Blood flow and oxygen extraction fraction in regions of oedema following head injury

Introduction

Cerebral blood flow (CBF) is reduced around areas of contused brain after head injury [1]. However, since the cerebral metabolism is also reduced this may represent appropriate flow-metabolism coupling, rather than ischaemia. Matching of CBF to metabolism is quantified as the oxygen extraction fraction (OEF). We performed magnetic resonance imaging (MRI) and Oxygen 15 positron emission tomography (O-PET) to quantify CBF and OEF in regions of pericontusional oedema after head injury.

Methods

After local ethical approval, five patients with severe head injury underwent structural MRI and O-PET, in the first week after injury. The two studies were performed in immediate succession, with every effort made to maintain stable physiology. The fluid attenuation inversion recovery (FLAIR) MRI sequence provides cerebrospinal fluid-nulled, T₂-weighted MRI images, on which oedema is hyperintense. The FLAIR images were coregistered and voxel resized to O-PET-derived maps of CBF and OEF, using a published methodology [2]. Each patient's coregistered FLAIR image was inspected for the largest and most apparent region of pericontusional oedema. These regions were then manually outlined and applied to PET maps, and CBF and OEF in these regions of oedema were calculated. Results were compared with unit reference data obtained from healthy volunteers [3].

Results

Normal values of CBF and OEF obtained from volunteer datasets for mixed grey-white regions were 35 ± 5 ml/100 g/min and 45 ± 5%, respectively. While pericontusional regions showed a significantly lower CBF (20.5 ± 8.3 ml/100 g/min), we observed a wide range of values both across subjects and within individual lesions (14.7–35 and 5.1–52.3 ml/100 g/min, respectively). However, mean OEF values were low (35.4 ± 2.1%), with a much smaller range of values across patients (31.8–37.5%) and individual image voxels (24.9–41.8%). The 95% confidence intervals for the population mean pericontusional CBF and OEF were 10.2 and 30.8 ml/100 g/min and 32.8 and 38.1%, respectively.

Conclusions

The wide range of CBF values should make clinicians wary of predicting the viability of tissue in regions of pericontusional oedema. The values are all above the threshold, of 8.4 ml/100 g/min [4], for irreversible ischemia from the stroke literature. Thresholds for irreversible ischemia after head injury are unknown. The upper 95% confidence interval for the OEF is only 38% and the maximum OEF in any voxel was 42%, suggesting that the reduction in CBF may be appropriate to the metabolic demands of the tissue.