Phased Array Transducer

Abbreviations: BSA = Body surface area; LVIDd = left ventricular internal diameter, end diastole; LVIDs = left ventricular internal diameter, end systole; FSLV = fractional shortening of left ventricle; PWV = posterior wall velocity; IVS = interventricular septum; PW = posterior wall; RVD = right ventricular dimension; LAD = left atrial dimension.

Source: Felner JM, Schlant RC. Echocardiography: A Teaching Atlas. New York: Grune & Stratton; 1976. Reproduced with permission from the publisher and authors.

Two-Dimensional Echocardiography

A number of technical approaches exist by which multiple individual B-mode scan lines can be rapidly transmitted, received, and displayed in appropriate spatial orientation to construct a 2D image of the heart. The initial approach simply utilized a linear array of 20 piezoelectric crystals placed side by side, each of which transmitted and received signals independendy8 Fig. 13-10/1). The resulting scan lines were displayed simultaneously to yield rectangular images. Unfortunately, transducer size and interaction between the elements resulted in images of unsatisfactory quality.

Current 2D scanners utilize B-mode scan lines that are independently transmitted and received and are directed through a wedge-shaped sector of cardiac anatomy by means of mechanical or electrical beam steering (BhH; Fig. 13-10.8 to D). A variety of motorized devices are available that, by rapidly oscillating or rotating one or more ultrasonic crystals through space, can mechanically direct multiple scan lines through a sector arc of the cardiovascular system.9,10 The position of the beam in space is derived by determining the orientation of the piezoelectric crystal. A majority of current 2D scanners utilize a phasedarray approach, where multiple ultrasonic crystals are employed in concert to create individual B-mode scan lines.ii The piezoelectric crystals are activated in a closely coordinated temporal sequence such that the individual wavelets produced by each element merge to form a single beam whose direction is determined by the sequence of crystal firing (Fig. 13-11). Since the direction of the resultant beam is determined by the sequence of activation of the individual elements, the beam can be electrically swept throughout a 90-degree sector arc. In addition to electronic beam steering, a firing sequence can be employed that results in dynamic focusing of the beam along its length to achieve minimal beam width and increased resolution. Phased-array 2D scanners employ small transducers without moving parts that could require repair. The increased complexity of these scanners, however, makes the systems more costly.

Phased Array Transducer

Figure 13-11: Electronic "steering" of a phased-array ultrasound beam. A. Elements are fired in sequence from left to right, resulting in a beam directed to the left. B. Elements are fired in sequence opposite to those in (A), producing a beam directed to the right. C. Elements are fired from the periphery toward the center, producing a beam that converges on a given focal point. (From Hagan AD, DeMaria AN. Clinical Applications of Two-Dimensional Echocardiography and Cardiac Doppler. Boston: Little Brown; 1989, with permission.)

Figure 13-11: Electronic "steering" of a phased-array ultrasound beam. A. Elements are fired in sequence from left to right, resulting in a beam directed to the left. B. Elements are fired in sequence opposite to those in (A), producing a beam directed to the right. C. Elements are fired from the periphery toward the center, producing a beam that converges on a given focal point. (From Hagan AD, DeMaria AN. Clinical Applications of Two-Dimensional Echocardiography and Cardiac Doppler. Boston: Little Brown; 1989, with permission.)

Originally, echocardiographic data were displayed in analog form on a standard oscilloscope, transferred to a video monitor by a television camera, and hard-copied onto videotape or paper. Currently, computerized analog-to-digital scan conversion is standard, so that the polar signals of individual scan lines are converted to a series of numerical gray-level values for individual box-like picture elements (pixels) aligned along X-Y coordinates.33 The ability of a digital step-gradation technique to reproduce the continuous gradation of analog methods is a function of the density of pixels in the matrix and the shades of gray levels available. No loss of data can be detected in current digitally converted images, and the digital format provides the opportunity for image processing, enhancement, and quantitation. More importantly, storage in digital format can avoid the image degradation inherent in videotape, provide random access and easy comparison of studies, enable rapid image transmission, and prevent deterioration with image copying and prolonged storage. Technology for fully digital echocardiography is now becoming available, and fully digital acquisition and storage of echocardiograms will be commonplace in the near future, replacing analog videotape recordings.

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