Transcript MRIsaad_ch8

Magnetic Resonance
Imaging (MRI)
Magnetic Resonance
Imaging (MRI)
• Non-invasively
provides high
resolution anatomical
images of human
structures, such as
brain, heart and other
soft tissues
How MRI Works
1. Put subject in big magnetic field
2. Transmit radio waves into subject [2~10 ms]
3. Turn off radio wave transmitter
4. Receive radio waves re-transmitted by subject
• Store measured radio wave data vs. time
• Now go back to 2) to get some more data
• Process raw data to reconstruct images
• Allow subject to leave scanner
How MRI Works
• Strong magnetic field, usually from
• superconducting magnets.
• RadioFrequency coils and sub-system.
• Gradient coils and sub-system.
• Shimming coils and sub-system.
• Computer(s) that coordinate all
subsystems.
Timeline of MR Imaging
1924 - Pauli
suggests
that nuclear
particles
may have angular
momentum (spin).
1937 – Rabi
measures
magnetic moment
of
nucleus. Coins
“magnetic
resonance”.
1944 – Rabi
wins
Nobel prize in
Physics.
1946 – Purcell shows
that matter absorbs
energy at a resonant
frequency.
1946 – Bloch
demonstrates
that nuclear
precession can be
measured in detector
coils.
1952 – Purcell and
Bloch share Nobel
prize in Physics.
1959 – Singer
measures blood flow
using NMR (in
mice).
1972 – Damadian
patents idea for large
NMR scanner to
detect malignant
tissue.
1973 – Lauterbur
publishes method for
generating images
using NMR gradients
1973 – Mansfield
independently
publishes gradient
approach to MR.
1975 – Ernst
develops 2D-Fourier
transform for MR.
NMR becomes MRI
MRI scanners
become clinically
prevalent.
1985 – Insurance
reimbursements for
MRI exams
1990 – Ogawa and
colleagues create
functional images
using endogenous,
blood-oxygenation
contrast.
Early Human MR
Images
(Damadian)
Transmit Receive
rf
coil
main
magnet
Shimming gradient
Control
Computer
How MRI Works
How MRI Works
MRI whole system
Shimming Coils
• Used to compensate for magnetic field
non homogeneities
• Types of shim systems
-Passive: Large number of metal rods w/
adjustable weights
- Superconducting: Coils surrounded by
cryogens
- Resistive: Coils at room-temperature
- Cheaper, adjusted for each subject
Magnetic and Electromagnetic
Fields
• Magnetic fields generate the substance we
“see” which is HYDROGEN molecules:
magnetization of the H protons in H2O
• Magnetic fields also let us manipulate
magnetization
- make a map [or image] of its distribution
inside the body’s tissue
• Static magnetic fields change slowly (< 0.1
ppm/ hr)
- main field; static inhomogeneities
Magnetic and Electromagnetic
Fields
• RF fields are electromagnetic fields that
oscillate at Radio Frequencies (tens of
millions of times per second)
􀂾 transmitted radio waves into subject
􀂾 received signals from subject
• Gradient magnetic fields change quickly
(switching up to thousands of times per
second)
Magnetic and Electromagnetic
Fields
How do we detect magnetization?
• We need to perturb the system
• RF coil transmits B1 field
Magnetic induction causes RF signal
generation (precessing) which is received
by RF receiving coil
Various coils used for transmit, receive,
both, volume and surface measurements
Magnetic and Electromagnetic
Fields
Relaxation/Decay Times when
particles return to
equilibrium have diagnostic
value
• T1 or spin-lattice decay
• T2 or spin-spin decay
• Long time constants
• Spin echo
MRI image formation
• Defines the spatial location of the proton
pools that contribute to the MR signal after
spin excitation
• A 3-D gradient field (dB/dx, dB/dy, dB/dz)
would allow a unique correspondence between
the spatial location and the magnetic field. Using
this information, we will be able to generate
maps that contain spatial information – images.
Readout Localization
(frequency encoding)
• After RF pulse (B1) ends, acquisition of MRI
RF signal begins
􀂾During readout, gradient field perpendicular to slice
selection gradient is turned on
􀂾Signal is sampled about once every few
microseconds, digitized, and stored
􀂾Computer breaks measured signal into frequency
components
􀂾Since frequency varies across subject in a known
way, we can determine where frequency component
comes from
• Also Phase Encoding measurements
Image formation
• Fourier analysis used to determine the
amplitude distribution of the returned
frequencies
• No rotation like CT
􀂾Direction of the magnetic gradient is rotated
slightly
• Reconstuction similar to CT
􀂾Back projection
• No ionizing radiation used
􀂾In theory, safer than CT, but…
Risks of MRI
• Projectile Effects:
External
• Projectile Effects:
Internal
• Radiofrequency
Energy
• Gradient field changes
• Claustrophobia
• Acoustic Noise
• Quenching
July 31, 2001 —
A 6-year-old boy died
after undergoing an
MRI exam at a New
York-area hospital
when the machine's
powerful magnetic field
jerked a metal oxygen
tank across the room,
crushing the child's
head. …