Despite vulnerability to microstructural tissue damage following mild traumatic brain injury (mTBI), key subcortical brain regions have been overlooked in quantitative susceptibility mapping (QSM) studies. Alterations to tissue composition in the functionally and structurally distinct hippocampal subfields and basal ganglia regions may reflect distinct symptomatology, and better characterisation of these regions is needed to increase our understanding of mTBI pathophysiology. To address this issue, we analysed differences in positive and negative QSM values between 25 males with acute ( < 14 days) sports-related mTBI (sr-mTBI) and 25 age-matched male controls across 10 hippocampal subfields and 16 basal nuclei. Additional variables of interest including age, injury severity, and days since injury at time of the magnetic resonance imaging (MRI) scan were also correlated with both positive and negative susceptibility values. Primary analyses indicated no significant difference in positive susceptibility values between sr-mTBI participants and controls for hippocampal and basal ganglia ROIs. For negative sign values, susceptibility was significantly less negative for sr-mTBI participants in the cornu ammonis 4 (CA4) region only (pFDR = 0.04). In line with the known linear relationship between iron deposition and age in deep grey matter, particularly within the first three decades of life, significant positive relationships were observed between net positive susceptibility and age in the putamen, caudate, red nucleus, parabrachial pigmented nucleus, and ventral pallidum (pFDR < 0.05). Positive relationships were also observed between absolute negative susceptibility values and age in the hippocampal fimbria, caudate, and extended amygdala (pFDR < 0.05), suggesting age-related calcifications in these regions. A negative relationship was observed between absolute negative values and age in the ventral pallidum (pFDR = 0.04), indicating potential changes to myelin content in this region. No significant associations were observed between any other variable and signed susceptibility values. The results of this study contribute to, and extend, prior literature regarding the temporal kinetics of biomagnetic substrates as a function of ageing. Decreased negative susceptibility after mTBI in the CA4 region also suggests potential injury-related effects on myelin content or neuron loss; a particularly interesting finding in light of the well-established vulnerability of cell populations in this region and susceptibility to pathology in chronic traumatic encephalopathy (CTE). The lack of other significant between-group differences suggest that alterations to tissue content may not be quantifiable at the acute stage of injury in subcortical ROIs or may be masked by age-related tissue susceptibility changes as a common feature across all participants in this young cohort. Future research should consider the use of longitudinal study designs to mitigate the influence of these factors. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This study was funded by a grant from the Health Research Council of New Zealand (HRC), grant \#21/622. The first author (CE) is supported by a Dame Dorothy Winstone Doctoral Completion Award from the Kate Edger Foundation of New Zealand. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The Health and Disabilities Ethics Committee (HDEC) gave ethical approval for this work The Auckland University of Technology Ethics Committee (AUTEC) gave institutional approval for this work I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors
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