The Monro-Kellie hypothesis (MKH) states that volume changes in any intracranial component (blood, brain tissue, cerebrospinal fluid) should be counterbalanced by a co-occurring opposite change to maintain intracranial pressure within the fixed volume of the cranium. In this feasibility study, we investigate the MKH application to structural magnetic resonance imaging (MRI) in observing compensating intracranial volume changes during hypercapnia, which causes an increase in cerebral blood volume. Seven healthy subjects aged from 24 to 64 years (median 32), 4 males and 3 females, underwent a 3-T three-dimensional T1-weighted MRI under normocapnia and under hypercapnia. Intracranial tissue volumes were computed. According to the MKH, the significant increase in measured brain parenchymal volume (median 6.0 mL; interquartile range 4.5, 8.5; p = 0.016) during hypercapnia co-occurred with a decrease in intracranial cerebrospinal fluid (median -10.0 mL; interquartile range -13.5, -6.5; p = 0.034). These results convey several implications: (i) blood volume changes either caused by disorders, anaesthesia, or medication can affect outcome of brain volumetric studies; (ii) besides probing tissue displacement, this approach may assess the brain cerebrovascular reactivity. Future studies should explore the use of alternative sequences, such as three-dimensional T2-weighted imaging, for improved quantification of hypercapnia-induced volume changes.

中文翻译：

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高碳酸血症期间动脉二氧化碳压力的变化与脑实质体积的变化有关。


门罗-凯利假说 (MKH) 指出，任何颅内成分（血液、脑组织、脑脊液）的体积变化都应该通过同时发生的相反变化来平衡，以将颅内压维持在颅骨的固定体积内。在这项可行性研究中，我们研究了 MKH 在结构磁共振成像 (MRI) 中的应用，以观察高碳酸血症期间补偿性颅内体积变化，从而导致脑血容量增加。 7 名健康受试者，年龄 24 至 64 岁（中位数 32），其中 4 名男性和 3 名女性，在正常碳酸血症和高碳酸血症下接受了 3-T 三维 T1 加权 MRI。计算颅内组织体积。根据 MKH，高碳酸血症期间测得的脑实质体积显着增加（中位数 6.0 mL；四分位距 4.5、8.5； p = 0.016），同时发生颅内脑脊液减少（中位值 -10.0 mL；四分位距 -13.5） ，-6.5； p = 0.034）。这些结果传达了几个含义：（i）由疾病、麻醉或药物引起的血容量变化会影响脑容量研究的结果； (ii)除了探测组织位移外，这种方法还可以评估脑血管反应性。未来的研究应探索使用替代序列，例如三维 T2 加权成像，以改进高碳酸血症引起的体积变化的量化。