Andersons pediatric cardiology 468

Types of Doppler Ultrasound There are three types of Doppler ultrasound, namely continuous wave, pulsed wave, and color flow Doppler Continuous Wave Doppler In continuous wave Doppler, one piezoelectric crystal is used for transmitting a continuous wave at a fixed frequency, and a second crystal is used continuously to record the reflected signals Both crystals are embedded within the same transducer with a slight angle toward each other, and the Doppler shift is continuously sampled As an ultrasonic wave is transmitted continuously, no spatial information is obtained Indeed, all velocities occurring anywhere within the ultrasound beam, in other words on the selected ultrasound line of interrogation, will contribute to the reflected signal and will appear in the spectrogram As the ultrasound signal weakens with depth due to attenuation, velocities close to the transducer will intrinsically contribute more than the ones occurring further away The major advantage of continuous wave Doppler is its ability accurately to measure high frequency shifts with no upper limit It can be used, therefore, to measure high-velocity jets To optimize the direction of the echo beam within the direction of the flow, cross-sectional imaging and color Doppler can be combined with continuous wave Doppler from the same transducer, producing so-called duplex scanning This gives a visual impression of being simultaneous but is achieved by rapid and automatic switching between Doppler and cross-sectional imaging Adding color Doppler imaging helps optimally to align the beam of interrogation with the direction of the flow of blood Pulsed Wave Doppler Pulsed wave Doppler has been developed to give spatial information on the detected velocities The technique is not based on the Doppler principle but provides an output in a spectral display that looks very similar to the way the continuous wave signal is represented However, the Doppler shift itself is not measured by the system When using the pulsed wave system, an image line is chosen along which ultrasonic pulses are transmitted at a constant rate This rate is the pulse repetition frequency Instead of continuously sampling the backscattered waves, only one sample of the reflected wave is taken at a fixed time after transmitting a certain pulse This time interval is the range gate The range gate will determine the exact depth where velocities are measured The transducer sends pulses and must receive that signal back before other pulses can be transmitted A sample volume is positioned at the area of interest The velocity of sound in soft tissue is a given constant of 1540 m/s, and the go and return time is used to determine the depth of the sample volume The velocities that can be measured are limited by the pulse repetition frequency, or the number of pulses that are emitted per second Aliasing of the Doppler signal occurs when the pulse repetition frequency is too low and the returning signals from one waveform are not received before the next waveform is sent The frequency at which aliasing occurs is also called the Nyquist limit Aliasing will result in velocities being displayed at the same time below and above the baseline The Nyquist limit will be lower the deeper the velocity is measured because the time for the wave to travel will be longer The Nyquist limit depends on the frequency of the probe, with higher-frequency probes having lower Nyquist limits and lower-frequency probes having higher Nyquist limits A 2.5-mHz transducer will display velocities of twice the magnitude of those produced by a 5.0-mHz transducer Appropriate selection of the probe therefore is important when performing pulsed Doppler measurements Different options are possible further to increase the Nyquist limits The first is to shift the baseline so that velocities are measured in only one direction A second method is to send out a new pulse before the previous one has returned This is called the high-repetition frequency method and results in measuring velocities in more than one site or sample volume, thus reducing the spatial resolution but increasing the Nyquist limit The size of the sample volume can also be adjusted A smaller sample volume will result in a sharper velocity profile because fewer velocities are sampled at the same time Color Flow Doppler Color Doppler displays the direction and flow velocities of the blood superimposed on the cross-sectional image In this technology, different pulses are sent across an image line, and the phase shift between the different signals is measured at two sampling points This phase shift is proportional to the velocity of the reflecting object A color is assigned to the direction of flow according to whether it is away from or toward the transducer In this respect, it may be helpful to remember the mnemonic BART (“blue away and red toward”) The velocity of flow is displayed in shades of these colors The brighter the color, the higher the velocity The color information represents the mean velocity of flow When the flow is disturbed, or not laminar, the pattern will show as a mosaic This mosaic pattern is produced only if the variance is on The variance is a color Doppler option that is present with all the cardiac presets It is important to appreciate that, because color flow mapping is a form of pulsed Doppler, it is subject to the same physical principles Increasing velocities are represented as increasingly bright forms of red or blue until they reach the Nyquist limit, when aliasing then superimposes new colors on the display Typical Nyquist velocity limits are from 0.6 to 1 m/s Therefore for flows at high velocity, multiple aliasing occurs The color image is then useful only from a qualitative point of view and does not permit the demonstration of directional flow Similarly to pulsed Doppler, the Nyquist limit is dependent on transducer frequency and depth Color flow Doppler provides information regarding the Doppler shift from an entire area, unlike pulsed Doppler, which samples from a specific point Therefore more time is required for color Doppler to compute the lines of information onto the screen Frame rate and line densities are reduced proportional to the time required Keeping the color sector small will provide a better frame rate and produce a flicker-free image When using color flow, the operator is able to visualize the flow of blood in relation to the surrounding structures, which provides a method for rapid interpretation of abnormal location and direction of flow, and helps to guide Doppler interrogation of abnormal flow Comparison of Doppler Methods The three types of Doppler interrogation described are complementary, each measuring the velocities of flow in different ways For the evaluation of high velocities, the method of choice is continuous wave Doppler because it does not give rise to aliasing Although continuous wave does not permit gating for precise localization of the target, a steerable cursor line and a focused beam allows precise alignment, thus assuring appropriate measurements of flow Pulsed Doppler, in contrast, enables measurements of flow at a known depth, allowing more precise calculations, but is limited by the maximal measurable velocity Color flow Doppler is a qualitative method but provides spatial information not obtained with other methods By permitting visualization of the disturbed jet, it facilitates the alignment of the continuous wave Doppler beam The visual effect of color flow Doppler provides a method of rapidly screening abnormal velocities within the heart, thus directing the more quantitative