DK2985_C000 1..28 - AlSharqia Echo Club

Basic Principles of Doppler Ultrasound 37cardiac output measurements (16). In turbulent flows,random swirls and eddies appear, where there are widefluctuations in direction and velocity of flow components.They are spread amongst a slow varying, forward motionof blood. Consequently, the measured velocity correspondsto the mean flow velocity.VII.DOPPLER TISSUE IMAGINGDoppler signals are generated not only by flowing redblood cells but also by cardiac walls and valvularmotion. Tissues induce stronger backscatter echoes atlower frequency but these are usually filtered to improvethe blood flow image. These echoes appear only whencolor gain is too high or when filters are set too low.However, this drawback can be used to identify parietalmovements and wall kinetics if the low-amplitude, highfrequencysignals of blood cells are properly filtered(high-pass filtering system and low-clutter filter setting).With this technique, called “tissue Doppler,” velocitiesas low as 0.1 cm/sec are recorded. Depth resolution isinferior to that of conventional Doppler because velocitymapping requires longer pulses to be transmitted andlonger gate times (6).A myocardial velocity mapping can be obtained. Itassesses the regional myocardial velocity contraction patterns(17). Subepicardial layers usually have velocitieslower than subendocardial ones. These instantaneous velocitygradients within the walls present different patternsfor normal, ischemic, or dysfunctional myocardialmuscle. Encoded in color, they appear in shades of redfor positive transmural gradients (thickening), and bluefor negative ones (thinning). Frame rate can be increasedby sampling a single line in color M-mode. A spectraldisplay of PW analysis of a single sample can be used toidentify local movements, such as mitral ring displacementor the evaluation of diastolic function (Fig. 2.17).VIII.ARTIFACTSNumerous artifacts, due to physical properties of ultrasoundsor instrument ajustments, can mislead the examiner(see Chapter 6). Aliasing and ghosting have already beenmentioned. Strong reflectors, like prosthetic material orFigure 2.18 Reverberation. (A–B) Mid-esophageal short axis view of the aorta (Ao). (C–D) When a strong reflector is close to thetransducer, the high-energy echo beam giving the image at the initial time (t1) is reflected on the front part of the transducer and thenrerouted toward the reflector for a second time (t2). It gives a second image interpreted by the computer as being at a double distanceof the first target, since t2 is twice t1.