Basics Flashcards
Define the term period (include units, formula)
The time per cycle. Time it takes for one compression and one rarefaction to occur.
May be expressed in any unit of time. Often millisecond (ms) or microsecond (µs).
Formula: T (us) = 1/frequency (MHz)
Define the term frequency (include units, ranges).
Number of cycles (compressions and rarefactions) per second.
Cycles per second may be written as Hertz (Hz).
Infrasound 20 kHz
Clinical imaging ultrasound 2-10 MHz
List the four acoustic variables
Acoustic pressure
Density
Temperature
Particle motion
Define acoustic pressure.
Force/area
Units are Pascals (Pa)
Define density.
Mass/volume
Units are grams(g)/cm^3
Define temperature.
The degree of heat or cold measured on a definite scale
Units are Kelvin (K), Centigrade (C), or Farenheit (F)
Define particle motion.
The displacement or movement of a particle by a wave
Units are length or distance (e.g. millimeters (mm), meters (m)
What is sound?
A mechanical, longitudinal wave.
Does not travel as a transverse (particles of the medium move perpendicular to the direction of sound travel) wave
Mechanical
Infers motion and a transfer of energy
Longitudinal
The particles of the medium are vibrating parallel to the direction of sound travel
Wave
Vibration which travels from one location to another
Define acoustic impedance
The opposition to sound propagation through a medium
Units are rayl
Formula: Z(Rayls) = p(kg/m^3) * c(m/s)
What is a decibel (dB)?
A qualitative method of expressing the difference between two sound intensities or amplitudes
Define intensity. (include units, formula)
The amount of energy transferred (power) to a particular area.
Units are Watts (W)/cm^2 or milliwatts (mW)/cm^2
Formula: Intensity (W/cm^2 or mW/cm^2) = power (W or mW) / area (cm^2)
Is intensity controlled by the sonographer?
Yes, via the transmit gain control. Increasing the transmit gain causes the pulser to strike the piezoelectric element with a greater voltage thus increasing the amplitude, power and intensity of sound to the patient
Define power (include units)
The amount of energy transferred from the source.
Units are Watts (W) or milliwatts (mW)
Is power controlled by the sonographer?
Yes, through the transmit gain control
Define amplitude (include units)
One way to express the strength (loudness) of a sound wave.
Maximum variation of an acoustic variable minus the mean value.
Units depend on the acoustic variable being measured
Define the term propagation speed (include units, average value for soft tissue)
The rate at which a vibration (sound) travels through a medium.
Units are distance/time (m/s or mm/µs)
Soft tissue = 1540 m/s
Define the term wavelength (include units, formula)
The length of one cycle. The distance covered by one compression and one rarefaction.
Units are length or distance, usually meters (m) or millimeters (mm)
Formula: λ = propagation speed (mm/µs) / frequency (MHz)
List the 6 “R’s” of ultrasound physics
Rarefaction Reflection Refraction Reverberation Range Ambiguity Range Discrimination (Resolution)
Define rarefaction
The area of low pressure, low density of a sound wave
Define reflection
The sound energy returned back to the sound source
Define refraction
The change in the direction of sound as it crosses a boundary.
Occurs when there is oblique incidence and the propagation speeds of the two media are not equal
Define reverberation
An ultrasound artifact involving two highly reflective structures (e.g. transducer and skin surface) which results in the display of multiple echoes equally spaced
Define range ambiguity
A disadvantage of CW Doppler. Since CW Doppler collects Doppler shift information all along the ultrasound beam, the precise location or depth of the returning Doppler shift is unknown
Define range discrimination (resolution)
The advantage of pulsed wave Doppler in that Doppler shift information is determined and displayed only from a particular depth or location chosen by the sonographer
