DK2985_C000 1..28 - AlSharqia Echo Club

38 Transesophageal Echocardiography(A) (B) (C)Figure 2.19 Color-coded mapping variations. (A) Enhancedcolor. Flow towards the transducer is red at low velocity, progressivelyyellow at higher velocity and blue when flow is awayfrom the transducer with increasingly clear color and increasingvelocity. The number 50 indicates that the upper limit of meanvelocity readable without aliasing is 50 cm/sec under thepresent conditions (Nyquist limit). (B) Variance: green (greenarrow) is superimposed on the previous colors to illustrate the presenceof turbulence, characterized by a large amount of variancewithin the velocities. (C) Increased Nyquist limit: by increasingthe pulse repetition frequency and/or decreasing the depth ofthe color field, the upper limit of velocity readable without aliasingcan be significantly increased (in the present case: 140 cm/sec).calcium deposits, can mask structures located behind themand create “shadowing.” Close to the transducer, an acousticnoise is generated by high-amplitude oscillations of thepiezoelectric elements and prevents visualization of reflectors;this is called “near-field clutter” (7). More importantare “reverberations”: when an ultrasound beam is stronglyreflected by a nearby object, the near side of the anatomicwall or the transducer itself may function as anotherreflecting surface. The echo beam is then sent back tothe object being hit a second time by the same ultrasound.This second wave travels with the same burst and producesa second signal. It is interpreted as a second object at twicethe distance from the first. It is displayed on the screen in astraight line from the transducer and the real object(Fig. 2.18). This pattern can be repeated two or threetimes and can also occur between strong reflectorsdeeper in the field. When the source of reverberation ismoving, its amplitude of motion may be twice as greatas that of the original echo. Color Doppler is subject tothe same artifact: the flow of a sclerotic descendingaorta, very close to the esophagus, might be reproduceda second time in front of the aorta, as if a second vesselwas present with the same arterial pulse.(A) (B) (C) (D)(E)RVRALALV(F)SVCRPAAoMPA(G)LVLAFigure 2.20 (A) Color reversal with aliasing. As soon as the velocity is above the Nyquist limit, the color suddenly jumps from clearblue to bright yellow. (B) and (E) Accelerating flow through a restricted mitral valve orifice. (C) and (F) Central acceleration zone in apulmonary artery. (D) and (G) Proximal flow acceleration convergence zone on the ventricular side of a mitral regurgitation jet (Ao, aorta;LA, left atrium; LV, left ventricle; MPA, main pulmonary artery; PV, pulmonic valve; RA, right atrium; RPA, right pulmonary artery;RV, right ventricle; SVC, superior vena cava).