Magnetic Resonance : The Basics

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Magnetic Resonance TheBasics
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MR is very simple
U
N
S
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Resonance Principle
Some nuclei are magnetic Hydrogen H -
1 Carbon C - 13 Fluorine F - 19 Phosphorus P -
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B
?
Precession Frequency ?????????B
? Gyromagnetic Ratio (depends on nuclei,
e.g. protons 42 MHz/T) B Magnetic Field
Strength
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Patient Magnet
Without magnetic field
With magnetic field
No net magnetization
Low net magnetization
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Patient Magnet Radio Frequency
Excitation
Signal
Resonance ? Energy transfer
Signal induction in coil
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Transverse Relaxation Time T2
Transverse Relaxation Decay of magnetization
by interaction between nuclei
(Spin-Spin-Relaxation)
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Longitudinal Relaxation Time T1
Longitudinale Relaxation Energy transfer
between excited spins and Tissue
(Spin-Lattice-Relaxation) ?? Reestablishing of
longitudinal magnetization with time constant T1
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Relaxation Times are Tissue Specific
Mz
S
Tissue 1
Tissue 1
Tissue 2
Tissue 2
Short TE Medium TE Long TE
TR
Transverse Relaxation
Longitudinal Relaxation
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Spin Echo
Inhomogeneities of the magnetic field shorten
relaxation process T2 ? T2
T2
T2
Transverse relaxation T2 is too fast to receive
spatially encoded signals ??Echo of signal by
180o pulse
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Spin Echo Pulse Sequence
TR
RF Pulse
Echo
FID
MR Signal
Slice Selection Gradient
Phase Encoding Gradient
Readout Gradient
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Slice Selection
B B0 Gz z
?
B
Slice Excitation at ?????0
B0
Weaker Field ?????0
Stronger Field ?????0
higher field, ?????0
lower field, ?????0
z
-z
z
0
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Spatial Resolution within the Slice
Raw Data
p256f1
p256f256
p2f1
p1f1
p1f2
p1f256
(Phase und Frequency Encoding)
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Raw Data Matrix (k-Space)
ky
1 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?2 ? ? ? ? ? ? ?
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?
? ? ? ? ? ? ? N ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?
Raw data matrix or k-space is filled line by line
by variation of the Phase Encoding Gradient
kx
Line Information Frequencies of the Readout
Gradient
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Fourier Transformation
y
Phase
x
Frequency
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Scan Time of Spin Echo Sequence
Tac TR ? NPh ? AC
TR Repetition Time NPh Number of Phase
Encoding Steps Matrix Size AC Number of
Acquisitions (to improve S/N)
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Spin Echo Contrasts
TR
2500 2000 1500 1000 500
PD
T2
T1
30
90
TE
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Spin Echo Contrasts
Left Cervical spine, sagittal, T1 weighted
Right Lumbar spine, sagittal, T2 weighted
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Further Sequence Developments
Scan Time Reduction ? Gradient Echo
Sequences Hybrid Sequences Contrast
Variations ? Gradient Echo Sequences Improved
Spatial ? 3D Gradient Echo Sequences
Resolution
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Scan Time Reduction
Tac TR ? NPh ? AC
Minimum 0.5 (Half Fourier)
More Lines per TR Turbo Spin Echo Hybrid
Sequences (Multi Shot Sequences) Single Shot
Sequences
Gradient Echo FLASH FISP PSIF DESS CISS
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Turbo Spin Echo Sequences
RF Pulse
MR Signal
Slice Selection Gradient
Phase Encoding Gradient
Readout Gradient
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Turbo Spin Echo Sequences
Head, axial TR 4500 ms, TEeff 96 ms 7 Echoes,
TA 812 min
Abdomen, coronal HASTE, TEeff 74 ms 128 Echoes,
TA 1 sec
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Gradient Echo Sequences
Spin Echo Sequence Diagram
Gradient Echo Sequence Diagram
TR
TE
Echo
Lower flip angle (lt 90o) Echo by gradient pulse
instead of 180o pulse
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Gradient Echo Sequences
Knee sagittal, 2D FLASH, 5122, TA 712 min
MR Myelography, 3D FISP, 1.5 mm, TA 641 min
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3D Sequences
2D
3D
No Slice Gaps Thinner Slices Rectangular Slice
Profiles Postprocessing with MPR
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3D Postprocessing
Multiplanar Reconstructions (MPR) Sagittal Data
Set, 51 Partitions, Slice Thickness 1.5 mm, TR
27 ms, TE 9 ms, TA 544 min Orthogonal
reconstructions Oblique and double oblique
recons Curved reconstructions
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Further Sequence Developments
Gradient Echo Percentage /
Echoes
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