Transcript Magnetic Resonance Imaging

Magnetic Resonance Imaging
Mary Holleboom
ENGR 302
May 7, 2002
Outline
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Overview & History
Imaging Principles
Advanced Techniques
Imaging Hardware
Safety
Applications
Overview & History
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Use strong magnetic field to scan an object and produce an
image
No radiation
Utilize magnetism of internal particles
1973 – Back projection imaging technique
1975 – Phase and frequency encoding, Fourier Transform
1977 – Imaging of the whole body,
Echo-planar imaging: real-time movie imaging
1993 – Functional MRI
Imaging Principles
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Spin
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Protons, Neutrons, Electrons
+/- 1/2
Particle behaves like a
magnet in presence of
magnetic field
Grouped in packets - create
magnetization vector
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Fourier transform
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Most common technique used
today
Sequence of applied gradients
Fourier Transform
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RF pulse
Slice selection gradient pulse
Phase encoding gradient pulse
Frequency encoding gradient
pulse
Signal recorded
Process repeated 128 – 256
times
Signal Fourier Transformed in 2
directions
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Frequency encoding direction
Phase encoding direction
Intensities of data peaks
converted into intensities of
pixels
Tomographic image
Advanced Techniques
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Volume (3-D) Imaging
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Flow Imaging (MR angiography)
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Group of slices (volume) used instead
of one slice at a time
Image blood flowing through arteries
& veins
Velocity of blood flow determined by
intensity of image
Echo Planar Imaging (functional MRI)
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Imaging relates body function or
thought to specific locations in the
brain
Tomographic images produced at
video rates
Imaging Hardware
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Magnets
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Superconducting
Resistive
Permanent
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Solid magnetic material
Strongest
Electromagnet
Electromagnet
Cooled
Weakest
bymagnetic
air
field
Current
Greater
Used
forresistance
flows
open in
MR
wire
scanners
coil to
create
amagnetic
magnetic
field
 Arranged
in anyfield
position
Weaker
 Nozero
needresistance
for patient to
Nearly
in be
wire
surrounded by
magnet
at temperature
close
to
absolute zero
Cooling achieved with liquid
helium or nitrogen
Coils
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Inductive & capacitive elements
allow it to resonate
Gradient coils
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RF coils
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Create gradients in equilibrium
magnetic fields
Room temperature coils
Transmit magnetic field
Receive RF signal from spins
Various coils for specific applications
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Surface
Bird cage
Saddle
Phased-array
Litz
Safety
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Patients
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No biological hazards from magnetic
fields yet discovered
Most pregnant women prohibited from
undergoing MR imaging to prevent
possible damage to fetus
Most people with metal implants
prohibited
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Pacemakers - malfunction due to
magnet
Cerebral aneurysm clip - magnet could
move clip
Most orthopedic implants safe
because they are firmly embedded in
bone
RF coil failure can severely burn
patient
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Equipment
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Extremely powerful magnets
No ferromagnetic objects
allowed near scanner
Example
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Bucket pulled off ground
and into magnet
Fully loaded pallet jacket
pulled into bore
Fix problem
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Pull object off magnet
Use forceful device such as
a winch
Turn off magnet completely
Applications
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Brain
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Spine
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Tumors
Aneurysms
Blood clots
Individual vertebrae
Knees
Shoulders
Hips
Prevent strokes
Diagnose Multiple Sclerosis
QUESTIONS
?