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The measurement of blood flow volumes in veins poses a significant challenge, particularly in the context of jugular and mesenteric veins. The conventional approach to quantifying blood flow in arteries cannot be directly applied to veins. Conventionally, the flow volume of an artery is calculated by multiplying the transsectional area of the artery by the mean flow velocity. However, this method exhibits fundamental drawbacks, both in arteries and even more so in veins.
The following objections must be addressed:

1. Differing flow velocities across the vessel lumen
2. Non-circular shape of veins
3. Pulsation of the vessel’s shape
4. Pulsation of flow velocities
5. Backflow in veins and arteries
6. Curving of blood vessels
7. Modulation of flow velocities and shape by the external forces as pressure from bowel loops or respiratory traction.

It is evident that these seven influences cannot be taken into account by a simple formula multiplying a static, circular vessel shape by a uniform, constant and evenly distributed flow velocity of all erythrocytes at a specific point in a blood vessel.

In the case of the mesenteric and the jugular veins, an additional problem needs to be resolved: The parallel course of the vessels and the skin (respectively, the surface of the ultrasound transducer applied to the skin) results in a Doppler angle near to 90°, subsequently leading to a very weak and unreliable Doppler signal.The reason for the unreliability is not only that the signal is weak. Of greater significance is the fact that minute alterations in the orientation of the transducer or the blood vessel, resulting from inadvertent movement of the transducer by the examiner, changes in blood vessel position due to breathing, patient movements or pulsation, have a substantial effect on the recorded signal. This is due to the fact that the Doppler signal is subject to the cosine function of the Doppler angle. The cosine values of angles close to 90° exhibit significant variability, even with minimal angle changes, making it challenging to obtain precise volume flow measurements from vessels running parallel to the skin using conventional techniques.This challenge can be addressed by employing a prismatic ultrasound transducer distancer, which serves to create an angle between the skin and the ultrasound transducer.

Such distancers are not commercially available, but they can be custom-manufactured by specialised enterprises.

The remaining seven objections can only be addressed by utilising the PixelFlux technique in conjunction with a 4D recording of the respective blood vessel employing a high-end matrix transducer. Further elucidation on this subject is available on a dedicated website (coming soon).