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In the same figure, one can notice noticeable the ultrasound beam being focused in its intensity poste- roid due to echogenic foci within colloid. With Ring down artifact is a unique artifact caused each reverberation some sound energy is released by resonance. It occurs when fluid is trapped by back up to the probe as an echo. The time delay tetrahedron of air bubbles. This entrapment due to the reverberations causes a diminishing allows for resonance to develop. If that occurs, even a densely reflective object may not be visible Fig.

Note the diminishing tail of of reverberation artifact and typically indicates a benign echoes extending posteriorly from the object.

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This is best lesion seen in the enlarged view inset, arrow. It is a special case. Philadelphia, PA: Elsevier Mosby; Stokes GG. On the theories of the internal friction in This artifact is relatively rare in the thyroid and fluids in motion, and of the equilibrium and motion of parathyroid. It may occasionally be seen in the elastic solids.

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Trans Cambridge Phil Soc. Comprehensive com- fluid are mixed. Hagen-Ansert SL. Textbook of diagnostic ultrasonog- References raphy.

Louis, MO: Mosby Elsevier; Zwiebel WJ. Basic ultrasound physics and instrumen- tation. Introduction 1. Zagzebski JA. Essentials of ultrasound physics. Philadelphia, PA: Saunders; Louis, MO: Mosby; Merritt CRB. Physics of ultrasound.

In: Rumack CM, 7. Cobbold RSC. Foundations of biomedical ultrasound. Strissel 6. Although the frequency of the sound Color Doppler imaging will then be discussed in emitted by the source is constant, the motion of detail, including acquisition methods, character- the sound source will affect the frequency istics of the images, and artifacts and pitfalls detected by the receivers. The received frequency commonly seen when scanning the neck. Power depends on the position of the receiver with Doppler mode and its relationship to color respect to the source and the direction of motion Doppler will also be described.

More detailed of the source. Motion of the source is directly away from receiver R2, causing wave fronts to be stretched out, resulting in a lower detected frequency by R2. In the special case where the receiver is at right N. Hangiandreou and N. Strissel Fig. The frequency transmitted by the source and the frequency detected by the receivers are generally all different, depending on the direction of motion and positions of the source and receiver shift and is directly proportional to the velocity of as though no source motion is occurring.

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The Doppler angle is spectra measured each second. A sample mea- defined as the angle between the line connecting surement of blood velocity in the superior thyroid the source and receiver and the velocity vector artery is shown in Fig. The horizontal axis of arrow showing the direction of motion.

The sample volume is defined est possible Doppler shifts but opposite signs, by the short yellow lines in the two-dimensional one positive and one negative. The red arrow indicates the Doppler the sample volume are both also adjusted by the display can be adjusted using the Doppler gain sonographer and define the Doppler angle, dis- control. The peak systolic value ms , with all pulses aimed through the sample from the first full waveform cycle is estimated to volume along the indicated pulse direction. The although specific velocity measurements are not entire cross section of the small artery in Fig.

Undesired low-velocity signals caused velocities. This fil- in calculated velocity due to intrinsic spectral ter is helpful in eliminating nonvascular artifacts, broadening effects. However, signals from very slow sonographer, as it runs contrary to best practice blood flow can also be removed if the wall filter for grayscale imaging, where incidence angles of setting is too high.

The spectrum arrow.

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  6. The color scale control which adjusts the is noisy and demonstrates low velocities repli- PRF determines the velocity measurement sen- cated on either side of the zero-velocity baseline, sitivity, and the specific velocity values that cor- as compared with the correctly acquired image in respond to the top and bottom of the color bar are Fig. Pixel color indicates estimated components exceed the upper bounds of the spec- mean blood velocity and direction of flow, with tral graph dashed yellow line and wrap around the upper portion of the color bar representing to appear on the bottom of the graph solid yellow flow toward the probe usually reds and yellows line.

    This artifact can be distinguished from true and the bottom half representing flow away from flow away from the probe by noting that it is only the probe usually blue. Zero velocity is indicated by above the baseline.

    This artifact may be corrected the black region in the center of the color scale. It is important to recognize that spectra in the upper part of the graph or both as these values are accurate only for flow directions shown in Fig. As flow direction and thus Doppler angle are not speci- 6. If accurate measurements of velocity This is a thin slice, tomographic image typically are needed, duplex Doppler mode should be obtained with a wide bandwidth, high-frequency used. Pixels outside of the color box are all shown linear array probe in this case a 6—15 megaHertz in grayscale regardless of the presence of flowing [MHz] linear probe.

    Color Doppler images of blood. Pixels inside the color box yel- accurate velocity information from multiple loca- low are either rendered in color where flowing tions across a large spatial region, while the blood is detected or in grayscale for static soft duplex Doppler mode measures accurate velocity tissues. Velocity is color-coded according to the information in a single small spatial location.

    The dashed yellow line indi- location of these high-velocity signals in the wrong loca- cates the part of the velocity spectrum cut off due to the tion in the spectral display Fig. The red arrow indicates the color scale value and the color bar. This limited mation, two different sets of ultrasound pulses are pulse-echo data set is not sufficient to determine used.

    These pulses use autocorrelation algorithms can be used to deter- the highest available transmit frequency that can mine velocity mean, direction, variance, and penetrate to the specified image depth. An entire set power. However, the limited number of velocity of grayscale pulses is used to span the entire field of pulses per color line allows higher color Doppler view and acquire the 2D grayscale image.

    However, this angles that better demonstrate flow, in which case also results in limited color spatial lateral reso- the grayscale and velocity pulses will not be parallel lution. Sets of velocity pulses are used to interro- to one another. The velocity pulses will typically gate all of the color lines across the full color box, have transmit frequencies that are significantly lower thus estimating mean velocity at all locations than those used for the grayscale pulses, in order to within the color box. Depending on the color minimize the effects of attenuation on the very weak Doppler frame rate and the heart rate of the echoes reflected from blood.

    Also shown are grayscale black arrow and velocity yellow arrow pulse directions. Aliasing appears as colored After acquisition of all grayscale and velocity regions that include a mix of colors from both echoes for an image, the scanner must analyze all sides of the color bar, as seen in the vessel indi- locations inside the color box to determine if cated by the red arrow as well as other vessels in flowing blood is present. If so, color-coded veloc- Fig. Reducing the color scale sensitivity can ity information is displayed.

    Otherwise soft tissue reduce the incidence of aliasing resulting in ves- is assumed to be present and grayscale informa- sels with consistent color appearance, as indicated tion is shown. This blood-tissue discrimination by the red arrow in Fig. Color dropout refers algorithm considers multiple variables that may to situations where color in a vessel should be include mean velocity, velocity variance, spectral demonstrated but is absent. This can be caused by power, and echo amplitude. Different aspects of several factors. The average of these velocity sig- color write priority.

    Improperly adjusted scan con- 6.

    One limitation that has can cause this lack of color signal. Color dropout already been alluded to involves low frame rates. The smaller color box in Fig.