-8 kAPulse width: 200–600 µs

PULSE SHAPE𝑒𝑚𝑓 = −

𝜕𝜙 𝐵

𝜕𝑡

MONOPHASIC

𝑒𝑚𝑓 = −𝜕𝜙 𝐵

𝜕𝑡

SPATIAL RESOLUTION ANDPENETRATION DEPTH

- Spatial resolution: up to 0.5 cm- Penetration depth: up to 3 cm

(Ilmoniemi, Ruohonen, and Karhu 1999; Rossi et al. 2009)

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5) MAGNETIC CIRCUIT DESIGN

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B δ

MAGNETIC CIRCUIT DESIGN

𝐵𝑔𝑎𝑝 =𝑁𝑖𝜇0

𝑙𝑐𝑜𝑟𝑒𝜇𝑟

+ 𝛿

+

-

MMF𝜙

ℛ𝑐𝑜𝑟𝑒

ℛ𝑔𝑎𝑝

DESIGN STEPS- Core material- Coil design

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4) MAGNETIC CIRCUIT – STATIC

STIMULATION CURRENT ⟶ 𝐼0 = 4 𝐴

AIR GAP DIMENSION ⟶ 𝑑 = 3𝑚𝑚

MAGNETIC FIELD SENSOR ⟶ Hall effect sensor

𝑩𝒎𝒆𝒂𝒔 𝑩𝒕𝒉𝒆𝒐

50 𝑚𝑇 55 𝑚𝑇

GOOD AGREEMENT

𝐼0 = 20 𝐴 → 𝐵𝑡ℎ𝑒𝑜 = 250 𝑚𝑇

Kanthal A-1

Hall effect sensor

Stimulation coil

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KANTHAL A-1 – STATIC PROPERTIES

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3) ELECTRIC FIELD EVALUATION

𝐸 =𝑒𝑚𝑓

𝑛 ∙ 2𝜋𝑟

𝑒𝑚𝑓 = −𝜕𝜙 𝐵

𝜕𝑡

INDUCTION’S LAW

PICK-UP COIL

𝑩

𝑬𝒅𝑩

𝒅𝒕

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3) PICK-UP COIL CALIBRATION

PICK-UP COIL

𝑛 = 100𝜙𝑖𝑛𝑡 = 10.1 𝑚𝑚𝜙𝑒𝑥𝑡 = 12.1 𝑚𝑚

𝑅 = 7 Ω

𝑒𝑚𝑓 = −𝜕𝜙 𝐵

𝜕𝑡= −𝐴𝑒𝑓𝑓

𝜕𝐵

𝜕𝑡→ 𝐴𝑒𝑓𝑓 = −

𝑒𝑚𝑝𝑒𝑥𝑝𝜕𝐵𝑡ℎ𝑒𝑜𝜕𝑡

Experimental Geometrical approximation

(9.123 ± 1.23) ∙ 10−3𝑚2 𝑛𝜋𝑟𝑚𝑒𝑎𝑛2 = 9.503 ∙ 10−3 𝑚2

CALIBRATION

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