Patients with acute brain pathology requiring ferromagnetic bio-medical implants for on-going invasive monitoring are largely excluded from the benefits of MRI scanning. We evaluated the behaviour of a thermal diffusion cortical blood flow (TD-CBF) sensor both in vitro (phantom gelatin model) and in vivo environments in a high field strength MRI system. Two baboons underwent cranial subdural implantation of 2 TD-CBF sensors/hemisphere and a single left parietal sensor was implanted subcortically to determine any deleterious effects. Using standard MRI sequences, artefact size, thermal effects, current generation, movement and reliability of recordings were assessed during scanning. The deflection forces were negligible, no observable thermal effects were demonstrated, while wide fluctuations in cerebral blood flow recordings were recorded. Mean image artefact size for implanted sensors was 6 times larger than in vitro. Patients with an implanted TD-CBF sensor may be safely imaged provided the device is disconnected. The MRI images obtained are of an acceptable quality.

BACKGROUND Previous studies have demonstrated that rhinogenic subdural empyema (SDE) generally has a good prognosis. Most patients are admitted with an altered level of consciousness or significant neurological deficit, but eventually have a good outcome. It is well known that intra-operative brain swelling may occur with subdural empyema. OBJECTIVE To define cerebral blood flow (CBF) dynamics and determine the role of cerebral hyperaemia, if any, in intracranial SDE. METHODS CBF dynamics were assessed in five patients (mean age 13.2 +/- 2.2 years) with unilateral rhinogenic convexity SDE documented on computer tomography (CT). Regional cortical blood flow (rCBF) was measured using a thermo-coupled sensor placed on the cortex at the time of surgery. Dynamic CT scans were performed to assess cerebral blood volume (CBV) quantitatively, while transcranial Doppler ultrasonography (TCD) was used to measure cerebral blood flow velocities (CBF velocities) both pre- and post-operatively for 21 days. The opposite 'normal' hemisphere served as a control for each patient. RESULTS Post-operative rCBF and CBF velocities in the pathological hemisphere progressively increased to plateau at 96 hours. Cerebral blood volume was increased bilaterally, but to a greater extent in the pathological hemisphere and more so in grey than white matter. These haemodynamic changes, though clinically significant did not reach statistical significance (p>0.05). CONCLUSION Our results suggest that the accompanying brain swelling in rhinogenic SDE is a complex event, with reactive cerebral hyperaemia possibly playing neuroprotective role. Furthermore, unilateral convexity empyema causes bilateral cerebral haemodynamic changes. Future studies are necessary to define the aetiology of brain swelling in intracranial SDE.