Quantifying the effect of posture on intracranial physiology in humans by MRI flow studies

Abstract

Purpose:To quantify the effect of posture on intracranial physiology in humans by MRI, and demonstrate the relationship between intracranial compliance (ICC) and pressure (ICP), and the pulsatility of blood and CSF flows. Materials and Methods:Ten healthy volunteers (29 Ϯ 7 years old) were scanned in the supine and sitting positions using a vertical gap MRI scanner. Pulsatile blood and CSF flows into and out from the brain were visualized and quantified using time-of-flight (TOF) and cine phase-contrast techniques, respectively. The total cerebral blood flow (tCBF), venous outflow, ICC, and ICP for the two postures were then calculated from the arterial, venous, and CSF volumetric flow rate waveforms using a previously described method. Results:In the upright posture, venous outflow is considerably less pulsatile (57%) and occurs predominantly through the vertebral plexus, while in the supine posture venous outflow occurs predominantly through the internal jugular veins. A slightly lower tCBF (12%), a considerably smaller CSF volume oscillating between the cranium and the spinal canal (48%), and a much larger ICC (2.8-fold) with a corresponding decrease in the MRI-derived ICP values were measured in the sitting position. Conclusion:The effect of posture on intracranial physiology can be quantified by MRI because posture-related changes in ICC and ICP strongly affect the dynamics of cerebral blood and CSF flows. This study provides important insight into the coupling that exists between arterial, venous, and CSF flow dynamics, and how it is affected by posture. BODY POSTURE strongly affects intracranial hydrodynamics and cerebral hemodynamics. Yet quantitative data on posture-related changes in parameters such as intracranial compliance (ICC), intracranial pressure (ICP), and cerebral blood flow (CBF) in humans is scarce because of the invasiveness and risk associated with measurements of ICC and ICP, and because most neuroimaging studies used for CBF measurement are constrained to the supine posture. Invasive measurements in a previous study of head-injury patients (1) documented lower ICP and mean blood pressure in the carotid arteries, and relatively unchanged cerebrovascular resistance and CBF when the patient's head was elevated at 30°. Studies on the effect of posture on cerebral venous outflow are more abundant. Angiographic studies in nonhuman primates demonstrated that the internal jugular veins (IJVs) are the main pathway for venous outflow in the supine position, while in the upright posture the IJVs collapse and venous outflow occurs mainly through secondary veins such as the vertebral, epidural, and deep cervical veins, which compose the vertebral venous plexus (2,3). Dilenge and Perey (3) postulated that the closing and opening of the vertebral plexus are related to changes in CSF pressure in the cervical region. Studies in humans utilized colorcoded duplex sonography to quantify the effect of posture on cerebral venous outflow (4 -7). Valdueza et al (4) reported lower total venous outflow in the up...

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