DK2985_C000 1..28 - AlSharqia Echo Club

Imaging Artifacts and Pitfalls 125Figure 6.6 Color Doppler aliasing in mitral stenosis. In colorDoppler mode, acceleration of the blood flow through a stenoticmitral valve reaches velocities beyond the upper limit of the velocityscale. Aliasing appears as an abrupt change of color fromlight blue to yellow on the display, which could suggest a flowgoing in the opposite direction (AoV, aortic valve; LA, leftatrium; LV, left ventricle).Figure 6.5 Aliasing. Pulsed-wave Doppler signal in the leftventricular outflow tract. (A) The peak negative velocity(,0 cm/sec or baseline) exceeds the upper limit of the velocityscale and a simultaneous aliasing signal appears on the oppositeside of the baseline at the top of the display. Altering the scalepeak velocity from 260 to 290 cm/sec (B) or shifting the baselineup with a maximum velocity of 2140 cm/sec (C) eliminatesthe aliasing artifact.E. Near-Field ClutterEchocardiographic transducers are designed to generate anultrasound beam providing optimal imaging in the centerof the display sector. Near-field clutter arises from thecomplex and nonuniform energy distribution in theportion of the sector adjacent to the transducer. Reverberationbetween near-field structures has also been involvedin producing such interference and wave cancellationduring transthoracic echocardiography (TTE) (10). Alsocontributing to limited visualization in the vicinity of theultrasound probe are high-intensity echoes from reflectorsin the proximal zone of the ultrasound beam where theintensity of the signal is maximal (6,11). Adjusting gaincontrols, use of the focal point and introduction ofmultifrequency probes, with the highest frequencies dedicatedto the near field, improve visualization close to thetransducer and decrease near-field noise (Fig. 6.7).F. Range AmbiguityAs discussed in the section on aliasing, range ambiguity isan expected limitation of high PRF Doppler. This phenomenonmay, however, occur during conventional PWDoppler recording and must be recognized to avoidinappropriate diagnosis. As previously explained, the PWtransducer acts as an echo receiver for a limited timedelay between each transmission of ultrasound burst. Thesystem assumes that the incoming echoes represent thebackscattered signal of the most recently transmitted ultrasoundpulse. This may not be the case and rather representthe sum of returning echoes that include backscattering ofpulses sent in previous cycles and having traveled to andfrom a target in the far field. These distal signals maydisplay higher energy and represent higher velocities thanthe signal from the chosen sample volume. This situationis misleading as the displayed PW Doppler signal mostlyrepresents echoes from blood flow farther in the acousticfield while the sampling gate is in a more proximal location(Fig. 6.8) (6,12). When range ambiguity is suspected, the