FEASIBILITY STUDY AND PROTOTYPING OF AN ELECTROMAGNETIC CORTICAL STIMULATOR FOR BRAIN MAPPING IN OPEN SKULL NEUROSURGERY

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1 FEASIBILITY STUDY AND PROTOTYPING OF AN ELECTROMAGNETIC CORTICAL STIMULATOR FOR BRAIN MAPPING IN OPEN SKULL NEUROSURGERY Thesis of: Supervisor: Giancarlo Ferrigno Co-supervisors: Elena De Momi, Riccardo Bertacco, Christian Rinaldi

2 INTRODUCTION BRAIN TISSUE REMOVAL Preserving brain functionalities during resection of the pathological area: - Brain tumors new cases each year: 21 people over ( - Epilepsy 300 operations in Italy each year ( BRAIN MAPPING To localize principal functional areas of the brain, such as: - Motor cortex - Areas related to memory - Areas related to speech - 2

3 EXISTING TECHNIQUES FOR BRAIN MAPPING PREOPERATIVE Functional Magnetic Resonance Imaging INTRAOPERATIVE Direct Cortical Stimulation Safe, non-invasive Not good for language mapping Low resolution Simple Invasive Seizures Low penetration depth (Gore 2003) (Matz, Cobbs, and Berger 1999; Hervey-Jumper et al. 2015) Transcranial Magnetic Stimulation Safe, non-invasive Big coils (not suitable for intraoperative mapping) Electromagnetic coil Pulsed magnetic field Stimulated cortical region Electromagnetic Cortical Stimulation Safe, non-invasive Coil dimensions (Ilmoniemi, Ruohonen, and Karhu 1999; Rossi et al. 2009) (Buzzi et al. 2015; Developed by NearLab, DEIB) 3

4 AIM OF THE WORK FEASIBILITY STUDY of an alternative tool for the intraoperative cortical mapping. Stimulation coil Pulse generator Magnetic wire Pulse generator B E B i B φ B t = V if V > V spike spike 4

5 REQUIREMENTS Pulse generator B 5

6 DIRECT CORTICAL STIMULATION AWAKE PATIENT PULSE SHAPE Penfield protocol: - Current: 2-6 ma - Pulse width: 1 ms - Frequency: Hz ANESTHETIZED PATIENT Train-of-five protocol: - Current: ma - Pulse width: µs - Frequency: Hz (Matz, Cobbs, and Berger 1999; Hervey-Jumper et al. 2015) 6

7 TRANSCRANIAL MAGNETIC STIMULATION PULSE SHAPE Electric field: V/m Current: 2-8 ka Pulse width: µs BIPHASIC φ B emf = t MONOPHASIC Electromagnetic coil Pulsed magnetic field Stimulated cortical region φ B emf = t (Ilmoniemi, Ruohonen, and Karhu 1999; Rossi et al. 2009) 7

8 ELECTROMAGNETIC CORTICAL STIMULATOR PULSE SHAPE Current: 32 A Pulse width: 80 µs Electric field: 1 V/m Coil current Electric field 80 µs 80 µs 80 µs 25 ms 25 ms 90 ms 140 ms NOT ENOUGH TO STIMULATE 8

9 REQUIREMENTS MAGNETIC MATERIAL - FLEXIBLE - HIGH PERMEABILITY - HIGH SATURATION KANTHAL A-1 (FeCrAl alloy) wires of ϕ = 0.4 mm Pulse generator B CIRCUIT PARAMETERS d = 3 mm L TOT = 20 cm φ core = 7 mm STIMULATION COIL l φ int φ ext l = 3 cm φ int = 0.9 cm φ ext = 2.7 cm N = 292 φ wire = 0.82 mm R = ρl S = 0.5 Ω L = 509 μh 9

10 PERFORMANCE EVALUATION EXPERIMENTAL PROTOCOL 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 Pulse generator B 10

11 1) STIMULATION COIL CURRENT 11

12 2) KANTHAL A-1 STATIC PROPERTIES 12

13 2) KANTHAL A-1 STATIC PROPERTIES Coercive field Virgin Positive Negative Retentivity Saturation Field along the wire H c = 5.2 Oe H c1 = 4.43 Oe H c2 = 5.95 Oe 50 mt M sat = 971 ka/m H sat = 100 Oe Loop squareness m r /m sat 3.74% 13

14 2) KANTHAL A-1 STATIC PROPERTIES Pulse generator B B = μ 0 H + M(H) = μ 0 μ r H Coil Magnetic material 14

15 2) KANTHAL A-1 STATIC PROPERTIES Maximum relative permeability 80 Saturation 100 Oe 15

16 3) KANTHAL A-1 DYNAMIC PROPERTIES B Kanthal A-1 Pick-up coil STIMULATION CURRENT f = 100 Hz I 0 = 3.2 A PICK-UP COIL emf = φ B t Stimulation coil Up to saturation 16

17 3) KANTHAL A-1 DYNAMIC PROPERTIES EXPERIMENTAL DATA SIMULATION DATA (MATLAB) 17

18 3) KANTHAL A-1 DYNAMIC PROPERTIES B = μ 0 H + M emf = emf = φ μ 0H t φ B t + φ μ 0M t 18

19 4) MAGNETIC CIRCUIT Stimulation coil Kanthal A-1 Pick-up coil 19

20 4) MAGNETIC CIRCUIT FREQUENCY f = 37 Hz, I 0 = 3.2 A EXPERIMENTAL DATA Stimulation coil Kanthal A-1 Pick-up coil COMSOL SIMULATION 20

21 4) MAGNETIC CIRCUIT STIMULATOR Stimulation coil Kanthal A-1 Pick-up coil E max = 0.06 V/m NOT ENOUGH TO STIMULATE OPTIMIZATIONS: - MAGNETIC MATERIAL - GEOMETRY 21

22 OPTIMIZATION OF THE MATERIAL MATERIAL μ r H sat [A/m] Metglas nano finemet 50 hz nofieldannealed Metglas nano nanocrystalline viproterm 50 Hz Nickel steel Nickel steel permalloy NGO Nickel steel molypermalloy Stainless steel 430 annealed Stainless steel annealed sus Stainless steel 455 annealed Stainless steel chrome 35% steel Kanthal A

23 OPTIMIZATION OF THE GEOMETRY E max = 30 V/m ~ E stimulation = V/m 23

24 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 FURTHER DEVELOPMENT - Perform new simulations - Core material - Geometry THANK YOU FOR THE ATTENTION! - Realize and test a new prototype 24

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26 DIRECT CORTICAL STIMULATION AWAKE PATIENT PULSE SHAPE - Current: 2-6 ma - Pulse width: 1 ms ANESTHETIZED PATIENT - Current: ma - Pulse width: µs MONOPOLAR BIPOLAR Amplitude Distance Amplitude Distance SPATIAL RESOLUTION AND PENETRATION DEPTH - Spatial resolution: up to 0.5 cm - Penetration depth: up to 0.8 cm (Matz, Cobbs, and Berger 1999; Hervey-Jumper et al. 2015) 26

27 TRANSCRANIAL MAGNETIC STIMULATION BIPHASIC PULSE SHAPE Electric field: V/m φ B emf = t MONOPHASIC Current: 2-8 ka Pulse width: µs φ B emf = t (Ilmoniemi, Ruohonen, and Karhu 1999; Rossi et al. 2009) SPATIAL RESOLUTION AND PENETRATION DEPTH - Spatial resolution: up to 0.5 cm - Penetration depth: up to 3 cm 27

28 5) MAGNETIC CIRCUIT DESIGN 28

29 MAGNETIC CIRCUIT DESIGN R core B δ B gap = Niμ 0 l core μ r + δ + - MMF φ R gap DESIGN STEPS - Core material - Coil design 29

30 4) MAGNETIC CIRCUIT STATIC STIMULATION CURRENT I 0 = 4 A AIR GAP DIMENSION d = 3 mm Stimulation coil Kanthal A-1 Hall effect sensor MAGNETIC FIELD SENSOR Hall effect sensor B meas B theo 50 mt 55 mt GOOD AGREEMENT I 0 = 20 A B theo = 250 mt 30

31 KANTHAL A-1 STATIC PROPERTIES 31

32 3) ELECTRIC FIELD EVALUATION db dt B E INDUCTION S LAW emf = E = φ B t PICK-UP COIL emf n 2πr 32

33 3) PICK-UP COIL CALIBRATION PICK-UP COIL n = 100 φ int = 10.1 mm φ ext = 12.1 mm R = 7 Ω CALIBRATION emf = φ B t = A eff B t A eff = emp exp B theo t Experimental Geometrical approximation (9.123 ± 1.23) 10 3 m 2 2 nπr mean = m 2 33