A new method for diffusion imagingusing Burst excitation

C. Wheeler-Kingshott1, D. Thomas2, M. Lythgoe2, S. Williams2 and S. J. Doran1

1University of Surrey (Guildford)

2Institute of Child Health (London)

Acknowledgements

• Steve Williams (Institute of Child Health, London)

• SMIS (Surrey Medical Imaging Systems, Guildford)

• Dave Guilfoyle (Nathan Klein Institute, New York)

Summary

• What is Burst ?

• Molecular diffusion, T2 and Burst

• A new single-scan Burst sequence Sequence Scheme Advantages and Problems

• Experimental work Repeatability of the method Comparison with DW-SE data and ME data

• Conclusions

• Burst: single-shot imaging method, first proposed by Hennig in 1988.

• A series of low angle pulses creates a train of echoes, which can be used to form an image.

• A variety of different types of Burst sequence exists.

Burst techniques

180° BURST excitation Echo acquisition

RF

READ G G

Burst signal decay

The echo train acquired during Burst experiments suffers from a combined decay due to T2 relaxation and Molecular Diffusion.

• Echo time increases with the echo number.

• Burst excitation/readout gradient acts as diffusion pair of different length for each pulse/echo couple

2/

0

TTEDb

jjj eeAA

Increasing echo timeIncreasing b-value

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0 5 10 15 20 25 30 35 40

Echo Number

A /

A0

Data for CuSO4

T2 and D double fit

Typical Spectroscopic Data Typical Image Data

Can we use the decay to get D and T2?

ADC&T2 Burst sequence

• A data array of n echoes is acquired for each PE step.

• Each echo, j, corresponds to the same k-space line of the same slice, but with a different ADC&T2 weight.

• Corresponding echoes in successive arrays are used to reconstruct a given image.

In one scan we collect n ADC&T2 weighted images.

Ph

ase

enco

de

Readout

D, T 2

j = 0

j = n-1

Pulse sequence diagram

RF

SLICE

READ

180 selective pulseBURST excitation Echo acquisition

j

TEj

G G

j

PHASE

Potential Advantages

• Method allows acquisition of ADC&T2 weighted data in one scan.

• Acquisition of n images in one scan eliminates registration artifacts arising from motion between successive scans.

• Different bj values are obtained by increasing the diffusion time, j and consequently j,

diffusion data are sensitive to restriction

Problems

• The choice of the parameters is governed by the rules of Burst imaging:

The range of b factors, bj , and echo times, TEj , are not independent of other parameters.

• Only in-plane ADC maps are possible because the diffusion gradient coincides with the readout gradient.

• Burst images have slightly lower signal-to-noise ratio than the corresponding DW-SE

• Double-parameter fit

Repeatability of the Burst experiment

• SMIS 360 MHz scanner at the Institute of Child Health

• ADC&T2 Burst and conventional DW-SE sequences with exactly the same 16 different b values

• Spin-echo experiment is 3 times longer (for equivalent SNR in images)

• Same cycle of experiments (Burst-SE-Burst-SE-Burst) on a rat before and after sacrifice

• Data show a good repeatability in both cases

The plot of the data obtained with ADC&T2 Burst sequence differs from the plot of the SE data only because of the extra T2 decay.

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Spin Echo 1

Spin Echo 2

Burst 1

Burst 2

Burst 3

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Spin Echo

Burst 1

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Alive Post-mortem

bjbj

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rmal

ized

ech

o a

mp

litu

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ech

o a

mp

litu

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Experimental results

• After optimization on phantoms, we performed the experiments on 4 animals, both before and after sacrificing them.

Parameters ADC&T2-Burst SE-DWI Multi-echo

Matrix size 128x64x25 128x64x4 128x64x4

Echo Time (s) 0.006 to 0.087 0.065 0.035 to 0.175

b values (s cm-2) 77 to 6.4x105 N/A

Experiment length 12’ 48” 12’ 48” 3’ 12”

Burst images SE images

ME images

Data analysis

ROI1 ROI2 ROI3

BURST ADC 6.5 0.6 5.5 0.5 5.3 0.5DW-SE ADC 7.2 0.4 6.1 0.4 6.3 0.4(10-6 cm2 s-1)

BURST T2 49 5 50 5 42 4

ME T2 40 2 42 2 40 2(ms)

Plot data and fitted function

Echo number Echo number

Echo number Echo number

Aj / A

0A

j / A

0

Aj / A

0A

j / A

0

ROI1

ROI2

ROI3

Typical single pixel fit

Conclusions

• The data obtained in the Burst experiment fit the theoretical model well.

• When T2 is known, the values of ADC obtained are accurate.

However …

• Fitting to two parameters causes problems.

• Extracting both ADC and T2 from a single data set appears to be difficult in rat brain.

• More work needs to be done to establish under what conditions the method can be used successfully.

T2

ADC

BurstDouble fit

Burst Single fit(given T2)

DW-SE

ME