Oxygen challenge magnetic resonance imaging in healthy human volunteers.

Oxygen challenge imaging involves transient hyperoxia applied during deoxyhaemoglobin sensitive (T2*-weighted) magnetic resonance imaging and has the potential to detect changes in brain oxygen extraction. In order to develop optimal practical protocols for oxygen challenge imaging, we investigated the influence of oxygen concentration, cerebral blood flow change, pattern of oxygen administration and field strength on T2*-weighted signal. Eight healthy volunteers underwent multi-parametric magnetic resonance imaging including oxygen challenge imaging and arterial spin labelling using two oxygen concentrations (target FiO2 of 100 and 60%) administered consecutively (two-stage challenge) at both 1.5T and 3T. There was a greater signal increase in grey matter compared to white matter during oxygen challenge (p < 0.002 at 3T, P < 0.0001 at 1.5T) and at FiO2 = 100% compared to FiO2 = 60% in grey matter at both field strengths (p < 0.02) and in white matter at 3T only (p = 0.0314). Differences in the magnitude of signal change between 1.5T and 3T did not reach statistical significance. Reduction of T2*-weighted signal to below baseline, after hyperoxia withdrawal, confounded interpretation of two-stage oxygen challenge imaging. Reductions in cerebral blood flow did not obscure the T2*-weighted signal increases. In conclusion, the optimal protocol for further study should utilise target FiO2 = 100% during a single oxygen challenge. Imaging at both 1.5T and 3T is clinically feasible.

Respiratory challenge MRI: Practical aspects.

Respiratory challenge MRI is the modification of arterial oxygen (PaO2) and/or carbon dioxide (PaCO2) concentration to induce a change in cerebral function or metabolism which is then measured by MRI. Alterations in arterial gas concentrations can lead to profound changes in cerebral haemodynamics which can be studied using a variety of MRI sequences. Whilst such experiments may provide a wealth of information, conducting them can be complex and challenging. In this paper we review the rationale for respiratory challenge MRI including the effects of oxygen and carbon dioxide on the cerebral circulation. We also discuss the planning, equipment, monitoring and techniques that have been used to undertake these experiments. We finally propose some recommendations in this evolving area for conducting these experiments to enhance data quality and comparison between techniques.

Magnesium in the management of catecholamine-secreting glomus tumours with intracranial extension.

PURPOSE: Catecholamine-secreting glomus jugulare tumours are uncommon and their anesthetic management can be challenging. The authors present the first description of the use of magnesium sulfate in the management of two patients with catecholamine-secreting glomus jugulare tumours where there was significant intracranial extension. CLINICAL FEATURES: Patient 1 underwent a transmastoid transoccipital excision of a catecholamine-secreting glomus tumour. He exhibited marked hemodynamic instability after handling of the tumour began, which was not controlled by sodium nitroprusside. Improved hemodynamic stability was seen after the patient received magnesium sulfate. Patient 2 also underwent a transmastoid transoccipital excision of a catecholamine-secreting glomus tumour. Magnesium sulfate was commenced prior to tumour handling and continued until the tumour was removed. The patient remained hemodynamically stable. Sodium nitroprusside was not required. CONCLUSION: Magnesium sulfate may be useful in preventing or minimizing the blood pressure changes associated with handling during excision of catecholamine-secreting glomus jugulare tumours. It may be of particular benefit in patients where there is significant intracranial extension.