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Quantitative Susceptibility Mapping of the Brain – A Comparative In vivo Study of Humans and Nonhuman Primates
Rakshit Dadarwal1,2, Luzia Hintz1, Amir Moussavi1, and Susann Boretius1,2

1Functional Imaging Laboratory, German Primate Center, Goettingen, Germany, 2Center for Systems Neuroscience, Goettingen Graduate School for Neurosciences, Biophysics, and Molecular Biosciences, University of Goettingen, Goettingen, Germany

Synopsis

Quantitative susceptibility mapping of the brain was performed in healthy humans and cynomolgus monkeys at comparable age using almost identical MR parameters, including the magnetic field strength. This comparative study revealed very similar values of magnetic susceptibility in gray matter structures between the two species, but a significantly lower magnetic susceptibility in parts of the corpus callosum of monkeys compared to the humans. This difference may be related to differences in the position of fiber tracts relative to the magnetic field lines, but it may also reflect differences in iron content, fiber density, and myelination.

Introduction: In humans1, quantitative susceptibility mapping (QSM) has been successfully used to study iron deposition and demyelination in a wide range of neurological disorders and in aging. Non-human primates (NHPs) are phylogenetically closely related to humans and share a very similar physiology and neuroanatomy with humans. Thus, research with monkeys is most important for basic neurosciences, for our understanding of cognitive, motor and mental illnesses, and brain aging, to mention just a few aspects. However, it is not always clear how well NHPs reflect the human situation. Using MRI as a noninvasive technique applicable both in humans and experimental animals we compared for the first time whole brain QSM in healthy volunteers and cynomolgus monkeys utilizing almost identical MR parameters and magnetic field strength.

Methods:

Subjects: 6 healthy young adult female volunteers with an age range of 23 - 29 years and 5 healthy young adult female cynomolgus monkeys with an age range of 7 - 9 years were included in the study.

Data Acquisition: QSM acquisitions were carried out at 3T (MAGNETOM Prisma, Siemens) using a 3D multi-echo gradient echo (GRE) sequence with TE1/ spacing/ TE9/ TE10 of 3.7/ 4.9/ 43/ 50.4 ms, TR=57 ms, and FA = 20°. The spatial resolutions for humans and monkeys were 0.75 x 0.75 x 0.75 mm³ and 0.312 x 0.312 x 0.312 mm³ respectively.

Data analysis: The data analysis pipeline used likewise for human and monkeys is illustrated in Figure 1. Multi-echo GRE magnitude data was corrected for bias field using ANTs tool2 and was averaged for all echo times. The respective averaged magnitude data was then utilized to create brain masks using FSL bet tool3. A study template was constructed by registering the magnitude images of all subjects. QSM reconstruction was performed on multi-echo phase data using the QSMbox toolbox1. Subject to template ANTs registration transformation files were used to build a QSM study template for human and monkey data, respectively (Fig. 2) on which regions of interests were drawn manually using the ITK-SNAP tool4 (Fig. 3).

Results: Humans and monkeys revealed similar values of magnetic susceptibility in all analyzed gray matter structures, even without referencing them to CSF or whole brain susceptibility (Fig. 4). In contrast, parts of the corpus callosum and, in particular, its body showed lower values of magnetic susceptibility in monkeys compared to the humans.

Discussion: To the best of our knowledge, this is the first study comparing QSM of the brain of monkeys and humans. For most analyzed structures, humans and monkeys revealed similar values of magnetic susceptibility, further supporting NHPs as a valuable model for a better understanding of the human brain. The observed difference in white matter structures may be related to differences in the position of fiber tracts relative to the magnetic field lines but may also reflect differences in iron content, fiber density, and myelination. Further studies are needed to clarify this point.

Acknowledgements

No acknowledgement found.

References

1. Acosta-Cabronero, J et al. A robust multi-scale approach to quantitative susceptibility mapping. NeuroImage, 2018.

2. Avants, B B et al. A reproducible evaluation of ANTs similarity metric performance in brain image registration. NeuroImage, 2010.

3. Smith, S M et al. Fast robust automated brain extraction. Human Brain Mapping, 2002.

4. Yushkevich, Paul A et al. User-guided 3D active contour segmentation of anatomical structures: Significantly improved efficiency and reliability. NeuroImage 2006.

Figures

Fig1. Data analysis pipeline used to reconstruct QSM maps. Study template was created using all subjects bias corrected magnitude data.

Fig2. Maps of magnetic susceptibly for human and monkey brain.

Fig3. Location and size of the analyzed brain regions.

Fig4. Comparison of the magnetic susceptibility given in parts per million for the ROIs shown in Fig. 3. (blue: monkey, orange: human)

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)
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