Neuroimaging

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Neuroimaging

• Bart Krekelberg
• vision.rutgers.edu
• bart_at_rutgers.edu

Foundations II 21 March, 2006
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The neural basis of love
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Outline

• Positron Emission Tomography (PET)
• Physics
• Possibilities and Limitations
• Examples
• Magnetic Resonance Imaging (MRI)
• Physics
• Possibilities and limitations
• Practical MRI

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PET Physics

• Principle
• Find a molecule used in the brain.
• Make it radioactive.
• Put it in the brain
• Measure where the radiation comes from.
• Example
• Deoxyglucose
• Taken up by neurons just like glucose, but not
  metabolized.
• Add 18F to deoxyglucose and inject.
• Measure radiation.

?
e-
N
?
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PET Physics
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PET

• Limitations
• Need synchrotron to make tracer
• Need to inject tracer
• Poor temporal resolution
• Poor spatial resolution (3-9mm)
• Promise
• Baseline measurement possible
• Receptor-ligand specificity

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PET Examples
Blood entering the brain
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PET Examples
Dopamine production in Parkinsons patients
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MRI
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MRI Physics
A changing magnetic field causes an electrical
current at a distance
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Magnets in the brain

• Protons, neutrons, and electrons are all magnetic
• On average, there is no magnetic field

M00
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The brain in a (big) magnet

• Two things happen in a magnetic field

B0 M0
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Relaxation
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Relaxation

• T1
• Relaxation time constant
• For the protons in H20 1s
• Only 1 out 105 protons align with B0
• Per cubic centimeter M0 protons
• Difficult to measure
• M0ltlt B0
• M0 B0
• M0 constant after a few seconds

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Precession

• Two things happen in a magnetic field

B0 M0
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Mechanical Precession
Video UCSD Physics Program
https//physics-blog.ucsd.edu/weblog/physics2avide
o/ Lecture Angular Momentum Conservation
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Magnetic Precession
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Precession

• Relaxation

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Precession

• View from above

B0
Time
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Precession

• Mechanics strong force -gt fast precession
• Magnetism strong B0
• -gt fast precession
• Larmor Equation ? ?B0
• ? Larmor frequency
• ? gyromagnetic ratio
• 42MHz/T for protons (1H)
• 11MHz/T for 13C
• 176GHz/T for electrons (e-)

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A Proton in a Strong Magnet

• relaxation precession

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Transverse Magnetization

• Mt is orthogonal to B0
• Mt changes -gt causes an electrical field
• But, Mt lasts only seconds (T1)
• Can we flip the magnets?

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Flipping with a static field
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Flipping with a static field

• Another huge magnet?
• Relaxation in B1 direction T1

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Flipping with a dynamic field
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Mechanical Resonance
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Magnetic Resonance
B1
Precession
Time
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The Flip Angle
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The B1 field

• Even a small B1 can produce a large flip angle in
  a short time if it oscillates at the Larmor
  frequency
• A large B1 at the wrong frequency does nothing
• Larmor 120MHz for protons in 3T -gt Radio
  Frequency Pulse
• After the RF pulse we have a magnetic field that
  is
• changing (precessing)
• macroscopic (in phase)
• transverse (flip angle)
• This causes a measurable electrical current at a
  distance.

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The basic MR experiment
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Density weighting

• Flip, then measure
• Insensitive to relaxation times (no time)
• Sensitive to proton density

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The basic MR experiment
Single proton
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Dephasing T2 relaxation

• Random events cause small changes in the magnetic
  field
• Changes in the magnetic field change the
  precession speed (Larmor)
• Some protons speed up, others slow down and they
  become out-of-phase
• The (small) currents caused by these protons no
  longer sum the signal decays.

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T2 transverse relaxation time

• Depends on tissue type
• White matter 70 ms
• Gray matter 90 ms
• CSF 400 ms
• T2 ltlt T1
• Measure the signal 200 ms after the RF pulse.
• White matter e-200/70 5
• Gray matter e-200/90 10
• CSF e-200/400 60

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What about T1?
M0,max
Time
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T1-weighted images

• If TR gtgt T1 then T1 has no influence
• Density weighted image
• T1 at 1.5T
• Gray matter 900 ms
• White matter 700 ms
• CSF 4000 ms
• TR 400ms, measure soon after RF pulse (ltltT2)

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MR Imaging

• Tissue Parameters
• T1
• T2 (T2)
• Proton density

• Scan parameters
• TR
• Measurement time after RF Pulse (TE)

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But

• How do I make an image?
• How does this relate to neural activity?

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Making an image

• Slice selection with a gradient field

z

• Set a z-gradient
• Choose the frequency of the RF pulse
• Switch of the z-gradient

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Making an image

• Frequency encoding with a gradient field

Bx
0
x
Faster precession fast changing signal
Slower precession slow changing signal

• When measuring the signal, set a gradient
• Measure only fast signals -gt back of head
• Measure only slow signals -gt front of head

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Making an image

• Phase encoding with a gradient field

By
0
y

• After the RF pulse, set a gradient for a brief
  time
• Measure the signal
• The phase of the signal depends on the y-position
  sin(y)
• Repeat, with ever stronger gradient
• The signal sin(2y), sin(3y), sin(4y)
• Signals that change rapidly with the repeat
  number have large y
• Signals that change slowly with the repeat number
  have small y

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Making an Image

• Magnetic Field Gradients for
• Slice selection
• Frequency encoding
• Phase encoding

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Neural activity

• Neural activity increases T2

Time
Measure here
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Neural activity
2005 Monkey, 7T
1992 Human, 1.5T
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Neural activity
Oxyhemoglobin (Hb02)
Deoxy-hemoglobin (Hb)
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The BOLD Signal

• Blood Oxygen Level Dependent
• Hb is paramagnetic and reduces T2
• Active neurons use the O2 from HbO2
• Neural activity increases Hb and therefore
  reduces T2
• Didnt I say activity increases T2 before??
• The vascular system overcompensates for the
  increased O2 consumption by increasing blood flow
  and volume.
• The net effect is that activity leads to a
  decrease in Hb and therefore an increase in T2

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The BOLD signal

• Limitations
• It takes 2-3 seconds before the HbO2
  overcompensation is maximal.
• Neurons draw HbO2 from a fairly wide region.
• Cannot distinguish between excitatory or
  inhibitory activity (or anything else that uses
  O2).
• Promise
• Non-invasive, works in humans

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MRI in practice
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The MR room
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Scanner Internals
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The Magnet

• Goal align the protons
• Coils
• Super conductance Helium
• 1.5T, 3T, 7T (Earth magnetic field 0.0005T)
• Side Effects (FDA lt8T, neonates lt4T )
• Nausea
• Vertigo
• Tingling
• Headache
• Pain in tooth fillings

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Not harmful?
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The Gradient Coils

• Goal
• Slice selection
• Frequency encoding
• Phase encoding
• Side Effects
• Induced currents (dynamo small)
• Nerve stimulation
• Phosphenes
• Acoustic Noise

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The RF Coil

• Goal
• Turn longitudinal magnetization into transverse
  magnetization
• Measure the signal generated by the precessing
  spins.
• Side Effects
• Induced currents Specific Absorption Rate (SAR)
  limits
• Heating avoid loops.

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Links to more information

• Handouts
• Principles of Neural Science Eric Kandel , James
  Schwartz, Thomas Jessel Pages 366-380
• Introduction to Functional Magnetic Resonance
  Imaging Principles and Techniques Richard B.
  Buxton Chapter 4 (Pages 64-85)
• Webpages with additional material and sources for
  these slides
• Jody Culha ms fMRI for Newbies
  http//www.ssc.uwo.ca/Jody_web/fmri4newbies.htm
• BrainTutor Learn about the human brain
  http//www.brainvoyager.com/BrainTutor.html