DK2985_C000 1..28 - AlSharqia Echo Club

Basic Principles of Doppler Ultrasound 33(A)(B)(C)Pulsed Wave Doppler Color Doppler Each Sample SiteFigure 2.12 Color Doppler principle. (A) Pulsed-wave Doppler: one site is sampled on one scan line. (B) Color flow imaging: severalhundred sites are sampled on many scan lines. (C) Each area of the scanning lines is interrogated successively eight times according to thepacket size. Each successive wave is slightly out of phase compared with the preceding one; one number, the mean velocity, is stored byan algorithm at each sample site. The algorithm processes the phase shift between the waves of the eight pulses, estimates the meanvelocity by derivation at each sample and assigns a color to each area. The larger the packet size, the more precise the results, but thelower the frame rate.two is ahead of pulse one, the target is moving towards thetransducer and if pulse two lags behind pulse one, thetarget is flowing away. Echoes from subsequent pulsesare correlated with echoes from previous pulses to determinethe mean Doppler shift and its “variance,” which isthe difference between the highest and the lowest returningfrequencies or the frequency spread of the spectrum. Averaging,by repetitive sampling, is performed to improve thestatistics. The modal frequency can be used in the Dopplerequation to determine mean velocities and variance. Forlaminar flows, the value of the mean velocity is approximatelythe same as the peak velocity (14).Figure 2.13 Color code displayed on the screen. (A–D) Mid-esophageal five-chamber view using color Doppler interrogation of themitral valve inflow. The upper limit of the color coded scale is progressively increased from 26 cm/sec (A), 50 cm/sec (B), and 94 cm/sec(C). Aliasing is reduced by increasing the Nyquist limit. (E) Color flow. Flow towards the transducer is red at low velocity, progressivelyyellow at higher velocity and blue when away from the transducer with increasingly clear color at higher velocity. The number 50 indicatesthat the upper limit of mean velocity readable without aliasing is 50 cm/sec under the present conditions (Nyquist limit). (F)Increased Nyquist limit. By increasing the pulse repetition frequency and/or decreasing the depth of the color field, the upper limit ofvelocity readable without aliasing can be significantly increased (in the present case: 140 cm/sec) (Ao, aorta; LA, left atrium; LV,left ventricle; RA, right atrium; RV, right ventricle).