Power (Angio) Mode of Ultrasound

In contrast to the more widely used velocity mode, the power mode determines the frequency shift of the reflected echoes from the ultrasound energy . In the power mode display, the sum of the Doppler signal intensities reflected by moving particles is represented by levels of brightness while the magnitude of the flow velocity and different velocities as well as flow direction (in older devices) are ignored. The color intensity in the power mode is determined by the density of the moving reflectors. The power mode is more sensitive to slow flow and flow with only few reflectors as compared with the velocity mode. The more gates sampled along each color Doppler line, the better the signal-to-noise ratio. Lighter shades indicate higher densities of reflecting flow ab a Types of color coding used to display flowing blood: In the velocity mode (left), red and blue represent the flow direction and lighter shades a higher Doppler shift. In the power/angio mode (right),higher amplitudes of the reflected ultrasound echoes are displayed in lighter shades irrespective of the frequency and flow direction.

b Depiction of the right kidney by color duplex ultrasound in the velocity mode on the left and in the power Doppler mode on the right. In the velocity mode, the color coding of the vessels indicates the blood flow direction with veins being shown in blue (flow away from the transducer) and arteries in red (toward the transducer).Vessels to the level of the interloper vessels are depicted. The power Doppler mode provides no information on the flow direction of blood but enables evaluation of slow flow and is less angle dependent, which is why this mode is superior in depicting parenchymal blood flow in small vessels. The kidney (N) is marked with plus signs. Part of the liver (L) is depicted near the abdominal wall with visualization of peripheral vessels in the power Doppler mode. (in blood cells). While the power mode is similar to other techniques in that it only registers reflected echo signals within a certain range of Doppler frequency shifts, it is largely independent of the angle between the ultrasound beam and the blood vessel. Since blood does not flow strictly in one direction, there will always be some reflection of echo pulses even at an unfavorable insounation angle but color intensity is reduced at an angle around 90°. The power Doppler mode is particularly suitable for evaluating slow flow in small vessels and can thus be used to examine peripheral perfusion as well as perfusion in small tumor vessels or in parenchymal organs.

The power mode is limited by the fact that it does not provide qualitative or semi quantitative information on flow velocity. Moreover, it is more susceptible to artifacts induced by organ movement and has a poorer temporal resolution. There is no aliasing because the power mode is independent of the magnitude of the Doppler frequency shift. The main advantage of the power mode lies in the fact that it uses very low pulse repetition frequencies (in the range of some 100 hertz), which in turn enables the resolution of very small Doppler frequency shifts (slow flow). Such low pulse repetition frequencies would be highly susceptible to aliasing in velocity-dependent color coding. The power mode represents the intensities of the signals reflected by moving blood by different levels of brightness based on their amplitudes. The so-called bidirectional power mode offered by the manufacturers of state-of-the-art scanners enables simultaneous color coding of flow direction (blue and red). This is achieved by the activation of some additional ultrasound beams which solely serve the purpose of sampling and processing flow direction information using the autocorrelation technique.

Angle of Insonation & B-flow Imaging

Angle of Insonation

Angle dependency of the Doppler shifted frequencies is also a critical factor in blood flow analysis. In sector scanning, multiple scan lines spread out from the transducer in a fanlike manner. When the sector scanner is used to interrogate circulator system in which the direction a flow is across these scan lines in a color window the angle of insonation between the flow axis and the ultrasound beam changes. The angle is smallest when flow stream enters in the sector filed and progressively rises to 90 0 as flow approaches the center of the field. Concurrently the Doppler shifted frequencies progressively decline and may become undetectable at the center of the color filed. A sector scanner may also create apparently. Contradictory directional information in a vessel transferring across the color encoding for flow toward to the transducer which usually is red. as the flow moves away, it will show bi directional flow. This paradox actually highlights the basic concept of representation of flow directionally by any Doppler system.

B-FLOW IMAGING
B-mode blood flow (B-flow) imaging is a new method that improves the resolution,frame rate, and dynamic range of B-mode to image blood flow and tissue simultaneously (Chiao et al., 2000).  The gray scale of an echo is adjusted by correlating the echo waveforms temporally. The correlation function measures the similarity of two echo waveforms and is determined by blood echogenicity, blood flow velocity, and beam width. A filter is designed to suppress large and slow or nonmoving echoes. The result is that the image of blood in an image is enhanced so as to allow the better visualization of blood flow, especially close to the vessel wall. A comparison of the B-mode and B-flow images of a carotid artery. It is evident that blood flow is better visualized by B-flow imaging. The vessel lumen where blood flows in the B-mode image is basically anechoic.

Frequency Processing Ultrasound

In a blood vessel, blood components move with different velocities which are represented in the Doppler spectrum  . Three-dimensional Doppler frequency spectrum showing the distribution of individual Doppler shifts (amplitudes), flow directions (above and below the time axis), and flow velocities (computed from Doppler frequency shifts). The heights of the boxes correspond to the amplitudes of the respective Doppler frequencies. A Doppler frequency spectrum represents amplitudes by different levels of brightness. In color-coded duplex ultrasound (black boxes), the averaged flow velocity at a specific point in time is displayed in color according to the flow direction and superimposed on the two-dimensional grayscale image in real time. (According to P.M. Klews, in Wolf and Fobbe 1993) a range of frequencies with different amplitudes reflecting the distribution of flow velocities in the vessel. The technique that has established itself for spectral analysis is an algorithm known as fast Fourier transform (FFT), which breaks down the waveform into a series of sinusoidal waveforms. For the individual frequency values, the corresponding amplitudes are calculated and displayed in different shades of gray. According to Fourier’s theorem, any periodic waveform can be reconstructed from its component waveforms. Conversely, in spectral analysis, a complex waveform of a given frequency (Doppler shift frequency) is decomposed into its frequency components. In this case, the fast Fourier transform yields the amplitudes of the individual frequencies of the respective sine and cosine functions, which together make up the waveform. The individual frequencies thus separated are continuously displayed over time in the Doppler frequency spectrum (spectral waveform).

The Doppler spectrum contains the following information on blood flow  : vertical axis representing different flow velocities as Doppler frequency shifts, horizontal axis representing the time course of the frequency shifts, density of points, or color intensity, on vertical axis representing the number of red blood cells moving at a specific velocity (may also be plotted in the form of a histogram). Flow toward and away from the transducer is processed simultaneously and respectively represented above and below the baseline (zero flow velocity line). Alternatively, some ultrasound devices display the magnitudes of the different velocity components in a separate power spectrum. This is done by measuring the signal intensities of the individual Doppler frequencies at a specific time in the cardiac cycle and displaying the spectral distribution in a histogram. Doppler frequency spectrum of the superficial femoral artery (left section).The histogram plotted on the vertical axison the left represents the distribution of the different Doppler frequency shifts during systole. In the Doppler waveform, this distribution is represented by different levels of brightness (laminar flow). The right section shows the corresponding distribution during systole in the common carotid artery, which has less pulsatile flow

Yolk Sac Vascularization and Volume Estimation by 3D Ultrasound

Recently, Kupesic and associates performed a transvaginal colour Doppler study of yolk sac vascularization and volume estimation by 3D ultrasound. They examined 150 patients whose gestation age range from 6 to 10 weeks from the last menstrual period during normal uncomplicated pregnancy. Transvaginal 3D and power Doppler examination was performed before the termination of pregnancy for psychosocial reasons. The highest visualization rates for yolk sac vessels were in the 7th and 8th weeks of gestation, reaching value of 90.71%. a characteristic waveform profile included low velocity, and the absence of diastolic flow was found in all the examined yolk sacs. The pulsatility index showed a mean value of 3.24 ± 0.94 without significant change between subgroups. The author found a positive correlation between gestational age and volumes of the yolk sac until 10 week gestation. At the end of the first trimester, yolk sac volume remained constant, while gestational sac volume continued to grow. 3D ultrasound may significantly contribute to in vivo observation of the yolk sacs “honeycomb” surface pattern.
Increased echogenicity of the yolk sac walls were reported as a sign of dystrophic change that occur in a nonviable cellular material indicating early pregnancy loss. Automatic calculation will allow us to estimate precise relationship between the yolk sac volume and CRL measurement. Kupesic and coworkers measured gestational sac volume and yolk sac volume and vascularity in eighty women with uncomplicated pregnancy between 5 and 12 weeks of gestation. Regression analysis revealed an exponential growth of the gestational sac volume throughout the first trimester of pregnancy. Gestational sac volume measurement can be used for estimation of the gestational age in early pregnancy. Abnormal gestational sac volume measurement could potentially be use as a prognostic marker for pregnancy outcome. Yolk sac volume was found to increase from 5 to 10 weeks of gestation. However, when the yolk sac reaches its maximum volume at around 10 weeks, it has already started to degenerate; which can be indirectly proved by a significant reduction in visualization rates of the yolk sac vascularity.
As suggested earlier, the disappearance of the yolk sac in normal pregnancies is, probably, the result of yolk sac degeneration rather than of a mechanical compression of the expanding amniotic cavity. These events suggest that the evaluation of the biological function of the yolk sac by measuring the diameter and/or the volume is limited. Therefore, a combination of functional and volumetric studies is necessary to identify some of the more important moment during early pregnancy.
Kurjak et al reported vascularization ot the yolk sac in normal pregnancies between 6 and 10 weeks of gestation. Pulsed Doppler signal characterized by low velocity and high pulsatility were obtained in 85.71 percent of the yolk sacs during 7th and 8th gestational weeks. Although the reports on yolk sac and vitelline circulation are very exiting, it should be noted that such studies are not ethically feasible in ongoing human pregnancies since secondary yolk sac is a source of primary germ cells and blood stem cells.
Three-dimensional ultrasound and power Doppler will allow as to study turgescent blood vessels withstanding from the surface of the yolk sac. The same technique can be used to study evolution from the embryo-vitelline towards embryo placental circulation. Since yolk sac and vitelline blood vessels are prerequisite for the oxygen transfer, absorptive and transfer processes during the first trimester, alteration in this early circulatory system may have some prognostic value for predicting pregnancy outcome.

Left Ventricle Showed by Ultrasound

To identify the cylinder in the image data, core atoms of an appropriate range of diameters Automated Identication and Measurement of Objects via Medial Primitives were collected in sample volumes on a regular lattice, and ellipsoidal voting was applied.  Crosses are shown in the cylindrical chamber of the ventricle. Due to the pre{selection of core atoms by scale, no other signi cant densities of core atoms were found. Next, the mitral valve was sought, by limiting the formation of core atoms to an appropriately smaller scale, and to orientations nearly perpendicular to the transducer. As shown in Fig. 10B, the strongest superdensities (short vertical line segments) were clustered around the center of the mitral valve, although weaker false targets were detected in the myocardium. To eliminate these false targets, a criterion was established for the formation of appropriate pairs of superdensities, in the spirit of core atoms. Only slab{like densities appropriately located and oriented with respect to cylindrical densities were accepted. These pairs were allowed to vote for their constituent superdensities, and the mean location of the winning superdensities used to establish a single mitral valve lo96 G. D. Stetten and S. M. Pizercation and a single LV cylinder location. The vector between these two locations was used to establish a cone for expected boundary points at the apex of the LV, and the mean distance to the resulting boundary points used to determine the location of the apical cap along that vector. Thus an axis between the apex and the mitral valve was established. Given this axis, LV volume was estimated by collecting boundary points around the axis. Only boundaries that faced the axis were accepted. The boundary points were organized into bins using cylindrical coordinates, in other words, disks along the axis and sectors within each disk. An average radius from the axis was established for the boundary points in each bin, creating a surface map of the endocardial surface.

The problem of empty bins was avoided by convolving the surface map with a binomial kernel in 2D until each bin had some contribution to its average radius. Volumes were then calculated by summing over all sectors. The entire procedure including identification and volume measurement of the LV was automated, and required approximately 15 seconds on a 200 MHz Silicon Graphics O2 computer. The automated volumes were compared to manual tracings performed on a stack of flat slices orthogonal to a manually {placed axis  ). This axis employed the same anatomical end{points (the ventricular apex and the center of the mitral valve) as the axis determined automatically above. The volumes and locations of the end{points were compared to those determined automatically.  They are very encouraging, particularly for the automated placement of the axis end points, which had an RMS error of approximately 1 cm. Volume calculations introduced additional errors of their own, but were still reasonable for ultrasound. Only four cases have been tried, and all are shown. The method worked in all cases.

We have described a new method for identifying anatomical structures using fundamental properties of shape extracted statistically from populations of medial primitives, and have demonstrated its feasibility by applying it under challenging conditions. Further studies are presently underway to establish reliability over a range of data. Future directions include introducing greater speficity and adaptability in the boundary thresholding, incorporating more than 2 nodes into the model, introducing variability into the model to reflect normal variation and pathologic anatomy, extending the method to the spatio {temporal domain, and applying it to visualization.

Aliasing on Doppler Ultrasound

The main forms of Doppler used pulse wave Doppler(PWD), Color Doppler (CD) and Power Doppler (PD). Modern ultrasound equipment with CD felicities usually offers conventionally B-mode real-time scanning. CD, which shown color representation of blood flow towards and away from probe, pulsed wave spectral analysis shown as waveforms above and below a baseline and often PD.

The Doppler waveforms is constructed from a series of samples. For a given sampling rate or pulse repetition frequency (PRF) , there is a maximum frequency. Above this , the sampled waveform cannot be constructed accurately and a lower frequency signal is produced.

The sample Doppler signal adequately , at least two samples must be taken on the shortest cycle for the correct frequency to be determined. This is called Nyquist limit. This can al,so be expressed as the sample rate being at least 2 x maximum frequency in the Doppler signal. If the sample rate, controlled by a PRF, is less than the essential twice per cycle, then a interpretation of the high frequencies above one half 0f the sampling frequency. Because the signals is not sampled quickly enough, it appears below the baseline. The PRF therefore should be at least twice the maximum values of Doppler frequency to be measured in order to avoid aliasing.

Doppler Mirror Artifact

The mirror image artifact can also occur with Doppler system. This means that an image of a vessel and a source of Doppler shifted echoes can be duplicated on the opposite side of a strong reflector such as bone. The duplicated vessel containing flow could be misinterpreted as additional vessel. A mirror image of a Doppler spectrum can be appear on the opposite side of the baseline when, indeed, flow is unidirectional and should appear only on one side of the baseline. This is an electronic duplication of spectral information. It can occur when receiver gain is set too high causing overloading in the receiver and cross talk between the two flows channels or with gain where the receiver has difficulty determining the sign of the Doppler shift. It can also occur when Doppler angle is near 90 o. Here the duplication is usually legitimate and this is because beams are focused and not cylindrical in shape. Thus, portions of the beam can experience flow toward while other portions can be experience flow away.
A prerequisite for optimal utilization of ultrasound in obstetrics and gynecology is an in depth knowledge of the principles and limitations of the dynamics technique. It is important to appreciate that the appearance of the Doppler images is influenced by the operational setting of the equipment that must be taken into account for any reable interpretation. Only persons with sufficient training and education should perform diagnostic ultrasound. One major reason for so many conflicting and controversial results in the ultrasound literature originates from technique complexity and rather limited education in physics and technique. With all artifacts, it is important not to let a superficial knowledge cause trouble. Once the cause and nature of an artifact are understood, it is important not to misinterpret a real lesion as a miss the true pathologic. This can happen, particularly with pelvic masses such as leiomaymas with poor through transmission in which the deep wall is not well seen. If there is also an artifact situated near where the deep wall would be, the actual mass might be dismissed as simply an artifact. One must pay attention at all times not only to identify artifacts but also not to let them interfere with the identification of true lesions. While the more common artifacts seen on ultrasound imaged frequency ignored and appreciated as such as, it is certainly interesting to know why they occur, on the other hand, the usefulness of the artifacts cannot be underestimated. Occasionally, the identification of an artifact may prevent the novice from making an important error in diagnosis or management. An appreciation and understand of how to avoid artifacts can help even the more experienced practitioner decide weather a structure is real. It is also important not to ignore real pathology under the assumption that it is caused by artifact.