Basics of MRI - uni-heidelberg.de...Basics of MRI Andreas Lemke and Prof. Dr. Lothar Schad MR Lab...

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RUPRECHT-KARLS-UNIVERSITY HEIDELBERG

Computer Assisted Clinical MedicineAndreas Lemke

12/22/2010 |

Basics of MRI

Andreas Lemke and Prof. Dr. Lothar Schad

MR Lab Rotation, Mannheim

Chair in Computer Assisted Clinical MedicineFaculty of Medicine Mannheim University HeidelbergTheodor-Kutzer-Ufer 1-3D-68167 Mannheim, GermanyLothar.Schad@MedMa.Uni-Heidelberg.dewww.ma.uni-heidelberg.de/inst/cbtm/ckm/



12/22/2010 | Page 2www.ma.uni-heidelberg.de/inst/cbtm/ckm/

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12/22/2010 | Page 3Literature

• Reiser, Semmler, Hricak: “Magnetic Resonance Tomography” Chapter 2, 2008

• Vlaardingerbroek and den Boer:“Magnetic Resonance ImagingTheory and Practice”, 2003

• Haacke:’’ Magnetic Resonance Imaging: Physical Principles and Sequence Design’’



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Brief Review

Brief Review

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12/22/2010 | Page 5

B0

M0m = -1/2

m = +1/2

B = 0 B = B0

splitting of energy levels(Zeeman effect)

Curie’s law: M0 = ρ • I(I+1)•γ2•h2•B03kT

Zeeman Effect



12/22/2010 | Page 6Quantum Mechanic ↔ Classical Mechanic

quantum mechanic classical mechanic

Schrödinger equation:

BMdt

tMd rrr

×⋅= γ)(

notice: M is a macroscopic quantity, all nutation angles are allowed → CMμ is a microscopic quantity, only +1/2 and -1/2 are allowed → QM

Bloch equation

Slichter. “Principles of Magnetic Resonance” 1978

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12/22/2010 | Page 7

90°-pulse (π/2-pulse) in the laboratory and rotating coordinate systemN-1/2 = N+1/2↑ to ↓ : 3x1018 spins per 1 mm3 at 1.5 T

source: Lissner and Seiderer. “Klinische Kernspintomographie” 1987

90°- and 180°- Pulses

180°-pulse (π-pulse) in the laboratory and rotating coordinate systemN-1/2 > N+1/2 = -M0↑ to ↓ : 6x1018 spins per 1 mm3 at 1.5 T

laboratory system rotating system



12/22/2010 | Page 8Movie: Spin Excitation

© Plewes DB, Plewes B, Kucharczyk W. The Animated Physics of MRI, University Toronto, Canada

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12/22/2010 | Page 9

M

magneticfield

excitation

MRF transmit coil

with ν = ω0 RF receive coil

B0 B0

detection

RF receive coil

NMR Excitation and Signal Detection



12/22/2010 | Page 10Faraday Induction

N S

x

y

z

My

bicycledynamo

loop withrotating magnet

rotating magnetic moments

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12/22/2010 | Page 11Movie: Free Induction Decay FID

timesign

al in

tens

ity

Mxy

free induction decay: FID




12/22/2010 | Page 12Radio Frequency Coils: Coil Arrays II

102 seamlessly integrated coil elements at 32 receiving channels

matrix coils:

head

neck

stem

leg

courtesy: Siemens AG, Erlangen

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12/22/2010 | Page 13

transmitter

receiver

gradient

shimim

age

proc

esso

r350 MHz

control panel

computer

350 MHz

radio- gradients Gxyz static field B0frequency RF shim coils

MRI Components: Physical Parameters

static field B0 M0

radiofreq. RF signal

gradients Gxyz image

technicalcomponent

physicalparameter



12/22/2010 | Page 14

Relaxation

Relaxation

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12/22/2010 | Page 15Magnetization: Mz and Mxy

longitudinal magnetization: Mz

transversal magnetization: Mxy

transversal magnetization: Mxy- phase synchronization after a 90°-pulse- the magnetic moments μ of the probe startto precede around B1 leading to a synchronizationof spin packages → Mxy

- after 90°-pulse Mxy = M0



12/22/2010 | Page 16Movie: Mz and Mxy

source: Schlegel and Mahr. “3D Conformal Radiation Therapy: A Multimedia Introduction to Methods and Techniques" 2007

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12/22/2010 | Page 17Longitudinal Relaxation Time: T1

after 90°-pulse:- N-1/2 = N+1/2 and Mz = 0, Mxy = M0

after RF switched off:- magnetization turns back to thermal equilibrium- Mz = M0, Mxy = 0

→ T1 relaxationlongitudinal relaxation time T1spin-lattice-relaxation time T1

thermal equilibrium excited state



12/22/2010 | Page 18Physical Model of T1 Relaxation

- in a real spin system (tissue) every nuclei is surrounded by intra- and intermolecularmagnetic moments

- thermal motion (rotation, translation, oscillation) leads to an additional fluctuatingmagnetic field Bloc(t) with typical spectral distribution J(ω)

- transversal components of J(ω) at ω0 allow energy transfer hω0 from the spin system to the “lattice”

→ T1 relaxation

source: Liang and Lauterbur. “Principles of Magnetic Resonance Imaging” 2000J(ω) ⊥ B0

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12/22/2010 | Page 19Phenomological Description of T1 || B0

the longitudinal magnetization Mz relaxes exponential to the equilibrium state Mz = M0 with a typical time constant T1

dMz/dt = (γ x B)z + (M0 – Mz)/T1 : Bloch equation with T1 with Mz = 0 at t = 0:

Mz(t) = M0 (1 – exp(-t/T1)) → solution of Bloch equation



12/22/2010 | Page 20Transversal Relaxation Time: T2

after 90°-pulse:- N-1/2 = N+1/2 and Mz = 0, Mxy = M0

after RF switched off:- magnetization Mxy starts to rotate in thex,y-plane at Larmor frequency

- all longitudinal components J(ω) of thefluctuating magnetic field Bloc(t) result in adephasing of Mxy → spin-spin interaction

- mainly static frequency components J(ω) of thefluctuating magnetic field Bloc(t) at ω = 0 are contributing

- no energy transfer in the spin system (entropy ↑)

→ T2 relaxation (also called spin-spin-relaxation)

- although technical in homogeneities of B0 causedephasing of Mxy → T2* (effective relaxation)

J(ω) || B0

J(ω=0)

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12/22/2010 | Page 21Movie: Spin Dephasing




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0,88 ± 0,0372 ± 41,08 ± 0,0469 ± 3white matter

1,03 ± 0,3440 ± 61,47 ± 0,0647 ± 11heart

1,01 ± 0,0244± 61,41 ± 0,0350 ± 4skeletal muscle

1,13 ± 0,0595 ± 81,82 ± 0,1199 ± 7 grey matter

0,58 ± 0,0346 ± 60,81 ± 6442 ± 3liver

T1 [s] at 3 T

T2 [ms] at 3 T

T1 [s] at 3 T

T2 [ms] at 3 T

tissue

T1 and T2 Relaxation Times in-vivo

- T1 increases with increasing B0- T2 decreases with increasing B0

Bottomley et al. Med Phys 1984

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12/22/2010 | Page 23

Saturation-Recovery Sequence

Inversion-Recovery Sequence

Spin-Echo Sequence

Standard Techniques for T1 and T2



12/22/2010 | Page 24

• saturation-recovery sequence

• 90°-pulse moves the longitudinal magnetization M0 to the x-, y – plane → FID

• transversal magnetization Mxy decays with T2*

• longitudinal magnetization starts to recover to thermal equilibrium → Mz↑ with T1

• after TR actual (reduced) magnetization Mz is moved to the x-, y – plane → FID

• repeat measurement with different TR→ T1 determination by

Saturation-Recovery Sequence

S ~ ρ [1 - exp(-TR / T1)] with TR >> T2*

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12/22/2010 | Page 25

• inversion-recovery sequence

• 180°-pulse invert the longitudinal magnetization M0 to –M0 at the z-axes

• longitudinal magnetization starts to recover to thermal equilibrium → Mz↑ with T1

• inversion time TI

• after TI 90°-pulse moves the actual (reduced) longitudinal magnetization Mz to the x-, y –plane → FID

• transversal magnetization Mxy decays with T2*

• repeat measurement with different TI→ T1 determination by

Inversion-Recovery Sequence

S ~ ρ [1 – 2 exp(-TI / T1)] with TR > 5 ·T1



12/22/2010 | Page 26

TI0

T1 Measurement: Inversion Recoveryinversion recovery (Mz(0) = -M0):

Mz(t) = M0 (1 – 2 exp(-TI/T1))

with Mz = 0 at TI = TI0:0.5 = exp(-TI0/T1)

→ T1 = -TI0 / ln(0.5) = TI0 / 0.7

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12/22/2010 | Page 27

a) RF impulse schema

b) timing of longitudinal magnetization Mz

c) induced measured signal: spin-echo SE

source: Schlegel and Bille. “Medizinische Physik Bd. 2” 2002

rephasing partof signal

dephasing partof signal

Spin-Echo Schema



12/22/2010 | Page 28Movie: Spin-Echo II


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12/22/2010 | Page 29T2 Measurement: Spin Echo

spin-echo (Mxy(0) = M0):

Mxy(t) = M0 exp(-t/T2)

T2 Measurement: Spin-Echo



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Imaging

Imaging

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12/22/2010 | Page 31

interaction of spins with static magnetic

field B0 → M0

ω = γ.B0

interaction of protons with resonant magnetic radiofrequency field

B1 → FID signal

linear magnetic field gradient

across the objectG → spatial encoding

ω(x) = γ.B(x)ΔE = hω

B0 B(x) = B0 + Gx·x

Magnetic Resonance Imaging: Principle

x



12/22/2010 | Page 32

typical value for G: 1 - 25 (40) mT/m

example:x = 30 cm, B0 = 1 T, G = 10 mT/m

B = 0.9985 T .... 1.0015 T

Gradient Coil: Principle I

coil for static B0 field

z-gradient

x-gradient

y-gradient

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12/22/2010 | Page 33

radiofrequency:ω(z) = γ (B0 + Gzz)

gradient Gz

magnetic field gradient, i.e. Gz

z

G

Gradient Field: Slice Selection



12/22/2010 | Page 34

zz Gf

Gd

⋅Δ

=⋅

Δ=

γπ

γω 2

)zGB()z( z ⋅+⋅= 0γω

1. slice thickness d = z2 - z1 can be varied by RF bandwidth (frequency width) or gradient strength Gz.

2. slice position can be changed by shifting the frequency spectra with constant RF bandwidth

ω ω

I(ω)

ω (z2)

ω (z1)

ω0

ω = γ (B0+Gzz)

zz1z2

d

frequency spectraof RF pulse

Slice Selective Excitation

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12/22/2010 | Page 35

“dephasing” of transversal magnetization after unipolar

gradient pulse

z-gradient with compensation and RF sinc-pulse for creating a homogeneous

transversal magnetizationsource: Dössel. “Bildgebende Verfahren in der Medizin” 2000

Gradient Compensation

excitedslice



12/22/2010 | Page 36

Gz

t

Gx

t

t

signalacquisition - superposition of a gradient field

Bx= Gx·x during the acquisition phase results in:

ω(x) = γ (B0+ Gx·x)

where the Lamor frequency islinked with the spatialinformation x

- spatial information is encodedin the precision frequency of thetransversal magnetization

Frequency Encoding: Gradient Schema

RF

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12/22/2010 | Page 37

- FID-signal S(t) is digitalized by using an “analog to digital converter” ADC and a discrete timing interval Δt in a total acquisition window taq

- for the Fourier-analysis of the FID-signal there are in total N = taq/Δt measured data points:

S(Δt), S(2Δt), S(3Δt), … , S(NΔt)

- spatial resolution in x-direction Δx is given by the sampling theorem:

Δx = X / N = 2π / γ Gx N Δt

with X: the maximum object diameter (FOV), N: number of sampling points,Gx: gradient strength, Δt: sampling interval (1/Δt = bandwidth BW in Hz/pixel)

Frequency Encoding: Spatial Resolution



12/22/2010 | Page 38

slice selectiongradient

phase encodinggradient

Gz

Gy

yTy

- phase encoding is performed before signalacquisition

- gradient field is switched on for a constant time Ty

-gradient strength is increased stepwise by ΔGy after every sequence passage

Phase Encoding: Gradient Schema

- phase encoding gradient includes aspatial dependency of the spin phase according to:

φp = – γ · Gy · y · ty

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12/22/2010 | Page 39k-Raum

the k-space construction is a relation between spatial encoding (phase and frequency encoding)

and the Fourier transformation

frequency encoded signal:

∫∞

∞−

→⋅⋅⋅⋅−

→

⋅⋅=→→

xdextS txGi γρ )()(

tGk ⋅⋅⋅

=→→

πγ

2

∫∞

∞−

→⋅⋅⋅⋅−

→→

⋅⋅=→→

xdexkS xki πρ 2)()(

K-Space: Definition



12/22/2010 | Page 40

constant gradient results in a straight trajectory

change in polarity inverts the trajectory

refocusing pulse (180°)inverts phase

(mirroring at origin)

read

phase

read

phase RF180°-pulse

Surfing through k-Space

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12/22/2010 | Page 41Movie: 2D K-Space X and Y




12/22/2010 | Page 42Movie: 2D K-Space X and Y


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12/22/2010 | Page 43

k-space

image

Fouriertransformation

kx

ky

y

x

hologram

K-Space: Summary

density



12/22/2010 | Page 44

xx W

k 1≤Δ

yy W

k 1≤Δ

samplingtheorem:

⎩⎨⎧

⋅Δ⋅=ΔΔ⋅⋅=Δ

peyy

xx

TGktGk

γγ

source: Liang and Lauterbur. “Principles of Magnetic Resonance Imaging” 2000

K-Space: Sampling Requirements I

Gx : frequency encoding gradientΔt : frequency encoding intervalΔGy : phase encoding incrementTpe : phase encoding interval

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12/22/2010 | Page 45

yNTWTG

xNGWGt

ypeypey

xxxx

Δ⋅⋅⋅⋅

=⋅⋅

⋅≤Δ

Δ⋅⋅⋅⋅

=⋅⋅

⋅≤Δ

γπ

γπ

γπ

γπ

22

22

K-Space: Sampling Requirements II

Nyquist - interval



12/22/2010 | Page 46

- during a MR measurement the signal S(t) is discretely sampled (frequency encoding interval Δt) in a total acquisition time taq (typical 5 - 30 ms)

→ number of measuring points N = taq / ΔtS(Δt), S(2Δt), ... S(NΔt)

⇒ spatial resolution Δx is limited by:

tNGNW

xxxx

x

Δ⋅⋅⋅==Δ

γπ2

Δx = 1.593 mm

Wx = N · Δx = 50 cm⇒

example: Nx = 256Δt = 30 µs Gx = 1,566 mT/m

Example

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12/22/2010 | Page 47

TR

TE

90

-

60ms

40

-

10ms

T1-weighted

T2-weighted

proton-weighted

300 - 800 ms 1500 - 3000 ms

no contrastSNR low

Spin-Echo Contrast: TR, TE



12/22/2010 | Page 48

kx

k-space: after 1. TR k-space: after 2. TR k-space: after 256. TRky

Spin-Echo Sequence

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12/22/2010 | Page 49

PwTR = 2775 ms

TE = 17 ms

T2wTR = 2775 msTE = 102 ms

T1wTR = 575 msTE = 14 ms

Spin-Echo Images

32143421factorT2

2TE/T

factorT1

1TR/TSE ]1[S

−

−

−

− ⋅−⋅= eeρ

- SE gold standard technique for T1 and T2 morphology



12/22/2010 | Page 50Spin-Echo: Multi-Slice

measurement #1

measurement #2

- interleaved SE measurement

- multi-slice SETR = 600 ms, TE = 10 ms→ about 60 slices can be measured

simultaneously !

1. slice

2. slice

3. slice

4. slice

5. slice

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12/22/2010 | Page 51Inversion Recovery Sequence

source: Reiser and Semmler. “Magnetresonanztomographie” 2002



12/22/2010 | Page 52Inversion Recovery Images

321444 3444 21factorT2

TE/T2

factorT1

TR/T1TI/T1IR ]21[S

−

−

−

−− ⋅+−⋅= eeeρ

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12/22/2010 | Page 53

α

- example: α = 20° Mz - reduction by 6%Mxy- value 34% of Mz!!

M

x´,y´

z

Mz

Mxy

Gradient-Echo Sequence

spoiler



12/22/2010 | Page 54

SE GE

α = 90° / 180°TE = 20 ms

TR = 600 ms

Taq: minutes

α = 25°TE = 7 ms

TR = 20 ms

Taq: seconds !!

Gradient-Echo: Measuring Time

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12/22/2010 | Page 55

Mansfield and Pykett. JMR 1978

Fast Gradient-Echo Imaging: EPI

- “echo-planar imaging” EPI- multi-gradient imaging technique- single-shot technique with strong requirementsto the gradient power

- strong T2* dependency → susceptibility artifacts !- fastest imaging technique

Sir Mansfield2003 Nobel prize in medicine



12/22/2010 | Page 56Fast Gradient-Echo Imaging: EPI Technique

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12/22/2010 | Page 57EPI Problem: Static Inhomogeneities

gradient echo EPI

2. echo shift of oddagainst even echoes

1. distortions in phase direction



12/22/2010 | Page 58

classical EPI

Fast Gradient-Echo Imaging: EPI Methods

spin-echo EPISE-EPI

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12/22/2010 | Page 59Exercise: Measurement of the ADC

)exp(0 bDSS −=

1. Calculate the apparent diffusion coefficient (ADC) from the DWI images in White Matter (WM), Grey Matter (GM), and Cerebrospinal Fluid (CF)

Experiment #1: 2 images were measured (DWI_b0.IMA, DWI_b1000.IMA, data at: http://www.ma.uni-heidelberg.de/inst/cbtm/ckm/lehre/„Medical Physics: Lab Rotation MR-Radiology“) with b = 0 and 1000 s/mm2. Plot signal intensity (= pixel mean value of ROI) semi-logarithm as a function of b-value and calculate ADC

- group 1: CF ( )

- group 2: WM ( )

- group 3: GM ( )



12/22/2010 | Page 60Exercise: Measurement of the FA

23

22

21

321 )²()²()²(23

λλλ

λλλλλλ

++

−+−+−=FA

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

zzyzxz

yzyyxy

xzxyxx

DDDDDDDDD

3λ1λ

2λ

2. Calculate the fractional anisotropy (FA) from the DTI images in White Matter (WM), Grey Matter (GM), and Cerebrospinal Fluid (CF)

Experiment #2: 6 images were measured (DTI_b1000_1.IMA … DTI_b1000_6.IMA, data at: http://www.ma.uni-heidelberg.de/inst/cbtm/ckm/lehre/„Medical Physics: Lab Rotation MR-Radiology“) with b = 1000 s/mm2 and 6 different directions. Evaluate signal intensity (= pixel mean value of ROI) for the different directions andcalculate FA

- group 2: CF ( )

- group 3: WM ( )

- group 1: GM ( )

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12/22/2010 | Page 61

Diffusion

Diffusion



12/22/2010 | Page 62Free Diffusion

Dtx 6² =• mean diffusion length

(D = diffusion coefficient, t = time)

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12/22/2010 | Page 63Principle of DWI I

without diffusion weighting:

with diffusion weighting:



12/22/2010 | Page 64Principle of DWI II

constant phase in the rotating reference frame

position of the spin in the rotating reference frame

after 90° pulse

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12/22/2010 | Page 65Principle of DWI III

1. gradient relative phase shift in the rotating frame

without diffusion:



12/22/2010 | Page 66Principle of DWI IV

without diffusion:

2. gradient rephasing of the phase gradient echo

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12/22/2010 | Page 67Principle of DWI V

with diffusion:

1. gradient relative phase shift in the rotating frame



12/22/2010 | Page 68Principle of DWI VI

with diffusion:

diffusion during the gradients spins on a different position

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12/22/2010 | Page 69Principle of DWI VII

2. gradient spins not completely rephased signal reduction

with diffusion:



12/22/2010 | Page 70Dependence of the Signal Decay

)exp(0 bDSS −=( )3/222 δδγ −Δ= Gbstrength of the diffusion weighting:

signal decay:

• b-value can be calculated from the sequence• two DWIs are necessary to calculate D

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12/22/2010 | Page 71Diffusion Measurement: Calculation of D

b-values = 0.5 – 1100 s/mm2

white matterD ~ 0.6 · 10-3 mm2/s

waterD ~ 2.4 · 10-3 mm2/s

Measurement of D in biological tissue is called ADC0 200 400 600 800 1000

b-value [s/mm2]

ln(s

igna

l) [a

.u.]

100

1

000

S = S0 · e-bD



12/22/2010 | Page 72

Axon

free diffusion along the fiber

restricted orthogonal to the fiber

0,5 μm

1 μm

Bealiue, NMR Biomed., 2002

Anisotropic Diffusion

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12/22/2010 | Page 73Diffusion Tensor Imaging (DTI)

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

DD

D

000000

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

zzyzxz

yzyyxy

xzxyxx

DDDDDDDDD

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

z

y

x

DD

D

000000

1λ

2λ3λ

1λ

diffusion ellipsoid

diffusion tensor

water molecule

3λ

1λ

2λ



12/22/2010 | Page 74

derived quantities

DTI: Quantification

apparent diffusion coefficient

( )3

321 λλλ ++=ADC

fractional anisotropy (FA)

23

22

21

321 )²()²()²(23

λλλ

λλλλλλ

++

−+−+−=FA

0 < FA < 1

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12/22/2010 | Page 75Fractional Anisotropy

FA-map FA-colormap

white matter: FA = 0.4 – 0.8

grey matter:FA = 0.05 – 0.2



12/22/2010 | Page 76

post mortem atlasin vivo DTI

- fibertracking method:using main eigenvector of diffusion tensor the main direction of fibers can be tracked pixel by pixel

3D Fibertracking

Stieltjes et al. Neuroimage 2001

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12/22/2010 | Page 77

Safety Aspects

Safety Aspects



| Page 78MRI System with Superconducting Magnet

Siemens MagnetomSymphony

• B0 = 1.5 Tesla• Quantum-gradient:

30 mT/m• slew rate: 125 T/m/s• removable patient couch• short magnet: 1.6 m• relatively open bore• 8 receiving channels

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| Page 79Warnings

warnings:

• strong magnetic field

• radiofrequencyinterdictions:

• cardiac pacemaker

• open fire

• metallic implants

• watch, camera

• magnetic fire extinguisher

• metal: scissor, key

• magnetic disc



| Page 80Static Magnetic Field: B0 I

control area: 0.5 mT - line

source: Reiser and Semmler. “Magnetresonanztomographie” 2002

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| Page 81

B0

• missile effect• strong forces to ferromagnetic

materials

• forces and torques to implants• stent• gunshot wound, rest of projectile

• failure of electromagnetic devices• cardiac pacemaker• computer• controlling system

• physiologic effects• vertigo when entering the magnet

(> 3 Tesla)

B0 - Danger



| Page 82

courtesy: Gross. Siemens Erlangen

B0 – Example II

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| Page 83B0: MR – Compatible Fire Extinguisher



| Page 84B0: Emergency Switch-Off Magnet

when ?• only in case of emergency

• if people have to be saved• and if saving is not possible due

to magnetic field forces

how ?• inducing magnet quench

• press “magnet stop” switch• all persons should be saved and

leave the magnet room quickly• acute danger of suffocation• protect magnet room

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| Page 85B0: Danger from Cold Gases and Fluids

magnet as pressure tank• quench

• evaporation of fluid helium• high pressure• quench pipeline to outdoor• cold gases can come into the

examination room• „burning“ at cold surfaces• displacement of oxygen• danger of suffocation



| Page 86

magnet stop• interrupts super-conduction,

magnet coil gets normal conducting

• current in magnet coil comes to rest

• coil wires get hot and helium evaporates (quench)

• magnetic field is switched off

emergency stop• all electric systems of the MR

system are switched off• magnetic field is still on• cooling system is off• magnet stop is still possible since

battery-operated

B0: Magnet Stop and Emergency Stop

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| Page 87B1: Gradient Field G(t)

G(t)• temporal variation of magnetic field

(dB/dt)• induction of eddy currents in the

object / body• eddy currents run through nerves• peripheral nerve stimulation• stimulation of myocardium ?• interaction with implants

• extreme noise• ear protection



| Page 88RF: Radiofrequency

radiofrequency field• is directly absorbed • critical value:

specific absorption rate (SAR)• can be resonant induced in

conductive structures• creates offset currents in tissue• massive heating• flashover• destroys not connected coils

Wagle and Smith. AJR 2000

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| Page 89RF: SAR Critical Values

→ patient weight and age are relevant for safety !

body part normal

temperature increase body stem

1. level 2. level

whole bodypart bodyheadlocal (head, body)local (extremities)

scales with the ratio of “exposed patient mass / total patient mass”



| Page 90RF: Flashover

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| Page 91B0 and RF: Implants

contra indication• cardiac pacemaker• neuro stimulators

(deep brain stimulators)• ferromagnetic implants• gun projectiles• paramagnetic stents up to 2 weeks

after implantation

information• http://www.mrisafety.com• Shellock FG.

Reference Manual for Magnetic Resonance Safety, Implants, and Devices: 2006 Edition.



| Page 92MR Safety Labeling

MR safe• an item which poses no known hazards in all MR environments

MR conditional• an item which has been demonstrated to pose no known hazards

in a specified MR environment with specified conditions of use. Field conditions that define the specified MR environment include

• field strength, • spatial gradient, • dB/dt (time rate of change of the magnetic field), • radio frequency (RF) fields, and • specific absorption rate (SAR).

• additional conditions, including specific configurations of the item, may be required

MR unsafe• an item which is known to pose hazards in all MR environments

ASTM International. Standard practice for marking medical devices and other items for safety in the magnetic resonance environment. F 2503-05

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| Page 93Light Vizier

patient localizing and referencing• ask patient for closing eyes before

laser switch on



| Page 94Danger of Crushing

couch movement• watch position of arms and fingers• save connections, ECG-wires, and

alarm ball• push “couch stop” – patient couch can

be moved manually

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Seite 48



| Page 95MR Safety Aspects: Summary

static magnetic field• missile effect• influencing equipments• physiological effects

gradients• peripheral nerve stimulation• noise

radiofrequency• heating• coils connected ?

cold gases / fluids• suffocation• “burning”

emergency procedures• magnet stop• emergency stop• couch stop

patient information• patient information sheet• alarm ball

control area• watch and take care about• unknown people (i.e. service and cleaning

people, anesthesia doctors and staff, ...)• unknown equipment and systems before

bringing into the magnet room

Basics of MRI - uni-heidelberg.de...Basics of MRI Andreas Lemke and Prof. Dr. Lothar Schad MR Lab...

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