US Flashcards
What is a sound wave?
Mechanical energy that produces vibrations when propagating through material.
Sound requires a medium to travel in.
These vibrations produce alternating areas of high pressure (compression) and low pressure (rarefaction).
What is frequency?
Rate of change between compression and rarefaction - given in Hertz.
Number of times the wave oscillates through a cycle each second.
What is wavelength?
Distance between areas of compression
Equation for speed
Speed = wavelength x frequency
Speed is thought to be constant (in a particular medium), so that an increase in frequency decreases in wavelength - and vice versa.
How does speed change in different materials?
Speed is based on compressibility of something.
Very compressible (air) will have a very low speed. Not very compressible (bone) will have a very fast speed.
US machine assumes everything travels at 1540 m/s in tissue.
What is the assumed speed of sound waves in tissue?
1540 m/s
What affect does speed have on frequency?
None. The frequency is the same, irrelevant of the sound speed in various media.
Wavelength changes in media.
How is relative intensity measured in US?
The dB.
A change of 10 in the dB scale corresponds to two orders of magnitude (100 times) and so forth. The dB is based on a log 10 scale.
Reducing the sound intensity to 10% is -10 dB
Reducing to 1% is -20 dB
Reducing to 0.1% is -30dB
Loss of 3 dB (-3 dB) represents a 50% loss of signal intensity (power)
The tissue that reduces the US intensity by 3 dB is considered “half-value” thickness.
What does a change in 10 in the dB scale correspond to?
Two orders of magnitude (100 times) and so forth. The dB is based on a log 10 scale.
Reducing the sound intensity to 10% is what in dB?
-10 dB
Reducing the sound intensity to 1% is what in dB?
-20 dB
Reducing the sound intensity to 0.1% is what in dB?
-30 dB
A loss of 3 dB represents what?
50% loss in signal intensity (power)
What is half value thickness in US?
The tissue that reduces the US intensity by 3 dB
What are the types of US interaction with matter?
Reflection
Refraction
Scattering
Absorption
What is reflection in ultrasound?
US energy gets reflected at a boundary between two tissues b/c of the differences in the acoustic impedance of the two tissues.
Large difference in “stiffness” results in a large reflection of energy.
What is impedance?
Z = density x speed of sound.
Compressibility of a spring.
What is refraction?
Bending of the sound wave caused by a change in speed.
Change in direction of transmitted US energy at a tissue boundary when the beam is not perpendicular to said boundary.
Frequency doesn’t change, but speed might.
Influenced by speed change - which is based on tissue compression and angle of incidence.
Hits straight - part will bound straight back and part goes straight through.
Strikes at an angle - part will be reflected and the other part will be refracted - with severity of this refraction depending on the speed difference of the two media.
What influences refraction?
Speed change - based on tissue compression
Angle of incidence
No refraction occurs if the speed of sound is the same in the two media or with perpendicular incidence.
Can you have total reflection?
If the speed difference and angle of incidence is great enough (exceeds the “critical angle”).
What are the two types of scattering in US?
Specular (smooth)- reflector dimensions are larger than the wavelength of the incident - strength of reflection is highly angle dependent.
Non-specular (diffuse)- scattering surfaces are about the size of a wavelength or smaller - angle has no effect on strength.
What are specular reflectors?
Smooth reflectors
Reflector dimensions are larger than the wavelength of the incident - strength of reflection is highly angle dependent.
What are non-specular reflectors?
Diffuse reflectors
Scattering surfaces are about the size of a wavelength or smaller - angle has no effect on strength.
High frequency = small wavelength = surfaces appear more rough = more scatter.
What is strength of reflection dependent on with specular reflectors?
Angle dependent