Musculoskeletal MRI The basics

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Musculoskeletal MRIThe basics

• Xiaojuan Li, PhD
• Dept of Radiology, UCSF
• BioE297, Sept 16, 2005

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MRI Overview

• No ionizng radiation
• Superior soft tissue contrast
• High resolution and multiplanar capability

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Questions

• How do we get MR signals?
• How do we reconstruct MR images?
• How do we interpret MR images?

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Spins

• Protons constantly spin around an axis
• The positive charge attached with protons also
  moves
• Electrical current lt--gt magnetic field

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MR Signals -- Spins
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Precession
N
S
Larmor frequency
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MR Signals -- Magnetic Vectors
B0
Equilibrium Status M0 aligned with B0
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Relaxation -- T1

• T1 relaxation (longitudinal relaxation)

z
M0

M0
y
x
0.63M0
Energy change btw spins and surroundings T1 time
depends on tissue composition, structure and
surroundings
tT1
Spin-lattice relaxation
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Relaxation -- T2

• T2 relaxation (transverse relaxation)

M0
0.37M0

• Loss of phase coherence due to
• Molecular interactions
• Inhomogeneity of B0
• 1/T2 1/T2 1/T2inhomo

tT2
Spin-spin relaxation
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Questions

• How do we get MR signals?
• How do we reconstruct MR images?
• How do we interpret MR images?

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Localization

• Slice selection
• Phase encoding
• Frequency encoding

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Slice Selection
Slice Selection Gradient
RF pulses with certain bandwidth
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Spatial Encoding
Frequency encoding
Phase encoding

• Spins with the same phase have different
  frequencies
• Spins with the same frequencies have different
  phases

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Image Reconstruction
K-space
Spatial domain
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Pulse Sequence
900
1800

RF
Gz
Gx
Gy
Echo
DA
TE
TR
TE time of echo TR time of repetition
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Questions

• How do we get MR signals?
• How do we reconstruct MR images?
• How do we interpret MR images?

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Image Contrast - T1 weight
T1B
Short TR give T1 weight. Tissues with short T1
are brighter
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Image Contrast - T2 weight
Long TE gives T2 weight. Tissues with long T2 are
brighter
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Image Contrast
Fluids normally have longest T1 and T2, so it
appears darker in T1-weighted images and brighter
in T2-weighted images. (Be careful to images
with fluid suppression though!)
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Factors affecting MR signals
PD Proton density at location (x,y,z) T1
Longitudinal relaxation time T2 Transverse
relaxation time TR Time of Repetition TE Time
of Echo TI Time of Inversion FA Flip
angle Pulse sequence, flow, contrast medium etc
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High resolution MR imaging
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MR Terminology

• Field of view (FOV)
• Matrix size
• In-plane resolution FOV/Matrix size
• Slice thickness
• SNR voxel size, acquisition time, receiving
  bandwidth

• Pulse sequence Fast spin-echo (FSE), Spoiled
  gradient-echo (SPGR)?
• TR/TE
• T1-, T2-, or PD-weighted?
• Fat-suppressed? Fluid suppressed?

• Field strength 1.5T, 3T or 7T?
• Coil body coil, knee coil, paddle coil

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References

• MR Made Easy, GE and BERLEX
• http//www.cis.rit.edu/htbooks/mri/
• http//www.cis.rit.edu/htbooks/nmr/