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
frequency ranges of medical US
2-15 MHz
range equation
c/2
(speed/2)
Factor of “2” results from the fact that the total round-trip path length of an ultrasound beam includes the path from the transducer to the reflector and then back to the transducer.
what happens to the ultrasound pulse if the reflector is smaller than the US wavelength
(d < wavelength)
scattering
echoes are reflected through a very wide range of angle
most of the signal visible in US images results from scatter interactions
what minimizes the acoustic impedance difference between the transducer and the patient?
matching layer
what is a main difference between element excition in linear vs phased arrays?
phased arrays are shorter and have fewer elements but excite a larger fracture of the elements for each beam
Slight offsets in excitation timing are used to electronically steer the beam off at nonperpendicular angles
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
is interference less or more with a broadband transducer?
interference is less with a broadband transducer
how do you affect the mechanical focus of an US beam?
with a concave face you can set a fixed focal depth/zone. or could use a concave acoustic lens
the three dimensions in US
axial
lateral
elevation (slice thickness)
what s the minimum required separation between two reflectors before they overlap
1/2 the spatial pulse length (# of cycles emitted per pulse by the transducer)
if 1/2 the length of the SPL is larger than the distance between two nodules:
will look like one big nodule
how does scan density affect lateral resolution
increased scan density (more individual scan lines) = better lateral resolution
dependent on:
axial resolution –>
lateral resolution –>
elevation resolution –>
dependent on:
axial resolution –> Spatial pulse length
lateral resolution –> Transducer element width
elevation resolution –> transducer element height
factors that improve axial resolution
- shorter pulses
- greater dampening (“low Q”)
- Higher frequency probe (Shorter wavelength)
factors that improve lateral resolution
- put the thing you want to look at in the focal zone
- phased array with multiple focal zones
- increasing the “line density”
- higher frequency probe (less beam spreading)
- narrow pulse WIDTH (narrow beam)
- larger diamter transducers
factors that improve elevation resolution
- use a fixed focal length across the entire surface of the array (downside is partial volume effects)
- use a thinnner crystal
- minimize slice thickness - done by phase excitation of outer to inner array
what is time gain compensation?
time gain compensation:A way to overcome ultrasound attenuation is time gain compensation (TGC), in which signal gain is increased as time passes from the emitted wave pulse. This correction makes equally echogenic tissues look the same even if they are located in different depths.
ideal angle of isonation
between 30-60
is spatial resolution better or worse in doppler US compared to gray scale imaging?
spatial res in doppler is WORSE compared to gray scale imaging
what does power doppler register?
number of frequency shifts. Will NOT give info about flow direction
does power doppler exhibit aliasing?
NOPE! only regular color doppler will demonstrate aliasing
what is the Nyquist limit?
the Nyquist limit represents the maximum Doppler shift frequency that can be correctly measured without resulting in aliasing in color or pulsed wave ultrasound.
Nyquist limit always equals Pulse Repetition Frequency (PRF)/2.
how does increasing US output power (transmit gain) affect lateral resolution?
lateral resolution decreases (think about it widening the beam
what is harmonic imaging
transmitting at one frequency and receiving it at another - “the second harmonic”
Advantages of harmonic imaging over conventional ultrasound:
decreased reverberation and side lobe artifacts
increased axial and lateral resolution
cyst clearing
increased signal to noise ratio
improved resolution in patients with large body habitus
Advantages of harmonic imaging over conventional ultrasound:
Advantages of harmonic imaging over conventional ultrasound:
decreased reverberation and side lobe artifacts
increased axial and lateral resolution
cyst clearing
increased signal to noise ratio
improved resolution in patients with large body habitus
two main effects of compounding imaging in US
- will sharpen the edges
- will cause loss of posterior shadowing
Does harmonic imaging improve lateral or axial resolution?
increased lateral resolution and reduction of side lobe artifacts and reverberation artifacts caused by tissues close to the transducer. Harmonic imaging utilizes harmonic frequencies generated by the interaction of the incident ultrasound beam with the patient’s tissues. The wave distortions generating harmonic frequencies increase with increasing depth and are localized within the central part of the beam. The transducer sends a lower frequency pulse, but only receives the 1st order harmonic (double the incident frequency) echoes. This leads to increased lateral resolution and reduction of side lobe artifacts and reverberation artifacts caused by tissues close to the transducer.
How is acoustic impedance defined?
Acoustic impedance is defined as the product of the density (in kg/m^3) and speed of sound (in m/s) in the tissue.
The ACR recommends quality control on US transducers be performed how frequently:
quarterly
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
How can beam width artifact be reduced?
- Dynamic receive focusing
- multiple transmit focal zones (FZs), FZ placement
- Increasing frequency
- local speed of sound selection
Which two tissues have the greatest impedance difference?
muscle and bone
what parameters produce longer near fields
larger transducer crystals and higher center frequencies produce longer near fields
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
the main drawback for using broadband transducers
spectral broadening
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)
