Compartmental anisotropy of filtered exchange imaging (FEXI) in human white matter: What is happening in FEXI?

Hyeong-Geol Shin; Xu Li; Hye-Young Heo; Linda Knutsson; Filip Szczepankiewicz; Markus Nilsson; Peter C M van Zijl

doi:10.1002/mrm.30086

Compartmental anisotropy of filtered exchange imaging (FEXI) in human white matter: What is happening in FEXI?

Magn Reson Med. 2024 Mar 25. doi: 10.1002/mrm.30086. Online ahead of print.

Authors

Hyeong-Geol Shin^{1

2}, Xu Li^{1

2}, Hye-Young Heo^{1

2}, Linda Knutsson^{1

2

3}, Filip Szczepankiewicz³, Markus Nilsson^{4

5}, Peter C M van Zijl^{1

2}

Affiliations

¹ F.M. Kirby Research Center for Functional Brain Imaging, Kennedy Krieger Institute, Baltimore, Maryland, USA.
² Russell H. Morgan Department of Radiology and Radiological Science, The Johns Hopkins University School of Medicine, Baltimore, Maryland, USA.
³ Department of Medical Radiation Physics, Clinical Sciences Lund, Lund University, Lund, Sweden.
⁴ Department of Radiology, Clinical Sciences Lund, Lund University, Lund, Sweden.
⁵ Department of Medical Imaging and Physiology, Skåne University Hospital, Lund, Sweden.

PMID: 38525601
DOI: 10.1002/mrm.30086

Abstract

Purpose: To investigate the effects of compartmental anisotropy on filtered exchange imaging (FEXI) in white matter (WM).

Theory and methods: FEXI signals were measured using multiple combinations of diffusion filter and detection directions in five healthy volunteers. Additional filters, including a trace-weighted diffusion filter with trapezoidal gradients, a spherical b-tensor encoded diffusion filter, and a T2 filter, were tested with trace-weighted diffusion detection.

Results: A large range of apparent exchange rates (AXR) and both positive and negative filter efficiencies (σ) were found depending on the mutual orientation of the filter and detection gradients relative to WM fiber orientation. The data demonstrated that the fast-diffusion compartment suppressed by diffusional filtering is not exclusively extra-cellular, but also intra-cellular. While not comprehensive, a simple two-compartment diffusion tensor model with water exchange was able to account qualitatively for the trends in positive and negative filtering efficiencies, while standard model imaging (SMI) without exchange could not. This two-compartment diffusion tensor model also demonstrated smaller AXR variances across subjects. When employing trace-weighted diffusion detection, AXR values were on the order of the R₁ (=1/T1) of water at 3T for crossing fibers, while being less than R₁ for parallel fibers.

Conclusion: Orientation-dependent AXR and σ values were observed when using multi-orientation filter and detection gradients in FEXI, indicating that WM FEXI models need to account for compartmental anisotropy. When using trace-weighted detection, AXR values were on the order of or less than R₁, complicating the interpretation of FEXI results in WM in terms of biological exchange properties. These findings may contribute toward better understanding of FEXI results in WM.

Keywords: anisotropy; diffusion MRI; filtered exchange imaging; water exchange; white matter.

Abstract

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