Directional Transverse Oscillation Vector Flow Estimation

Abstract

Abstract-A method for estimating vector velocities using transverse oscillation (TO) combined with directional beamforming is presented. In Directional Transverse Oscillation (DTO) a normal focused field is emitted and the received signals are beamformed in the lateral direction transverse to the ultrasound beam to increase the amount of data for vector velocity estimation. The approach is self-calibrating as the lateral oscillation period is estimated from the directional signal through a Fourier transform to yield quantitative velocity results over a large range of depths. The approach was extensively simulated using Field IIpro and implemented on the experimental SARUS scanner in connection with a BK Medical 8820e convex array transducer. Velocity estimates for DTO are found for beam-to-flow angles of 60 • , 75 • , and 90 • , and vessel depths from 24 to 156 mm. Using 16 emissions the Standard Deviation (SD) for angle estimation at depths ranging from 24 to 104 mm are between 6.01 • and 0.93 • with a mean SD of 2.8 • . The mean relative SD for the lateral velocity component is 9.2% and the mean relative bias -3.4% or 4 times lower than for traditional TO. The approach also works for deeper lying vessels with a slight increase in SD to 15.7%, but a maintained bias of -3.5% from 126 to 156 mm. Data for a pulsating flow has also been acquired for 15 cardiac cycles using a CompuFlow 1000 pump. The relative SD was here 7.4% for a femoral artery waveform.

A Vector Flow Imaging Method for Portable Ultrasound Using Synthetic Aperture Sequential Beamforming

“…The expression in (1) is strictly valid only for continuous wave excitation and at the transmit focus, hence it represents an approximation for pulsed wave emissions. Inaccurate frequencies yield biased velocities, and for this reason, f x has to be directly estimated from the acquired ultrasound data using DTO, as previously shown in [21]. Also, DTO does not require the calibration of the beamformers at each depth as in conventional TO.…”

“…The velocity vectors can be estimated from the HRIs produced by the second-stage beamformer based on DTO [21]. The estimator's equations are reviewed here.…”

“…Liebgott et al [19] and Sumi [20] synthesized a TO by using a synthetic aperture (SA) for the optimization of the lateral oscillation frequency. A directional TO (DTO) approach was also proposed by Jensen [21] using directional beamforming in the lateral direction.…”

Real-time implementation of synthetic aperture vector flow imaging on a consumer-level tablet

“…The beamformed HRIs are Hilbert transformed in the lateral direction, and the lateral frequency is estimated through a Fourier transform [9]. For each velocity point, M lateral samples are selected, and the velocity is estimated from N HRIs using an autocorrelation approach [11]. The method provides velocity estimates everywhere in the image, and continuous data acquisition is obtained.…”

“…For these reasons, the integration of VFI in hand-held devices has the potential to improve the clinical workflow. A 2-D SA VFI method for portable ultrasound has been recently proposed in [9] combining SA sequential beamforming (SASB) [10] and directional transverse oscillation (TO) [11]. The method enables the wireless transmission of the ultrasound data and uses a relatively inexpensive 2-D phase-shift approach for the velocity estimation.…”