Brian Hargreaves

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Introduction to Magnetic Resonance Imaging

• Brian Hargreaves
• Stanford University

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Magnetic Resonance Imaging

• Non-invasive medical imaging method, like
  ultrasound and X-ray.
• Clinically used in a wide variety of specialties.

Abdomen
Spine
Heart / Coronary
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Magnetic Resonance Imaging

• Advantages
• Excellent / flexible contrast
• Non-invasive
• No ionizing radiation
• Arbitrary scan plane
• Challenges
• New contrast mechanisms
• Faster imaging

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MRI Systems

• At 2 million, the most expensive equipment in
  the hospital

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Topics

• Magnetic Resonance
• MR Image Formation
• Contrast
• Applications of MRI

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Magnetic Resonance

• Certain atomic nuclei including 1H exhibit
  nuclear magnetic resonance.
• Nuclear spins are like magnetic dipoles.

1H
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Polarization

• Spins are normally oriented randomly.
• In an applied magnetic field, the spins align
  with the applied field in their equilibrium
  state.
• Excess along B0 results in net magnetization.

No Applied Field
Applied Field
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Static Magnetic Field
Longitudinal
z
x, y
Transverse
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Precession

• Spins precess about applied magnetic field, B0,
  that is along z axis.
• The frequency of this precession is proportional
  to the applied field

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Relaxation

• Magnetization returns exponentially to
  equilibrium
• Longitudinal recovery time constant is T1
• Transverse decay time constant is T2
• Relaxation and precession are independent.

Precession
Decay
Recovery
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Signal Reception

• Precessing spins cause a change in flux (F) in a
  transverse receive coil.
• Flux change induces a voltage across the coil.

z
B0
y
F
Signal
x
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Excitation

• Excite spins out of their equilibrium state.
• Transverse RF field (B1) rotates at gB0 about
  z-axis.

B1
Magnetization
B0
Rotating Frame
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MR Image Formation

• Gradient coils provide a linear variation in Bz
  with position.
• Result is a resonant frequency variation with
  position.

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Selective Excitation
1
Slope
g
G
Position
Frequency
(a)
(b)
Magnitude
RF Amplitude
Frequency
Time
(d)
(c)
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Image Acquisition

• Gradient causes resonant frequency to vary with
  position.
• Receive sum of signals from each spin.

Frequency
Position
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Image Reconstruction

• Received signal is a sum of tones.
• The tones of the signal are the image.
• This also applies to 2D and 3D images.

Fourier Transform
Image
Received Signal
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Pulse Sequences

• Excitation and imaging are separate.
• Pulse sequence controls
• RF excitation
• Gradient waveforms
• Acquisition
• Reconstruction information as well.

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1D-Pulse Sequence
RF
Gz
Gx
Acq.
Excitation
Imaging
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1D-Pulse Sequence Detailed!
Phase, Modulation Frequency
RF
Finite amplitude, slew rate
Gz
Gx
Acq.

• Demodulation frequency, phase
• Sampling rate and duration

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MR Signal
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k-space
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k-Space Trajectories
2D Fourier Transform
Echo-Planar
Spiral
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2DFT - Pulse Sequence
RF
Gz
Gx
Gy
Acq.
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Spiral - Pulse Sequence
RF
ky
Gx
kx
Gy
Gz
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2D Image Reconstruction
Frequency-space (k-space)
Image space
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Resolution

• Image resolution increases as higher spatial
  frequencies are acquired.

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Image Noise and SNR
Low Signal-to-Noise Ratio
High Signal-to-Noise Ratio
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Contrast

• Contrast is the difference in appearance of
  different tissues in an image.

X-ray contrast is based on transmission.
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Contrast in MRI

• Hydrogen (water) density results in contrast
  between tissues.
• Many other mechanisms, some based on relaxation.

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T2 Contrast
Long Echo-Time
Short Echo-Time
CSF
Signal
White/Gray Matter
Time
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T1 Contrast
Short Repetition
Long Repetition
White/Gray Matter
Signal
Signal
Time
Time
CSF
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Some Applications of MRI

• Brain / Spine imaging
• Knee Imaging
• Cardiac Imaging

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Knee Imaging - Ligaments

• MRI is 97 accurate in diagnosing an ACL tear.

??
Healthy ACL
Full-Thickness ACL Tear
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Knee Imaging - Menisci

• MRI is the best non-invasive method of diagnosing
  meniscal tears

FSE
DEFT
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Knee Imaging - Cartilage

• High resolution images begin to show cartilage
  structure
• 0.4 x 0.4 x 2 mm3 resolution
• 5 minute scan time

Cartilage
Bone
(from Erickson 1997)
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Real-Time Interactive MRI

• Shows live images.
• Useful when there is motion, such as in the
  chest.
• Imaging is very fast, but SNR is lower.
• Interactive imaging allows us to move the scan
  plane in real-time.

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Coronary Artery Imaging
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EXTRAS
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Field of View

• Sampling density determines FOV.
• Sparse sampling results in aliasing.

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Off-Resonance

• Practically, the magnetic field strength is not
  perfectly uniform.
• Resonant frequency is proportional to field
  strength

z
z
Off-resonance
y
y
x
x
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Spin Echoes

• 180 RF tip can reverse the dephasing effects of
  off-resonance.
• Spins realign at some time to form a spin echo

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Imaging Sequences

• Image acquisition usually requires multiple
  repetitions.

90x
90x
90x
90x
TR
TR
TR
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Image Noise and SNR
Low Signal-to-Noise Ratio
High Signal-to-Noise Ratio
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SNR Efficiency

• Can improve SNR by simply averaging.
• Use SNR efficiency, hSNR, as a fair comparison of
  SNR between different imaging methods.

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Summary

• B0 polarizes atomic nuclei
• Spins precess and relax to align with B0.
• B1 allows manipulation of magnetization.
• Excitation sequences provide image contrast.

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Motivation for Cartilage Imaging

• Osteoarthritis has a high incidence
• (15 of the Canadian population)
• Assess new treatments for cartilage degeneration
• Alternative to arthroscopy
• (invasive and expensive)

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Cartilage Imaging Knee Anatomy
Patella
Patellar Cartilage
Femur
Femoral Cartilage
Synovial Fluid
Tibia
Femur
Axial Image
Sagittal Image
(T1-weighted)
(T2-weighted with Fat Suppression)
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Steady-State Sequences

• Fast imaging
• High SNR efficiency, useful contrast
• Short repetition time, incomplete recovery
• Steady-state evolves with time

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Transient Response Reduction

• Imaging during transient response can produce
  artifacts (A)
• Steady-state imaging is delayed until the
  transient decays (B)
• Can we catalyze the steady-state?

A
m0
B
Signal
0
B
A
Time
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Steady-State Free Precession
(Freeman 1971, Oppelt - 1986)
SSFP Sequence
ax
ax
ax
. . .
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Steady State SSFP Signal

• Sensitive to resonant frequency.
• Periodic with nulls every 1/TR.

M0 / 4
Signal Magnitude
0
0
1/TR
2/TR
-1/TR
-2/TR
Resonant Frequency
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Refocused SSFP
2000 ms
8 ms