Volumetric Flow Rate Measurement

Background

Direct measurement of volumetric flow in blood vessels has always been a big challenge in the clinical practice and research although knowing the flow volume is beneficial to a range of clinical applications. Most ultrasound techniques which used a 1D array transducer must assume an analytical or symmetrical velocity profile, based on which the flow volume was estimated either from the velocity measurements at a point by Spectral Doppler or from 2D velocities by vector flow techniques in the longitudinal scanning plane.  Phase-contrast MRI could obtain the volumetric flow, but its application is limited by the low temporal resolution and by its poor accessibility compared to ultrasound. We proposed the speckle decorrelation method to overcome those limitations with a conventional 1D array ultrasound transducer, meaning it can be easily implemented with the current clinical ultrasound hardware.

Methods

The proposed ultrasound speckle decorrelation (SDC) method can estimate the through-plane flow velocity in blood vessels. With the through-plane velocity, the flow volume can be calculated by integrating the velocity over the luminal area when the vessel is scanned from the transverse view. Imaging frame rate must be high enough to capture the fast signal decorrelation due to rapid flow passing through the rather small elevational dimension of the transmitted acoustic beam. Microbubble contrast agents are required to enhance the scattered ultrasound signal from the flow.

In-vitro and in-vivo results

The through-plane velocity estimations were demonstrated both in a flow phantom (Figure 1) and on a rabbit’s abdominal aorta (Figure 2). A movie showing the 4D (in-plane and through-plane) velocity profile in the scanning plane on the phantom was given in the video at the bottom, where the in-plane velocities were measured through the ultrasound imaging velocimetry (UIV) with the same ultrasound data. In Figure 3(a), the decorrelation method was compared with the UIV method from the longitudinal view, and with a commercial invasive catheter, which demonstrated the high accuracy of the proposed decorrelation method in vivo.  A movie showing the 2D through-plane velocity profile from the rabbit’s aorta was given in the video at the top.

Conclusions

A new method using high-frame-rate plane-wave imaging, microbubble contrast agents, UIV, and decorrelation analysis is able to accurately estimate through-plane flow velocity and volumetric flow rate both in vitro and in vivo using a 1-D imaging probe. The technique has a range of potential clinical applications.

References

  1. X. Zhou et al., “3-D velocity and volume flow measurement in vivo using speckle decorrelation and 2-D high-frame-rate contrast-enhanced ultrasound,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control, vol. 65, no. 12, pp. 1–12, 2018.
  2. X. Zhou, X. Zhou, C. H. Leow, and M.-X. Tang, “Measurement of Flow Volume in the Presence of Reverse Flow with Ultrasound Speckle Decorrelation,” Ultrasound Med. Biol., 2019.