# Numerical Modeling of Venous Outflow from the Cranial Cavity in the Supine Body Position
## 4 Discussion
Thus, addressing these venous abnormalities may have a prophylactic or therapeutic potential. Yet, the hemodynamic relevance of different types of strictures in the IJVs is difficult to assess in living subjects. Firstly, all investigations aimed at such an evaluation are more or less invasive and not free from severe side effects. Therefore, clinical investigations are very difficult to conduct, considering bioethical issues. Secondly, proper interpretation of these investigations (catheter venography, Doppler ultrasonography, or magnetic resonance imaging) is not obvious [32]. Therefore, in silico flow modeling can provide a surrogateevaluation of the hemodynamic significance of particular types of these structures. This was the primary aim of our current CFD study. From the physical point of view, it may seem obvious that a narrow or eccentrically positions stricture should evoke flow impairment. Still, from the clinical perspective, this problem is not so straightforward. In the neck, there are many alternative outflow routes, and some of the veins can be temporarily squeezed by adjacent bones, muscles, and ligaments without clinical consequences. There is a general thinking among clinicians that irrespective of these narrowings, there are still enough veins to provide adequate cerebral venous drainage. In the case of a suspected clinically relevant stenosis, doctors either perform non-invasive MR imaging or invasive catheter angiography, while color Doppler ultrasonography primarily serves the purpose of a screening test. Still, there are problems with interpretations of all these diagnostic modalities. While MR can reveal decreased flow velocity and reduced vein diameter (e.g., caused by external compression), this diagnostic method poorly shows abnormal valves. On the contrary, standard catheter angiography cannot demonstrate flow abnormalities caused by stenoses located at the level of the jugular foramen, unless the diagnostic catheter is advanced to the intracranial venous sinuses, which is associated with a risk of life-threatening bleeding; therefore, it is not performed routinely. Moreover, during catheter venography, doctors actually assess the flow of the contrast and not the whole blood stream. Consequently, such phenomena as flow separation or flow reversal can remain undetected. Theoretically, these flow phenomena could be detected non-invasively using color Doppler ultrasonography; yet, for the time being, no clinically validated criteria exist that can be used for such a purpose [32]. Moreover, usually color Doppler ultrasonography cannot be used at the level of the jugular foramen because of the bones in this area. This lack of understanding of relevant vs. irrelevant flow disturbances in the IJV is probably responsible for discordant results of the diagnostic studies in these veins [33]. Currently, the significance of irregular flow in blood vessels in the medical literature is not adequately understood. Doctors acknowledge that in most of blood vessels, the energy losses resulting in a drop of pressure come from friction. But in some blood vessels, frictional energy losses are accompanied by other types of losses, in physics commonly referred to as minor losses [34; 35]. These energy losses primarily result from flow separation that typically develops when a blood vessel bends or suddenly expands. Except for aneurysms (pathologically enlarged arteries), the significance of flow separation and their associated minor losses has been largely ignored by medical research. In the case of abdominal aortic aneurysms, numerical simulations explained that flow separation and stagnation inside the aneurysm, and not an increased tension of the aneurys wall, are primarily responsible for ruptures [36; 37; 38; 39; 40]. Similarly, CFD studies on cerebral artery aneurysms helped in the proper understanding of flow phenomena occurring in these pathological dilatations. It subsequently resulted in designing special stents that currently are routinely used for the treatment of cerebral aneurysms [41; 42; 43; 44].