Generation of an Ultrasound Wave
An ultrasound wave is generated when an electric field is applied to an array of piezoelectric crystals located on the transducer surface.
Electrical stimulation causes mechanical distortion of the crystals resulting in vibration and production of sound waves (i.e. mechanical energy).
The conversion of electrical to mechanical (sound) energy is called the converse piezoelectric effect (Gabriel Lippman 1881).
Each piezoelectric crystal produces an ultrasound wave. The summation of all waves generated by the piezoelectric crystals forms the ultrasound beam. Ultrasound waves are generated in pulses (intermittent trains of pressure waves) and each pulse commonly consists of 2 or 3 sound cycles of the same frequency.
The pulse length (PL) is the distance traveled per pulse. Waves of short pulse lengths improve axial resolution for ultrasound imaging. The PL cannot be reduced to less than 2 or 3 sound cycles by the damping materials within the transducer.
Pulse Repetition Frequency (PRF) is the rate of pulses emitted by the transducer (number of pulses per unit time). Ultrasound pulses must be spaced with enough time between pulses to permit the sound to reach the target of interest and return to the transducer before the next pulse is generated. The PRF for medical imaging ranges from 1-10 kHz. For example, if the PRF = 5 kHz and the time between pulses is 0.2 msec, it will take 0.1 msec to reach the target and 0.1 msec to return to the transducer. This means the pulse will travel 15.4 cm before the next pulse is emitted (1,540 m/sec x 0.1 msec = 0.154 m in 0.1 msec = 15.4 cm).