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Spectral principal axis system (SPAS) and tuning of tensor-valued encoding for microscopic anisotropy and time-dependent diffusion in the rat brain

Lasič, Samo LU ; Just, Nathalie ; Nilsson, Markus LU ; Szczepankiewicz, Filip LU orcid ; Budde, Matthew and Lundell, Henrik (2025) In Imaging Neuroscience 3.
Abstract

Tensor-valued encoding in diffusion MRI allows probing of microscopic anisotropy in tissue, however, time-dependent diffusion (TDD) can bias results unless b-tensors are carefully tuned to account for TDD. We propose two novel strategies for tuning b-tensors to enable accurate measurements without interference from TDD due to restricted diffusion. The first strategy involves identifying encoding tensor projections that yield equal mean diffusivities (MD), providing robust tuning across a wide range of diffusion spectra. The second strategy uses geometric averaging of signals, ensuring tuning regardless of the diffusion spectra. Importantly, the same encoding waveforms used for geometric averaging to probe microscopic anisotropy (µA) can... (More)

Tensor-valued encoding in diffusion MRI allows probing of microscopic anisotropy in tissue, however, time-dependent diffusion (TDD) can bias results unless b-tensors are carefully tuned to account for TDD. We propose two novel strategies for tuning b-tensors to enable accurate measurements without interference from TDD due to restricted diffusion. The first strategy involves identifying encoding tensor projections that yield equal mean diffusivities (MD), providing robust tuning across a wide range of diffusion spectra. The second strategy uses geometric averaging of signals, ensuring tuning regardless of the diffusion spectra. Importantly, the same encoding waveforms used for geometric averaging to probe microscopic anisotropy (µA) can also generate an independent contrast due to TDD. This is enabled by considering spectral anisotropy of encoding and defining the spectral principal axis system (SPAS), which unfolds TDD as an additional independent dimension in tensor-valued encoding. Projections of encoding waveforms along the SPAS axes allow for the simultaneous acquisition of independent contrasts due to both µA and TDD within a single multidimensional diffusion encoding protocol. Additionally, SPAS projections inherit useful properties from the reference tensor, such as optimized b-value, motion nulling, and minimal concomitant field effects. This framework is demonstrated through simulations of various restricted diffusion compartments. Experimental validation on perfusion-fixed and in vivo rat brains highlights the method’s potential for enhanced microstructural specificity. In addition to mapping MD, fractional anisotropy, and unbiased microscopic fractional anisotropy, we propose a model-free approach to independently map µA and TDD. This approach uses a minimal yet highly specific protocol, enabling the identification of distinct µA-TDD contrasts across different brain regions, including details in cortical gray matter, choroid plexus, dentate gyrus of the hippocampus, and white matter.

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author
; ; ; ; and
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
diffusion MRI, gradient waveform, power spectrum, restricted diffusion, tensor-valued encoding, time-dependent diffusion
in
Imaging Neuroscience
volume
3
article number
IMAG.a.35
publisher
MIT Press
external identifiers
  • scopus:105011479693
  • pmid:40800797
ISSN
2837-6056
DOI
10.1162/IMAG.a.35
language
English
LU publication?
yes
additional info
Publisher Copyright: © 2025 The Authors. Published under a Creative Commons Attribution 4.0 International (CC BY 4.0) license.
id
a19294fc-8ceb-43bd-bfbe-ed6343c7ba5f
date added to LUP
2025-12-17 14:37:32
date last changed
2025-12-18 03:00:04
@article{a19294fc-8ceb-43bd-bfbe-ed6343c7ba5f,
  abstract     = {{<p>Tensor-valued encoding in diffusion MRI allows probing of microscopic anisotropy in tissue, however, time-dependent diffusion (TDD) can bias results unless b-tensors are carefully tuned to account for TDD. We propose two novel strategies for tuning b-tensors to enable accurate measurements without interference from TDD due to restricted diffusion. The first strategy involves identifying encoding tensor projections that yield equal mean diffusivities (MD), providing robust tuning across a wide range of diffusion spectra. The second strategy uses geometric averaging of signals, ensuring tuning regardless of the diffusion spectra. Importantly, the same encoding waveforms used for geometric averaging to probe microscopic anisotropy (µA) can also generate an independent contrast due to TDD. This is enabled by considering spectral anisotropy of encoding and defining the spectral principal axis system (SPAS), which unfolds TDD as an additional independent dimension in tensor-valued encoding. Projections of encoding waveforms along the SPAS axes allow for the simultaneous acquisition of independent contrasts due to both µA and TDD within a single multidimensional diffusion encoding protocol. Additionally, SPAS projections inherit useful properties from the reference tensor, such as optimized b-value, motion nulling, and minimal concomitant field effects. This framework is demonstrated through simulations of various restricted diffusion compartments. Experimental validation on perfusion-fixed and in vivo rat brains highlights the method’s potential for enhanced microstructural specificity. In addition to mapping MD, fractional anisotropy, and unbiased microscopic fractional anisotropy, we propose a model-free approach to independently map µA and TDD. This approach uses a minimal yet highly specific protocol, enabling the identification of distinct µA-TDD contrasts across different brain regions, including details in cortical gray matter, choroid plexus, dentate gyrus of the hippocampus, and white matter.</p>}},
  author       = {{Lasič, Samo and Just, Nathalie and Nilsson, Markus and Szczepankiewicz, Filip and Budde, Matthew and Lundell, Henrik}},
  issn         = {{2837-6056}},
  keywords     = {{diffusion MRI; gradient waveform; power spectrum; restricted diffusion; tensor-valued encoding; time-dependent diffusion}},
  language     = {{eng}},
  month        = {{06}},
  publisher    = {{MIT Press}},
  series       = {{Imaging Neuroscience}},
  title        = {{Spectral principal axis system (SPAS) and tuning of tensor-valued encoding for microscopic anisotropy and time-dependent diffusion in the rat brain}},
  url          = {{http://dx.doi.org/10.1162/IMAG.a.35}},
  doi          = {{10.1162/IMAG.a.35}},
  volume       = {{3}},
  year         = {{2025}},
}