Magnetic Resonance Imaging

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Magnetic Resonance Imaging
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Magnetic field gradients
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Gradient echo
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Gradient echo
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Maxwell Pair
a
3
dB
Maxwell Pair for
z
dz
Net B field from coils
a
Local fields
Current paths
B
o
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Straight wires
dB
Straight wires for
z
dy
Reduces B
o
Increases B
o
y
B
o
z
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Gradient field Acoustic noise

• One of the biggest challenges at high field
• Acoustic noise
• Ear defenders and foam to reduce bone conduction
• Gradient system decoupled from bed (and room)
• Modification of gradient waveforms
• Novel gradient coil designs

Conventional Waveform
Minimum slew rate waveforms
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Gradients can be combined to produce gradients in
arbitrary directions
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Slice selection using gradients
Sample
Bz
B(x1)
B(x2)
x
x1
x2
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Slice selection
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Imaging in gradients
Sample
Bz
B(x1)
B(x2)
x
x1
x2
Signal (spectrum)
Frequency
w1
w2
PROJECTION
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Projection Reconstruction
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Projection Reconstruction
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Basic reconstruction
X-ray beam is attenuated forming a projection
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Projections are collected from various directions
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The projection tells you that the beam was
attenuated with equal probability anywhere along
its path
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Projection Reconstruction

• Diffusion imaging
• Radial Spin Echo and TSE Scan PR
• Low sensitivity to motion
• Low sensitivity to susceptibility artifacts

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Projection Reconstruction

• Sodium Imaging
• Human Skeletal Muscle Sodium MR Imaging and
  Quantification-Potential Applications in Exercise
  and Disease
• Constantinides, Gillen, Boada, Pomper, Bottomley,
  
• Radiology

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K-space
Image
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K space
Units rad/ m
Increasing spatial frequency
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Projection Reconstruction
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With MRI we can control how we sample k-space
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Phase encoding
Phase encoding
Readout gradient
Initial phase during readout depends on phase
encoding amplitude
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Phase encoding
Phase encoding
Readout gradient
Initial phase during readout depends on phase
encoding amplitude
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With MRI we can control how we sample k-space
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Spin warp imaging
Wicklow, Washington University, St. Louis
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Roger Ordidge, UCL
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Ultrashort TE- Tendons
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Motion artefacts
Anesthetic
Steve Keevil UMDS
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EPI
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Diffusion EPI
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Diffusion
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EPI- fetus
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EPI- gut
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EPI - fMRI
Cochlear Stimulation
Ludman, Mason, ODonoghue, Morris, Nottingham