US Flashcards
velocity of sound wave =
speed = wavelength x frequency
speed an US machine assumes for a soundwave (regardless of medium)
1540 m/s in tissue
does frequency or wavelength change in different media?
wavelength changes
frequency stays the same
a loss of 3 dB represents
a 50% loss of signal intensity (power)
“half-value” thickness for US
tissue thickness that reduces the US intensity by 3 dB (50% loss of power) is considered the “half value” thickness
difference in what results in reflection of US waves?
impedance. US E gets reflected at a boundary between two tissues because of the difference in acoustic impedances
what influences refraction?
- speed change (difference in densities of material) and 2. angle of incidence
is the speed of the US wave greater in media 1 or media 2

media 2
When the velocity of an ultrasound wave in media 1 is greater than that in media 2 (C1>C2), the angle of reflection is decreased, and when velocity of an ultrasound wave in the media 1 is less than in media 2 (C1 Use Snell’s law

Snell’s law is as follows:
sin θ1/c1 = sinθ2/c2
or
sin θ1/sinθ2 = c1/c2
Where:
θ1 is the incident angle and θ2 is the refracted angle
c1 is the speed of propagation in the first material and c2 is the speed of propagation in the second material
is the speed of the US wave greater in media 1 or media 2

media 1

Snell’s law
Snell’s law is as follows:
sin θ1/c1 = sinθ2/c2
Where:
θ1 is the incident angle and θ2 is the refracted angle
c1 is the speed of propagation in the first material and c2 is the speed of propagation in the second material

Do high frequency US beams cause more or less scatter?
more scatter
high frequency = small wavelength = surfaces appear more rough = more scatter
general rul of thumb for attenuation of US beam in soft tissue
0.5 (dB/cm)/MHz
how does frequency affect HVT in US?
as frequency increases, HVT decreases
what determines the strength of the echoes in US?
angle and impedence
what is impedance?
degree of stiffness in a tissue. the differences in tissue impedence determines the strength of reflection
what unit is used for impedance
Rayl
you will get a big reflection if?
there is a large difference in impedance. example - skin and air. that’s why yuou have to lube it up - otherwise you can’t transmit any sound
how does the US machine know what the speed is in various tissues the sound has traveled through?
trick questions. it doesn’t. it just assumes its always 1540 m/s - which can lead to artifacts
what makes the sound have “bend”?
changes in the speed of sound - which occur as it travels through different media - creating a “bending” or “refraction”, as described by Snell’s law
more or less scatter with high frequency probes?
more - smaller wavelength makes surfaces look rougher (non-specular). Causes scatter
more or less attenuation with high frequency probes?
more
components of US probe
- Matching layer
- piezoelectric material (crystal or lead-zinc-titanate)
- damping layer

Thickness of the PZT is generally ______ the wavelength of the ultrasound produced.
Thickness of the PZT is generally 1/2 the wavelength of the ultrasound produced.
matching layer is generally _______the wavelength of the ultrasound produced
matching layer is generally 1/4 the wavelength of the ultrasound produced































