Physics Principles of Diagnostic Ultrasound Flashcards
The speed at which a wave travels through a medium is determined by:
the density and compressibility of the medium
Absorption of the ultrasound beam is highest in …..
bone
A longitudinal wave requires a
medium to propogate
The propagation speed of ultrasound waves is slowest in
air (330 ms-1)
One cycle of a wave is known as
the period
Which medium can a sound wave not travel through?
A vacuum
What determines the speed of sound?
The medium
If the frequency of the wave is 1MHz the period is:
0.000001s
In a sound wave regions where pressure is lower than normal are called regions of
rarefaction.
What is sound?
A vibration that propagates as an acoustic wave through a transmission medium
What type of wave is a sound wave
A longitudinal wave, the vibrations are in a parallel direction to wave travel. They require a medium to propogate
What is the audible sound range?
20Hz - 20KHz
Diagnostic ultrasound range?
2MHz - 20MHz
What is the propagation speed in normal soft tissue?
1540 ms-1
What happens to the wavelength as Frequency increases?
Wavelength gets smaller. Higher resolution
What happens to wavelength as frequency decreases
Wavelength gets larger, deeper penetration.
What happens as US passes through an interface with identical impedences z1 = z2
Total transmission, no energy is reflected, no echo produced. Interface will not been seen on US
When does partial reflection occur?
When two tissues have difference impedances (eg. liver vs fat). A fraction of the energy is reflected and the rest is transmitted.
When does total reflection occur?
When there is a large difference in acoustic impedance of tissues. (Soft tissue vs air) All energy is reflected and none is transmitted into the second tissue. The interface is seen as a strong linear structure on US.
What is the equation for acoustic impedance?
Z = p x c (Ryals)
p = density of the tissue
c = US propogation speed
Define attenuation
As an US wave travels through tissue it becomes progressively weaker.
