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MRI History and Hardware Basic Safety Issues Introduction to fMRI

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Title: MRI History and Hardware Basic Safety Issues Introduction to fMRI


1
MRI History and HardwareBasic Safety
IssuesIntroduction to fMRI
  • John VanMeter, Ph.D.
  • Center for Functional and Molecular Imaging

2
Outline
  • History of MRI
  • Hardware components of an MRI scanner
  • Basic MRI Safety
  • Introduction to fMRI

3
Chapter 1 History of MRI
4
Pauli, Stern and Gerlach - 1920s
  • 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
  • Shot beam of gas through a static magnetic field
  • Produced multiple smaller beamlets

5
Rabi - 1937
  • 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

6
Edward Purcell - 1945
  • 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

7
Felix Bloch - 1945
  • Similar experiment to Purcells 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

8
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

9
Paul Lauterbur - 1973
  • Used GRADIENTS to distinguish spatially localized
    signals ? PHASE ENCODING
  • 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

10
First MR Image - 1973
  • Lauterbur created an image by applying gradients
    at different angles to produce 1D projections
  • Combining projections forms image
    (back-projection reconstruction technique)
  • Inefficient as time needed for each angle
    equivalent to a single acquisition

11
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

12
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

13
Sir Peter Mansfield - 1976
  • 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!

14
Damadian - 1977
  • 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!

15
FDA Clears First MRI Scanner - 1985
  • Minicomputers such as the PDP-11 and VAX become
    widely available
  • GE develops first high-field (1.5T) commercial
    MRI scanner (1982)
  • Medicare starts paying for MRI scans (1985)

VAX 11/750 (1982)
16
1990s
  • Functional imaging using MRI is first
    demonstrated initially by injecting a contrast
    agent and later using properties of the blood
    itself

17
5 Nobel Laureates for MRI
Rabi (1944)
Bloch, Purcell (1952)
Lauterbur, Mansfield (2003)
18
Nobel Controversy - 2003
  • Damadian took out full page ads in NY Times and
    Washington Post protesting award to Lauterbur and
    Mansfield
  • This Years 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"

19
Chapter 2 MRI Hardware
20
Basic MRI Hardware
  • Magnet
  • Large magnetic field that is homogeneous over a
    large area
  • Aligns protons in the body
  • Radiofrequency (RF) coils
  • Transmit and Receive RF energy into and from the
    body
  • Gradients
  • Induce linear change in magnetic field
  • Spatial encoding
  • Computer System and Console
  • Patient Handling System

21
Types of Magnets
  • Permanent Iron Core
  • Low Field Open
  • Resistive Electromagnet
  • Up to 0.2T
  • Superconducting Magnet
  • Cools wire coil with cryogens
  • 0.5T to 35T

22
Electromagnets
  • 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

23
Properties of Superconducting Magnets
  • Very high field strengths generated
  • Cool magnets 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

24
RF (Radiofrequency) Coils
  • Used to transmit and receive RF energy
  • Needed to create images

25
Coil Designs
  • Closer coil is to object being imaged the better
    signal
  • Variety of coils designed for specific body parts

26
Coil Design Affects Images
27
Gradient Coils
  • 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

28
Under the Hood of an MRI Scanner
Cyrostat
Gradients
Body RF Coil
Passive Shims
29
Under the Hood of Our MRI Scanner
Quench Pipe
Cold Head
30
Computer System and Console
  • 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

31
Patient Handling System
  • 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

32
MRI Safety
33
MRI Safety
  • Static B0 Field
  • Projectiles
  • Implants/other materials in the body
  • RF Field
  • tissue heating
  • Gradient fields
  • peripheral nerve stimulation
  • acoustic noise

34
Forces on Ferrous Objects
Crash cart meets a 1.5T magnet
35
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36
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37
Welding tank
38
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39
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40
Preventing Accidents Due to Ferrous Metallic
Objects
  • 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

41
MRI Safety
  • Static B0 Field
  • projectiles
  • RF Field
  • tissue heating
  • Gradient fields
  • peripheral nerve stimulation
  • acoustic noise

42
RF Exposure Standards
  • The FDA limits RF exposure to less than a 1
    degree C rise in core body temperature

43
RF Exposure Standards
  • 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/Kg in any gram of tissue in the extremities
    for 15 min

44
MRI Safety
  • Static B0 Field
  • projectiles
  • RF Field
  • tissue heating
  • Gradient fields
  • peripheral nerve stimulation
  • acoustic noise

45
Stimulation Caused by the SwitchingGradient
Fields
  • Nerve stimulation
  • Acoustic trauma
  • Burn from looped cables
  • be careful when using anything with electrical
    wires or cables in the scanner

46
Introduction to Functional MRI
47
Difference BetweenMRI fMRI
From Daniel Bulte Centre for Functional MRI of
the Brain University of Oxford
48
Tools Necessary for fMRI
  • High-field MRI (1.5T or greater) scanner
  • BOLD effect (fMRI signal) increases with field
    strength though not linearly
  • Fast imaging sequence
  • Echo Planar Imaging (EPI)
  • Stimulus presentation equipment
  • Projector to show visual stimuli
  • Response devices such as button box to record
    subjects response
  • Headphones for auditory stimuli (and hearing
    protection)

49
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50
Functional Brain Mapping with MRI
  • 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

51
Basic fMRI Experiment
Fixation
Thumb movement
time
52
Data Analysis
  • Identify voxels with signal changes matched to
    the timing of experiment

Tapping Tapping Tapping
Rest Rest Rest
53
Unimanual Thumb Flexion
Right Thumb Left Thumb
L R
54
fMRI Compared to Other Functional Techniques
55
Examples of fMRI
56
Activity in a Vegetative State
57
Super Bowl Ads
  • Marco Iacoboni at UCLA used fMRI to examine the
    brains 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

58
Brain Activity During Disney Ad
Mirror Neurons
59
Brain Activity During FedEx Ad
Fear response in Amygdala during scene where the
human is squashed by the dinosaur
60
Caution Needed
  • Interpretation of the signal changes depends on a
    lot of factors
  • Communication of results with public needs to be
    approached with care
  • McCabe Castel (2008, Cognition) brain imaging
    increased perceived credibility of research
    compared to bar graphs
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