Transcript MAGNETIC RESONANCE IMAGING

MAGNETIC RESONANCE
IMAGING
• Jack E. Peterson, Ph.D.
Summary Slide
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I. HISTORICAL PERSPECTIVE
II. MAGNETIC NUCLEI
III. TISSUE MAGNETIZATION AND RELAXATION
IV. RF PULSE SEQUENCES AND IMAGING METHODS
V. SPATIAL CHARACTERISTICS OF THE MR IMAGE
VI. PROTOCOL FACTORS FOR CONTRAST CONTROL
VII. IMAGE DETAIL AND NOISE
VIII. BASIC MRI EQUIPMENT
1.1 Historical Perspective
– 1.1.1
NMR Spectroscopy - 1946
• 1.1.1.1 Felix Bloch - Stanford
• 1.1.1.2 Edward M. Purcell - Harvard
• 1.1.1.3 Joint Nobel Prize - 1952
– 1.1.2
MRI Imaging
• 1.1.2.1 Paul Lauterbur - 1973 - Water Image
• 1.1.2.2Raymond Damadian - 1974 - Patent Granted
• 1.1.2.3Peter Mansfield - 1975 - Finger Image
II. MAGNETIC NUCLEI
•2.1Magnetic Moments
•2.2
Characteristics
• 2.3
Nuclides
•2.4
Magnetic Fields
•2.5
Magnetic Resonance
2.1 Magnetic Moments
2.2 Characteristics
• 2.2.1 Sensitivity to a Magnetic Field
– 2.2.1.1 Needs odd number of protons/neutrons
• 2.2.2 Isotopic Abundance
– 2.2.2.1 Abundant in nature?
• 2.2.3 Tissue Concentration
– 2.2.3.1 Abundant in the body?
2.3 Nuclides
• 2.3.1
• 2.3.2
• 2.3.3
– 2.3.3.1
• 2.3.4
• 2.3.5
• 2.3.6
– 2.3.6.1
• 2.3.7
Hydrogen-1
Phosphorus-31
Carbon-12
Carbon-13
Sodium-23
Potassium-39
Oxygen-16
Oxygen-17
Fluorine-19
2.4 Magnetic Fields
– 2.4.1
Magnets
– 2.4.2
Field Characteristics
•2.4.2.1
•2.4.2.2
•2.4.2.3
Size and Shape
Direction
Strength Units
– 2.4.2.3.1 The Tesla = 10,000 Gauss
– 2.4.2.3.2 The Gauss – Measure of Earth’s field
• 2.4.2.3.2.1 Atlanta = 0.6 Gauss
2.5 Magnetic Resonance
• 2.5.1
Resonant (Larmor) Frequency
– 2.5.1.1 Field Strength
– 2.5.1.2 Nuclide - Gyromagnetic Ratio
(Magnetogyric Ratio)
• 2.5.2
Radio Frequency (RF) Absorption
• 2.5.3
Radio Frequency (RF) Emission
III. TISSUE MAGNETIZATION
AND RELAXATION
• 3.1 Material Magnetization
– 3.1.1
Production
• 3.2 Magnetic Characteristics
– 3.2.1
– 3.2.2
Direction (Vector)
Strength
• 3.2.2.1Nuclear Concentration
• 3.2.2.2
Field Strength
– 3.2.3
Stability
• 3.2.3.1Permanent
• 3.2.3.2
Temporary
3.3 Magnetic Directions
– 3.3.1
Longitudinal
•3.3.1.1
•3.3.1.2
– 3.3.2
Stable
Unstable
Transverse
•3.3.2.1
•3.3.2.2
Unstable
Produces RF Signal
3.4 Magnetic Excitation
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–
–
3.4.1
3.4.2
3.4.3
3.4.4
Radio Frequency Energy
90° Excitation
180° Rephasing
Excitation Method
•3.4.4.1
•3.4.4.2
RF Coils
RF Generator
3.5 Magnetic Relaxation
– 3.5.1
Energy Exchange
– 3.5.2
Relaxation Rates
3.6 Longitudinal Relaxation
• 3.6.1
Mechanism
• 3.6.2
Relaxation Time – T1
3.7 Transverse Relaxation
– 3.7.1
Mechanism
– 3.7.2
Relaxation Time - T2
3.8 Tissue Values
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3.8.1
3.8.2
3.8.3
3.8.4
3.8.5
Field Strength
Temperature
Water Content
Protein Content
Paramagnetic Ions (Contrast Media)
Some Neural Tissue
Characteristics
Tissue
Proton
Density
T1
T2
100
290
60
Gray Matter
84
520
95
White
matter
72
380
85
CSF
100
2700
160
Fat
Other Tissue Characteristics
Tissue
Proton
Density
T1
T2
Liver
91
290
50
Muscle
100
630
40
Blood
90
820
180
Water
100
2700
2700
IV. RF PULSE SEQUENCES
AND IMAGING METHODS
• 4.1
• 4.2
• 4.3
• 4.4
• 4.5
• 4.6
• 4.7
RF Functions
Coils (RF Antennas)
RF Energy Generator (Transmitter)
RF Receiver
RF Shielding
Magnet Types
Field Strength Issues
4.1
RF Functions
•4.1.1
Excitation
•4.1.2
Signal Detection
4.2 Coils (RF Antennas)
• 4.2.1
• 4.2.2
• 4.2.3
Body Coils
Head Coils
Surface Coils
– 4.2.3.1 Homogeneous – send & receive
– 4.2.3.2 Inhomogeneous – receive only
• 4.2.4
Quadrature Coils
– 4.2.4.1 Detect both halves of signal
– 4.2.4.2 Think stereo – better location of source
4.3 RF Energy Generator
(Transmitter)
4.4 RF Receiver
• 4.4.1
Need for Quadrature Reception
– 4.4.1.1 Localize the signal
– 4.4.1.2 Receive both halves of the signal
4.5 RF Shielding
• 4.5.1
Keep RF out of the Bore!
• 4.5.2
Shield the Room
– 4.5.2.1 Phosphor-bronze screens
– 4.5.2.2 Difficult and expensive
– 4.5.2.3 Easy with mobile units
• 4.5.3
Shield the Bore
– 4.5.3.1 Pull-out shield
4.6 Magnet Types
• 4.6.1
Permanent
– 4.6.1.1 Very heavy (~100 tons)!
– 4.6.1.2 Limited field strength (0.3 T)
– 4.6.1.3 Field is vertical
• 4.6.2
Resistive
– 4.6.2.1 Low field strength
– 4.6.2.2 Uses much electrical power
– 4.6.2.3 Needs cooling
• 4.6.3
Superconducting
– 4.6.3.1 Uses niobium-titanium conductors
– 4.6.3.2 Needs liquid helium and nitrogen
4.7
• 4.7.1
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4.7.1.1
4.7.1.2
4.7.1.3
4.7.1.4
• 4.7.2
Field Strength Issues
Image Quality
Contrast
Detail
Noise
Artifacts
Imaging Time
– 4.7.2.1 Number of Averages (NSA)
– 4.7.2.2 Number of Slices
• 4.7.3
Safety
V.
SPATIAL CHARACTERISTICS
OF THE MR IMAGE
• 5.1 The Imaging Space
• 5.2 Gradients
• 5.3 Imaging
5.1 The Imaging Space
• 5.1.1
The Magnetic Field
• 5.1.2
RF Coils
• 5.1.3
Gradient Coils
5.2
• 5.2.1
Purpose
• 5.2.2
Gradient
• 5.2.3
Gradient Directions
– 5.2.3.1 Selection - Z
– 5.2.3.2 Preparation - Y
– 5.2.3.3 Detection/Measurement - X
Gradients
5.3 Imaging
• 5.3.1
Slice Selection
– 5.3.1.1 Thickness
– 5.3.1.2 Position
– 5.3.1.3 Orientation
• 5.3.2
Image Matrix (Pixel) Formation
– 5.3.2.1 Phase Encoding
– 5.3.2.2 Frequency Encoding
VI. PROTOCOL FACTORS
FOR CONTRAST CONTROL
• 6.1
• 6.2
• 6.3
• 6.4
• 6.5
• 6.6
Image Gray Scale
Tissue Contrast Factors
Procedure Parameters
Spin Echo
Transverse Magnetization
Longitudinal Magnetization
6.1
Image Gray Scale
• 6.1.1
RF Signal Intensity
• 6.1.2
Tissue Magnetization
6.2 Tissue Contrast Factors
• 6.2.1
• 6.2.2
• 6.2.3
• 6.2.4
• 6.2.5
Nuclear (Proton) Concentration
Longitudinal Relaxation Time - T1
Transverse Relaxation Time - T2
Flow and Motion
Chemical Shift
6.3 Procedure Parameters
• 6.3.1
Pulse Sequences
– 6.3.1.1 Spin Echo
– 6.3.1.2 Inversion Recovery
– 6.3.1.3 Gradient Echo
• 6.3.2
Pulse Intervals
– 6.3.2.1 Repetition Time (TR)
– 6.3.2.2 Echo Time (TE)
– 6.3.2.3 Inversion Time (TI)
6.4 Spin Echo
• 6.3.1
Purpose
• 6.3.2
Method
6.5 Transverse Magnetization
• 6.5.1
Reason
– 6.5.1.1 Obtain RF Signal
– 6.5.1.2 Observe T2 Characteristics
• 6.5.2
Production - 90 Pulse
• 6.5.3
Use of TE
6.6 Longitudinal Magnetization
• 6.6.1
Observe T1 Characteristics
• 6.6.2
Production
– 6.6.2.1 90 Pulse - SE Sequence
– 6.6.2.2 180 Pulse - IR Sequence
• 6.6.3
Use of TR
6.7 Spin Echo Image “Weighting”
T1
T2
P.D.
TR
Short
Long
Long
TE
Short
Long
Short
Brightest
Tissue
Shortest
T1
Longest
T2
Greatest
P.D.
CSF
Dark
Bright
Bright
Fat
Bright
Dark
Bright
6.8 Gradient Echo Image
“Weighting”
TR
(ms)
TE
(ms)
Flip
Angle
T1
LowT2*
High T2*
P.D.
200 - 400
20 - 50
200 - 400
200 - 400
12 - 15
12 - 15
30 - 60
12 - 15
45 - 90°
30 - 60°
5 - 20°
5 - 20°
VII. IMAGE DETAIL AND NOISE
• 7.1 Detail (Voxel Size)
• 7.2 Image Noise
• 7.3 Signal-to-Noise Ratio
• 7.4 Trade-Offs
7.1 Detail (Voxel Size)
• 7.1.1
Slice Thickness
– 7.1.1.1 RF Bandwidth
– 7.1.1.2 Gradient Slope
• 7.1.2
– 7.1.2.1
– 7.1.2.2
Pixel Size
Matrix (64, 128, 179, 256, 512, 1024)
Field of View (FOV)
7.2 Image Noise
• 7.2.1
RF Noise Sources
– 7.2.1.1 The Human Body
– 7.2.1.2 Effect of RF Coils
7.3 Signal-to-Noise Ratio
• 7.3.1
• 7.3.2
Nuclear (Proton) Concentration
Magnetic Field Strength
– 7.3.2.1 Slice Thickness
– 7.3.2.2 Pixel Size
• 7.3.3
• 7.3.4
• 7.3.5
TR/T1
TE/T2
Averaging (NSA)
7.4 Trade-Offs
• 7.4.1
Image Detail
• 7.4.2
Image Acquisition Time
VIII.
• 8.1
• 8.2
• 8.3
• 8.4
BASIC MRI EQUIPMENT
Magnets
Radiofrequency (RF) Equipment
Operator Controls
Computers
8.1 Magnets
• 8.1.1
Permanent
• 8.1.2
Resistive
• 8.1.3
Superconducting
• 8.1.4
Conclusions
8.2
• 8.2.1
Radiofrequency (RF) Equipment
Receiver
– 8.2.1.1 Antennas (Coils)
• 8.2.2
Transmitter
– 8.2.2.1 Antennas (Coils)
– 8.2.2.1.1
– 8.2.2.1.2
Power Output
Frequency Requirements
8.3 Operator Controls
• 8.3.1
Console
• 8.3.2
Terminal
8.4 Computers
• 8.3.1
Types
– 8.3.1.1 Mainframes – DEC, IBM
– 8.3.1.2 Work Stations – DEC, Sun
• 8.3.2
Storage
– 8.3.2.1 Memory
– 8.3.2.2 Disk/Tape Storage
– 8.3.2.3 Laser Disk Storage
• 8.3.3
Array Processor
684 x 362 Image