introduction to magnetic resonance imaging (mri) · 1 of 105 introduction to magnetic resonance...
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Introduction to Magnetic
Resonance Imaging (MRI)
Hsiao-Wen Chung (鍾孝文), Ph.D., Professor
Dept. Electrical Engineering, National Taiwan Univ.
Dept. Radiology, Tri-Service General Hospital
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What is MRI (磁振造影) ?
• M : magnetic 磁
• R : resonance 共振
• I : imaging 影像
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Generation of MRI
• Human body = magnet ensemble ??
• Magnet motion = induced current
signals ??
• With spatial encoding and decoding
= imaging ??
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Generation of MRI
• Human body = magnet ensemble ??
• Quantum physics phenomena
• How to strengthen the magnetic
property for human then ?
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“Human Body Magnets”
Protons in the human body behave like tiny magnets
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Generation of MRI
• Magnet motion = induced current signals
• Just like the electrical power generators
• How to set the human magnets into
motion ?
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Radiofrequency (RF) Excitation
RF coil excites the human magnets so that they rotate
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Detection of MRI Signals
Electromagnetic induction (Faraday’s law)
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Generation of MRI
• Encode the signals according to their
originating locations
– Location-dependent signal
• With decoding computation of the
current signals = imaging
• Fourier transform
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Generation of MRI
• Human body = magnet ensemble
• Magnet motion = induced current
signals
• With spatial encoding and decoding
= imaging
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What is MRI (磁振造影) ?
• M : Source of signals
• R : Principles of excitation and
detection
• I : Turning signals into images
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Generation of MRI
• M : Source of signals
• R : Principles of excitation and detection
• I : Turning signals into images
• As the human magnet gets stronger, the
signal gets better ?
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Effects of Human Magnet
Signal obviously larger with a stronger magnet
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Generation of MRI
• M : Source of signals
• R : Principles of excitation and detection
• I : Turning signals into images
• What hardware equipments do we need to
achieve these functions ?
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MRI Instrumentation
• Equipments to:
– Turn human body into a magnet
– Set the human magnet into motion
– Receive the signals
– Turn signals into images
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MRI Instrumentation
• Equipments to:
– Turn human body into a magnet :
Strong magnetic field
– Set the human magnet into motion
– Receive the signals
– Turn signals into images
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Typical MRI Systems
General Electric Signa Siemens Magnetom
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MRI Instrumentation
• Equipments to:
– Turn human body into a magnet
– Set human magnet into motion : RF coil
– Receive the signals : RF coil
– Turn signals into images
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Radiofrequency Coils for MRI
Head Coil Surface Coils
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Image Comparison : Different Coils
Body coil Head coil Surface coil
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MRI Instrumentation
• Equipments to:
– Turn human body into a magnet
– Set human magnet into motion
– Receive the signals
– Turn signals into images :
Gradient coils
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Gradient Coil for MRI
The z gradient coil
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Major Hardware Components of MRI
Shim coils Gradient coils
RF coil
Magnet
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Control of the Hardware
• Of course using computers !
• Control the timing of “ON” and “OFF”
for all the coils for scanning
• The software handling the timing
control : Pulse sequence
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MRI Pulse Sequence (Gradient Echo)
x gradient
y gradient
RF coil
z gradient
t
t
t
t
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How to Accelerate Scan ?
• Well, just accelerate the ON and OFF
timing for the coils
• Just change the pulse sequence
• Of course within the hardware limits
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The Gradient Echo Pulse Sequence
x gradient
y gradient
RF coil
z gradient
t
t
t
t
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The Fractional Echo Function
x gradient
y gradient
RF coil
z gradient
t
t
t
t
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The Half Fourier Function
x gradient
y gradient
RF coil
z gradient
t
t
t
t
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Real Examples (Fast MRI)
• Gradient echo
• Fast spin-echo
• Half Fourier
• Echo planar imaging ...
• Oh, come on! Too many …
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Well, so ...
• “As long as I can use the pulse
sequence to control the coils, a
faster or slower scanning can be as
easily adjusted ?”
• But the image appearance … ?
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The Different Contrast for Brain MRI
PDWI T1WI T2WI
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MRI Appearance
• Proton density, T1, T2 ...
• Why different brightness/darkness, if
all based on the same principles of
image formation ?
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What is MRI ?
• M : human body = magnet
• R : magnet motion = induced current
… something must be missing …
• I : spatial encoding and decoding
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What is MRI ?
• M : human body = magnet
• R : magnet motion = induced current
• Change of signal before detection
• I : spatial encoding and decoding
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T2 Decaying of Tissue Magnet
Time
Sig
na
l In
ten
sit
y
Magnet becoming
smaller and smaller !
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Effects of TE on T2 Contrast
TE = 30 TE = 90 TE = 150
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Contrast of MRI
• Signal intensity at detection
directly affects image contrast
• Then can we artificially alter the
signal behavior before detection ?
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Types of MRI Contrast
• Proton density, T1, T2 ...
• Angiography ?
• Molecular diffusion ?
• Sources of useful diagnostic
information !
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How to Control Contrast?
• Pulse sequence !
– playing a major role in MRI
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Gradient Echo
x gradient
y gradient
RF coil
z gradient
t
t
t
t
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Gradient Echo with Inversion Recovery
x gradient
y gradient
RF coil
z gradient
t
t
t
t
1800
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Real Examples (Contrast)
• IR (inversion recovery)
• STIR (short TI inversion recovery)
• FLAIR (fluid-attenuated inversion
recovery)
• MP (magnetization preparation)
• Oh, come on! Too many …
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T1 & T2 Contrast to Depict Tumor
low-grade glioma
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Multiple Sclerosis
Pre-Contrast Post-Contrast
reactivation
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Water and Fat Images
In-phase Water only Fat only
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Selective Suppression of CSF Signals
T1WI T2WI FLAIR
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3D Time-Of-Flight MRA
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Flow Quantification (Reversed in SVC)
SVC flow profile in one cardiac cycle
SVC flow
cardiac phase
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Dynamic Perfusion Imaging
Dynamic scans CBV map
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Perfusion MRI in Glioma
F/U at 7 months
F/U at 1 year
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Diffusion MRI in Ischemic Stroke
Depicting infarct as early as 2 hr after symptom onset
T2 weighted Diffusion weighted
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Hyperacute Ischemic Stroke
5 hours after symptom onset
T2 weighted Diffusion weighted
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Anisotropy of Molecular Diffusion
Fastest diffusion along fiber direction
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Appearance of Diffusion Anisotropy
Neural fiber orientation in human brain
b = 0 slice read phase
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Tracking of Fibers in Myocardium
Fiber direction from DTI
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Diffusion Tensor Fiber Tracking
Tracts in the corpus callosum
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T2* Image in Rat Brain
normal air pure oxygen
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Visual Stimulation Experiments
Increase of signals upon stimulation
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T2* MRI with Visual Stimulation
Kwong et al., PNAS 1992
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Brain Functional MRI (Left Auditory)
gray : anatomy ; color : neural activation
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Chemical Shift MRI (Ischemic Stroke)
T1 weighted local MR spectra
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MR CSI of Glioma
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NAA Map of Glioma
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Choline Map of Glioma
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How about outside brain?
• Certainly a lot !
• Gastrointestinal, genitourinary tract,
musculoskeletal, cardiopulmonary,
fetus, animal, plant, rocks …
• Depending on how you manipulate …
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MRI of Liver Tumor
Pre Gd Arterial phase
VIBE MIP Portal venous phase
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MR Cholangiopancreatography (MRCP)
T2 FSE images MIP projection
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Contrast-Enhanced 3D Body MRI
intestine image joint image
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Morphological & Functional Heart MRI
Morphology
Viability
Coronary arteries
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Dynamic MR Angiography
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Myelination Process in Fetal Brain
2D TrueFISP (1 sec scan)
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in utero Fetal Brain MRI and Histology
22 weeks gestation
Immature cortex
Intermediate zone
Row of migrating neurons
Germinal matrix
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Plant MRI
scallion (蔥) loofah (絲瓜)
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植物 MR 影像
cucumber (黃瓜) balsam pear (苦瓜)
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Double Quantum Filter (?? what ?)
Absolutely non-conventional MRI contrast
Tendon in rat tail (highly structured collagen fibers)
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Hey ! It’s too much !
• Sorry, there will be more in the future
• Morphology + function + metabolism
= integrated diagnosis
• “No ionizing radiation” is not the
most important advantage of MRI
• “Multiple information” is !
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After Fancy MR Course …
• Great ! I got excellent research !
• Beautiful images come out after
hitting the “Return” key … ?
• There is no free lunch in the world
• Otherwise you don’t need to come to my classes at all …
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What is Wrong with My MRI ?
Expected ! The result ?
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What is Wrong with My MRI ?
Expected ! The result ?
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You Said Scan Can Be Accelerated ?
FSE T2WI EPI T2WI
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Not My Business ?
• I’m an M.D. responsible for diagnosis only ?
• I’m a technologist only responsible for
routine operation of the stupid scanner ?
• I’m only a student. I just need a thesis ?
• I’m not an EE major. MRI is just a tool ?
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Very Simple Multi-Echo ?
1st 2nd 3rd 4th
Water in container
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Water Image and Fat Image
In-phase Water only Fat only
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Scanner Has Built-in Fat Saturation …
Is it due to the scanner or the operator ?
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Selective Suppression of CSF Signals
T1WI T2WI FLAIR
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What about FLAIR in Fourth Ventricle ?
Hemorrhage (IVH) ? Just normal ?
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3D Time-Of-Flight MRA
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Is There Stenosis ?
MRA XRA
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The Same MRA Method …
Basically normal ? Blood vessel gone !
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Brain Functional MRI (Left Auditory)
gray : anatomy ; color : neural activation
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Then Which fMRI is Correct ?
Operator A Operator B
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Great Finding ! CSF Can Think !
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Perfusion MRI & MRS in Glioma
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MRS from the Same Scanner
What a great difference from the same data !
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After Your Graduation …
• You still want to work on MRI ?
– You’ll encounter all these difficulties !
• No longer work on MRI ?
– It’s even harder to master this tough
topic within one single semester !
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MRI : Powerful Yet Tough
• Try asking yourself :
– Are those experimental errors
really not my faults ?
– Don’t I need to learn more ?
• I think you need to study harder …
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Look at the Speed of Developments
Brain MRI (1980 JCAT) Common MRI 1991
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MRI in Taiwan (1999) : Sensory fMRI
Left-ear auditory fMRI at 1.5T
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MRI in Taiwan (1999) : Sensory fMRI
Cortical surface Cortical inflation
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MRI in Taiwan Today : Cognitive fMRI
professional amateur
corrected p-value 0.05 corrected p-value 0.05
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MRI in Taiwan Today : Neural tracts
Corpus callosum
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Future of MRI ?? (Example of CSI)
Multi-dimensional spectrum for every voxel ?
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Future of MRI ?? (Example of CSI)
Protein structure and function in vivo ?
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Wake Up, Guys !
• You need to study hard,
don’t you ? • Welcome to this course …
• But don’t expect to master this topic
within one single semester !
• It is never too late to drop this course