PURPOSE

Mild traumatic brain injury (mTBI) affects over 1.7 million people each year in the United States¹. The most common imaging for mTBI is routine MRI exam. However, in most cases, MRI exams appear normal. Previous studies have shown that an abnormal cerebral venous oxygen saturation (SvO₂) value could reflect changes in the energy demand of traumatized cerebral tissues². Therefore, the early detection of cerebral SvO₂ may play an important role in better diagnosing mTBI. Recently, susceptibility mapping (SWIM) has been introduced to evaluate cerebral SvO₂³. However, no existing literature has reported the relationship between cerebral SvO₂ levels and different periods of elapsed time post trauma (ETPT), and between SvO₂ levels and neurophysiological function. In this study, we aim to assess the changes in regional cerebral SvO₂ in mTBI patients using SWIM and to investigate the correlation between this parameter, ETPT, and neurophysiological function.

METHODS

48 mTBI patients and 32 healthy controls (HCs) participated in this study. The inclusion criteria were made based on the definition of mTBI by the American Congress of Rehabilitation Medicine⁴. The patients were then divided into an axonal injury (AI) group (16 patients) and non-AI group (32 patients) depending on the imaging-based evidence of AI. MRI data were collected on a MAGNETOM Trio Tim 3T MR scanner (Siemens, Erlangen, Germany) and included a routine head scan and susceptibility-weighted imaging (SWI). The parameters of SWI were: TR/TE=27/20ms; field of view=230×200mm2; receiver bandwidth=120 Hz/pixel; flip angle=15°; number of slices=56; acquisition time=179 sec; voxel resolution=0.5×0.5×2mm³. SWIM was reconstructed based on both the phase and magnitude data using an in-house-developed SMART (Susceptibility Mapping and Phase Artifacts Removal Toolbox, Detroit, Michigan, USA) software. The susceptibility of bilateral cerebral veins (cortical veins, thalamostriate veins, septum pellucidum veins, internal veins, basal veins) and the straight sinus were measured using another in-house-developed software SPIN (signal processing in nuclear magnetic resonance, Detroit, Michigan, USA) (Fig. 1). The changes of regional cerebral SvO₂ were calculated using the susceptibility of veins in mTBI patients and HCs according to the equations [ ∆X_vein-tissue=∆X_do.Hct (1-Y) and ΔY_np = -(1-Y_n)ΔX_np/X_n ]³. The agreement between the vein susceptibility from the HCs in our study and that of the normal subjects as reported by Doshi et al.²was calculated using Pearson’s correlation. The differences in vein susceptibility among AI, non-AI, and HC groups were explored using non-parametric testing. The difference in the ETPT and the Rivermead Post-Concussion Symptoms Questionnaire (RPQ) scores between the AI and non-AI groups was calculated using the Mann-Whitney U test and an independent-sample t-test. The correlations between the susceptibility of the cerebral veins in the AI and non-AI groups and either ETPT or RPQ scores were calculated using a Pearson’s or Spearman’s correlation.

RESULTS

The susceptibility of cerebral veins was highly consistent with that of normal controls as reported by Doshi et al.’s study² (r=0.914, P=0.011) (Fig. 2). There was a significant difference in the susceptibility of the straight sinus among the three groups (p=0.000) (Fig. 3). The susceptibility of the straight sinus in the non-AI (256±35, P=0.000) and AI (265±61, P=0.004) groups was significantly lower than that in the HC group (311±43) (Fig. 4). The SvO₂ of the straight sinus in non-AI and AI patients increased by 5.32% and 4.42%, respectively. In the non-AI group, the susceptibility of the straight sinus significantly positively correlated with ETPT (r=0.514, P=0.006) and in the AI group, the susceptibility of the straight sinus negatively correlated with the RPQ scores (r=-0.582, P=0.018) (Fig. 5).

DISCUSSION

Our results do not indicate a significant difference in the susceptibility of the straight sinus between the AI and non-AI patient groups. The reason for this may be that the time at which the change is measured is critical. Since the ETPT for the non-AI group was 51 hours and was much shorter than the 110 hours in the AI group, the oxygenation levels may have recovered to a level similar to that of the AI group. This is supported in part by the fact that we found that susceptibility in the non-AI group increased with increasing ETPT. In other words, in the non-AI group, the SvO₂ decreased with increasing ETPT. Our study also suggests that increased cerebral SvO₂ indicates impaired neurophysiological functioning. This may be due to reduced consumption of oxygen at the tissue level after injury which may translate into impaired neurophysiological function.

CONCLUSION

The results of this study suggest that SWIM is a useful tool to assess cerebral SvO₂. A decreased susceptibility of the straight sinus (indicating increased SvO₂) appears to correlate with brain function deficits in mTBI patients.

Acknowledgements

The contribution of each author to the work was listed as followed: Chao Chai (C.C.): substantial contributions to conception and design, acquisition of data, or statistical analysis and interpretation of data; drafting the abstract; Chao Zuo (C.Z.): acquisition of data, or analysis and interpretation of data; Linlin Fan (L.L.F.): contributions to conception and design; statistical analysis and interpretation of data; Tianyi Qian (T.Y.Q.): analysis and interpretation of data; E Mark Haacke (E.M.H.): substantial contributions to conception and design; analysis and interpretation of data; revising it critically for important intellectual content; Wen Shen (W.S.) and Shuang Xia (S.X.): substantial contributions to conception and design; agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.