Chaos and COSMOS-Considerations on QSM methods with multiple and single orientations and effects from local anisotropy

Dimitrios G Gkotsoulias; Carsten Jäger; Roland Müller; Tobias Gräßle; Karin M Olofsson; Torsten Møller; Steve Unwin; Catherine Crockford; Roman M Wittig; Berkin Bilgic; Harald E Möller

doi:10.1016/j.mri.2024.04.020

Chaos and COSMOS-Considerations on QSM methods with multiple and single orientations and effects from local anisotropy

Magn Reson Imaging. 2024 Apr 16:110:104-111. doi: 10.1016/j.mri.2024.04.020. Online ahead of print.

Authors

Affiliations

¹ Nuclear Magnetic Resonance Methods & Development Group, Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany. Electronic address: gkotsoulias@cbs.mpg.de.
² Department of Neurophysics, Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany.
³ Nuclear Magnetic Resonance Methods & Development Group, Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany.
⁴ Epidemiology of Highly Pathogenic Microorganisms, Robert Koch-Institute, Berlin, Germany.
⁵ National Veterinary Institute, Uppsala, Sweden.
⁶ Kolmården Wildlife Park, Norrköping, Sweden.
⁷ Wildlife Health Australia, Canberra, Australia.
⁸ Department of Human Behavior, Ecology and Culture, Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany; The Ape Social Mind Lab, Institut des Sciences Cognitives Marc Jeannerod, Bron, France; Taï Chimpanzee Project, Centre Suisse de Recherches Scientifiques, Abidjan, Côte d'Ivoire.
⁹ Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Boston, MA, USA; Harvard Medical School, Boston, MA, USA; Harvard/MIT Health Sciences and Technology, Cambridge, MA, United States.

PMID: 38631534
DOI: 10.1016/j.mri.2024.04.020

Abstract

Purpose: Field-to-susceptibility inversion in quantitative susceptibility mapping (QSM) is ill-posed and needs numerical stabilization through either regularization or oversampling by acquiring data at three or more object orientations. Calculation Of Susceptibility through Multiple Orientations Sampling (COSMOS) is an established oversampling approach and regarded as QSM gold standard. It achieves a well-conditioned inverse problem, requiring rotations by 0°, 60° and 120° in the yz-plane. However, this is impractical in vivo, where head rotations are typically restricted to a range of ±25°. Non-ideal sampling degrades the conditioning with residual streaking artifacts whose mitigation needs further regularization. Moreover, susceptibility anisotropy in white matter is not considered in the COSMOS model, which may introduce additional bias. The current work presents a thorough investigation of these effects in primate brain.

Methods: Gradient-recalled echo (GRE) data of an entire fixed chimpanzee brain were acquired at 7 T (350 μm resolution, 10 orientations) including ideal COSMOS sampling and realistic rotations in vivo. Comparisons of the results included ideal COSMOS, in-vivo feasible acquisitions with 3-8 orientations and single-orientation iLSQR QSM.

Results: In-vivo feasible and optimal COSMOS yielded high-quality susceptibility maps with increased SNR resulting from averaging multiple acquisitions. COSMOS reconstructions from non-ideal rotations about a single axis required additional L2-regularization to mitigate residual streaking artifacts.

Conclusion: In view of unconsidered anisotropy effects, added complexity of the reconstruction, and the general challenge of multi-orientation acquisitions, advantages of sub-optimal COSMOS schemes over regularized single-orientation QSM appear limited in in-vivo settings.

Keywords: Anisotropic magnetic susceptibility; COSMOS; Gradient-recalled echo; Quantitative susceptibility mapping.