Transcript Introduction to fMRI - Georgetown University

MRI History and Hardware
Basic Safety Issues
Introduction to fMRI
John VanMeter, Ph.D.
Center for Functional and Molecular
Imaging
Outline
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History of MRI
Hardware components of an MRI
scanner
Basic MRI Safety
Introduction to fMRI
Chapter 1: History of MRI
Pauli, Stern and Gerlach 1920’s
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Pauli postulated that atomic nuclei (e.g. H, C,
etc) have two properties: spin and magnetic
moment
Further, the rate of spin occurs at a given
frequency depending on the nuclei
Stern & Gerlach demonstrate this in pure
gases
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Shot beam of gas through a static magnetic field
Produced multiple smaller beamlets
Rabi - 1937
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Rabi showed that nuclei absorb energy if the
frequency matched the “resonant frequency”
of the nuclei
Showed resonance frequency is dependent
on static magnetic field strength
Measured resonance frequency of the lithium
nucleus
Edward Purcell - 1945
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Detected resonance frequency in bulk matter
Used current passing through paraffin wax in a strong
magnetic field
Changed strength of magnetic field over time
At first did not see any change in current but
hypothesized it would take some time for relaxation
of the spins to occur
Repeated experiment after leaving wax in magnetic
field overnight and had success
Fundamental basis of Nuclear Magnetic Resonance
Spectroscopy and MRI
Felix Bloch - 1945
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Similar experiment to Purcell’s except using water in
a brass box inside a magnetic field
Used a transmitter coil to send electromagnetic
energy into the box and receiver coil to measure
changes in energy absorbed by the water
Was also able to measure magnetic resonance effect
This basic setup is the basis of NMR spectrometers
used in biochemistry
With some additional refinements it is also the basis
modern MRI scanners
Raymond Damadian - 1971
Discovered tumors in a rat had
relaxation time longer than normal
tissue
Differences in relaxation time provides
one form of tissue contrast - T1
Paul Lauterbur - 1973
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Used GRADIENTS to distinguish spatially
localized signals  PHASE ENCODING
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Also, used GRADIENTS to manipulate the
frequency of the spins to localize signals. He
referred to this as Zeumatography 
FREQUENCY ENCODING
Both techniques needed to encode spatial
location of signals
First MR Image - 1973
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Lauterbur created an
image by applying
gradients at different
angles to produce 1D
projections
Combining projections
forms image (backprojection reconstruction
technique)
Inefficient as time
needed for each angle
equivalent to a single
acquisition
Sir Peter Mansfield - 1974
Devised selective excitation of a slice
again using gradients 
Slice Select
Identifies where in a 3D object
to collect signal from
Richard Ernst - 1975
Used 2D-FT 
Two-Dimensional Fourier
Transformation
Needed to reconstruct images, which are
encoded with frequency and phase
Faster alternative to back-projection technique
Sir Peter Mansfield - 1976
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Developed very efficient way to collect
data using technique called echo planar
imaging (EPI)
Transmits 1 RF pulse per slice
Rapidly switches gradients and records
signal
EPI used today in fMRI!
Damadian - 1977
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First ever MRI image
of the human body
Created using the
“Indomitable” scanner
Field strength was
0.05T
Homogeneous part of
field very limited so
patient table was
moved to collect each
voxel!
Took 4hrs to collect
single slice!
FDA Clears First MRI Scanner - 1985
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Minicomputers such as
the PDP-11 and VAX
become widely available
GE develops first “highfield” (1.5T) commercial
MRI scanner (1982)
Medicare starts paying
for MRI scans (1985)
VAX 11/750 (1982)
1990’s
Functional imaging using MRI is
first demonstrated – initially by
injecting a contrast agent and
later using properties of the
blood itself
5 Nobel Laureates for MRI
Rabi (1944)
Bloch, Purcell (1952)
Lauterbur,
Mansfield (2003)
Nobel Controversy - 2003
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Damadian took out full page ads in NY Times and
Washington Post protesting award to Lauterbur and
Mansfield
“This Year’s Nobel Prize in Medicine. The
Shameful Wrong That Must Be Righted”
“The Nobel Prize Committee for Physiology or Medicine chose to
award the prize, not to the medical doctor/research scientist who
made the breakthrough discovery on which all MRI technology
is based, but to two scientists who later made technological
improvements based on his discovery”
"I know that had I never been born, there would be no MRI today"
Chapter 2: MRI Hardware
Basic MRI Hardware
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Magnet
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Radiofrequency (RF) coils
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Transmit and Receive RF energy into and from the body
Gradients
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Large magnetic field that is homogeneous over a large area
Aligns protons in the body
Induce linear change in magnetic field
Spatial encoding
Computer System and Console
Patient Handling System
Types of Magnets
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Permanent Iron Core
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Resistive Electromagnet
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Low Field “Open”
Up to 0.2T
Superconducting Magnet
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Cools wire coil with cryogens
0.5T to 35T
Electromagnets
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Field proportional to
number of loops
relative to cross-section
area of each loop
Increases in current
also increases field
strength
Field highest and most
homogenous at center
of coil
Properties of Superconducting
Magnets
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Very high field strengths generated
 Cool magnet’s wire coil using cryogens (liquid
helium and in older scanners nitrogen) to near
absolute zero
 Reduces resistance to zero for certain metals
Provides stable and homogeneous magnetic field
over a relatively large area
Once ramped up no electricity used (relatively cheap)
MAGNET ALWAYS ON!
New dangers specific to these types of magnets
RF (Radiofrequency) Coils
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Used to transmit and receive RF energy
Needed to create images
Coil Designs
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Closer coil is to object being imaged the
better signal
Variety of coils designed for specific body
parts
Surface Coil
Volume Coil
(aka Birdcage Coil)
Coil Design Affects Images
Gradient Coils
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Induce small linear changes in magnetic field
along one or more dimensions
Produces two types of spatial encoding
referred to as Frequency and Phase
Encoding
Under the Hood of an
MRI Scanner
Cyrostat
Gradients
Body RF Coil
Passive Shims
Under the Hood of Our
MRI Scanner
Quench Pipe
Cold Head
Computer System and
Console
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Image reconstruction and
post processing is
computationally intensive
Standard workstation
sufficient for basic clinical
MRI system
Multi-processor systems
with gigabytes of memory
needed for functional MRI
and DTI (Diffusion Tensor
Imaging) scanning
Console computer
coordinates everything
Patient Handling System
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Methods to get patient in and out of the
scanner
Alignment of the body part to be
scanned with isocenter of the scanner
Labeling of scans with appropriate
identifiers and anatomic labels
MRI Safety
MRI Safety
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Static B0 Field
Projectiles
Implants/other materials in the body
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RF Field
tissue heating
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Gradient fields
peripheral nerve stimulation
acoustic noise
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Forces on Ferrous Objects
Crash cart meets a 1.5T magnet
Welding tank
Preventing Accidents Due to
Ferrous Metallic Objects
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Train ALL personnel who work in the facility
Perform MRI safety screening on everyone
prior their entering the MRI scanner room
Limit access to the scanner facility based on
training and need
ACR guidelines establish 4 MRI Safety Zones
and limit access to each zone
MRI Safety
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Static B0 Field
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RF Field
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projectiles
tissue heating
Gradient fields
peripheral nerve stimulation
acoustic noise
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RF Exposure Standards
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The FDA limits RF exposure to less
than a 1 degree C rise in core body
temperature
RF Exposure Standards
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4W/Kg whole body for 15 min
3W/Kg averaged over head for 10 min
8W/Kg in any gram of tissue in the
head or torso for 15 min
12W/Kgin any gram of tissue in the
extremities for 15 min
MRI Safety
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Static B0 Field
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RF Field
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projectiles
tissue heating
Gradient fields
peripheral nerve stimulation
acoustic noise
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Stimulation Caused by the Switching
Gradient Fields
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Nerve stimulation
Acoustic trauma
Burn from looped cables
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be careful when using anything with electrical wires or
cables in the scanner
Changing B field
Creates voltage,
current and heat
V ~ (Area) x (dB/dt)
Introduction to Functional MRI
Difference Between
MRI & fMRI
From: Daniel Bulte
Centre for Functional MRI of the Brain
University of Oxford
Tools Necessary for fMRI
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High-field MRI (1.5T or greater) scanner
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Fast imaging sequence
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BOLD effect (fMRI signal) increases with field
strength though not linearly
Echo Planar Imaging (EPI)
Stimulus presentation equipment
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Projector to show visual stimuli
Response devices such as button box to record
subject’s response
Headphones for auditory stimuli (and hearing
protection)
Functional Brain Mapping with MRI
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Basic concept - changes in neuronal
activity produces a measurable change in
MR signal
Collect 100-500 MRI scans continuously (1
every 2-3s; each typically cover 30-50 slices)
Experimenter induces changes in activity at
known points in time by having subject
perform some cognitive or motoric task
Analyses statistically tests for MR signal
changes that corresponding to experimental
task
Basic fMRI Experiment
Fixation
Thumb movement
time
Data Analysis
Identify voxels with signal changes
matched to the timing of experiment
Tapping
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Rest
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Tapping
Rest
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Tapping
Rest
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Tim e
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Unimanual Thumb Flexion
Right Thumb
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Left Thumb
R
fMRI Compared to Other
Functional Techniques
Examples of fMRI
Activity in a Vegetative State
Super Bowl Ads
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Marco Iacoboni at UCLA used fMRI to
examine the brain’s response to
different super bowl ads
Ranked ads based on brain responses
Found differences in the ads that
stimulated the brain most and those
people reported as liking the most
Brain Activity During
Disney Ad
Mirror
Neurons
Brain Activity During
FedEx Ad
Fear response in Amygdala during scene where
the human is squashed by the dinosaur
Caution Needed
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Interpretation of the
signal changes
depends on a lot of
factors
Communication of
results with public
needs to be
approached with care
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McCabe & Castel (2008,
Cognition) brain imaging
increased perceived
credibility of research
compared to bar graphs