Transducers Chapter 8

Transducers Chapter 8 What is a transducer? A device that converts one form of energy into another Transducer Converts Converts energy from: energy to: Utrasound Electrical Acoustic transmitting Ultrasound Acoustic Electrical Reception Ca...

Transducers Chapter 8 What is a transducer? A device that converts one form of energy into another Transducer Converts Converts energy from: energy to: Utrasound Electrical Acoustic transmitting Ultrasound Acoustic Electrical Reception Car engine Chemical Motion Electric motor Electrical Motion Light bulb Electrical Light + heat Muscle Chemical Motion Ultrasound Transducers Perform two functions During transmission, electrical energy from system is converted into sound During reception, reflected sound pulse is converted to electricity Transducers are made with Piezoelectric Materials These can convert sound into energy and vice versa Piezoelectric materials - crystal, ceramic, PZT, element, active element  Transducers Piezoelectric Effect: a property of certain materials to create a voltage when they are mechanically deformed. Mechanic al Crystal Voltage Wave  Transducers Reverse Piezoelectric Effect: the material changes shape when a voltage is applied, creating a mechanical wave. piezoelectric crystal Mechanic Voltage Crystal al Wave  Polarization Polarization the piezoelectric properties of a crystal are created by exposing the material to a strong electrical field while being heated to a substantial temperature Curie Temperature the temperature at which PZT is polarized Depolarization piezoelectric properties of PZT can be destroyed by heating above the Curie temperature  Cleaning Transducers Cleaning Terms ○ Sterilization: the destruction of all microorganisms by exposure to extreme heat, chemical agents, or radiation ○ Disinfection: the application of a chemical agent to reduce or eliminate infectious organisms on an object Basic Cleaning Guidelines: ○Transducers should be disinfected with CidexTM or other cold germicides (or you may use a Trophon device) Should not be exposed to extreme temperatures (may effect piezoelectric properties even if not above Curie temp, may damage adhesives & bonding agents We should routinely use ‘single use’ disposable covers on endocavitary probes Basic Transducer Construction Case Electrical Shield Acoustic Insulator Crystal / Active Element Matching Layer Backing Material You must be know the parts and be able to label an image of a transducer! he crystal or Active Element Piezoelectric material: Active element, Called the Ceramic / Crystal / Element/ PZT (Lead Zirconate Titanate) ½ wavelength thick (λ of sound in the PZT) In pulsed wave ultrasound, characteristics of the sound beam are related to the characteristics of the crystal (more on this later!)  Matching Layer – Impedance Easer  What happens when two materials have different impedances?  Large reflections occur with large differences in impedance. There is a large difference in impedance between the crystal and the skin.  Matching Layer Positioned in front of the crystal at the transducer face and is the portion of the probe that is in contact with the skin. 1/4 wavelength thick (λ of sound in the matching layer) Evens out impedance between active element and skin. Impedance: PZT>matching layer>gel>skin Backing Material Also called Damping material Plays an essential role in pulse creation Without backing material, excited PZT will ring for a long time The emitted pulse would be increase in length & duration (not good!) Backing material enhances resolution by limiting the ringing of PZT, shortening the pulse length & duration Characteristics of backing material: High degree of sound absorption Acoustic impedance similar to PZT  Consequences of using backing material Decreased Sensitivity Reduces crystal vibration during transmission but also during reception This inhibits its ability to convert low-level sound reflections into meaningful electrical signals during reception  Consequences of using Backing Material Creates a Wide Bandwidth Pulses are not one single pure frequency. A Pulse is a sound ‘click’ or ‘beep’ that is made up of a range of frequencies above and below the main frequency. Backing material restricts PZT from vibrating freely causing wave of many different frequencies both above & below the resonant frequency  Bandwidth Bandwidth the range of frequencies between the highest and lowest frequency emitted from the transducer Resonant Frequency the main frequency emitted by the transducer aka: center frequency, primary frequency, natural frequency The transducer may be a 3 MHz transducer, but the 3 MHz is the center or main frequency. The transducer may also have frequencies above and below 3 MHz  Consequences of Using Backing Material Low Quality Factor Quality Factor: Unitless number related to the purity of the frequency Quality Factor = main frequency bandwidth Quality factor is directly related to pulse length. A shorter/dampened pulse has a low Q factor A longer/undampened pulse has a high Q factor Imaging Vs. Non-Imaging Transducers IMAGING TRANSDUCERS NON-IMAGING TRANSDUCERS Pulses with short duration and length Creates continuous wave or pulses with long duration and length Uses backing material to limit ringing No backing material Reduced sensitivity Increased sensitivity Wide bandwidth or broadband Narrow bandwidth Lower Q-factor High Q-factor Improved axial resolution Cannot create an image  Determining Frequency – Continuous wave Transducers The Machine Produces a continuous electrical signal that excites the crystal in the transducer. The frequency of sound emitted by the probe is the SAME frequency as the electrical stimulus Electrical frequency = transducer frequency Determining Frequency - Pulsed Wave Transducers PW transducers create a short duration electrical spike that excites the PZT. Transducer frequency, however, is NOT determined by the electrical stimulus. The frequency is determined by the properties of the crystal  Pulsed Wave Transducer Frequencies The MAIN or CENTER frequency is determined by two things: Thickness of the crystal Speed of Sound in the crystal Frequency (MHz)=  Speed of Sound and Transducer Frequency The speed of sound in the crystal and the frequency of Speed of sound in Produced the sound are directly related crystal Frequency When the speed of sound is higher, the frequency produced will be higher When the speed of sound is lower, the frequency produced will be lower. High speed High frequency most range from 4 to 6 mm/µs characteristic of each PZT, cannot be changed Low speed Low frequency  How does the thickness of the PZT (crystal) impact frequency? Thickness of the crystal and Crystal Produced frequency are inversely related Thickness Frequency Thin high Thin crystals create high frequency sound Thick low Thick crystals create low frequency sound PZT thickness = 1/2 wavelength of sound in the PZT ranges from 0.2 to 1 mm  Pulsed Wave Transducer Frequencies Higher Frequency Lower Frequency Transducer Transducer Faster PZT propagation speed Slower PZT propagations speed Thinner crystal Thicker Crystal