meg systems

Transcription

meg systems

Lecture 1
Magnetic Fields & MEG Systems
Maher Quraan, PhD
Toronto Western Research Institute
April 16, 2013
April-16-13
Quraan, MEG lecture 1
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Outline
 PART I: Electromagnetism
 PART II: MEG systems
 PART III: The future of MEG systems
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Part I
ELECTROMAGNETISM
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Forces in the Universe
Electromagnetism
There are four forces:
1. Gravitational
2. Electromagnetic
3. Weak nuclear
4. Strong nuclear
Maxwell’s Equations
  E  4
B  0
1 B
0
c t
1 E 4
 B 

J
c t
c
 E 
James Clerk Maxwell
Born:
1831, Edinburgh
Died:
1879, Cambridge
Electricity, magnetism and light are manifestations of the same phenomenon
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Magnetic Fields
Magnetic Fields
Magnetic field lines
South pole
Strong field
North pole
Weak field
Magnetic fields
are always dipolar
Concentration of field lines
indicates field strength
N
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S
Newton Henry Black, Harvey N. Davis (1913) Practical Physics
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Magnetic Fields
From Electric Currents
I
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Magnetic Fields
Neural Currents
sink
A
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source
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Magnetic Fields
Neural Currents
MEG Topology Map
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EEG Topology Map
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Magnetic
Fields
Neural Activity
Primary
Currents
Electric &
& Volume
Magnetic
Fields
Electric currents
Magnetic fields
Measure with EEG
Measure with MEG
Primary
current
Volume
currents
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July-11-13
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Volume Currents
Modeling
Shallow Source
Left thalamic source
300 mA/m2
Wolters et al. NeuroImage 2006.
Ramon et al. BioMedical Engineering OnLine 2006.
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EEG vs MEG
Spatial Resolution
 Signals detected by MEG arise from primary currents
and volume currents.
 Volume conduction is much stronger near the
source resulting in “good” spatial resolution for MEG.
 EEG requires the transmission of signal to the scalp
resulting in poor spatial resolution.
 Particularly that the skull is a poor conductor.
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Magnetic Fields
Current Sources
current I
dl
r
dB
Magnetic field dB
corresponding to
wire length dl
constant
o Idl
dB 
4 r 2
Source
Field to be
measured
Idl
dB 
2
r
 dB is proportional to I and the length of wire element dl
 dB is inversely proportional to the square of the distance between
the wire observation point r
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Magnetic Fields
Units
current I
dl
r
dB
Magnetic field dB
corresponding to
wire length dl
constant
o Idl
dB 
4 r 2
 Idl   Amperes  meters  A  m
 dB   tesla  T
15
fT  10 T
pT  1012 T
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nA  m  109 A  m
Idl
dB 
2
r
 r   meters  m
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Magnetic Fields
Examples
Example 1
Idl  50nA  m  50 109 Am
r  3cm  0.03m
 Idl
dBo  o 2
4 ro
It takes tens of thousands
of neurons activating in
synchrony to generate
such a signal!
1
 50 109 
 (10 ) 

 0.03  0.03 
 5.56 1012 T  5.56 pT
7
r  6cm  0.06m
dB 
o Idl
4 r 2
0.9
current I
0.8
0.7
dB / dBo
Example 2
Idl  50nA  m  50 109 Am
ECD: Equivalent Current Dipole
dBo
dl
0.6
ro
0.5
0.4
r
0.3
 50 10 
 (107 ) 

 0.06*0.06 
 1.39 1012 T  1.39 pT
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9
dB
0.2
0.1
0
1
Quraan,
MEG2 lecture31
4
5
r/r
6
7
8
9
14 10
Magnetic Fields
Equivalent Current Dipole
The current dipole
q  Idl  I r2  r1

dB  o
4

 o
4

 o
4
Idl
r2
I r2  r1
r2
q
r2
current I
r2
r2
dl
dB
r
r1
r1
ECD: Equivalent Current Dipole
Point current dipole
q  I (r2  r1 )
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Magnetic Fields
Magnetic Flux
  BA
   B dS
S
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   B  dS
S
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Magnetic Fields
Summary
Electric and Magnetic fields





Magnetic fields are generated by moving charges (currents).
EEG measures the electric field whereas MEG measures the magnetic field.
Electric and magnetic fields are orthogonal.
The magnetic field strength decreases as 1/r2 with distance (r) from the source.
Magnetic fields are measured in units of tesla (T, fT, pT).
Current Dipoles
 Current dipoles generate magnetic fields.
 The right hand rule is used to determine field lines.
 Current dipoles have units of amperes*meters (Am, nAm).
Magnetic fields of the brain
 Magnetic fields generated by the human brain result from primary currents and
volume currents.
 A 10 nAm source magnitude requires tens of thousands of neurons to be activated
in synchrony.
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Part II
MEG SYSTEMS
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MEG Chronology
1968
1969
1970’s and 80’s
1990’s
1 st MEG measurement
()
David Cohen
Nuclear physicist
Winnipeg
SQUIDs developed
by
James Zimmerman
Multi-channel systems
Whole-head systems
Zimmerman et al. Journal
of Applied Physics 1970.
Cohen D, Edelsack EA,
Zimmerman JE. Applied
Physics Letters 1970.
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MEG
Technology
~160 MEG systems installed worldwide
CTF
Neuromag
4D Neuroimaging
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MEG Systems
Worldwide
~160 MEG systems installed worldwide
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MEG Systems
North America
7 MEG systems installed in Canada
• 3 in Toronto (CTF -151)
• 2 in Montreal (CTF-151 and CT-275)
• 1 in Halifax (Neuromag)
• 1 in Vancouver (CTF-151)
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Sensor Design
Magnetometers
Change in magnetic flux through the
pickup coil wires induces a current
  BA
http://blogs.cas.suffolk.
edu/ekprime/files/2011
Demo
/04/faradyanim.gif
Pickup Coil
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Sensor Design
Pickup Coils
<
<
<
<
<
<
Less sensitive to brain field
More sensitive to brain field
Equally sensitive to distant sources
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MEG Systems
Vendors & Bankruptcy
CTF
4D
Neuroimaging
Elekta
Neuromag
OR
AND
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Sensor Design
Pickup Coils
 Magnetometers are very susceptible to noise but (in
the absence of noise) have highest sensitivity to deep
sources.
 Radial (axial gradiometers) have better noise
subtraction but less sensitive to deep sources.
 Planar gradiometers have the best noise subtraction
but are least sensitive to deep sources.
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MEG Systems
The Dewar
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MEG Systems
Seated & Supine Positions
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MEG Systems
Elekta Neuromag Vectorview
Each card has
3 pickup coils
102 Cards
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306 Channels
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MEG Systems
Magnetometer
measures
y
z
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Bz
x
Planar gradiometer
measures
Bz ( x2 )  Bz ( x1 ) Bz

x2  x1
x
B
 z
x
Planar gradiometer
measures
Bz ( y2 )  Bz ( y1 ) Bz

y2  y1
y
B
 z
y 30
MEG Systems
Front
L
R
Back
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CTF
275-channel System
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Elekta Neuromag
EEG System
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MEG Systems
Summary II
 Most MEG systems in North America and Europe
are CTF, Neuromag or 4D systems
 CTF systems use radial gradiometers (151 or 275
channels).
 Neuromag systems use 102 cards with three channels
coils one each card (1 magnetometer and 2 orthogonal
planar gradiometers).
 4D systems use magnetometers or radial gradiometers.
 All three systems are SQUID based
 They require cryogenics to operate.
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PART III
THE FUTURE OF MEG SYSTEMS
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Atomic Magnetometers
SERF
 A high degree of spin polarization is
established in a high-density alkali-metal
vapor.
 Sensitivity is often limited by
decoherence caused by spin-exchange
collisions.
 Operating at high-density and near-zero
ﬁeld it is possible to operate in a spinexchange-relaxation-free SERF regime.
Spin-exchange-relaxation-free magnetometer
(SERF)
Sensitivity of 5fT/Hz
Possible to arrange in array surrounding head
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Comparison with SQUID-based Systems
April-16-13
Johnson et al. App. Phys. Lett. 2010 37
CSAM
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Comparison with SQUID-based Systems
Sanders et al.,
Biomed
Optics 2012
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Cost
Current estimates:
 SERF-2: $11k/sensor
 Laser system: $23.8k
 Electronics: $47k
April-16-13
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meg systems

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