Test 1 Flashcards
Define sound
Sound is a travelling pressure / density variation
Compressions and rarefactions
What are acoustic variables? Give examples
Quantities that vary in a sound wave
- Pressure
- Density
- Particle motion/vibration
Compression
Regions of high pressure / density
Rarefaction
Regions of low pressure / density
Longitudinal wave
Vibration parallel to direction of wave
Frequency
Cycles per second
Hz
KHz
MHz
Audible sound
20Hz - 20 KHz
Ultrasound range
20 KHz - 160 KHz
Period
Seconds per cycle
Unit: us (microsecond)
Wavelength
Cycle length
Unit: mm
Acoustic velocity /
Prop speed
The speed at which a wave moves through a medium
Unit:
m/s
mm/us
Propagation speed is dependent on
The medium through which the wave travels
Wavelength formula
Lambda = c/f
c= mm/us f= MHz
Propagation speed units
mm/us
What is stiffness
Resistance of a material to compression
AKA Bulk modulus (B)
Propagation speed formula
Square root of :
Bulk modulus (N/m^2)
/
Density (kg/m^3)
Propagation speed through solid liquid gas - what is the order
Solid is highest
Gas is lowest
Acoustic velocity in soft tissue
1540 m/s
1.54 mm/us
Propagation speed in:
bone
Fat
Bone: 3-5 times higher
Fat: slightly lower than ST
Compressibility and elasticity are related how?
How are they related to stiffness
They represent the same thing. Both inversely related to stiffness
Stiffness is ____ to propagation speed
Directly related to
How is stiffness related to bulk modulus
Directly proportional
Describe pulse echo technique in the use of ultrasound
Pulse separated by a gap because ultrasound needs to receive and process the information
Describe harmonics production
You send out one frequency, and you get back several higher frequencies.
Therefore harmonics allows us to achieve higher frequency waves and which allows for better image quality
Continuous wave
Sinusodal waveform where cycles repeat indefinitely
Pulse repetition frequency
Number of pulses per second
Unit: KHz
What does PRF depend on?
Pulse repetition frequency depends on the instrument
Pulse repetition period
Time from beginning of one pulse to beginning of next
Unit: ms
If PRF increases, what happens to PRP
Inversely proportional, so PRP decreases
What is pulse duration and it’s unit
Time for one pulse to occur
Unit: us
Typical numbers of cycles in an ultrasound pulse
2-3 cycles long
Typical number of cycles in a Doppler pulse
5-30 pulses
If frequency increases, what happens to PD? Why?
Frequency increases
Period decreases because inversely proportional
Therefore pulse duration decreases because period and PD are directly proportional :
PD = n x T
PD units
us (because T is us)
What is the advantage of shorter PD?
Improves the quality of images
What is DF? It’s unit? It’s equation?
Fraction of time the pulsed ultrasound is ON
Unit is a decimal or %
DF = PD (us) / PRP (us)
If pulse duration increases what happens to DF?
Increases because the sound is on more % of the time
DF = PD (us) / PRP (us)
What is SPL? Unit? Significance?
SPL is the length of the pulse
Unit: mm
SPL(mm) = n x wavelength (mm)
If frequency increases, what happens to SPL?
Frequency increases therefore wavelength decreases (because wavelength is inversely proportionate). So if wavelength decreases SPL decreases
Anechoic
Without internal echoes
Echogenic
With internal echoes
Heterogeneous
Not smooth
Homogeneous
Smooth
Hyperechoic
Brighter echoes than surrounding tissue
Hypoechoic
Darker echoes than surrounding tissues
Isoechoic
Similar echoes to surrounding tissues
Grey scale US
Pulse-echo technique
Doppler US
Pulse-echo + Doppler effect
Bandwidth
Range of frequencies produced by a transducer
Bandwidth is seen only in
Pulsed ultrasound
What happens to bandwidth with a shorter pulse?
Larger bandwidth
Thus lower Q factor
All of these improve image quality
What is the range of fractional bandwidth in most transducers
50-100%
What happens to quality factor if fractional bandwidth goes down
It increases
Amplitude is
The strength of a sound wave
Unit: pressure (N/m^2) or voltage
Power is
The rate at which energy is transferred
Unit: W or mW
Intensity is
The rate at which energy passes through a unit area
Unit:
mW/cm^2
Power/area
What is the relationship between intensity and amplitude
I = Amp^2 Amp = 1/I^2
What is the relationship between power and amplitude
P = Amp^2 Amp = 1/P^2
Waves
Transfer energy from one location to another
Cyclical
Amplitude units
Pressure - N/m2
Density - kg/m^3
Voltage - V
Nonlinear propagation
Propagation in which speed depEnds on pressure and the shape of the wave changes
