Introduction

There is increasing evidence that neurovascular dysregulation contributes to the persistent symptoms in patients with traumatic brain injury (TBI). In a recent animal study, the cerebral vasculature was reported to be more sensitive to blast-induced TBI than other elements in the brain ¹. Damaged microvasculature may disrupt neurovascular coupling, especially under physiological stress, where the local cerebral blood flow no longer matches the metabolic requirements of the tissue. The negative or abnormally delayed blood oxygenation level dependent (BOLD) signal changes under hypercapnic challenge, recorded in our published ² and recent preliminary findings in both patients ³ and animals ⁴ with mTBI, suggest a potentially novel imaging marker with a sensitivity to localize subtle abnormal vascular function in individual TBI patients. Such an imaging marker may be used to characterize the persistent post-traumatic symptoms that persist for a long time after TBI. In the present study, instead of administering low-dose exogenous carbon dioxide, we applied breathhold challenge for the hypercapnic fMRI on patients with moderate TBI within 48 hours, 2 weeks and 3 months after injury.

Subjects and Methods

Participants: Eleven patients with moderate TBI (5 males, 6 females, aged from 21 to 79 years) were included. All the patients required hospital admission because of the head injury. The Glasgow Coma Scale scored above 9 at admission and the patients also had abnormal imaging findings. Methods: MRI scanning was performed on a 3-Tesla scanner (Siemens Medical Germany) in the Athinoula A. Martinos Center for Biomedical Imaging at the Massachusetts General Hospital. Each patient was scanned within 48 hours (Scan1), 2 weeks (Scan2) and 3 months (Scan3) after the injury All the experimental procedures were explained to the subjects, and signed informed consent was obtained prior to participation in the study. Whole brain MRI datasets were acquired for each subject: 1) standard high-resolution sagittal images acquired with volumetric T1-weighted 3D-MEMPRAGE (TR=2530ms, TE=1.69ms/3.55ms/5.41ms/7.27ms, flip angle=7º, FOV=256mm, matrix=256´256, slice thickness=1mm); 2) SMS BOLD-fMRI images acquired with gradient-echo echo planar imaging (EPI) sequence (TR=1250ms, TE=30ms, flip angle=90º, FOV=256mm, matrix=108´108, thickness=2.4mm) while the subject had breathhold challenge. The paradigm consisted of 2 consecutive phases (resting and breathhold) repeated 5 times. The resting phase lasted no less than 60 seconds, while the breathold phase lasted 30 seconds. The challenge lasted 10 minutes. Data analysis: All the BOLD-fMRI data were imported into the software Analysis of Functional NeuroImage (AFNI) ⁵ (National Institute of Mental Health, http://afni.nimh.nih.gov) for time-shift correction, motion correction, normalization and detrending. Individual maps of neurovascular response with percent BOLD signal change were derived. Breathhold index (BHI) was derived using multiple regression with a regressor of breath duration. BHI was defined by the maps of total percent BOLD signal changes over per unit time of breath. Analysis of statistical parametric maps were corrected at the overall threshold of p<0.05. The amplitude of positive BHI change in each brain region was derived based on the parcellation using Freesurfer ⁶ (http://surfer.nmr.mgh.harvard.edu). To increase the sensitivity of detecting negative or abnormally delayed BOLD signal changes, Hilbert transform analysis ⁷ was applied to derive individual maps with BOLD response delay relative to the changes of breath duration.

Results

Reduced amplitude of BHI was observed in all brain regions in 2-week follow-up (Scan2) relative to the 48-hour initial (Scan1) and 3-month follow-up scan (Scan3) (Figure 1), which is consistent with the reduced cerebral perfusion that we reported in the same group of patients. Figure 2 shows abnormally delayed BOLD signal changes to the breathhold challenge in multiple brain regions of a representative patient in the 48-hour scan. In the 2-week and 3-month follow-ups, the BOLD responses to the breathhold challenge in some of these brain regions restored to normal.

Discussion

The abnormally delayed BOLD responses in prior hypercapnic studies have been reported in patients with steno-occulsive disease ⁷ as well as in patients with mild traumatic brain injury ³; in those individuals, it was suggested that the delayed response was due to a relative “steal phenomenon” of compromised tissue away from intact areas. The parallel reduction of cerebral perfusion and BHI suggest the potential disruption of neurovascular coupling due to reduced cerebral blood flow. Our findings suggest that the neurovascular response to the hypercapnic breathhold challenge may be used as an imaging marker to localize and follow subtle abnormal vascular function.

Acknowledgements

This research was supported by Congressionally Directed Medical Research Program W81XWH-13-2-0067.