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Tuned exchange imaging : Can the filter exchange imaging pulse sequence be adapted for applications with thin slices and restricted diffusion?

Lasič, Samo LU ; Chakwizira, Arthur LU ; Lundell, Henrik ; Westin, Carl Fredrik and Nilsson, Markus LU (2024) In NMR in Biomedicine 37(11).
Abstract

Filter exchange imaging (FEXI) is a double diffusion-encoding (DDE) sequence that is specifically sensitive to exchange between sites with different apparent diffusivities. FEXI uses a diffusion-encoding filtering block followed by a detection block at varying mixing times to map the exchange rate. Long mixing times enhance the sensitivity to exchange, but they pose challenges for imaging applications that require a stimulated echo sequence with crusher gradients. Thin imaging slices require strong crushers, which can introduce significant diffusion weighting and bias exchange rate estimates. Here, we treat the crushers as an additional encoding block and consider FEXI as a triple diffusion-encoding sequence. This allows the bias to be... (More)

Filter exchange imaging (FEXI) is a double diffusion-encoding (DDE) sequence that is specifically sensitive to exchange between sites with different apparent diffusivities. FEXI uses a diffusion-encoding filtering block followed by a detection block at varying mixing times to map the exchange rate. Long mixing times enhance the sensitivity to exchange, but they pose challenges for imaging applications that require a stimulated echo sequence with crusher gradients. Thin imaging slices require strong crushers, which can introduce significant diffusion weighting and bias exchange rate estimates. Here, we treat the crushers as an additional encoding block and consider FEXI as a triple diffusion-encoding sequence. This allows the bias to be corrected in the case of multi-Gaussian diffusion, but not easily in the presence of restricted diffusion. Our approach addresses challenges in the presence of restricted diffusion and relies on the ability to independently gauge sensitivities to exchange and restricted diffusion for arbitrary gradient waveforms. It follows two principles: (i) the effects of crushers are included in the forward model using signal cumulant expansion; and (ii) timing parameters of diffusion gradients in filter and detection blocks are adjusted to maintain the same level of restriction encoding regardless of the mixing time. This results in the tuned exchange imaging (TEXI) protocol. The accuracy of exchange mapping with TEXI was assessed through Monte Carlo simulations in spheres of identical sizes and gamma-distributed sizes, and in parallel hexagonally packed cylinders. The simulations demonstrate that TEXI provides consistent exchange rates regardless of slice thickness and restriction size, even with strong crushers. However, the accuracy depends on b-values, mixing times, and restriction geometry. The constraints and limitations of TEXI are discussed, including suggestions for protocol adaptations. Further studies are needed to optimize the precision of TEXI and assess the approach experimentally in realistic, heterogeneous substrates.

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author
; ; ; and
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
cumulant expansion, diffusion MRI, diffusion spectrum, encoding power spectrum, exchange, filter exchange imaging, restricted diffusion, time-dependent diffusion
in
NMR in Biomedicine
volume
37
issue
11
article number
e5208
publisher
John Wiley & Sons Inc.
external identifiers
  • pmid:38961745
  • scopus:85197420516
ISSN
0952-3480
DOI
10.1002/nbm.5208
language
English
LU publication?
yes
id
d47cff4e-c69b-42c2-9191-1d36374d92b8
date added to LUP
2024-12-03 11:34:52
date last changed
2025-07-16 05:51:27
@article{d47cff4e-c69b-42c2-9191-1d36374d92b8,
  abstract     = {{<p>Filter exchange imaging (FEXI) is a double diffusion-encoding (DDE) sequence that is specifically sensitive to exchange between sites with different apparent diffusivities. FEXI uses a diffusion-encoding filtering block followed by a detection block at varying mixing times to map the exchange rate. Long mixing times enhance the sensitivity to exchange, but they pose challenges for imaging applications that require a stimulated echo sequence with crusher gradients. Thin imaging slices require strong crushers, which can introduce significant diffusion weighting and bias exchange rate estimates. Here, we treat the crushers as an additional encoding block and consider FEXI as a triple diffusion-encoding sequence. This allows the bias to be corrected in the case of multi-Gaussian diffusion, but not easily in the presence of restricted diffusion. Our approach addresses challenges in the presence of restricted diffusion and relies on the ability to independently gauge sensitivities to exchange and restricted diffusion for arbitrary gradient waveforms. It follows two principles: (i) the effects of crushers are included in the forward model using signal cumulant expansion; and (ii) timing parameters of diffusion gradients in filter and detection blocks are adjusted to maintain the same level of restriction encoding regardless of the mixing time. This results in the tuned exchange imaging (TEXI) protocol. The accuracy of exchange mapping with TEXI was assessed through Monte Carlo simulations in spheres of identical sizes and gamma-distributed sizes, and in parallel hexagonally packed cylinders. The simulations demonstrate that TEXI provides consistent exchange rates regardless of slice thickness and restriction size, even with strong crushers. However, the accuracy depends on b-values, mixing times, and restriction geometry. The constraints and limitations of TEXI are discussed, including suggestions for protocol adaptations. Further studies are needed to optimize the precision of TEXI and assess the approach experimentally in realistic, heterogeneous substrates.</p>}},
  author       = {{Lasič, Samo and Chakwizira, Arthur and Lundell, Henrik and Westin, Carl Fredrik and Nilsson, Markus}},
  issn         = {{0952-3480}},
  keywords     = {{cumulant expansion; diffusion MRI; diffusion spectrum; encoding power spectrum; exchange; filter exchange imaging; restricted diffusion; time-dependent diffusion}},
  language     = {{eng}},
  number       = {{11}},
  publisher    = {{John Wiley & Sons Inc.}},
  series       = {{NMR in Biomedicine}},
  title        = {{Tuned exchange imaging : Can the filter exchange imaging pulse sequence be adapted for applications with thin slices and restricted diffusion?}},
  url          = {{http://dx.doi.org/10.1002/nbm.5208}},
  doi          = {{10.1002/nbm.5208}},
  volume       = {{37}},
  year         = {{2024}},
}