DK2985_C000 1..28 - AlSharqia Echo Club

44 Transesophageal Echocardiography2. What are the main characteristics of an ultrasoundbeam (structure and focus)?3. How can a good quality ultrasound image be generatedby combining a set of focused beams (theprinciple of scanning and steering)?A. The Emission of Ultrasound EnergyTo understand the fundamental principles of transducerfunction, one has to keep in mind the important characteristicsof an ultrasound wave as illustrated in Fig. 3.5 (seealso Chapter 1).1. Mode of Generation of the Electrical ImpulseTransducers can operate in two different generationmodes: burst excited or shock excited. In the burstexcited mode, transducers convert an alternating electricalvoltage burst into an ultrasound pulse by inducing conformationalchanges in the piezoelectric elements. The transducercan also convert incoming ultrasound echoes intoalternating electrical voltage bursts. Most modern transducersoperate in burst excited mode because this providesthe possibility of selecting the operating frequency fromthe bandwidth. Typically, transducers are driven by oneto three cycles of alternating current in order to generatean image. In the shock excited mode, a transducer convertsa uniphasic voltage impulse into ultrasound pulses whilereturning echoes are converted into alternating electricalvoltage bursts.2. Operating FrequencyThe operating, or resonant, frequency of the system is thepreferred vibrating frequency of the transducer that providesthe maximum efficiency of operation. This frequencydepends on two factors:1. the piezoelectric element thickness which correspondsto one-half of the pulse wavelength and2. the propagation speed of the crystal.As wavelength and frequency are reciprocal, the thinner theelement the higher the transducer frequency. The typicalthickness for a piezoelectric element is between 0.2 and1 mm (1).Propagation speed ¼ wavelength frequencyElement thickness ¼ 1 2 wavelengthThereforepropagation speedFrequency ¼(3:1)2 element thickness3. Frequency BandwidthUnder ideal circumstances, when a transducer produces apulse, its frequency will equal the operating frequency.In reality, however, a pulse contains different frequencies.The bandwidth refers to the range of frequencies in whichamplitude exceeds a given value, contained in the ultrasoundpulse. The bandwidth is inversely related tospatial pulse length. Therefore, sound pulses with a shortduration (shorter spatial pulse length) are composed ofFigure 3.5 The generation and transmission of ultrasound waves. (A–C) This figure illustrates the important characteristics of ultrasoundwaves leading to the generation of an image (LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle).