Abstract
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.
Highlights
The cerebral vasculature plays a critical role in maintaining an optimal supply of cerebral blood flow (CBF) to Dilation of the cerebral arteries, arterioles and capillaries prompts an increase in cerebral blood volume (CBV) and CBF [2, 3]
An increase in CBV is accompanied by an outflow of cerebrospinal fluid (CSF) to the spinal canal in order to maintain intracranial pressure within the fixed volume formed by the cranium and dura mater as postulated by Burrow’s correction of the MonroKellie doctrine (Fig. 1) [4]
Following the Monro-Kellie hypothesis, we hypothesise that compensating volume changes should be observed in brain parenchymal and CSF volumes on structural magnetic resonance imaging (MRI) in response to cerebral blood volume increases during hypercapnia
Summary
This study illustrates the application of the MonroKellie hypothesis to structural MRI. Hypercapnia induced an increased segmented brain volume and decreased cerebrospinal fluid volume. Blood volume changes can affect the results of brain volumetric studies. Hypercapnia volume changes could serve as a volumetric measure of cerebrovascular reactivity
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