Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b)...

63
Quiz spin warp or FT MR scan, aliasing should only occur the slice select direction the readout direction the phase encoding direction adjacent phase encodes, the larger phase encode has ional phase wrap across the FOV in the y direction. k-space samples in the readout direction, there is a 2 phase wrap across the FOV in the x direction.

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Transcript of Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b)...

Page 1: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

QuizIn a 2D spin warp or FT MR scan, aliasing should only occur

a) in the slice select direction

b) in the readout direction

c) in the phase encoding direction

Between adjacent phase encodes, the larger phase encode has

an additional phase wrap across the FOV in the y direction.

Between k-space samples in the readout direction, there is a

relative 2 phase wrap across the FOV in the x direction.

Page 2: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Imaging Considerations

We have touched earlier on some the effects that make the imaged slice magnetization look complex. Let’s quantify some of them.

Main field inhomogeneity

Bo inhomogeneity ( also know as shim)

a) magnet

1.5 T shim specifications to 1/3 ppm (parts per million)

over a 20 cm diameter sphere

~ 20 Hz at 1.5 T

Is this significant?

Page 3: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Patient-Induced Inhomogeneity

Main field inhomogeneity

Tissue’s bulk magnetic susceptibility, , can vary.

varies by 10-6 to 10-5

1 ppm to 12 ppm

Worst areas: tissue-air interface

lung / abdomen

sinus

intestines

Chemical Shift:

Electronic shielding reduces the magnetic field seen by the fat molecules.

?f

fatfor ppm 5.3 )1(B

oshielded B

Page 4: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Patient-Induced InhomogeneityT2*

z component of magnetic field

is the field inhomogeneity term.

)(EBo r

)(E r

dVeermts rTttr

volume

)(/)(iω 2E )()(

- unintended

- due to local

neighborhood

inhomogeneity

- T2 effect

T2*

Varying magnetic field in immediate neighborhood (within a voxel) causes in-plane spins to dephase earlier than intended T2 decay. The faster decay has a decay constant referred to as T2*

Page 5: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

'22

*2

111

TTT

T2* Equation

• T2* is actual decay in gradient echo

experiment.• T2 is actual spin-spin decay of material. We

will see how to easily measure this today.• Last term is decay due to local magnetic

environment

Page 6: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Causes of Phase ErrorMain field inhomogeneity,

patient-induced inhomogeneity,

gradient non-linearity,

and blood flow

all cause phase errors (or frequency errors, depending on your point of view). That is, spins will resonate faster or slower than our basic formula predicts.

Chemical shift is another type of phase error.

Let’s study the effects of these phenomena on the two major classes of MR imaging sequences.

)B(γω o rG

Page 7: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Gradient EchoThis is what we have studied to date:

RF t

Gx

s(t)

90º

20

t

t

Where does the gradient echo form?

What happens when we push the echo time out?

drttzyx

dzyxtzyx

tcs

t

0E

0

)(Gγ),,(ω

),,,(ω),,,(

t

Page 8: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

2D Fourier Transform (2)2D Fourier Transform: (2D FT or Spin Warp)

1)

t

Gx(t)

RF

6GRADECH.AVI

Page 9: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Susceptibility Artifacts

Short echo time

Longer echo time

Water /plastic interface

has large susceptibility.

Notice dropout of signal

around water discs in bottom image.

Note: Not exactly the same slice and so bottom slice has some water signal also around discs.

Page 10: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Perils of Gradient Echo Imaging and T2*

TE = 8 ms TE = 24 ms

0.17 T GE Orthopedic Scanner

Page 11: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Spin Echo Phenomenon

Page 12: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Spin Echo Generation

Image Copyright Nishimura

90º 180º

(a) (b) (c) (d) (e)

Spin Echo Generation: Following a 90º excitation pulse, (a -b) the spin vectors begin to fan out and dephase because of

precessional frequency differences (c) at time , a 180º excitation rotates all the spins about the x’ axis (d) The spin vectors continue to precess at their slightly different

frequencies, rephasing at time 2 (e).

Page 13: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Tip Bulk Magnetization

x'

y'

z'

M

Page 14: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Tip Bulk Magnetization

x'

y'

z'

M

B1

Page 15: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Tip Bulk Magnetization

x'

y'

z'

B1

Page 16: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Tip Bulk Magnetization

x'

y'

z'

B1

Page 17: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Tip Bulk Magnetization

x'

y'

z'

B1

Page 18: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Transverse Magnetization

x'

y'

z'

Mtrans

Page 19: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

T2 Decay

x'

y'

z'

T2 relaxation is dephasing of transverse magnetization

f

s

Page 20: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

T2 Decay

x'

y'

z'

T2 relaxation is dephasing of transverse magnetization

f

s

Page 21: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

T2 Decay

x'

y'

z'

T2 relaxation is dephasing of transverse magnetization

f

s

Page 22: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Refocusing Pulse

x'

y'

z'

f

s

B1

Page 23: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Refocusing Pulse

x'

y'

z'

f

s

B1

Page 24: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Refocusing Pulse

x'

y'

z'

f

s

B1

Page 25: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Rephasing

x'

y'

z'

f

s

Page 26: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Rephasing

x'

y'

z'

f

s

Page 27: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Rephasing

x'

y'

z'

f

s

Page 28: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Echo Formation

x'

y'

z'

Mtrans

Page 29: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Spin Echoes

The phase errors E are for the most part out of our control, but there is a method around them called a spin echo.

Chemical shift, CS , can be compensated with techniques beyond the scope of this class. But these require more scan time also..

However, we can use a “spin echo” to correct for both effects. Spin echoes form the second major class of pulse sequences.

Spin echoes use a 90º pulse as we have seen, but also a second RF pulse, a 180º pulse. Let’s first consider just the effects of off-resonance and ignore the imaging gradients.

drttzyxdzyxtzyxt

cst

0E0)(Gγ),,(ω),,,(ω),,,(

RF t

90º

20

180º

Page 30: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Spin Echo Generation in Rotating Frame

The phase error just prior to the 180º pulse isJust after the 180º pulse, it is

Then we let the spins progress for another .

),,(ω)( E_ zyx

Image Copyright Nishimura

90º 180º

(a) (b) (c) (d) (e)

),,(

)()(

E

_

zyx

)2)(,,(),,,(

'),,,()()2(

EE

2E

zyxzyx

dttzyx

A “spin echo” is said to be formed at t=2

Page 31: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Effect of echo delay on signal lossEffect of echo delay on susceptibility-induced signal losses:

(a) TE = 15 ms

(b) TE = 10 ms

(c) TE = 5 ms Note artifactual signal reduction in the

region of the nasal and mastoid sinuses.

Notice homogeneity of water discs

and air bubble appearance.

Page 32: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Spin Echo Pulse Sequence

Z grad

RFX Grad

Y Grad Gy

90180

Echo Time (TE) = 2

TE

TR

90

Page 33: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Spin Echo Signal Plus X Readout Gradient

Left plot experiences no refocusing pulse. Right plot experiences

refocusing pulse half way through time series. Sum of magnitude of

spins given on top. Time series ends at echo time.

6SPINECH.AVI

Page 34: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Spin Echo RF Sequence

T2

T2*

TE

TR

T2*

1 =T2

1 + B0

Page 35: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Spin Echo Sequence with 180 y pulse

Page 36: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Spin Echo Sequence – Long Versus Short T2

long T2 short T2

long T2

short T2

T2-weighting: long TE, long TRPD-weighting: short TE, long TR

Page 37: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

T1, T2, and Density-Weighted Images

T1-weighted T2-weighted -weighted

Page 38: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Scan Duration

Scan Time = TR PE NEX

TR = repetition timePE = number of phase encoding valuesNEX = number of excitations (averages)

Page 39: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Spin Echo Formation• For spins to be refocused, they must

experience the same magnetic field after the 180 refocusing pulse as they experienced before the 180 refocusing pulse.

• Thus, only dephasing due to macroscopic inhomogeneities (B0) is refocused. Dephasing due to microscopic inhomogenieties (T2) is not refocused.

Page 40: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Spin Echo Parameters

TR TE

T1-weighting short (400 msec) short (20 msec)

T2-weighting long (3000 msec) long (100 msec)

-weighting long (3000 msec) short (20 msec)

Page 41: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Signal vs Weighting

T1-weighting long T1, small signal short T1, large signal

T2-weighting long T2, large signal short T2, small signal

-weighting high , large signal low , small signal

T1-weighted T2-weighted -weighted

Page 42: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Images of the Knee

-weighted T2-weightedNeeds longer TE

Page 43: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

T2 & T2* Relaxation: Image Contrast Sources

Mxy

Time

T2*

T2

T2*

1 =T2

1 + B0

Page 44: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

FSE Pulse Sequence Timing Diagram

SliceSelect

PhaseEncode

Freq.Encode

rf

Signal

90° 180° 180° 180° 180°

ESP

ETL=4

Page 45: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

PhaseDirection

Frequency Direction

Filling k-space FSE

Page 46: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Scan Duration

Scan Time = TR PE/ETL NEX

TR = repetition timePE = number of phase encoding valuesNEX = number of excitations (averages)ETL = echo train length

Page 47: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

T2 Weighting (Various Sequences)

FSE SE

TR = 2500TE = 116ETL = 16NEX = 224 slices17 slices/pass2 passesTime = 2:51

TR = 2500TE = 112

ETL = N/ANEX = 124 slices

20 slices/pass2 passes

Time = 22:21

Page 48: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Inversion Recovery RF Sequence

Page 49: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Inversion Recovery (IR) Sequence & Spin Behavior

Page 50: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Inversion Recovery

SliceSelectPhaseEncode

Freq.Encode

rf

Signal

90° 180°180°

TI

Page 51: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

IR Sequence – Short Versus Long T1

Page 52: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Inversion Recovery Signal

longer T1

short T1

Mlong

TI

Short T1 = brighter

Long T1 = darker

Page 53: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Inversion Recovery Signal

longer T1

short T1

Mlong

TI

Short T1 = brighter

Long T1 = null

Page 54: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Short Tau Inversion Recovery (Fat Nulled)

longer T1

short T1 (fat)Mlong

TI

Short T1 = null

Long T1 = brighter

STIR

Page 55: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Spine: T1 versus T2 versus STIR

T1 T2 STIR

Page 56: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Inversion Recovery II

Coronal Tumor

Here the inversion recovery delay is very long ( ~2 s). Here the idea

is to null CSF so one can distinguish tumor from CSF. Notice tumor

still has positive contrast.

IR pulse precedes

T2-weighted sequence

Page 57: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

MR: SNR vs Field Strength

Signal

oo BM

ooreceive Bt

Ms

- energy separation of Zeeman levels

Signal amplitude is 2oB

Noise Power

Receiver oB

Patient 2oB

- electronics

- patient electrons in Brownian motion

Patient noise dominates at most field strengths T) 1.( oB

o

o

o BB

BSNR

2

2

- Faraday’s Law of Induction

Page 58: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

SNR

• We will first consider the case where we ignore T1 and T2 of the tissue involved. – leave sequence dependent effects for later

Page 59: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

SNRNoise Power

Study for impulse at x=0, y = 0

Each k-space point has signal A, noise power 2

Each sampled point is independent with respect to noise.

kx

ky

Ny

Nxsampling interval in time

y xN N

2

AA yx

yx

yxNN

NN

NNSNR

T

12

T

Amplitude of Signal = NxNyA

Noise Power = NxNy 2

on A/D TimeTNNSNR yx AA

Page 60: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

SNR

voxel size since this determines the # of protons per voxelReplace A above by voxel size

SNR

Together,

on A/D TimeTNNSNR yx AA

on A/D Timesize voxelSNR

SNR

Time Resolution

Page 61: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Scan Time

Image b) has twice the SNR as a)

at an expense of four times the scan time

Page 62: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

Axial Forearm T2-Weighted

TE = 15 ms TE = 45 ms TE = 75 ms

TE = 105 ms

TR = 4000 ms for all

Notice appearance of supinator

Muscle ( arrow) on all images.

One arm had an exercise protocol.

Which one? What is in between arms?

Page 63: Quiz In a 2D spin warp or FT MR scan, aliasing should only occur a) in the slice select direction b) in the readout direction c) in the phase encoding.

TR

1,nzM

zMnzM ,

During each TR, Mz will recover.

When in steady-state,

nznz MM ,1,

Where n-1 is the n-1th RF pulse

and n is the nth RF pulse.

)1(

/2,

)sin()cos1(

)1(

)cos1(

)1(

)1()cos1(

)1(cos

condition statesteady- Use

)1(cos

for in Plug

)1(

equationrecovery

cos

cosby reduced is , ingAfter tipp

condition stateSteady

1/0

1/0

1/0

,

1/0,

1/0

1/,,

1/0

1/1,,

,

1/0

1/,,

1,,

,1,

TTR

TTR

TTR

nz

TTRnz

TTRTTRnznz

TTRTTRnznz

nz

TTRTTRnznz

nznz

nznz

eMMxy

For

eMMxy

eMM

eMM

eMeMM

eMeMM

M

eMeMM

Mz

MM

Mz

MM


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Item code: BH153. MIG-29 - shop.pichler.de · MIG-29 Item code: BH153. Instruction Manual 3 Wing warp: Hold the panel twisted gently in the opposite direction to the warp, and apply

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