Chapter 17 - Using Ultrasound In Medicine Flashcards
Define transducer?
A general term used for any device that changes one form of energy into another.
Define piezoelectric crystal?
A material that produces an induced e.m.f. when it is compressed. When a voltage is applied across it in one direction it drinks slightly. Reverse voltage, expands slightly.
Explain how a piezoelectric transducer works to produce US?
An alternating voltage is applied across the crystal causing it to expand and shrink, acting as a vibrating source of ultrasound waves.
Explain how the piezoelectric transducer can be used to detect US?
The piezoelectric effect works in reverse. Therefore the US waves cause varying stress on the crystal leads to a varying voltage.
What are three features of an ultrasound transducer?
1) crystal made of polyvinylidene protected by a case
2) at the base there is an acoustic window to transmit US
3) damping material behind crystal to prevent crystal vibrating too much due to resonance
Define acoustic impedance and give equation?
The product of density and speed of sound of the substance
Z=pc
Explain what happens to an incident Ultrasound wave at a boundary?
The wave is partially refracted and partially reflected. We detect the reflected part of the wave
What is the equation to show the ratio of reflected intensity to incident intensity?
Ir/Io = ((z2-z1)^2) / ((z2+z1)^2)
When will a large fraction be reflected?
When there is a big change in the acoustic impedance
Why do you hospitals use a gel when doing ultrasounds?
There is a big difference in acoustic impedance between skin and air, so the transducer is coupled to the skin using a gel who’s impedance matches that of the skin to prevent a big reflection at the air-skin boundary.
Define impedance matching?
Reduction in intensity of reflected ultrasound at boundary between substances, achieved by two substances having similar acoustic impedances.
What is ultrasound good for scanning and bad for scanning?
Good - tissues/bones
Bad - lungs (due to air-tissue boundary)
What is ultrasound?
A sound wave with frequency above upper limit of human hearing (>20kHz)
Explain what an A scan is?
- Pulse of US sent into body, reflected waves detected, displayed as a V/t graph
- Each partial reflection is shown as a peak on the graph
Note for reading A-scan graphs?
The change in time often had to be divided by 2 since US wave goes there and back
Thickness = cΔt/2
When is an A scan used?
Simple procedures like measuring thickness of eye lens
Explain what a B-scan is and how it works?
It’s a detailed is image of a patient’s cross-section, using a number of sensors in different positions and at different angles to build up an image.
1) US transducer moves across area of interest
2) each pulse analysed to determine depth from surface (time) and nature of surface (amplitude of reflected wave).
3) 2-D image built up by positioning dots
Disadvantage of B-scans?
Image can blur if organs of interest are moving
Define Doppler effect?
The change in frequency and wavelength of a wave caused by the relative movement between source and detector.
Explain the five steps for Doppler ultrasound?
1) US probe help against something moving (eg. Blood)
2) if blood moving away from source, each wave is reflected from slightly diff. position
3) result - reflected waves have greater wavelength, and since v is constant, f decreases
4) tf greater the speed of receding reflector, greater the decrease in freq.
5) speed of reflector can be deduced from change in freq of waves
2 uses of Doppler ultrasound?
1) determining steady bloodflow
2) kidney transplant patients have it to see how well blood flows through their new kidney
Why is it desirable to have a short wavelength for an US scan?
Shorter wavelength will give finer detail in the picture
