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