DK2985_C000 1..28 - AlSharqia Echo Club

14 Transesophageal EchocardiographyFigure 1.16 Matrix representation of an ultrasound image. (A–C) A matrix is a 2D arrangement of rows and columns of pixels, easilyevident on higher magnification in (C). (D) Anatomical correlation (LA, left atrium; LAA, left atrial appendage; LVOT, left ventricularoutflow tract). (Photo (D) courtesy of Dr Michel Pellerin.)tissue that returns an echo) is confined laterally for a givenscan line. By moving the scan line mechanically or electronically,2D maps of echo strength can be obtained.During the receiving of a given scan line, all echoes areassumed to originate from points along that line, whoseprecise distance from the transducer can be determinedusing Eq. (1.12). The vast majority of contemporary B-mode systems now direct the beam by using a nonmechanicaltechnique in a manner that will be described in detailin Chapter 3.image. This mode is commonly used for dimensionalmeasurements at specific time of the cardiac cycle andfor the analysis of valve leaflets, wall and abnormal structuremotions.4. Other ModesOther modes of imaging, such as C-mode, duplex,Doppler, and 3D imaging are not described here and arebeyond the scope of this chapter.3. M ModeIn M-mode (motion mode), an A line is displayed on a 2DB-mode map (image matrix) with brightness indicating theecho strength or the amplitude [Fig. 1.14(D)]. However,the other dimension direction corresponds to time ratherthan lateral distance, as in A- or B-mode. Consequentlythis mode is used to show the motion of reflectors as afunction of time by continuously retracing a line such asis acquired in B-mode, but maintaining the location towhich this line corresponds with respect to the patient,hence insonifying the same column of tissue every time(in a true B-mode image acquisition, that line in sweptin the patient). Each of these lines is traced vertically inthe image, and shifted laterally on the screen from oneline to the next. Once the full width of the image hasbeen swept in this way, the process is repeated for a newB. Instrumentation1. TransducersWhatever the type of transducer used, a 2D scanner imageinvolves the gathering of multiple B-mode lines to constructthe final image. Modern transducers are actuallymade of many crystal elements disposed as an arraywithin a portable assembly. Each element is selectivelyfired using a scheme described in Chapter 3. It iscommon to refer to the assembly as the probe. Moderntransducers can generate a narrow beam of ultrasound,and direct it in a selectable direction, in a manner compatiblewith the description of B-mode acquisition givenearlier. The term “narrow” in the previous sentenceapplies to both the lateral (or “in-plane”) and axial (or“in the beam”) directions.