Arterial CO2 pressure changes during hypercapnia are associated with changes in brain parenchymal volume

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

MRI quantitative hemodynamic evaluation of the brain

The cerebral blood flow (CBF) or the delivery of nutrients to the brain tissue is essential for the viability of brain cells and is a necessity for the human body to perform physical and mental activities. Both under-and overperfusion of the brain tissue can cause substantial harm wherefore the CBF is very tightly regulated. Dilatation and constriction of arterioles contribute to this regulation which can be expressed by the cerebrovascular reactivity (CVR). Through the cerebral blood flow, essential nutrients such as glucose and oxygen are delivered to the brain tissue. The consumption of these nutrients can either be expressed by the cerebral metabolic rate of glucose (CMRGlc), the oxygen extraction fraction (OEF) or the cerebral metabolic rate of oxygen (CMRO2). The CBF, the CVR, the CMRGlc, the OEF, and the CMRO2, or the brain hemodynamic parameters, provide important information of the brain tissue’s health. For instance, in patients with steno-occlusive disease, they provide insight into the amount of stress the brain tissue is opposed to, and, they are reflective of maturation and aging processes. Fluorodeoxyglucose positron emission tomography (PET) and oxygen-15 PET are the gold standards to evaluate these parameters. But, the invasiveness of these techniques and the requirement of an onsite cyclotron in case of oxygen-15 PET have limited their application in clinical practice. This thesis focused on the development and application of alternative non-invasive techniques to evaluate the brain hemodynamic parameters. We demonstrated that arterial spin labeling (ASL) obtained CBF values in neonates reflect maturation and can predict adverse outcome in case of a perinatal ischemic event. Similar, T2-TRIR obtained OEF values were lower in these latter infants and could thus be indicative of adverse outcome. In the adult population we applied calibrated MRI, which combines ASL and blood oxygenation level-dependent (BOLD) measurements performed at hypercapnic and hyperoxic breathing, to obtain a full evaluation of all brain hemodynamic parameters. Although some technical issues still prohibit its translation to clinical practice, the technique seems to be promising in the evaluation of patients with steno-occlusive disease and in the investigation of the aging process. At last, as the amount of brain tissue in adults also provides important information regarding (pathological) aging, a fast volumetric CSF mapping technique was developed. This technique was consequently shown to be able to act as a surrogate for time-and post-processing intensive imaging (tools) which are now commonly used to evaluate brain atrophy. In conclusion, non-invasive imaging techniques to evaluate brain hemodynamics show great promise. The application of these techniques in clinical practice will be possible when some of the limitations, encountered in our clinical studies, are dealt with. Once optimized they will be of great value in the investigation of maturation and ageing processes, in therapeutic decision making in patients with ischemic or cerebrovascular disease, and in the prediction of outcome in patients with ischemic disease

In vivo blood T(1) measurements at 1.5 T, 3 T, and 7 T

The longitudinal relaxation time of blood is a crucial parameter for quantification of cerebral blood flow by arterial spin labeling and is one of the main determinants of the signal-to-noise ratio of the resulting perfusion maps. Whereas at low and medium magnetic field strengths (B(0) ), its in vivo value is well established; at ultra-high field, this is still uncertain. In this study, longitudinal relaxation time of blood in the sagittal sinus was measured at 1.5 T, 3 T, and 7 T. A nonselective inversion pulse preceding a Look-Locker echo planar imaging sequence was performed to obtain the inversion recovery curve of venous blood. The results showed that longitudinal relaxation time of blood at 7 T was ∼ 2.1 s which translates to an anticipated 33% gain in the signal-to-noise ratio in arterial spin labeling experiments due to T(1) relaxation alone compared with 3 T. In addition, the linear relationship between longitudinal relaxation time of blood and B(0) was confirmed. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.Neuro Imaging Researc