Ultrasound Flashcards
Describe the properties of ultrasound.
They are longitudinal waves with a frequency higher than humans can hear.
Describe the piezoelectric effect.
The application of of a pd across a crystal causes vibration/expansion/contraction.
Explain how ultrasound transducers emit
and receive high-frequency sound.
A piezoelectric crystal inside the transducer (usually PZT) can either convert ultrasound waves into an alternating voltage, or convert an alternating voltage into ultrasound.
When do we receive maximum voltage from an ultrasound transducer?
The largest voltage will be produced when the wavelength it produces is double the thickness of the crystal.
Describe the principles of ultrasound
scanning.
When an ultrasound wave meets the boundary between two materials, some of it is reflected. The amount reflected depends on the change in acoustic impedance - the bigger the change, the more is reflected.The reflected waves are detected by an ultrasound scanner and are used to generate an image.
Describe the difference between an A-scan and a B-scan.
An A-scan simply shows the amplitude of each reflected ultrasound wave, which allows to calculate how deep the reflection occurred. A B-scan, however, produces a 2-dimensional image that uses the intensity of the reflected to wave to translate into brightness.
How do we calculate the acoustic impedance of a medium?
Multiply the density of the medium by the speed of light in the medium.
Describe the importance of impedance matching.
It allows ultrasound waves to pass through a boundary with minimal reflections.
Why is gel required for effective ultrasound imaging techniques?
To reduce the change in acoustic impedance between the skin and what it is next to. If it was air, all the ultrasound would be reflected at the air-skin boundary, so we use gel to reduce this reflection.
