MRI Basics Lecture 9 26 slide 0

MRI Basics Lecture 9 26

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Magnetic Resona esonance Imaging: Physical Pr ical PrinciplesLewis Center for NeuroImaging,Physics of MRI, An OverviewNuclear Magnetic Resonance Nuclear spins Spin precession and theFourier Transforms Continuous FourierLarmor equation Static B0 RF excitation RF detectionTransform Discrete Fourier Transform Fourier properties k-space representation in MRISpatial Encoding Slice selective excitation Frequency encoding Phase encoding Image reconstru

Transcript

Magnetic Resona esonance Imaging: Physical Pr ical Principles

Lewis Center for NeuroImaging

,

Physics of MRI, An Overview

Nuclear Magnetic Resonance Nuclear spins Spin precession and the

Fourier Transforms Continuous Fourier

Larmor equation Static B0 RF excitation RF detection

Transform Discrete Fourier Transform Fourier properties k-space representation in MRI

Spatial Encoding

Slice selective excitation Frequency encoding Phase encoding Image reconstruction2

4/4/2010

Physics o MRI sics of

Echo formation Vector summation Phase dispersion Phase refocus

Medical Applications Contrast in MRI Bloch equation

Tissue properties T1 weighted imaging T2 weighted imaging Spin density imaging

2D Pulse Sequences Spin echo Gradient echo Echo-Planar Imaging

Examples 3D Imaging Spectroscopy

3

4/4/2010

Many s ins in aspins in stepRotating frame Lamor precession

x l:

ct r s

ati n

spins not in step

4/4/2010

4

Phase dispersion du to perturbing B ion due fieldsSpin Phase J w KBt KB B = B0 + HB0 + HBcs + HBppsampling Immediately after RF excitation sometime after RF excitation

4/4/2010

5

Refocus spin phase echo formation hase

Echo Time (TE) cho Ti Invert perturbing field: HB B Phase 0 HBt (gradient echo, k-space sampling) ling) Invert spin state: Phase 0 (spin echo)4/4/2010

time

-HB J-HB(t-TE/2)

0

HBt

J -J -J+HB(t-TE/2)

0

6

Spin Echo pin EcSpins dephase with time Rephase spins with a 180 pulse Echo time, TE Repeat time, TR (Running analogy)

1. E uili ium 1. Equilibrium

2. 90 Pulse ulse t=0 t 0 3. Spin Dephasing ephasing

. Spin echo t=TE t TE4/4/2010

4. 180 Pulse 180 ulse t TE/2 7 t=TE/2

Frequency encodin - 1D imaging ncodingSpatial-varying resonance frequenc during RF detection quency

B = B0 + Gxx S(t) ~ eiK&t S(t) ~ m(x)eiKGxxtdx

m(x)x S(t) = m(x)eikxxdx = S(kx), (k4/4/2010

kx = KGxtm(x) = FT{S(kx)}8

Slice sele e selectionSpatial-varying resonance frequenc during RF excitation equency

[B1 freq band

[ = [0 + KGzz

zExcited location Slice profile4/4/2010

m+ = mx+imy ~ K b1(t) -iKGzztdt = B1(KGzz) b (t)e

Gradient Echo FT imaging35000

x Gradient Amplitude (arb)

0

kyReadout

-35000

35000

y Gradient Amplitude (arb)

0

-35000

35000

z Gradient Amplitude (arb)

kx

0

-35000

35000

Amplitude (arb)

RF

0

K k (t ) ! G (t )dt 2T10000

-35000

0

2000

4000

Time (us)

6000

8000

4/4/2010

Repeat with different phase-encoding Repe amplitudes to fill k-space amp 10

Pulse sequen design equence35000

x Gradient

prewinder spoiler

Amplitude (arb)

0

35000 -35000

y Gradient Amplitude (arb)

rephasor rewinder spoiler

0

35000 -35000

z Gradient

Amplitude (arb)

0

Amplitude (arb)

35000 -35000

RF

0

-35000

4/4/2010

0

2000

4000

Time (us)

6000

8000

0000 0000

11

X

EPI (echo plana imaging) planarky

Y

Z

kx

RF time Quick, but very susceptible to artifacts, particularly B0 field inhomogeneity. tifacts, Can acquire a whole image with one RF pulse single shot EPI4/4/2010 12

Spin Echo FT imaging hox ( lit-

(

lit

-

)

(

lit

-

(

)

lit

-

4/4/2010

$$$%'

$$$$'

i

(

)

)

#"

i i

)

!

$$$%& 10 ) ( $$$$&

$ $$%

$

t

i

t

kyReadout

t

K k (t ) ! G (t )dt 2T

kx

Repeat with different phase-encoding R amplitudes to fill k-space am 13

Spin Rela ation nSpins do not continue to precess forever ue p Longitudinal magnetizatio returns to equilibrium tization due to spin-lattice interact nteractions T1 decay Transverse magnetization is reduced due to both zation spin-lattice energy loss and local, random, spin an dephasing T2 decay y Additional dephasing is introduced by magnetic g in field inhomogeneities within a voxel T2' decay. es with This can be reversible, un le, unlike T2 decay

4/4/2010 14

Bloch Eq ch Equation The equation of f

MR physics

T T T 1 1 T dM ! KM v B M 0 M z z M B dt T1 T2 Summarizes

the interaction of a nuclear e inter spin with the externa magnetic field B xternal and its local environm (relaxation vironment effects)15

4/4/2010

Contrast - T1 Decay t TLongitudinal relaxation due to spin-lattice interaction Mz grows back towards its equilibrium value, M0

1.0 180 0.5o

e

Mz/M0

0.0 Inversion ecovery I on overy

M z t ! M 0 1 e

t T1

-0.5

For short TR, equilibrium moment is reduced

-1.0 0 1 2 3 4 5

t/T1

4/4/2010

16

Contrast - T Decay stTransverse relaxation due ue to spin dephasing T2 irreversible dephasing ng T2/ reversible dephasing g Combined effect

1.0 0.

M t /M 0

0.6 0. 0.

T

* 2

!

T2

T

/ 2

0.0 0 1

M B (t ) ! M B ( )e4/4/2010

t / T2*

t/T

*

3

5

17

Free Induction Decay duction Gradient ech (GRE) nt echoExcite spins, then measure decay Problems:

Rapid signal decay Acquisition must be

MR M signal

e-t/T

*

disabled during RF Dont get central echo data4/4/2010

time0

90 9 RF18

Spin echo (SE) n e-t/T e-t/T*

MR signal

time

90 RF4/4/2010

0

1 0 RF

0

19

MR Parameters: TE and TR ters:Echo time, TE is the time from the RF excitation e to the center of the echo being received. Shorter b echo times allow less T2 s ss signal decay Repetition time, TR is the time between one acquisition and the next. Short TR values do not allow the spins to recover their longitudinal cover magnetization, so the net magnetization available e m is reduced, depending on the value of T1 ng t Short TE and long TR give strong signals R giv

4/4/2010 20

Contrast, Imaging Parameters agingS(TR , TE ) w V(1 e RR TR / T1

)e

TE / T2* / T2

(SE)

R or V(1 e TR / T1 ) TE

GR )

TE TR Image Weighting Short Long Proton Short Short T1 * Long Long T2, T24/4/2010 21

Properties of Bo Tissues s BodyTissue Grey Matter (GM) White Matter (WM) Muscle Cerebrospinal Fluid (CSF) CSF) Fat Blood T1 (ms) T2 (ms) 950 600 900 4500 250 1200 100 80 50 2200 60 100-200

MRI has high contrast for different tissue types! trast fo4/4/2010 22

MRI of the Brain - Sagittal e Brai

T1 Contrast TE = 14 ms TR = 400 ms4/4/2010

T2 Contra Contrast TE = 100 m ms TR = 1500 ms

Proton Density TE = 14 ms TR = 1500 ms23

MRI of the Brain - A ial he Bra

T1 Contrast TE = 14 ms TR = 400 ms4/4/2010

T2 Contra Contrast TE = 100 m ms TR = 1500 ms

Proton Density TE = 14 ms TR = 1500 ms24

Brain - Sagittal M gittal Multislice T1

4/4/2010

25

Brain - A ial M Multislice T1

4/4/2010

26

Brain Tu ain TumorT1 T2

4/4/2010

Post-Gd T1

27

3D Ima D ImagingInstead of exciting a thin slice, excite a thick slab s and phase encode along both ky and kz long b Greater signal because more spins contribute to use mo each acquisition Easier to excite a uniform thick slab than very iform, thin slices No gaps between slices ices Motion during acquisition can be a problem isition

4/4/2010 28

2D Sequence (G ce (Gradient Echo)acq Gx Gy Gz b1 TE TR4/4/2010

ky

kx

Scan time = NyTR29

3D Sequence (G ce (Gradient Echo)acq Gx Gy Gz b1

kz

kx

ky

Scan time = NyNzTR4/4/2010 30

3D Imaging - e ample gingContrast-enhanced MRA of the carotid arteries. Acquisition time ~25s. 160x128x32 acquisition (kxkykz). 3D volume may be reformatted in post-processing. Volume-ofinterest rendering allows a feature to be isolated. More on contrast-enhanced MRA later

4/4/2010

31

Spectrosc ctroscopyPrecession frequency depend on the chemical epends environment (HBcs) e.g. Hydrogen in water and hydrogen . Hydr in fat have a (f = fwater ffat = 220 Hz Single voxel spectroscopy excites a small (~cm3) volume py ex and measures signal as f(t). Different frequencies D (chemicals) can be separated using Fourier transforms arated Concentrations of chemicals other than water and fat tend icals o to be very low, so signal stren al strength is a problem Creatine, lactate and NAA are useful indicators of tumor AA types

4/4/2010 32

Spectroscopy - E ample scopyIntensity

4/4/2010

Frequency

33

Future lectures ure leMagnetization preparation Perfusion and diffusion tion (phase and magnitude, Functional imaging pelc) (fMRI) Fast imaging (fast Cardiac imaging ) sequences, epi, spiral) (coronary MRA) Motion (artifacts, compensation, correction, on, navigator) MR angiography (TOF, , PC, CE)

4/4/2010 34

3rd dimension ph n phase encodingBefore frequency encoding and after slice selection, oding apply y-gradient pulse that makes spin phase e varying linearly in y. Repeat RF excitation and detection with different d gradient area. S(ky, t) = m+(x,y,z)d ikyyeiKGxxtdxdy x,y,z)dz)e4/4/2010 35

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