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Flow and oxygenation dependent (FLOOD) MR images of GH3 prolactinomas display large intensity increases in response to carbogen (5% CO 2 /95% O 2 ) breathing. To assess the relative contributions of carbon dioxide and oxygen to this response and the tumour oxygenation state, the response of GH3 prolactinomas to 5% CO 2 /95% air, carbogen and 100% O 2 was monitored by FLOOD MRI and pO 2 histography. A 10-30% image intensity increase was observed during 5% CO 2 /95% air breathing, consistent with an increase in tumour blood flow, as a result of CO 2 -induced vasodilation, reducing the concentration of deoxyhaemoglobin in the blood. Carbogen caused a further 40-50% signal enhancement, suggesting an additional improvement due to increase blood oxygenation. A small 5-10% increase was observed in response to 100% O 2 , highlighting the dominance of CO 2 -induced vasodilation in the carbogen response. Despite the large FLOOD response, non-significant increases in tumour pO 2 were observed in response to the three gases. Tissue pO 2 is determined by the balance of oxygen supply and demand, hence increased blood flow/oxygenation may not necessarily produce a large increase in tissue pO 2 . The FLOOD response is determined by the level of deoxygenation of blood, the size of this response relating to vascular density and the potential of high-oxygen content gases to improve the oxygen supply to tumour tissue.

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