[1] An echocardiogram uses high frequency sound waves to create an image of the heart while the use of Doppler technology allows determination of the speed and direction of blood flow by utilizing the Doppler effect.

Velocity measurements allow assessment of cardiac valve areas and function, any abnormal communications between the left and right side of the heart, any leaking of blood through the valves (valvular regurgitation), calculation of the cardiac output and calculation of E/A ratio[2] (a measure of diastolic dysfunction).

Contrast-enhanced ultrasound-using gas-filled microbubble contrast media can be used to improve velocity or other flow-related medical measurements.

An advantage of Doppler echocardiography is that it can be used to measure blood flow within the heart without invasive procedures such as cardiac catheterization.

The combination of flow and tissue velocities can be used for estimating left ventricular filling pressure, although only under certain conditions.

2D velocity is useful even if complex flow conditions such as stenosis and bifurcation exist.

There are two major methods of 2D velocity estimation using ultrasound: Speckle tracking and crossed beam Vector Doppler, which are based on measuring the time shifts and phase shifts respectively.

The phase shift is found by taking the autocorrelation between echoes from two consecutive firings.

The phase shifts measured from left and right apertures are combined to give the axial and lateral velocity components.

[5] Speckle tracking, which is a well-established method in video compression and other applications, can be used to estimate blood flow in ultrasound systems.

The decorrelation is mainly caused by the different velocity of pixels within a speckle, as they do not move as a block.

This is less severe when measuring the flow at the center, where the changing rate of the velocity is the lowest.

pair that gives the lowest D for SAD and SSD, or the largest ρ for the cross correlation, is selected as the estimation of the movement.

The velocity is then calculated as the movement divided by the time difference between the frames.

Usually, the median or average of multiple estimations is taken to give more accurate result.

Therefore, sub pixel resolution is needed to improve the accuracy of the estimation in the lateral dimension.

In the meantime, we could reduce the sampling frequency along the axial dimension to save computations and memories if the sub pixel movement is estimated accurately enough.

[9] As shown in the right figure, parabolic fit can help find the real peak of the cross correlation function.

[9] The right figure illustrates the procedure of creating the synthetic lateral phase, as a first step.

Basically, the lateral spectrum is split in two to generate two spectra with nonzero center frequencies.

They have the same magnitude, and the integer peak is found using traditional cross correlation methods.

[9] Both methods could be used for 2D Velocity Vector Imaging, but Speckle Tracking would be easier to extend to 3D.

Also, in Vector Doppler, the depth and resolution of the region of interest are limited by the aperture size and the maximum angle between the transmit and receive apertures, while Speckle Tracking has the flexibility of alternating the size of the kernel and search region to adapt to different resolution requirement.

[citation needed] Velocity estimation from conventional Doppler requires knowledge of the beam-to-flow angle (inclination angle) to produce reasonable results for regular flows and does a poor job of estimating complex flow patterns, such as those due to stenosis and/or bifurcation.

2D Doppler data can be used to calculate the volumetric flow in certain integration planes.

This method does not require prior knowledge of the Doppler angle, flow profile and vessel geometry.

The complete measurement of 3D velocity vectors makes many post-processing techniques possible.

Not only is the volumetric flow across any plane measurable, but also, other physical information such as stress and pressure can be calculated based on the 3D velocity field.

However, it is quite challenging to measure the complex blood flow to give velocity vectors, due to the fast acquisition rate and the massive computations needed for it.

Plane wave technique is thus promising as it can generate very high frame rate.