Transcript Ultrasound Probe

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.
A Typical Transducer for
Medical Applications
A Typical Transducer for
Medical Applications
• 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.