A new method for diffusion imaging using Burst excitation C. Wheeler-Kingshott 1, D. Thomas 2, M. Lythgoe 2, S. Williams 2 and S. J. Doran 1 1 University of Surrey (Guildford) 2 Institute 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, T 2 and Burst A new single-scan Burst sequence u Sequence Scheme u Advantages and Problems Experimental work u Repeatability of the method u Comparison with DW-SE data and ME data Conclusions

Burst: single-shot imaging method, first proposed by Hennig in 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 T 2 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 Increasing echo time Increasing b-value

Echo Number A / A0 Data for CuSO 4 T 2 and D double fit Typical Spectroscopic Data Typical Image Data Can we use the decay to get D and T 2 ?

ADC&T 2 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&T 2 weight. Corresponding echoes in successive arrays are used to reconstruct a given image. In one scan we collect n ADC&T 2 weighted images. Phase encode Readout D, T 2 j = 0 j = n-1

Pulse sequence diagram RF SLICE READ 180 selective pulse BURST excitation Echo acquisition   j j TE j G G  j j  PHASE

Potential Advantages Method allows acquisition of ADC&T 2 weighted data in one scan. Acquisition of n images in one scan eliminates registration artifacts arising from motion between successive scans. Different b j 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, b j, and echo times, TE j, 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&T 2 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&T 2 Burst sequence differs from the plot of the SE data only because of the extra T 2 decay.

Alive Post-mortem bjbj bjbj Normalized echo amplitude

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

Burst images SE images ME images

Data analysis ROI1 ROI2 ROI3 BURST ADC 6.5    0.5 DW-SE ADC 7.2    0.4 (10 -6 cm 2 s -1 ) BURST T 2 49  5 50  5 42  4 ME T 2 40  2 42  2 40  2 (ms)

Plot data and fitted function Echo number A j / A 0 ROI1 ROI2 ROI3 Typical single pixel fit

Conclusions The data obtained in the Burst experiment fit the theoretical model well. When T 2 is known, the values of ADC obtained are accurate. However … Fitting to two parameters causes problems. Extracting both ADC and T 2 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.

T2T2 ADC Burst Double fit Burst Single fit (given T 2 ) DW-SE ME