2023 11 30
Four-dimensional (4D) colour Doppler sonographic flow volume measurements-a new promising tool to clarify unresolved cases
Disturbed blood flow towards or from an organ may cause substantial pain, dysfunction of the organ and may require collateral circulation which can be responsible for symptoms in remote areas.
These situations can only be properly diagnosed with a quantitative blood flow volume measurement in all affected part of the circulation.
The conventional technique is to multiply the local flow velocity with the area of the vessel which is examined. The ultrasound machine is equipped with a virtual measurement device – the so-called sample volume – which is placed in the center of the vessel. There it records continuously the flow velocity. Then, the transverse diameter of the vessel at the examination site is measured and the area of the vessel is calculated with the formula for a circle. By multiplying area and flow velocity the flow volume is calculated.
This approach has some inherent fundamental drawbacks.
- The flow velocity which is measured in the center of the vessel is by no means representative for the entire transsectional area of the vessel since in the center of the vessel the flow is always faster than in the periphery. The friction of the blood on the wall of the vessel is producing a velocity decline from the center to the periphery. The distribution of the velocity within the vessel is thus difficult to predict. With this traditional method many assumptions are necessary which cannot be checked during the examination. The decline of velocity from center to periphery is dependent on the size of the vessel, the velocity of the flow, the viscosity of the blood, the shape of the vessel and curves of the vessel .
- The consequence is an inherent unpredictable error which may substantially influence the measurement result and thus the diagnostic conclusions.
These imponderabilities are even more pronounced in veins than in arteries. While arteries are usually round veins are frequently not. Tissue volume flow measurements are still far behind the diagnostic horizon.
Venous flow volume measurement is thus a major challenge. Subsequently, the quantitative diagnostic of venous disorders is often uncharted territory.
We developed a solution for this problem-the PixelFlux technique.
The PixelFlux technique is measuring flow velocities pixel by pixel and is thus taking into account in a very realistic manner the individual distribution of flow velocities inside the vessel. Thus, the application of PixelFlux opens the window to many diagnosis which are overseen with conventional imaging techniques.
One problem but still remained: The colour of a pixel encodes the velocity of the blood stream at the very location of the pixel. But since the Doppler effect which is background for calculating the colour hue by the ultrasound machine is angle dependent the colour of the pixel on the ultrasound screen usually does not reflect exactly the flow velocity but only the partial vector of the full flow vector which (the partial vector) is directed towards the ultrasound transducer.
In most cases, the blood vessel is not running perpendicular to the ultrasound transducer but in varying angles. It is thus necessary to correct the angle between the beam of ultrasound waves and the interrogated vessel. Moreover, a transverse section of the vessel is required to depict not only the blood flow velocity in the central plane of the vessel but also in the peripheral layers left and right is imaging plane. But if the vessel is cut transversally, it is not clear in which direction the vessel is running. Thus, no angle correction can be done with the traditional colour Doppler measurement technique.
This dilemma is now solved with the 4D-flow volume measurements with the PixelFlux technique.
We prepared for years for this diagnostic breakthrough, but ultrasound companies only recently were able to deliver sufficiently evolved ultrasound transducers to apply this revolutionary technique.
The unrivalled pioneer in this field is Philips® with the introduction of so-called matrix transducers.
Matrix transducers in their latest stage of development now allow a simultaneous recording of multiple layers which lie side-by-side-comparable to the simultaneous use of many transducers at the same time.
Video
This 4D-sonographic video shows the frontal view (left), sagittal view (upper right) and the most important and technically challenging horizontal view (lower right) of the mesentric vein. The color encodes the partial velocity vector which is directed to the ultrasound transducer. The main vector is the direction of the vessel in space. So the horizontal cut is the only one that depicts the true velocity values since the vessel in this view runs straight towards the transducer. PixelFlux thus can directly calculate all color pixels’ area and velocity to calculate the real flow volume in a vessel which is not round. For round vessels we already managed true flow volume measurements with a data extraction from 2D-videos. But veins are usually not round. To quantify venous flow volumes the new 4D-technique is a real breaktrough.
The ultrasound signal is displayed in real-time and heartbeat-triggered simultaneously in 3 orthogonal imaging planes. For the first time in medical diagnostics, it is thus possible to record a blood vessel in a transverse cut in the horizontal plane which represents just those velocity vectors which are directed towards the ultrasound transducer.
This means, the colour displayed in the horizontal plane of the vessel can be directly translated into the velocity values. moreover, PixelFlux is now able to monitor in real-time every single tiny speckle of colour which reflects the very flow velocity at exactly this location inside the vessel. Thus, the limitations of the traditional measurements now disappear.
4D-PixelFlux measurements now offer unprecedented possibilities to diagnose complicated diseases with disturbed blood flow no matter which kinds of vessels are involved. The diagnostic of complex venous disorders, tissue perfusion measurements in ml/s, demanding arterial diseases and more is now at our fingertips.
