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Sonographic Physics

Chapter 9: Transducers

QuestionAnswer
What is a transducer? Any device that converts one form of energy into another
What two functions do U/S transducers perform? transmission: electrical energy from system is converted into sound reception: sound pulse is converted into electrical energy
What is the Piezoelectric Effect? property of certain materials to create voltage when they are mechanically deformed
What is reverse piezoelectric effect? when materials deform (change shape) when voltage is applied
What are piezoelectric materials? material capable of converting sound into electricity and vice versa
What is another name for piezoelectric materials? ferroelectric materials
Name some natural and man-made piezoelectric materials. Natural: quartz, tourmaline Man-made: lead zirconate titanate (PZT)
What piezoelectric matterial is commonly used in clinical transducers? PZT
PZT is also known as what? ceramic, active element, crystal
What are the 7 components of a basic transducer? active element (PZT) case electrical shield acoustic insulator wire matching layer damping element (backing material)
What is the case made of and what is its purpose? constructed of metal or plastic; protects the internal components from damage and insulates the patient from electricalshock
What is the electrical shield and what is its function? thin metallic barrier lining the inside of the case; prevents false electrical signals unrelated to diagnostic information from entering the transducer
What is the acoustic insulator and what is its function? thin barrier of cork or rubber that isolates internal components of the TD from the case; prevents case vibrations from inducing an electrical voltage in the PZT
What is the matching layer and what is its function? positioned in front of the PZT at the face of the transducer; has an impedance between that of the skin and the active element to increase percentage of transmission
What between the matching layer and skin also helps with transmission of the sound beam? Gel
What is the backing material (damping element) and what is its funtion? bonded to the active element to limit "ringing"; commonly composed of epoxy resin impregnated with tungsten
What are the advantages and disadvantages of the backing material? Advantages: shortens SPL and pulse duration, increased bandwidth with decreased Q factor, increases axial resolution (picture quality) Disadvantages: decreases sensitivity,
Greater differences in what result in more reflection at a boundary? impedance
PZT impedance is about how much more than skin impedance? 20 times
What impedance is between that of skin and PZT? matching layer
The impedance of what is between that of the matching layer and skin? gel
The thickness of the matching layer is what the wavelength of sound in the matching layer? 1/4
The thickness of the active element (PZT) is what the wavelength of sound in the active element? 1/2
What are some characteristics of backing material? high degree of sound absorption; acoustic impedance similar to PZT so sound energy moves away from patient into backing material
What does it mean when its said that backing material causes decreased sensitivity of the TD? the TD is less able to convert low-level sound reflections into meaningful electrical signals (won't display on monitor); reduces the vibration of the PZT during transmission and reception phases; These make the transducer less responsive
What is resonant frequency and what are some other names for it? a long pure tone of single frequency; operating frequency, single, main, primary, pure frequency, TD frequency
What happens to the PZT when backing material is present? PZT cannot vibrate freely so the pulse becomes short duration "click" instead of a long steady tone
The short click caused by the backing material contains what? many frequencies above and below the resonant frequency
what is bandwidth? the range between the highest and lowest frequencies emitted from the TD
What is the relationship between bandwidth and pulse duration? long pulse = narrow bandwidth short pulse = wide bandwidth
An imaging TD has what kind of pulse and bandwidth? CW probe? Imaging TD: short pulse, wide bandwidth CW probe: no backing: long pulse, narrow bandwidth
In CW transducers, the sound wave's frequency equals what? the frequency of the voltage applied to the PZT
With CW transducers, electrical frequency equals what? acoustic frequency
With PW transducers, the frequency produced depends on what? characteristics of the PZT
What are the two characteristics of the PZT that determine the frequency of sound from pulsed wave transducer? Speed of sound in the PZT Thickness of the PZT
The speed of sound in the PZT means what? the propagation speed of the PZT
With PW TDs, when sound's speed in PZT is faster, the frequency created is what? higher
With PW TD's the speed of sound in the PZT is related how to frequency of sound? directly related
How much greater is the speed of sound in the PZT of PW transducers than in soft tissue? Whats the range? about4 times greater than soft tissue. the range: 4-6mm/us
With pulsed wave transducers, thinner PZT crystals create what kind of frequency sound pulse? higher frequency
With PW transducers, a thinner PZT crystal creates what kind of wavelength? short wavelength pules
With PW transducers, the thickness of the PZT crystal is related how to frequency? inversely related
What is the thickness range of PZT crystals in PW transducers? thickness range: 0.2-1mm
What is the equation for frequency for pulsed wave transducers? frequency(MHz)= speed of sound in PZT(mm/us)/(2 x thickness(mm))
Created by: quietone055 on 2010-03-29



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