Presentation on theme: "Piezoelectricity Medical Physics Notes: Ultrasound."— Presentation transcript:
Piezoelectricity Medical Physics Notes: Ultrasound
The Piezoelectric Effect The piezoelectric effect describes an interchange between mechanical and electrical energy which occurs in certain crystals known as piezoelectric crystals. Examples - quartz or the synthetic ceramic, lead zirconate titanate.
Unstressed crystal In an unstressed state, the centres of symmetry of both the positive and negative ions of such a crystal lattice coincide. Therefore no effective charge appears on electrodes attached to either side of the crystal. The charge is balanced.
Stressed Crystal When the crystal is compressed or extended and, the centres of symmetry of the charges move apart. They no longer coincide, and give rise to net charge on the electrodes. Therefore there is a voltage across them.
Applying a voltage to the crystal If a voltage is applied across an unstressed piezoelectric crystal, the centres of symmetry move. This deforms the crystal. An applied alternating voltage gives rise to mechanical vibrations in the crystal. A maximum response (or resonance) occurs when the applied frequency matches a natural frequency of vibration of the crystal If these frequencies of vibration are higher than 20,000 Hz they are in the ultrasonic range
The Piezoelectric Transducer A piezoelectric transducer can operate as: (a) an ultrasound generator, by applying a ‘stimulating voltage’ of suitable frequency, thereby causing crystal vibrations and the emission of ultrasonic waves, (b) an ultrasound detector, by monitoring the piezoelectric voltage developed across the crystal when it is forced to vibrate by incoming ultrasonic waves.
The electrodes must be light and they usually consist of thin layers of silver. The one nearest the patient is connected to the earthed metal case for safety. The other is called the 'active' or 'live' electrode and is connected via a coaxial cable to either a source of power (when used as a transmitter) or an amplifier and cathode ray tube (when used as a receiver).
Clarity of Signal Clear distinct echoes are obtained using short pulses. To generate such pulses, the vibrations must be damped out as quickly as possible. This is achieved by bonding a damping material (araldite or other epoxy resin) to the back face of the crystal.