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
what is the purpose of the “matching layer”
The purpose of the matching layer is to try to maximize transmission of ultrasound from the PZT to the patient.
Recall that when there is a large difference in impedance between two media, most sound wil be reflected and little will be transmitted at the interface. There is a large difference in impedance between the PZT and skin. The matching layer and ultrasound gel have an impedance inbetween that of the PZT and the skin to reduce the amount of reflection at this interface and improve transmission into the patient.
what is rarefaction
area of low pressure in the sound wave
which image (A or B) was obtained with a tranducer that has a shorter SPL?
B.
B demonstrates better axial resolution which is dependent on sptaial pulse length (SPL). The shorter the spatial pulse length the higher the axial resolution.
SPL is calulated as the number of cycles emitted per pulse x wavelength.
Factors that improve axial resolution:
- higher frequency tranducer
- greater dampening of the transducer element (Reduces pulse duration)
- broader bandwidth pulse
solve for spatial pulse length if n =3
In US, what does bandwith refer to?
the range or difference between the highest and lowest frequencies in the pulse
In US, what is quality factor and how is it mathematically defined?
Quality factor is a unitless number than is related to bandwith
mathematically defined as:
Q = main frequency/bandwith
Q factor is DIRECTLY RELATED to the pulse length.
thin dampening block (long pulse length) has a “High Q” while a thick dampening block (short pulse length) has a “Low Q”
for Doppler do we use low dampening or high dampening
low dampening (high Q). This preserves velocity information
describe the components of the US sinusoidal wave
peaks = compression
troughs = rarefaction
describe the shape of a normal US beam
narrows at the focal zone.
Beam converges at Near field = Fresnel zone
Beam diverges in far field = Fraunhofer zone
what is the Fresnel zone?
near field of the beam. occurs due to multiple constructive and destructive interference patterns
Near field length = D2/4wavelength.
D= diameter of the transducer
Near field length =
Near field length = D2/4wavelength.
higher frequency = longer near field
shorter wavelength = longer near field
bigger diameter of the transducer = longer near field
what parameters can reduce beam spread?
high frequency, big diameter probe
best lateral resolution is found in what part of the US beam?
focal zone
how do you electronically focus an US beam?
shallow focus achieved by firing the inner transducers in a symmetrical pattern
longer focus can be achieved by reducing the delay time differences among transducer elements
if 1/2 the length of the SPL is larger than the distance between two nodules:
will look like one big nodule
ideal angle of isonation
between 30-60
When does reverberation artifact occur?
When the US signal reflects repeatedly between highly reflective interfaces. It has horizontally positioned linear echoes that are spaced evenly.
What does the focal point on the US machine represent?
Depth of highest lateral resolution
Lateral resolution (resolution perpendicular to the ultrasound beam) is determined by the beam diameter. For unfocused or single element transducers, beam diameter decreases with increasing depth in the near field, being the smallest at the junction between near and far field (about 1.2 the transducer diameter or the effective transducer diameter) and increases with increasing depth in the far field. Lateral resolution can be changed by not activating all transducer elements at one time, decreasing the effective transducer diameter. In addition, with phased array transducers, transmit and receive focusing allows to maximize lateral resolution. The focal zone is indicated by a little triangle at the side of the image and can be changed by the user. Elevational resolution is the slice thickness and depends on the transducer element heights.
What does the damping block on the nonpatient side of the piezo crystal do?
The damping block on the non-patient side of the piezo crystal absorbs backwards directed ultrasound waves. More dampening results in a larger bandwidth of the ultrasound pulse but decreased pulse duration.
What is beam width artifact?
Beam-width artifact refers to the lateral blurring of a point target that occurs as echoes from the same target are insonated at adjacent beam positions
Beam width varies with depth and is narrowest at the focal zone. Beam-width artifact is a manifestation of lateral resolution, which refers to the ability to discriminate two closely spaced points at the same depth within the imaging plane as distinct.
US artifact Radiographics article
reverberation artifact
Which two tissues have the greatest impedance difference?
muscle and bone
most likely focus of the transducer was set at:
B
beam intensity is strongest in the focal region
For this transducer, one would expect that the narrowest part of the beam would be in the
near field
What is the difference between these two images?
A 5-12MHz tranducer was used for both images
left = Penetration
(lower frequencies in the range used)
right = Resolution
(higher frequences used)
