Impact of sequence parameters on the sensitivity of DDE and DODE sequences to microscopic anisotropy

Andrada Ianuș^{1}, Ivana Drobnjak^{1}, Noam Shemesh^{2}, and Daniel C. Alexander^{1}

We use a model of randomly oriented finite cylinders with diameter d and eccentricity L/d to represent diffusion substrates featuring microscopic anisotropy. We investigate the difference between parallel and perpendicular measurements for DODE and DDE sequences with different varying parameters for a wide range of substrates with pore diameters d between 0.5 and 10μm and elongations L/d between 1 and 10. For DODE sequences we vary independently gradient strength G, number of periods N and pulse duration δ, while for DDE we vary G, δ and diffusion time ∆. The maximum gradient strength used in this study is G=300mT/m and the maximum duration is 140ms, values that can be achieved in practical applications. We also analyse the effect of noise and label the regions where the difference is larger than the standard deviation of the noise for different levels of SNR={20,50,100,1000}. This highlights which substrates can be distinguished from isotropic pores, given the diffusion sequence and SNR level. The time interval between the first and second gradient waveforms is fixed to 20ms for all sequences. All simulations are performed using the MISST software [13].

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Figure
1. a) Typical double diffusion encoding sequence with gradient
amplitude G_{1,2}, gradient duration δ_{1,2} , diffusion time ∆_{1,2 }and
mixing time τ_{m}. b) Double oscillating diffusion encoding sequence
(DODE) with oscillation frequency δ_{1,2}/N_{1,2} where N is the number of
periods.

Figure
2. Difference between parallel and perpendicular measurements of a)
DODE and b) DDE sequences as a function of pore size and
eccentricity. In each row a different parameter is varied, while the
others are constant. The white contours indicate where the difference
is equal to the noise standard deviation with SNR={20,50,100,1000}.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)

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