Abstract
Being able to measure 3-D flow velocity and volumetric flow rate effectively in the cardiovascular system is valuable but remains a significant challenge in both clinical practice and research. Currently, there has not been an effective and practical solution to the measurement of volume flow using ultrasound imaging systems due to challenges in existing 3-D imaging techniques and high system cost. In this study, a new technique for quantifying volumetric flow rate from the cross-sectional imaging plane of the blood vessel was developed by using speckle decorrelation (SDC), 2-D high-frame-rate imaging with a standard 1-D array transducer, microbubble contrast agents, and ultrasound imaging velocimetry (UIV). Through SDC analysis of microbubble signals acquired with a very high frame rate and by using UIV to estimate the two in-plane flow velocity components, the third and out-of-plane velocity component can be obtained over time and integrated to estimate volume flow. The proposed technique was evaluated on a wall-less flow phantom in both steady and pulsatile flow. UIV in the longitudinal direction was conducted as a reference. The influences of frame rate, mechanical index (MI), orientation of imaging plane, and compounding on velocity estimation were also studied. In addition, an in vivo trial on the abdominal aorta of a rabbit was conducted. The results show that the new system can estimate volume flow with an averaged error of 3.65% ± 2.37% at a flow rate of 360 mL/min and a peak velocity of 0.45 m/s, and an error of 5.03% ± 2.73% at a flow rate of 723 mL/min and a peak velocity of 0.8 m/s. The accuracy of the flow velocity and volumetric flow rate estimation directly depend on the imaging frame rate. With a frame rate of 6000 Hz, a velocity up to 0.8 m/s can be correctly estimated. A higher mechanical index (MI = 0.42) is shown to produce greater errors (up to 21.78±0.49%, compared to 3.65±2.37% at MI = 0.19). An in vivo trial, where velocities up to 1 m/s were correctly measured, demonstrated the potential of the technique in clinical applications.
Highlights
V OLUMETRIC flow rate is an important indicator for diagnosing many diseases, such as heart failure, carotid stenosis, and renal failure [1], [2]; it is useful for monitoring the changes caused by aging, remodeling, and surgical or pharmacological interventions [3], [4]
Values of beam correlation width (BCW) ranged between approximately 200 and 500 μm and increased linearly with depth even though the L12-3 probe has an elevational focal at 20 mm
The calibrated BCW values start to fluctuate, which is caused by the low signal-to-noise ratio (SNR) in the deeper area and the incoherence of the beam [34]
Summary
V OLUMETRIC flow rate is an important indicator for diagnosing many diseases, such as heart failure, carotid stenosis, and renal failure [1], [2]; it is useful for monitoring the changes caused by aging, remodeling, and surgical or pharmacological interventions [3], [4]. Ultrasound imaging, especially Doppler ultrasound, is currently the most common noninvasive modality for volumetric flow measurement, both in clinical practice and in research Other techniques, such as ultrasound imaging velocimetry, vector Doppler, transverse oscillation beamforming, and variants of these [5]–[8], have been widely investigated in the past two decades. Most of these techniques estimate only the 1-D or 2-D velocities in the scanning plane, called “in-plane,” from which the volumetric flow is derived by assuming the blood vessels have a circular cross section with symmetric flow profiles. The challenges for 3-D ultrasound imaging are the huge amount of data and the demanding hardware and computational requirements, which make it difficult and costly to implement [12]
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