Magnetic Resonance
Imaging (MRI)
Outline of Presentation
• Background & Brief Description
• Sample MRI Images
• Understanding Technology
(Procession, Larmor Frequency,
Radio Frequency, Gradient Magnets)
• Advantages & Disadvantages
• Distortions
• Figures of Merit
• Current Research & Future Works
What is MRI?
• Imaging technique used
primarily in medical
settings to produce high
quality images of the inside
of the human body
• application of nuclear
magnetic resonance (NMR)
• Produce anatomical images
(organs and soft tissues )
• physical-chemical state of
tissues
• flow diffusion (blood vessel)
• motion information (brain)
How Does MRI look like?
• a giant cube
• horizontal tube
running through the
magnet from front
to back. “bore of
the magnet.”
• The patient, lying
on his or her back,
slides into the bore
on a special table.
MR Image
Sagittal section through a
normal human face
Sagittal section through a
normal human knee
MR Image
Sagittal
section
through
a normal
human
What are its
components?
MRI system consists of:
• Radio Frequency (RF) Transmitter
• Main Magnet 0.5 to 2.0-tesla or 5,000 to 20,000 gauss
(Resistive, Permanent, or Superconducting)
• RF coils
• 3 Gradient Magnets 18 to 27-millitesla or 180 to 270
gauss
• RF amplifier
• Fourier Transformer
• Computer
How Does it Work? –
Brief Outline
1.
2.
3.
Orient all the nuclear spins in a patient’s
body in parallel with a strong magnetic
field. (Main Magnet)
Locate the point to be examined
(Gradient Magnets) and flip the spins of
the hydrogen atom in the point in the
other direction with a strong pulse of
radio frequency of exactly the right
frequency.
Collect the electromagnetic signal when
the spins relax to their original state and
transform the signal to produce image.
Spin (Precession)
• Hydrogen Atom –
Magnetic Dipole
Moment (MDM)
• High water content of
non-bony tissues
• A symmetric body with
spin angular
momentum and some
torque that is
perpendicular to the
angular momentum
precesses.
Spin
• All of the hydrogen
protons will align
with the magnetic
field in one direction
or the other.
• Vast majority of
these protons will
cancel each other
out
• The excess nuclei in
the lower energy
state give a net MDM
component along the
field
Larmor (Resonance)
Frequency
• Frequency of procession
• Depends on Magnetic field and Gyromagnetic ratio –
ratio of the MDM to the nuclear spin angular
momentum
• L = H/2 where L = Larmor frequency,  =
gyromagnetic ratio, and H = applied magnetic field
• Unique value for each type of nucleus - each type of
nucleus will precess at a unique frequency in a
given magnetic field -> we can distinguish between
nuclear types!
• exactly equal to the frequency of radiation absorbed
in a transition from one spin state to another.
Radio Frequency
Magnetic Field H1
• We want to displace M (tiny magnet:
Hydrogen atom) from its direction along H
and watch M as it tries to go back to its
alignment along H
• Apply a second magnetic field H1 to
displace M
• Little dipole magnets realigning themselves
and beginning to precess about the net
magnetic field (vector sum of H1 and H)
• H1 turned on and off quickly (90º pulse) ->
get a small displacement of M
• M precessing about H and gradually
realigning itself along H
Free Induction Decay (FID)
& Fourier Transformation
• FID: signal produced by the free
return of M to H direction
• MR signal (the FID) -> amplitude
vs. time
• FT of the FID -> signal strength
vs. frequency
BUT..How do we select a
“slice”?
• Selection: apply gradient magnets so that protons in
one slice precess at a unique Larmor frequency,
different from all other protons in the imaging field.
• Apply a range of frequencies to the RF coil to
observe slightly different Larmor frequencies in
selected slice.
NOTE:
A proton precessing at a certain Larmor frequency
will respond to an RF pulse only if the RF pulse
frequency is exactly the same as the Larmor
frequency.
Local environmental magnetic fields cause the dipoles
to precess at slightly different frequencies (L =
H/2)
Basis of NMR - identical nuclei in slightly different
magnetic fields having different Larmor frequency
Advantages
• Non-Invasive: MRI does not depend
on potentially harmful ionizing
radiation, as do standard x-ray and
CT scans.
• MRI scans are not obstructed by
bone, gas, or body waste, which can
hinder other imaging techniques
• Can see through bone (the skull) and
deliver high quality pictures of the
brain's delicate soft tissue structures
• Images of organs and soft tissues
Drawbacks
•
•
•
•
Pacemakers
Claustrophobic
Tremendous amount of noise during a scan
MRI scans require patients to hold very still
for extended periods of time. MRI exams
can range in length from 20 minutes to 90
minutes or more.
• Orthopedic hardware (screws, plates,
artificial joints) in the area of a scan can
cause severe artifacts
• High cost
Distortions
•
•
•
•
•
•
•
•
Poor magnetic field homogeneity
Imperfect gradient coil design
Radio frequency coil inhomogeneity
Pulsatile flow artifact – blood flowing
perpendicular to the slice direction
Respiratory motion
Cardiac motion – need motion
correction
Random bulk motion
Chemical shift effects
Current Research & Future
Works
• Still in its infancy - in widespread use for less
than 20 years (compared with over 100 years
for X-rays)
• Very small scanners for imaging specific
body parts are being developed
• Functional brain mapping
• Imaging ventilation dynamics of the lungs
through the use of hyperpolarized helium-3
gas
• development of new, improved ways to
image strokes in their earliest stages is
ongoing.
• MRA (Angiography) was developed to study
blood flow
Figures of Merit
• Producing and holding H constant – most
difficult job
• H-field value should be as large as
possible because the signal-to-noise ratio
(S:N) of the output information depends
on H
• Repetitive pulsing improves the S:N ratio
• Detection Limit? – How large a tumor has
to be to be detected by MRI?
• A point (horizontal points build up slice) is
a cube that is about half a millimeter on
each side.
Reference
•
•
•
•
•
•
•
•
•
•
•
•
•
Bankman, Isaac N.. Handbook of Medical Imaging: Processing and Analysis. The United
States of America: Academic Press, 2000.
Curry, Thomas S. III et al. Christensen’s Physcis of Diagnostic Radiology. The United States
of America: Lea & Febiger, 1990.
Mattson, James and Simon Merrill. The Pioneerrs of NMR and Magnetic Resonance in
Medicine: The Story of MRI. The United States of America: Bar-Ilan University Press, 1996.
Rubinson, Kenneth A. and Rubinson, Judith F.. Contemporary Instrumental Analysis. The
United States of America: Prentice-Hall, Inc., 2000.
Gould, Todd A.. <http://www.howstuffworks.com/mri.htm>
<http://www.ehendrick.org/healthy/00055770.html>
<http://www.vh.org/Providers/TeachingFiles/NormalRadAnatomy/Images/LowerEx/MRICorA
nkleLwrEx14.html>
<http://www.sciencephoto.com/html_tech_archive/magnet.html#>
<http://www.rwc.uc.edu/koehler/biophys/5c.html>
<http://www.varianinc.com/cgibin/nav?varinc/docs/nmr/nmr_work&cid=975JILOOLLOLPGKMJKQPO>
<http://imasun.lbl.gov/~budinger/medTechdocs/MRI.html>
<http://www.mc.uky.edu/mrisc/what.asp>
<http://www.cis.rit.edu/htbooks/mri/inside.htm >