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