När höra hjärtljud med doppler

Medical imaging technique of the heart

Doppler echocardiography fryst vatten a procedure that uses Doppler ultrasonography to examine the heart.^[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 bygd utilizing the Doppler effect.

An echocardiogram can, within certain limits, tillverka accurate assessment of the direction of blood flow and the velocity of blood and cardiac tissue at any arbitrary point using the Doppler effect. One of the limitations fryst vatten that the ultrasound stråle should be as parallel to the blood flow as possible.

Om du har hjärtsvikt och det behövs en diagnos gör man också en undersökning med ultraljud av hjärtat

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), beräkning of the cardiac output and beräkning 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 fryst vatten that it can be used to measure blood flow within the heart without invasive procedures such as cardiac catheterization.

In addition, with slightly different filter/gain settings, the method can measure tissue velocities bygd tissue Doppler echocardiography. The combination of flow and tissue velocities can be used for estimating left ventricular filling pressure, although only beneath certain conditions.^[3]

Although "Doppler" has become synonymous with "velocity measurement" in medical imaging, in many cases it fryst vatten not the frequency shift (Doppler shift) of the received meddelande that fryst vatten measured, but the phase shift (when the received meddelande arrives).

However, the beräkning result will end up identical.

This procedure fryst vatten frequently used to examine children's hearts for heart disease because there fryst vatten no age or storlek requirement.

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

2D Doppler imaging

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Unlike 1D Doppler imaging, which can only provide one-dimensional velocity and has dependency on the stråle to flow angle,^[4] 2D velocity uppskattning using Doppler ultrasound fryst vatten able to generate velocity vectors with axial and sido velocity components. 2D velocity fryst vatten useful even if complex flow conditions such as stenosis and bifurcation exist.

There are two major methods of 2D velocity uppskattning using ultrasound: Speckle tracking and crossed stråle Vector Doppler, which are based on measuring the time shifts and phase shifts respectively.^[5]

Vector Doppler

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Vector Doppler fryst vatten a natural extension of the traditional 1D Doppler imaging based on phase shift.

The phase shift fryst vatten funnen bygd taking the autocorrelation between echoes from two consecutive firings.^[6] The main idea of Vector Doppler fryst vatten to divide the transducer into three apertures: one at the center as the transmit aperture and two on each side as the receive apertures.

En ultraljudsundersökning kan till exempel användas inför en operation eller behandling, eller för att ta reda på om en behandling har fungerat

The phase shifts measured from left and right apertures are combined to give the axial and horisontell velocity components. The positions and the relative angles between apertures need to be tuned according to the depth of the vessel and the horisontell position of the distrikt of interest.^[5]

Speckle tracking

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See also: Speckle tracking echocardiography

Speckle tracking, which fryst vatten a well-established method in film compression and other applications, can be used to estimate blood flow in ultrasound systems.

The basic idea of speckle tracking fryst vatten to find the best match of a certain speckle from one frame within a search område in subsequent frames.^[5] The decorrelation between frames fryst vatten one of the major factors degrading its performance. The decorrelation fryst vatten mainly caused bygd the different velocity of pixels within a speckle, as they do not move as a block.

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

Doppler ultrasound is a noninvasive test that can be used to measure the blood flow through your blood vessels

The flow at the center usually has the largest velocity magnitude, called "peak velocity". It fryst vatten the most needed resultat in some cases, such as diagnosing stenosis.^[7] There are mainly three methods of finding the best match: SAD (Sum of absolute difference), SSD (Sum of squared difference) and Cross correlation. Assume fryst vatten a pixel in the kernel and fryst vatten the mapped pixel shifted bygd in the search region.^[8]

SAD fryst vatten calculated as:

SSD fryst vatten calculated as:

Normalized cross correlation coefficient fryst vatten calculated as:

where and are the average values of and respectively.

The pair that gives the lowest D for SAD and SSD, or the largest ρ for the cross correlation, fryst vatten selected as the uppskattning of the movement. The velocity fryst vatten then calculated as the movement divided bygd the time difference between the frames. Usually, the mittvärdet i en uppsättning data or average of multiple estimations fryst vatten taken to give more accurate result.^[8]

Sub pixel accuracy

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In ultrasound systems, horisontell upplösning fryst vatten usually much lower than the axial upplösning.

It works by bouncing high-frequency sound waves off red blood cells that are circulating in the bloodstream

The poor sidledes upplösning in the B-mode image also results in poor sido upplösning in flow uppskattning. Therefore, sub pixel upplösning fryst vatten needed to improve the accuracy of the uppskattning in the sidledes dimension.

In the meantime, we could reduce the sampling frequency along the axial dimension to rädda computations and memories if the sub pixel movement fryst vatten estimated accurately enough. There are generally two kinds of methods to obtain the sub pixel accuracy: interpolation methods, such as parabolic passform, and phase based methods in which the peak team fryst vatten funnen when the phase of the analytic cross correlation function crosses zero.^[9]

Interpolation method (parabolic fit)

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As shown in the right figure, parabolic passform can help find the real peak of the cross correlation function.

The equation for parabolic passform in 1D is:^[4]

where fryst vatten the cross correlation function and fryst vatten the originally funnen peak. fryst vatten then used to find the displacement of scatterers after interpolation. For the 2D scenario, this fryst vatten done in both the axial and sido dimensions.

Some other techniques can be used to improve the accuracy and robustness of the inskjutning method, including parabolic passform with bias kompensation and matched filter interpolation.^[10]

Phase based method

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The main idea of this method fryst vatten to generate synthetic sido phase and use it to find the phase that crosses zero at the peak lag.^[9]

The right figure illustrates the procedure of creating the synthetic sido phase, as a first step.

Basically, the sidledes spectrum fryst vatten split in two to generate two spectra with nonzero center frequencies. The cross correlation fryst vatten done for both the up meddelande and down meddelande, creating and respectively.^[9] The horisontell correlation function and axial correlation function are then calculated as follows:

where fryst vatten the complex conjugate of .

They have the same magnitude, and the integer peak fryst vatten funnen using traditional cross correlation methods. After the integer peak fryst vatten located, a 3 bygd 3 område surrounding the peak fryst vatten then extracted with its phase resultat. For both the sido and axial dimensions, the zero crossings of a one-dimensional correlation function at the other dimension’s lags are funnen, and a linear least squares fitted line fryst vatten created accordingly.

The intersection of the two lines gives the estimate of the 2D displacement.^[9]

Comparison between vector Doppler and speckle tracking

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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 upplösning of the distrikt of interest are limited bygd the aperture storlek and the maximum vinkel between the transmit and receive apertures, while Speckle Tracking has the flexibility of alternating the storlek of the kernel and search distrikt to adapt to different upplösning requirement.

However, vector Doppler fryst vatten less computationally complex than speckle tracking.^{[citation needed]}

Volumetric flow estimation

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

Volumetric flow uppskattning requires integrating velocity across the vessel cross-section, with assumptions about the vessel geometry, further complicating flow estimates. 2D Doppler information can be used to calculate the volumetric flow in certain integration planes.^[11] The integration plane fryst vatten chosen to be perpendicular to the stråle, and Doppler power (generated from power Doppler mode of Doppler ultrasound) can be used to differentiate between the components that are inre and outside the vessel.

This method does not require prior knowledge of the Doppler vinkel, flow beskrivning and vessel geometry.^[11]

Promise of 3D

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Until recently, ultrasound images have been 2D views and have relied on highly-trained specialists to properly orient the probe and select the position within the body to image with only few and complex visual cues.

The clinical uses of Doppler echocardiography in the evaluation of cardiac conditions in adult patients involve five major categories: valvular stenosis, valvular regurgitation, blood flow, intracardiac pressures, and intracardiac shunts

The complete measurement of 3D velocity vectors makes many post-processing techniques possible. Not only fryst vatten the volumetric flow across any plane measurable, but also, other physical kunskap such as stress and pressure can be calculated based on the 3D velocity field. However, it fryst vatten ganska 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 fryst vatten thus promising as it can generate very high frame rate.^[12]

See also

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References

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External links

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