title: 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.
authors.affiliation: Champalimaud Neuroscience Programme, Champalimaud Centre for the Unknown, Lisbon, Portugal.
authors.forename: Noam
authors.identifier: http://orcid.org/0000-0001-6681-5876
authors.initials: N
authors.lastname: Shemesh
authors.affiliation: CFIN/MindLab, Aarhus University, Aarhus, Denmark.|Department of Physics and Astronomy, Aarhus University, Aarhus, Denmark.
authors.forename: Sune N
authors.initials: SN
authors.lastname: Jespersen
authors.affiliation: Centre for Medical Image Computing, Department of Computer Science, University College London, London, United Kingdom.
authors.forename: Daniel C
authors.initials: DC
authors.lastname: Alexander
authors.affiliation: School of Chemistry, the Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, Israel.|Sagol School of Neurosciences, Tel Aviv University, Tel Aviv, Israel.
authors.forename: Yoram
authors.initials: Y
authors.lastname: Cohen
authors.affiliation: Centre for Medical Image Computing, Department of Computer Science, University College London, London, United Kingdom.
authors.forename: Ivana
authors.initials: I
authors.lastname: Drobnjak
authors.affiliation: Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital Hvidovre, Hvidovre, Denmark.
authors.forename: Tim B
authors.initials: TB
authors.lastname: Dyrby
authors.affiliation: Department of Systems Neuroscience, University Medical Center Hamburg-Eppendorf, Hamburg, Germany.|Neuroimage Nord, University Medical Centers Hamburg-Kiel-Lübeck, Germany.
authors.forename: Jurgen
authors.initials: J
authors.lastname: Finsterbusch
authors.affiliation: Institute of Medical Engineering, University of Lübeck, Lübeck, Germany.
authors.forename: Martin A
authors.initials: MA
authors.lastname: Koch
authors.affiliation: Medical Physics in Radiology, German Cancer Research Center, Im Neuenheimer Feld 280, Heidelberg, Germany.
authors.forename: Tristan
authors.initials: T
authors.lastname: Kuder
authors.affiliation: Medical Physics in Radiology, German Cancer Research Center, Im Neuenheimer Feld 280, Heidelberg, Germany.
authors.forename: Fredrik
authors.initials: F
authors.lastname: Laun
authors.affiliation: Department of Systems Neuroscience, University Medical Center Hamburg-Eppendorf, Hamburg, Germany.
authors.forename: Marco
authors.initials: M
authors.lastname: Lawrenz
authors.affiliation: Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital Hvidovre, Hvidovre, Denmark.
authors.forename: Henrik
authors.initials: H
authors.lastname: Lundell
authors.affiliation: Cold Spring Harbor Laboratory, Cold Spring Harbor, New York, USA.
authors.forename: Partha P
authors.initials: PP
authors.lastname: Mitra
authors.affiliation: Lund University Bioimaging Center, Lund University, Lund, Sweden.
authors.forename: Markus
authors.initials: M
authors.lastname: Nilsson
authors.affiliation: Department of Physics, Boğaziçi University, Bebek, Istanbul, Turkey.
authors.forename: Evren
authors.initials: E
authors.lastname: Özarslan
authors.affiliation: Division of Physical Chemistry, Department of Chemistry, Lund University, Lund, Sweden.
authors.forename: Daniel
authors.initials: D
authors.lastname: Topgaard
authors.affiliation: Department of Radiology, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts, USA.
authors.forename: Carl-Fredrik
authors.initials: CF
authors.lastname: Westin
mesh_terms: D017408:Guidelines as Topic; D008279:Magnetic Resonance Imaging / Q000145:classification* / Q000592:standards*; D009682:Magnetic Resonance Spectroscopy / Q000145:classification* / Q000592:standards*; D012815:Signal Processing, Computer-Assisted; D009626:Terminology as Topic
keywords: diffusion; diffusion mri; double diffusion encoding; double pfg; double wave vector; microstructure