DK2985_C000 1..28 - AlSharqia Echo Club

42 Transesophageal EchocardiographyFigure 3.1 The concept of an ultrasound transducer. (A–C) An ultrasound transducer converts electrical energy into ultrasound energyand vice versa. The heart represents the reflective surface. The image is generated from the incoming electrical energy (LA, left atrium;LV, left ventricle; RA, right atrium; RV, right ventricle).that was used in early electrical studies (1). The piezoelectriceffect is the phenomenon whereby some materials(quartz, ceramics, lithium sulfate, and others) are able togenerate an electric current when deformed by mechanicalpressure or stress (1,2). The same materials, whensubmitted to an alternating electrical current, changeshape and can be made to vibrate and deform, that is,the opposite of the first principle (Fig. 3.2). This is thebasis of the transducer’s construction and function.When we apply this principle to clinical echocardiography,we can understand how a piezoelectric element canbe used to generate ultrasound waves from electricalstimulation and how an electrical current can be generatedfrom the mechanical stimulation caused by the returningultrasound echoes.Figure 3.2 The piezoelectric principle. The arrows representpressure, while (þ) and (2) represent electrical charges.II. THE STRUCTURE OF THE TRANSDUCERA. The Structure of a Single ElementTransducerThe most simple transducer is composed of a single piezoelectricelement or crystal. In practice, single crystals areused for A- and M-mode recordings. The transducer containsseven important components (5) (Fig. 3.3):1. The piezoelectric element which generates acousticpulses and electrical signals when submittedto electrical or mechanical stimulation.2. The electrodes which transmit the electric currentto and from the piezoelectric element and recordthe voltage generated by the returning echoes.3. The backing material which shortens the timeduring which the crystal vibrates following excitation,thus shortening the spatial pulse length,and improving axial resolution. It is usually composedof a mixture of tungsten and rubber.4. The matching layer which reduces the acousticimpedance mismatch between the patient and thetransducer thereby attenuating reflected ultrasoundwaves at the transducer–patient interface.5. The acoustic insulation which prevents thetransmission of vibration to the housing of thetransducer.