Title: MRI History and Hardware Basic Safety Issues Introduction to fMRI
1MRI History and HardwareBasic Safety
IssuesIntroduction to fMRI
- John VanMeter, Ph.D.
- Center for Functional and Molecular Imaging
2Outline
- History of MRI
- Hardware components of an MRI scanner
- Basic MRI Safety
- Introduction to fMRI
3Chapter 1 History of MRI
4Pauli, 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
5Rabi - 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
6Edward 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
7Felix 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
8Raymond 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
9Paul 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
10First 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
11Sir Peter Mansfield - 1974
- Devised selective excitation of a slice again
using gradients ? - Slice Select
- Identifies where in a 3D object
- to collect signal from
12Richard 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
13Sir 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!
14Damadian - 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!
15FDA 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)
161990s
- Functional imaging using MRI is first
demonstrated initially by injecting a contrast
agent and later using properties of the blood
itself
175 Nobel Laureates for MRI
Rabi (1944)
Bloch, Purcell (1952)
Lauterbur, Mansfield (2003)
18Nobel 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"
19Chapter 2 MRI Hardware
20Basic 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
21Types 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
22Electromagnets
- 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
23Properties 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
24RF (Radiofrequency) Coils
- Used to transmit and receive RF energy
- Needed to create images
25Coil Designs
- Closer coil is to object being imaged the better
signal - Variety of coils designed for specific body parts
26Coil Design Affects Images
27Gradient 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
28Under the Hood of an MRI Scanner
Cyrostat
Gradients
Body RF Coil
Passive Shims
29Under the Hood of Our MRI Scanner
Quench Pipe
Cold Head
30Computer 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
31Patient 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
32MRI Safety
33MRI Safety
- Static B0 Field
- Projectiles
- Implants/other materials in the body
- RF Field
- tissue heating
- Gradient fields
- peripheral nerve stimulation
- acoustic noise
34Forces on Ferrous Objects
Crash cart meets a 1.5T magnet
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37Welding tank
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40Preventing 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
41MRI Safety
- Static B0 Field
- projectiles
- RF Field
- tissue heating
- Gradient fields
- peripheral nerve stimulation
- acoustic noise
42RF Exposure Standards
- The FDA limits RF exposure to less than a 1
degree C rise in core body temperature
43RF 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
44MRI Safety
- Static B0 Field
- projectiles
- RF Field
- tissue heating
- Gradient fields
- peripheral nerve stimulation
- acoustic noise
45Stimulation 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
46Introduction to Functional MRI
47Difference BetweenMRI fMRI
From Daniel Bulte Centre for Functional MRI of
the Brain University of Oxford
48Tools 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)
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50Functional 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
51Basic fMRI Experiment
Fixation
Thumb movement
time
52Data Analysis
- Identify voxels with signal changes matched to
the timing of experiment
Tapping Tapping Tapping
Rest Rest Rest
53Unimanual Thumb Flexion
Right Thumb Left Thumb
L R
54fMRI Compared to Other Functional Techniques
55Examples of fMRI
56Activity in a Vegetative State
57Super 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
58Brain Activity During Disney Ad
Mirror Neurons
59Brain Activity During FedEx Ad
Fear response in Amygdala during scene where the
human is squashed by the dinosaur
60Caution 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