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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