Week 7 Lecture 3 PDF - Ultrasound Transducers

Course code: BMIS-LHR A  The objectives of this lecture are as follows:  Ultrasound transducers  Piezoelectric effect  Natural and synthetic crystals  A transducer is a device that converts one form of energy into another. A loudspeaker converts electrical energy t...

Course code: BMIS-LHR A  The objectives of this lecture are as follows:  Ultrasound transducers  Piezoelectric effect  Natural and synthetic crystals  A transducer is a device that converts one form of energy into another. A loudspeaker converts electrical energy to sound energy. Amicrophone converts sound energy to electrical energy. A light bulb converts electrical energy into light and heat.  An ultrasound transducer uses the piezoelectric effect to convert electrical energy into ultrasound, and convert ultrasound back into electrical energy.  Modern transducers are all multi-element transducers. Element is a synonym for crystal (these two words can be interchanged). Multi-element transducers are known as arrays.  Historically, stand-alone A-mode and M- mode machines, as well as all static B- scanners, used a transducer that contained a single, disk-shaped piezoelectric crystal. As well, old mechanical real-time systems used a single piezoelectric crystal transducer.  Today,all modern transducers are electronic arrays that contain an arrangement of multiple small piezoelectric crystal elements  The piezoelectric effect is the cornerstone of the diagnostic ultrasound process. All ultrasound transducers possess piezoelectric properties which permit them to generate and detect ultrasound waves.  Thedirect piezoelectric effect is the formation of an electrical charge on the surfaces of the crystal when pressure is applied.  Inpulse-echo imaging, this effect occurs on reception when echoes return to the transducer and are converted into electrical signals.  Theindirect (reverse) piezoelectric effect occurs on transmission. In response to an electric voltage applied to the crystal, the crystal undergoes mechanical deformation, rapidly contracting and expanding.  When coupled to the body with gel, this causes a high frequency mechanical pressure wave to propagate through the tissue in the form of an ultrasound pulse.   Piezoelectriccrystals contain dipolar molecules (dipoles) that are geometrically aligned within the crystal. Dipoles are molecules that have a positive charge at one end and a negative charge at the other end.   The positive and negative ends are organized in a lattice fashion (A) so that an applied electric signal (B) will cause them to geometrically realign, resulting in a small physical change in the thickness of the crystal. This is the basis of the reverse piezoelectric effect.   (The diagram demonstrates what appears to be a considerable change in the crystal thickness, but this is for visual effect only. The actual change in the thickness of the transducer element is very, very tiny)  On reception, echoes hitting the crystal surface result in the physical deformation of the crystal.  Thisdeformation forces the tiny dipoles to change their resting orientation which, in turn, induces an electric signal between the electrodes.  Thissmall electrical signal is then amplified, processed, and displayed as a dot of light on the screen to represent that echo.  Stronger echoes produce stronger electrical signals, weaker echoes produce weaker electrical signals. Thus the amplitude of each electrical voltage is proportional to the strength of the echo that generated it.  The first ultrasound crystals discovered to possess piezoelectricity were natural crystals such as quartz. Today, our modern, efficient transducer crystals are "grown" in a manufacturing process and are known as synthetic crystals.   There are many different types of synthetic crystals in use today. Many are made from a mixture of lead, barium, titanate, and zirconate. These types of synthetic crystals are also known as ceramic crystals.  One of the more popular synthetic crystals is made from a combination of lead, zirconate and titanate, and is abbreviated PZT. Several types of PZT are available, with minor variations in chemical additions and thermal treatment producing slightly different crystal properties.  Ingeneral, ceramic crystals are much more sensitive than natural crystals. This means that they respond more easily to small electric signals and to weak echoes (small pressure waves.  Another type of transducer material is a type of hard plastic known as polymer.  Note: it is quite difficult to obtain detailed information about new transducer crystals because the transducer manufacturers consider the information proprietary.  After the completion of this lecture, students will be able to:  Know about the ultrasound transducers.  Explain energy conversion in different devices like light bulb, microphone and loud speaker.  Describe the piezoelectric effect and its types including direct and indirect piezoelectric effect.  KimLT. Principles of ultrasound and applied ultrasound physics relevant for advanced sonographers. In Advanced Ultrasound 2017 (pp. 37- 47). Springer, Cham.