Conventions and nomenclature for double diffusion encoding NMR and MRI

Abstract

Stejskal and Tanner's ingenious pulsed field gradient design from 1965 has made diffusion NMR and MRI the mainstay of most studies seeking to resolve microstructural information in porous systems in general and biological systems in particular. Methods extending beyond Stejskal and Tanner's design, such as double diffusion encoding (DDE) NMR and MRI, may provide novel quantifiable metrics that are less easily inferred from conventional diffusion acquisitions. Despite the growing interest on the topic, the terminology for the pulse sequences, their parameters, and the metrics that can be derived from them remains inconsistent and disparate among groups active in DDE. Here, we present a consensus of those groups on terminology for DDE sequences and associated concepts. Furthermore, the regimes in which DDE metrics appear to provide microstructural information that cannot be achieved using more conventional counterparts (in a model-free fashion) are elucidated. We highlight in particular DDE's potential for determining microscopic diffusion anisotropy and microscopic fractional anisotropy, which offer metrics of microscopic features independent of orientation dispersion and thus provide information complementary to the standard, macroscopic, fractional anisotropy conventionally obtained by diffusion MR. Finally, we discuss future vistas and perspectives for DDE. Magn Reson Med 75:82-87, 2016. V C 2015 Wiley Periodicals, Inc.

Machine Learning Based Compartment Models with Permeability for White Matter Microstructure Imaging

“…Even though these limitations have been known for over 20 years, there have been no improvements to the model due to the mathematical intractability of the problem. AXR imaging (Lasič et al, 2011; has recently been introduced as an alternative to estimating τ i via Kärger model; however it requires specialised double diffusion encoding imaging sequences (DDE) (Shemesh et al, 2016) and it again relies on strong assumptions about the compartmentation of water into a 'fast' and 'slow' pool. The estimated AXR parameter also conflates τ i with intra-axonal volume fraction f, making it difficult to disentangle the origin of any measured change.…”

Experimental validation of a bias in apparent exchange rate measurement

“…Diffusion Δ1 and Δ2 are the times between the diffusion gradient pulses in q (1) and q (2) , respectively. and are the corresponding gradient strengths.…”

“…Homospoil gradient pulse during longitudinal storage shown in solid black. All diffusion gradients (G (1) and G (2) ) have equal duration and are trapezoidal in shape.…”

“…Double diffusion encoding (DDE) [1] employs two diffusion weightings per acquisition. It can be used to acquire microstructural information that is not easily available using non-invasive techniques [2].…”

“…conditions δ  0 and Δ  ∞) for randomly oriented water filled pores, the diffusion-weighted signal S(q,ψ) will be related to the size of the compartment when  0 as in (1) at small values of (γ: gyromagnetic ratio, δ: gradient duration; G: gradient strength), where denotes the compartment's mean squared radius of gyration. Notice that Eq (1) is slightly different from the original approximation [2] since in this paper a stimulated echo sequence is used together with the conventions from Shemesh et al [1].…”