Sequence/Acquisition Overviewweb.stanford.edu/class/rad229/Notes/A2-Overview-Extras.pdf · Mag Prep •“Prepare” contrast •Image rapidly before steady-state evolves . Section

B.Hargreaves - RAD 229Section A2

Sequence/Acquisition Overview

• Gradient Echo, Spin Echo

• Sampling and Trajectories

• Parallel Imaging

1


Pulse Sequences and k-spaceRF

Gz

Gx

Acq.

Gy

kx

ky

kx

kz

ky

3D k-space


Sequence Questions• Which Gradient parts in 2DFT can overlap?

• Generally ramps • “k-space area” (min time) vs. “frequency mapping” (plateaus)

• Which is usually bigger, x-dephaser or y-phase-encode?

• x-dephaser, accounts for readout ramps

3

RF

Gz

Gx

Acq.

Gy

• For 50mT/m and 200mT/m/ms gradients, what is the duration where the fastest gradient to achieve a given area changes to trapezoidal?

• 0.5ms

• (0.25 ms full-scale rise time)


Gradient Echo Pulse Sequence

RF

Gz

Gy

Gx

Signal

TE ~ 1+ ms

?

?

?

Gradient Echo

Flip Angle

4

Refocusing Gradient

Slice-Select Gradient

Dephaser Gradient

Readout Gradient

Phase- Encode Gradient


Gradient Echo Contrasts

Balanced SSFP

Gradient Spoiled

RF-Spoiled

5∘ 10∘ 20∘ 30∘ 40∘ 50∘5


Spin Dephasing and Spin Echoes• Frequency variations cause “dephasing” (T2’) • Results in signal loss (T2*) • Refocus spins to spin-echo (T2)

6

Gradient-Echo Image

Spin-Echo Image

B1


Spin Echo Pulse Sequence

RF

Gz

Gy

Gx

Signal

180º TE ~ 8+ ms

7




Basic Spin Echo Considerations

Pros:

• Refocusing pulse reverses dephasing

• Image acquired at spin echo increases signal

Cons:

• Increased RF power deposition (SAR)

• Longer echo times than gradient echo (GRE)

8


Fast Recovery (FR) or Driven Equilibrium

RF

Gz

Gy

Gx

Signal

...

...

...

...

...

180º180º90º -90º

180º

Fast-Recovery

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Magnetization Preparation

• Examples: • Fat Saturation

• Inversion - Recovery

• Myocardial Tagging

• T2-prep

• Magnetization Transfer

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Mag PrepImaging Sequence

Mag Prep...

• “Prepare” contrast

• Image rapidly before steady-state evolves


Fat-Saturated FSE

RF

Gz

Gy

Gx

Signal

...

...

...

...

...

180º180º90º

Fat-Sat

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Fat Saturation (Magnetization Preparation)

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Fat Saturated T1w FSE


Sampling & Point-Spread Functions• PSF = Fourier transform of sampling pattern

• Just 1’s as samples, mostly a matter of scaling

• Lots more you can do with this...!

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Fourier Transform

k-space Sampling Point-Spread Function

Extent WidthSpacing FOV


Partial Fourier Acquisition/Reconstruction

ky

kx

kx

ky

ky

kx

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Alternate k-Space Trajectories

kx

ky

Cartesian

kx

ky

Spiral

kx

ky

Radial

kx

ky

EPI

kx

ky

Projection15


Parallel Imaging

• Coils have limited sensitivity

• Unalias based on known sensitivities (SENSE)

• Limited sensitivity results in k-space correlations

• Fill in missing k-space (GRAPPA)

• Build up FOV with coil arrays

16

kx

ky


Basic Parallel Imaging: PILS (Parallel Imaging with Localized Sensitivities)

• Consider 2 coils

• Each sensitive to exactly 1 breast

• Each coil uses a reduced FOV

• …but simultaneous acquisition

• Combination allows full image in less time

17

Rea

dout

Griswold 2000


SENSE: Unalias ImageSENSE Image

When it fails…

Sensitivity 1 (S1) Sensitivitiy 2 (S2)

Coil 1 Signal (C1) Coil 2 Signal (C2)

A A

BB

A

B

Pruessmann 1999

A

B

A

B

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SENSE: Brief Mathematics• At each pixel

• Using Coil 1: S1 = C1A x A + C1B x B

• Using Coil 2: S2 = C2A x A + C2B x B

• If we know C1 and C2 at A,B and signals S1 and S2,A = C1B S2 - C2B S1

C2AC1B - C2BC1A

B = C2A S1 - C1A S2 C2AC1B - C2BC1A

• More complicated with more than 2 coils

• If denominator is small, noise amplification

• Just a matrix inversion

A

B

19


SENSE Calibration

• Low-resolution images from each coil

• Divide images by RMS image or body coil image

• Challenge: coil sensitivity in area of low signal

kread

kphase

Low Resolution Image20


Parallel Imaging: k-space Approaches

• Acquire reduced FOV, and some “calibration” lines

• Fill in missing lines to extend the FOV

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Sodickson 1997 (SMASH), Griswold 2002 (GRAPPA)

kx

ky


GRAPPA: Coil Sensitivities and k-space

ky

kx

ky

kx

ky

kx

Correlated Pixels

Correlated k-Space

Reduced Image Extent

Reduced k-Space Extent22


GRAPPA Calibration• Fully-sampled central k-space

• Find “data correlation” between lines/coils

• Note: data-driven vs model (SENSE)

• Not just image vs k-space!

kread

kphase

Repeat for all calibration points and all coils

Coil 1

Coil 2Coil 3

Griswold 2002

23


GRAPPA Synthesis• Use kernel information to synthesize data

• Repeat for all coils

• Combine coils and reconstruct

kread

kphase

Coil 1Coil 2

Coil 3

Griswold 2002

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Parallel Imaging & Noise

25

Full

Acceleration is in Left-Right Direction in Images

2x 3x 4x

Full 2x 3x 4x

Acc

eler

ated

Con

stan

t Sca

n Ti

me


2D Parallel Imaging (for 3D Acquisitions)

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Fully Sampled 4x A/P2x A/P and 2x L/R

00

8-Channel Phased-Array Coil

Note: Readout is in S/I (head-foot) direction!

• 3D imaging uses 2 phase-encode directions

• Can apply parallel imaging in 2 directions


Parallel Imaging Questions

• For synthesis which takes more multiplies to fill in one missing pixel, SENSE or GRAPPA? • GRAPPA: kernel-size x (#coils)2

• SENSE: (#coils)

• Which direction(s) do we want coil sensitivity variations most?

• Phase-encode (y, and if 3D, z as well)

• Readout ~ helps GRAPPA a little

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Coil 1Coil 2

Coil 3


Summary of Sequence/Acq Overview

• Gradient Echo Sequence

• Spin Echo sequences

• Imaging Readouts / Sampling

• Parallel Imaging

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Documents

Sequence/Acquisition Overviewweb.stanford.edu/class/rad229/Notes/A2-Overview-Extras.pdf · Mag Prep •“Prepare” contrast •Image rapidly before steady-state evolves . Section