Module 1 Physics Flashcards
What is the equation for medium impedance? (and measurement)
AI = density x velocity (raylz)
What is the speed of EM waves?
3.0x10^8 m/s = FAST
What does it mean to be ionizing?
it can knock of electrons from atoms resulting in free radicals in the body which is not good
EM waves from short wavelength/high freq –> long wavelength/lowfreq
gamma ray, x-ray, UV, visible, infrared, microwave, radio
What are the acoustic variables?
- Pressure
- Density
- distance
Diagnostic Ultrasound Hz levels
1-15 MHz
What are the 4 sound wave parameters
- Freq and period
- Wavelength
- Amplitude (power/intensity)
- Prop. velocity
Define stress and strain
Stress - a change in pressure applied
Strain - % of change in volume from the stress
What is an acoustic variable
A mechanical change in a medium
What happens when we increase our overall gain?
we are increasing the voltage of already returned echoes that have been converted back into the electrical signal = increased amplitude = increased brightness and contrast
What is the measurement of attenuation? Why?
dB - large non linear energy changes occur
Relationship between frequency and amplitude?
No direct relationship but related through attenuation. Increase freq = increase attenuation = decrease amplitude
What has the longest pulse duration?
Continuous wave - continuous never stops
Smallest FOV
Sector
Lowest DF vs Highest DF
Lowest - B mode
Highest - Continuous wave, has a DF of 100
What is duty factors relations to bioeffects
Increased DF = increased bioeffects
Why wont we lose a lot of energy when going through skin even though it has a high attenuation?
because its so thin
What is the attenuation calculation
medium coefficient x frequency x distance (2)
What does 3dB of intensity attenuation represent?
1/2
What does 6dB of amplitude attenuation represent?
1/2
3 types of reflectors
Specular - large, flat, and smooth
Scatter - small and rough
Rayleigh scatter - very small
Specular reflections are highly dependent on what? And therefore independent from what?
highly angular dependent, and frequency independent
What could cause a loss of the bright reflection in a specular reflector?
loss of the perpindicular angle (angular dependent)
Scatter reflectors are dependent on what?
frequency dependent - increase freq = smaller wavelength = smoother reflector
degree of rayleigh scatter =
f^4
What kind of frequency does Doppler use?
Very low frequency to scan blood as an increased freq would increase friction and therefore loss of energy to heat and absorption
so wouldnt be able to see rayleigh
Impedence formula
impedence = density x prop speed
What is the Z value of Crystals
38 MRayls
Z value of tissue?
2 MRayls
In what cases will retraction occur
- an oblique angle of incidence
- when the speeds at the boundary are different on either side
What is the unit of amplitude
Pascals (Pa)
What is the unit of power
Watts (W) or mW
Relationship between power and amplitude
Power= amplitude squared
Relationship of power and intensity
equal Power = intensity therefore intensity also = amplitude squared
What is the Duty Factor
Displays the % of time the ultrasound system is producing sound
Its is the reinging period over the ringing and listening period
PD/PRP
what does dampening the sound beam do?
it reduces the length of the “ring” therefore decreases the SPL
What is the unit of density
kg/cm3
What is snells law?
describes the angle of transmission at an interface based on the angle of incidence and the propagation speeds of the two media (refraction)
What is the total attenuation equation? How do you get this to amplitude form
medium coefficient (dB/cm-Mhz) X freq (MHz) X Distance (2) (cm)
Amplitude form - dB (attenuation amount) X 20 X log (Af/Ai)
