Chapter 15
Magnetic Resonance
Imaging
Spatial Localization
•Uniform magnetic field
•Magnetic gradient
•RF energy of specific sequence
MR Signal
Object/Patient
Excitation and
relaxation
Spatial localization
f0=(/2)B0
Magnetic Field Gradient
• By gradient coil in x, y, z direction
Precession Frequency Variation
Left end Center Right end Range
Location x=-7.5cm x=0 x=7.5cm 15cm
Main magnet field strength 1.5 T 1.5T 1.5T
Gradient range over FOV -0.225mT 0 0.225mT 0.45mT
Maximum magnetic field 1.499775T 1.5T 1.500225T 0.45mT
Larmor frequency 63.86042MHz 63.87000MHz 63.87958MHz 19.2KHz
Gradient field strength 3mT/m=0.3G/cm
Frequency bandwidth/pixel 19.2KHz/256=74.85Hz/pixel
Slice Select Gradient
• RF pulse
– No selecting ability
• Selected Slice
– RF BW
– Gradient strength
• For same thickness
– SNR(BW)-1/2
RF pulse profiles
• Ideal pulse: Sinc pulse Rectangular BW
– Practically impossible approximation
• Small(Large) BW Long(Short) sinc pulse
Frequency Encoding Gradient • Readout gradient
– Gradient along x-axis different precession frequency signals of different frequency receiving FFT position dependent information
– Signal from tissue column not from voxel
• Rotating gradient – Many different angles
– Changing coil current
– For each TR
– Reconstruction • backprojection
Phase Encode Gradient
• Third dimension
• Temporal gradient
– Along y axis
– Before FEG
– Change precession frequencies
• Turn-off gradient
– Return to base precession frequency
– But in differently advanced phase angle
• Repeat with different gradient strength
- Decoding by Fourier transform
Gradient Sequencing
MR Data Acquisition
1. SEG: 90° pulse
2. PEG
3. Echo: 180° pulse
4. FEG
5. Data receiving – Fill one matrix row
6. Repeat from 1
7. 2D FFT
8. Image
MR Images
k-space image
MR image
Low-pass filtered
High-pass filtered
2D FFT
Slice Selection
• Directly Axial, Coronal, Sagittal
• By changing gradient coils appropriately
Acquisition Time • TR PEG steps Average No.
– 600ms 192 2 = 3.84min (for 256 192 image)
• Need faster imaging techniques
• Multi-slice Data acquisition
• Fast Spin Echo
Multi-Slice FSE
Image Characteristics
• Spatial resolution
– FOV, gradient strength, receiver coil characteristics, sampling bandwidth
– Image matrix: 128128~ 1024512
• Contrast resolution: SNR
– Voxel volume, RF bandwidth, RF coil factor , Magnetic field strength , Cross excitation , reconstruction algorithm
MR Angiography
• Flow related enhancement – Unsaturated blood into
imaging volume – Large contrast to non-moving
blood – Becomes partially saturated – Undesirable in some cases
• Pre-saturation – Above of below imaging
volume – Helpful to reduce motion
artifact
Time of Flight Angiography
• Tagging blood in one region
– Spin saturation
– Inversion
• Detecting in other slices
– Stack images
– Differentiate moving blood
– MIP projection
Phase Contrast Angiography
• Application of bipolar gradient – 2 Excitation with
reversed sequence
– Subtracting two
• Magnitude image – magnitude of flow
• Phase image – Direction of flow
Artifacts
• Magnetic field inhomogeneity – Distortion or misplacement of anatomy
• Focal field inhomogeity – By ferromagnetic objects: implants, clips, makeup,..
• Gradient field artifact – Compressed anatomy
• RF Coil artifact – Non ideal BW
• RF artifact – TV, Motors, fluorescent lights
Artifacts • k-space errors
– By bad pixel
• Slice-to-slice interference – Inter-slice gap
– Interleaving • Time gap reduce saturation
in adjacent slice
Artifacts
• Motion artifacts
– Long acquisition time
– Voluntary/involuntary
– Several reducing techniques
• ECG gating
• Chemical shift
– Molecular structure produce shield
– Different resonance
different location
Magnets
• Field strength – 0.1 T ~ 7.0T
• Field homogeneity – 1ppm ~ 50ppm
• Temporal stability
• Air core/ Solid core
• Magnet types – Resistive
– Superconductive
– Permanent
Magnet Types
Resistive Superconductive Permanent
Magnet Electrical magnet Super conductive
material Permanent
Magnet
Core Solid Air Air
Direction Vertical Horizontal Horizontal
Filed Strength 0.1~0.3T 0.3~3.0, 4, 7T 0.1~0.3T
Field Off Immediately Difficult Impossible
Homogeneity 30~50 ppm 1 ppm >1ppm
Merit “open” design High performance Low running
cost
Disadvantage High electric cost Helium for cooling,
high siting cost Heavy
Ancillary Equipments • Shim coil:
– Adjust main magnetic field to improve homogeneity
• Gradient coil: – Superimposed gradient
– Source of banging noise
• RF coil: – Create B1 field, rotate
magnetization
– Detect magnetization signal
• Proximity coil – To image extremities
Magnet Siting
• Extensive magnet fringe field
– Potential hazard in adjacent area
– Common to commercial FM band
• Magnetic shielding
– Magnet Local environment
• Pacemaker, clip
• Elevator, car
– Passive / active
• 0.5mT(5Gauss)
– Administrative control
Safety and Bioeffect
• Safety – Strong magnetic field, RF energy, varying gradient,
cryogen liquids, confining devices, nosy operation
– Implants, aneurysm clips, pacemaker, heart valves
– Considerable torque, adverse effect, heating, misdiagnosis due to distorted image
• Bioeffect – Generally no effect in diagnostic range(< 10T)
– Tissue heating, dizziness, disorientation, membrane permeability change, visual phosphene in higher magnetic fields
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