Ultrasound Physics Flashcards
(155 cards)
Sound is a traveling variation in ______.
Pressure
Sound involves ______ motion in the medium through which it travels.
Mechanical
What are the 3 acoustic variables?
Pressure
Density
Particle vibration
- they are the quantities that vary in a sound wave
What are rarefactions & compressions?
Rarefactions = regions of low pressure & density
Compressions = regions of high pressure & density
What is density?
Density = concentration of matter (mass per unit volume)
Sound is a _______, _________ wave in which back-and-forth particle motion is parallel to the direction of travel
Mechanical, longitudinal
If the up and down motion of a water surface is perpendicular to the direction of wave travel it is what type of wave?
Transverse or shear wave
What is the definition of frequency? And what is it’s unit?
Freq = # of cycles that occur in 1 second
Hertz (1 Hz =1 cycle/sec)
1 kHz = 1000Hz
1 MHz = 1,000,000Hz
The positive and negative halves of a pressure cycle correspond to what, respectively?
Regions of compression & rarefaction
(Higher pressure = denser medium & vice versa)
What is the definition of period? And what is it’s unit?
Period = time it takes for 1 cycle to occur
Common unit for period = microseconds ( 1 microsecond = 0.000001 second) e.g. the period for 5MHz U/S = 0.2 microseconds
What is the equation for period?
T (period in microseconds) = 1/ freq (Mhz)
If the period decreases, what happens to freq?
If period decreases –> freq increases.
If freq inc –> period dec
What is the definition of wavelength? what is the common unit of wavelength?
Wavelength = length of SPACE/DISTANCE that 1 cycle takes up
Aka the length of cycle from front to back
Millimeters (1 mm = 0.001m)
For a propagation speed of 1.54 mm/s and a freq of 5MHz, what is the wavelength?
0.31mm
(Eq:: wavelength = propagation speed [mm/us] / freq [MHz])
What is propagation speed (c)? What is it’s unit?
The speed with which a waves moves through a medium
Units = m/s and mm/microsecond
What is the relationship between wavelength, propagation speed, & freq?
Wavelength = propagation speed (mm/microsecond) / freq (MHz)
If wavelength increases —> freq ________
Decreases
What primarily determines propagation speed?
The medium, specifically it’s stiffness (hardness)
What is stiffness?
Stiffness = resistance of a material to compression
Stiffer media –> HIGHER sound speeds (gas < liquid < solid)
What is the average propagation speed in soft tissues?
1540 m/s or 1.54 mm/microsecond
Shear waves travel ______ in comparison to longitudinal waves.
More slowly
The average propagation speed in fat is approximately what percentage in comparison to soft tissue?
~6% lower
Propagation speed in any medium depends on the ______ and ______ of the medium.
density (p) and stiffness
Density = mass density = mass per unit volume
Stiffness aka hardness or bulk modulus of elasticity (B)
What is the equation to for propagation speed?
c (m/s) = [ B (N/m^2) / density (kg/ m^3) ] ^ 0.5
*In general, stiffness differences dominate the effect on propagation speed more than mass density (therefore solids have higher propagation speeds than liquids)