US Flashcards
What is a sound wave?
Mechanical energy that produces vibrations when propagating through material.
Sound requires a medium to travel in.
These vibrations produce alternating areas of high pressure (compression) and low pressure (rarefaction).
What is frequency?
Rate of change between compression and rarefaction - given in Hertz.
Number of times the wave oscillates through a cycle each second.
What is wavelength?
Distance between areas of compression
Equation for speed
Speed = wavelength x frequency
Speed is thought to be constant (in a particular medium), so that an increase in frequency decreases in wavelength - and vice versa.
How does speed change in different materials?
Speed is based on compressibility of something.
Very compressible (air) will have a very low speed. Not very compressible (bone) will have a very fast speed.
US machine assumes everything travels at 1540 m/s in tissue.
What is the assumed speed of sound waves in tissue?
1540 m/s
What affect does speed have on frequency?
None. The frequency is the same, irrelevant of the sound speed in various media.
Wavelength changes in media.
How is relative intensity measured in US?
The dB.
A change of 10 in the dB scale corresponds to two orders of magnitude (100 times) and so forth. The dB is based on a log 10 scale.
Reducing the sound intensity to 10% is -10 dB
Reducing to 1% is -20 dB
Reducing to 0.1% is -30dB
Loss of 3 dB (-3 dB) represents a 50% loss of signal intensity (power)
The tissue that reduces the US intensity by 3 dB is considered “half-value” thickness.
What does a change in 10 in the dB scale correspond to?
Two orders of magnitude (100 times) and so forth. The dB is based on a log 10 scale.
Reducing the sound intensity to 10% is what in dB?
-10 dB
Reducing the sound intensity to 1% is what in dB?
-20 dB
Reducing the sound intensity to 0.1% is what in dB?
-30 dB
A loss of 3 dB represents what?
50% loss in signal intensity (power)
What is half value thickness in US?
The tissue that reduces the US intensity by 3 dB
What are the types of US interaction with matter?
Reflection
Refraction
Scattering
Absorption
What is reflection in ultrasound?
US energy gets reflected at a boundary between two tissues b/c of the differences in the acoustic impedance of the two tissues.
Large difference in “stiffness” results in a large reflection of energy.
What is impedance?
Z = density x speed of sound.
Compressibility of a spring.
What is refraction?
Bending of the sound wave caused by a change in speed.
Change in direction of transmitted US energy at a tissue boundary when the beam is not perpendicular to said boundary.
Frequency doesn’t change, but speed might.
Influenced by speed change - which is based on tissue compression and angle of incidence.
Hits straight - part will bound straight back and part goes straight through.
Strikes at an angle - part will be reflected and the other part will be refracted - with severity of this refraction depending on the speed difference of the two media.
What influences refraction?
Speed change - based on tissue compression
Angle of incidence
No refraction occurs if the speed of sound is the same in the two media or with perpendicular incidence.
Can you have total reflection?
If the speed difference and angle of incidence is great enough (exceeds the “critical angle”).
What are the two types of scattering in US?
Specular (smooth)- reflector dimensions are larger than the wavelength of the incident - strength of reflection is highly angle dependent.
Non-specular (diffuse)- scattering surfaces are about the size of a wavelength or smaller - angle has no effect on strength.
What are specular reflectors?
Smooth reflectors
Reflector dimensions are larger than the wavelength of the incident - strength of reflection is highly angle dependent.
What are non-specular reflectors?
Diffuse reflectors
Scattering surfaces are about the size of a wavelength or smaller - angle has no effect on strength.
High frequency = small wavelength = surfaces appear more rough = more scatter.
What is strength of reflection dependent on with specular reflectors?
Angle dependent
Does angle have effect on non-specular reflectors?
No. Surfaces appear more rough = more scatter
What is absorption?
Sound energy gets turned into heat.
Increases with frequency.
What is attenuation?
Loss of intensity of the US beam from both absorption and scattering in the medium.
Degree of attenuation varies widely depending on the type of tissue involved
Rule of thumb for “soft tissue” is 0.5 dB per cm per MHz or 0.5 (db/cm)/MHz
What is the rule of thumb for “soft tissue” attenuation?
0.5 (dB/cm)/MHz
A 2 MHz US beam will have twice the attenuation of a 1 MHz beam
A 10 MHz beam will have 10 times the attenuation per unit distance
It is logarithmic- the beam intensity is exponentially attenuated with distance.
What is half value thickness in US?
Thickness of tissue necessary to attenuate the incident intensity by 50% which is equal to a 3 dB reduction in intensity.
As the frequency increases, the HVT decreases.
What is the relation between frequency at HVT?
As frequency increases, the HVT decreases.
What determines the strength of the echoes?
Angle and impedance
What is impedance?
The degree of stiffness in a tissue. The differences in tissue impedance (stiffness) determines the strength of surface reflection.
What is the unit used for impedance?
The Rayl
You will get a big reflection if?
There is a large difference in impedance.
Example skin and air - thats why you gotta lube it up (gel) otherwise you can’t transmit any sound.
Is the speed of sound constant in tissue?
No - changes (via wavelength) depending on the compressibility of the tissue (slow in air, fast in bone).
How does the machine know what the speed is in the various tissues the sound traveled through?
It doesn’t.
It just assumes it’s 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 Snells Law.
More or less scatter with high frequency probes?
More - smaller wavelength makes surfaces look rougher (non-specular).
Causes scatter
More or less attenaution with high frequency probes?
More
What are Piezoelectric Materials?
A crystal (or ceramic) and is the functional component of the transducer.
Can be quartz, but is usually lead-zinc-titanate (PZT).
Has a well-deined molecular arrangement of electrical dipoles. When mechanically compressed their normally organized alignment gets disturbed from equilibrium - can be measured and recorded.
What are the transducer crystals made of?
Piezoelectric materials
Can be quartz, but is usually lead-zinc-titanate (PZT).
What are resonance transducers?
Made to operate in a “resonance” mode, where short durations of voltage (usually 150 V) are applied, causing the PZT to vibrate at a natural resonance frequency.
What determines the frequency of a probe?
Determined from the speed of sound in and the thickness of the piezoelectric material.
Only way to change frequency is to change the probe. Wavelength changes to accomodate changing velocity in different media.
What determines the thickness of the transducer?
Thickness of the transducer is 1/2 the wavelength.
Lower frequency is seen with thicker crystals and higher frequency is seen with thinner crystals.
How do you change frequency in ultrasound?
Change probe
What is a dampening block?
Sits behind the crystal and absorbs the backward directed US energy.
Also dampens the transducer vibration to create a pulse with a short spatial pulse length - needed to preserve detail along the beam axis (axial resolution).
The process of dampening introduces a broadband frequency spectrum.
What are the characteristics of a Thin Dampening Block?
Light Damping
High Q
Long spatial pulse length
Narrow Bandwidth
What are the characteristics of a thick dampening block?
Heavy Damping
Low Q
Short Spatial pulse length
Broad bandwidth
What are the “Q”s of thin and thick dampening blocks?
Thin bock = high Q
Thick block = low Q
What are the spatial pulse lengths of thin and thick dampening blocks?
Thin block = long spatial pulse length
Thick block = short spatial pulse length
What are the bandwidths of thin and thick blocks?
Thin block = narrow bandwidth
Thick block = broad bandwidth
What dampening is used for Doppler?
Low dampening (high Q) - narrow bandwidth - preserve velocity information.
What dampening gives you high spatial (axial) resolution?
Heavy dampening (low Q) - broad bandwidth - fewer interference effects and therefore more uniformity.
What is the matching layer and what is it’s purpose?
Gives the transducer an interface between the transducer element and the tissue.
Minimizes the acoustic impedance differences between the transducer and the patient.
Made of stuff that has an acoustic impedance intermediate to soft tissue and the transducer material.
What is the optimal matching layer thickness?
1/4 the the wavelength.
What are the two types of transducer arrays?
Linear (which include curved) - sequenced
Phased - “activation/reactivation” types
What are linear (sequenced) array transducers?
Each element is on it’s own. Fire and receive on their own - no use of interference patterns/steering.
What is the width of the transducer?
Sum of all the individual elements
What are linear array transducers good for?
Peds and superficial things (carotids, leg veins, testicles, thyroids).
What is a curved probe?
Still a “linear” probe - operates with individual elements firing on their own.
Face is curved - scan lines diverge deeper into the image - gives you a wider FOV for deeper structures. Used for abd.
What are phased array transducers?
Groups of elements fire in multiples using interference patterns to steer the beam - operating like a search light scanning a dark room.
B/c of this steering, the probes can be made smaller.
What are the advantages of phased array transducers?
Probes can be made smaller
Good for limited acoustic windows (in between ribs, transvaginal etc…)
Characteristics of Linear Transducer Arrays?
256-512 elements
Large
Sequenced firing of small group of adjacent elements (20ish)
A rectangular FOV is produced (trapezoidal with durved)
Characteristics of Phased Transducer Arrays?
64-128 Elements
Small
Elements are activated and reactivated - in a phased pattern
Time delays in electrical activation can make it possible to steer and focus, without moving the probe.
What are the two beam components?
Converging beam - near field (Fresnel Zone)
Diverging beam - far field (Fraunhofer zone)
What is the near field?
Fresnel Zone
Convergence of the near field occurs b/c of the multiple constructive and destructive interference patterns
Length is dependent on transducer frequency and transducer diameter
What is near field length dependent on?
Transducer frequency and transducer diameter
In the near field, higher transducer frequency =
Longer near field
In the near field, large diameter element =
Longer near field
What does a longer near field result in?
Less divergence
What is the far field?
Fraunhofer Zone
Where the beam diverges - as beam diverges the ability do distinguishes two objects close to one another is reduced.
Divergence is less with a high frequency big probe and more with a low frequency small probe.
US intensity in the far field gradually decreases with distance.
What is the Intensity of the beam?
Power (measured in watts) flowing through a unit area.
The power of a beam is not uniform along its length or width. Caused by a variety of factors:
Beam lacks clearly defined edges and the intensity will decrease from the center outward - “beam spread”
Max sound pressure is always found along the acoustic axis (centerline)
Divergence of the beam in the far field causes the power to be spread over a larger area.
Interference occurs from numerous point sources interacting in the near field - less with a broadband transducer.
Reasons why power of an US beam is not uniform along its length
Beam lacks clearly defined edges and the intensity will decrease from the center outward - “beam spread”
Max sound pressure is always found along the acoustic axis (centerline)
Divergence of the beam in the far field causes the power to be spread over a larger area.
Interference occurs from numerous point sources interacting in the near field - less with a broadband transducer.
What is the Focal Depth/Zone?
Point at which the beam is at its narrowest and the area of maximum intensity.
Spot between the converging and diverging beams.
Get best echoes and best lateral resolution.
What are the two main strategies of “focusing”?
Mechanical - using a concave face - get a fixed focal depth/zone
Electronic - narrow focus can be achieved by firing the inner transducers in a symmetrical patterns. A longer focus can be achieved by reducing the delay time differences among the transducer elements (resulting in more distal beam convergence).
What is mechanical focusing?
Using a concave face - get a fixed focal depth/zone
What is electronic focusing?
Narrow focus can be achieved by firing the inner transducers in a symmetrical patterns. A longer focus can be achieved by reducing the delay time differences among the transducer elements (resulting in more distal beam convergence).
What is a stand off pad?
Block of US gel - or something with low impedance that you can scan through.
Moves things in the superficial region (near zone) into the focal zone.
What are the three dimensions in US?
Axial
Lateral
Elevation (slice thickness)
What is axial resolution?
Ability to tell two closely spaced objects apart in the direction of the beam.
For optimal axial resolution, need the returning echoes to be separate and not overlap.
Minimum required separation between two reflectors is 1/2 the spatial pulse length (SPL), otherwise returning echoes will overlap.
What is spatial pulse length?
Number of cycles emitted per pulse by the transducer multiplied by the wavelength.
What is the minimum required separation between two reflectors for axial resolution?
1/2 the spatial pulse length.
What is lateral resolution?
Ability of the system to resolve objects in a direction perpendicular to the beam direction.
Separate things must fit into separate beams.
Thinner beam = more likely each will fall into a separate beam.
Beam is thinnest at the end of the near field (at the focal zone). Worst close to and far from the transducer surface.
Is lateral resolution constant at different depths?
No. Beam diameter varies with the distance from the transducer in the near and far field, the lateral resolution is depth dependent.
Best lateral resolution occurs at the focal zone.
Lateral resolution worsens in the deeper field.
Is axial resolution constant at different depths?
Yes. Has nothing to do with depth or zones. Only dependent on the spatial pulse length.
Does increasing the gain improved lateral resolution?
No. Gain widens the beam.
What does elevation resolution depend on?
AKA - slice thickness.
Same as lateral resolution but measuring in plane orthogonal to the image plane. Usually the worst measure of resolution.
Depends on the transducer element height.
This type of resolution is why you can get volume averaging of acoustic details in regions that are close to the transducer and in the far field.
What things will improve axial resolution?
Shorter pulses - smaller spatial pulse length
Greater damping - “low Q” - shorter pulses
Higher frequency - shorter wavelength
What things will improve lateral resolution?
Narrowing the beam in the proximal field (adding an acoustic lens). Minimal necessary gain (gain widens the beam).
Put the thing you want to look at in the focal zone.
Phased array with multiple focal zones
Increasing the “line density” or lines per cm.
What things improve elevation resolution?
Use a fixed focal length across the entire surface of the array (downside is partial volume effects).
Minimize slice thickness - done by phase excitation of the outer to inner arrays.
Axial resolution is dependent on?
Spatial pulse length
Lateral resolution is dependent on?
Transducer element width
Elevation resolution is dependent on?
Transducer element height
What are the assumptions in US?
All echoes originate from w/in the main beam
All echoes return to the transducer after a single reflection
The amount of time an echo takes to return to the probe directly reflects on the object depth.
The speed of sound in human tissue is constant - 1540 m/s
The sound beam and its echo travel in a straight path
Acoustic energy is uniformly attenuated.
What are the beam related artifacts?
Side lobe
Beam width
What are the artifacts associated with multiple echoes?
Reverberation
Comet tail
Ring down
Mirror image
What are the artifacts associated with velocity errors?
Speed displacement artifact
Refraction artifact
What are the artifacts associated with attenuation errors?
Shadowing
Increased through transmission
What is side lobe artifact?
Normal US beam is a bow tie - central main beam and off axis low energy beams on the side - “side lobes” - caused by radial expansion of piezoelectric crystals - happens more with linear arrays.
Have strong enough reflector, bounce back this low energy side lobe which will be received by the transducer- violates the assumption that echoes originate from the main beam and incorrectly placed
Typically seen when the incorrectly placed echoes overlap an anechoic structure (bladder or GB) - “pseudosludge”
Side lobe energy is seen with what types of transducers?
Linear array.
What is beam width artifact?
US beam exits with same width as the transducer then narrows to the focal zone, then diverges in the far field past the original margins of the transducer.
If the diverged beam encounters a strong reflector, it could send back a signal - assumed to be from the main beam - misplaced.
Bladder is the classic example - adjust focal zone to level of interest and place transducer at the center of image.
What is the basis of artifacts associated with multiple echoes?
Violate the assumption that an echo returns to the transducer after a single reflection and that the depth of the object is related to the time for the round trip.
What is reverberation artifact?
Sound wave encounters two parallel highly reflective surfaces - echoes generated from a primary sound beam may be repeatedly reflected back and forth before returning to the transducer for detection - recorded as multiple echoes with increasing distance.
What is Comet Tail artifact?
Form of reverberation- the two parallel highly reflective surfaces are closer together which means the sequential echoes are closely spaced - less than 1/2 the spatial pulse length (SPL) - which is the minimal distance needed for axial resolution.
Displayed as a triangle, not a square - later echoes get attenuated and have decreased amplitude. Decreased amplitude is manifested on the display as decreased width - tapering.
Why is comet tail artifact a triangle?
Displayed as a triangle, not a square - later echoes get attenuated and have decreased amplitude. Decreased amplitude is manifested on the display as decreased width - tapering.
What is Ring Down Artifact?
Instead of two parallel highly reflective surfaces - the sound wave encounters fluid trapped between a tetrahedron of air bubbles. The vibrations create a nearly continuous sound wave transmitted back to the probe.
See as a line or series of parallel bands extending posterior to a collection of gas.
What is mirror image artifact?
Created by false assumption that an echo returns to the transducer after a single reflection.
US beam passes through a highly reflective surface then gets repeatedly reflected between the back side of the reflector and the adjacent structure.
Displayed as a duplication equidistant from but deep to the strongly reflective interface.
Liver/lung interface.
How does change in speed affect the depth of an object on US?
Travels slower - takes longer to return = depth is more
Travels faster - comes back faster = more shallow.
What is Speed Displacement Artifact?
Speed of sound slows down in fat relative to liver. Beam takes longer to return and is perceived by the machine as being further away.
Creates the appearance of a discontinuous and focally displaced liver border.
What is Refraction Artifact?
Speed difference in tissues causes refraction.
Change in direction violates the idea that the beam is going straight and can cause:
- object can appear wider than it actually is.
- object can be misplaced to the side of the returning echo
- object can appear duplicated
Classic location- deep to rectus muscles and midline fat - “duplicated SMA”
What is compensation amplification?
Type of processing US machines use on echoes that take longer to return to the transducer - echoes that return later are amplified more than echoes that return earlier.
Image appears more uniform in the deep field.
What is shadowing?
US beam runs into a material that attenuates sound to a larger degree than the surrounding tissue - strength of beam distal to this structure appears weaker (darker) than in surrounding field.
What is increased through transmission?
Opposite of shadowing - beam runs into material that attenuates sound less than the surrounding tissue - strength of the beam distal to the structure appears stronger (brighter) than the surrounding field.
What is A-mode?
“A” stands for amplitude
Gives processed information from the receiver vs time. An echo’s return from tissue boundaries and reflectors creates a digital signal proportional to echo.
“A”mplitude is produced as a function of time.
What is B-mode?
“B” stands for brightness.
This is the conversion of A line information to brightness-modulated dots on a display.
Proportional relationship of brightness to the echo signal amplitude.
What is M-mode?
“M” stands for motion
B-mode information is used to display the echoes from a moving organ (like heart valves) from a fixed transducer and beam position on the patient.
M-mode is how many times greater than B mode?
4x
Pulsed Doppler is how many times greater than B Mode?
20x
What is Doppler?
The frequency of sound changes with moving objects.
The sound wave gets crowded at the front and will sound different depending on where you are standing in relationship to the moving object (origin of the sound wave).
You can measure these changes (or shifts) in frequency to tell what direction things are moving (and how fast they are getting there).
What should the Doppler angle be? Why?
Between 30-60 degrees
Based on Cosign of the angle. If 0 (at 90 degrees), then won’t get flow. Also creates mirror image at 90.
What angles produces greatest Doppler flow?
0
Use 30, b/c angles less than 20 cause refraction and loss of signal. Aliasing also becomes an issue.
Why not use angle less than 30?
0 is best angle.
Use 30, b/c angles less than 20 cause refraction and loss of signal. Aliasing also becomes an issue.
What is pulsed wave (spectral) Doppler?
Utilizes a single transducer for both reception and transmission.
Blood flow velocity varies yielding a spectrum of doppler shifts instead of a single frequency.
Can obtain the direction of blood flow (plotted above and below the baseline) and velocity.
What is color Doppler?
Uses the gray scale image with a superimposed color blood flow image.
Gives direction of flow - intensity varies depending on flow intensity.
Obtains samples of each pixel multiple times then displays the average shift.
How is pulsed wave (spectral) Doppler performed?
Utilizes a single transducer for both reception and transmission.
Blood flow velocity varies yielding a spectrum of doppler shifts instead of a single frequency.
Can obtain the direction of blood flow (plotted above and below the baseline) and velocity.
How is Color Doppler obtained?
Obtains samples of each pixel multiple times then displays the average shift.
How is the spatial resolution different in gray scale vs Doppler?
Spatial resolution in gray scale is better (although smaller vessels are better seen with Doppler)
In which Doppler technique is angle more important?
More important in pulsed wave (spectral) Doppler.
Imformation is semi-quantitative with Color Doppler.
What is Power Doppler?
More sensitive for presence of flow without information on direction.
Still get color but instead each pixel registers the total number of “frequency shifts”
Is Power Doppler susceptible to aliasing?
No.
Color and spectral are.
Is Power Doppler dependent on Doppler Angle?
No - can measure at any angle.
What are the Doppler Artifacts?
Aliasing Tissue Vibration Mirror Image Twinkle Artifact Pseudoflow (pseudoblood) artifact Flash Artifact Color Bleed
What is Aliasing?
Super high velocities are displayed as low (negative) - wrapped around the baseline
Occurs when the doppler shift is greater than a threshold called the “Nyquist Frequency”
Nyquist Limit (kHz) = 1/2 x pulse repetition frequency (PRF)
When is aliasing produced?
Occurs when the doppler shift is greater than a threshold called the “Nyquist Frequency”
Nyquist Limit (kHz) = 1/2 x pulse repetition frequency (PRF)
How do you reduce or eliminate aliasing?
Decrease Doppler shift - either by using a lower frequency transducer or using a Doppler angle closer to 90 (increasing the angle).
Increase the pulse repetition frequency (which will increase your Nyquist) or selecting a sample volume at a lesser depth or increasing the scale.
What is Tissue Vibration?
Occurs secondary to turbulent blood flow.
Doppler shows a mixture of red and blue colors.
A-V fistula in a kidney post biopsy.
What is Mirror Image in Doppler?
Occurs secondary to a vessel adjacent to a highly reflective surface, such as the lung- just like gray scale.
Results in duplication of the structure being evaluated.
What is Twinkle Artifact?
Occurs behind strongly reflecting surfaces such as calcifications- Manifests as a noisy spectrum with rapid fluctuation of red and blue colors.
Has a greater sensitivity for detection of small stones than acoustic shadowing.
Highly dependent on machine settings and how round the reflecting surface is (more rough = more twinkle).
What artifact has increased sensitivity for detection of small stones?
Twinkle artifact - greater than shadowing.
What is Twinkle Artifact dependent on?
Highly dependent on machine settings and how round the reflecting surface is (more rough = more twinkle).
What is Pseudoflow (pseudoblood) artifact?
Things that move like blood but aren’t.
Ureteral jets.
What is flash artifact?
Burst of color filling the screen- secondary to transducer or patient motion
What is Color Bleed?
Looks like color extending beyond the vessel wall. Can decrease sensitivity to thrombus or stenosis.
Decreasing color gain makes it better.
What does increasing output power do?
Transmit gain - increases the brightness by adjusting the strength of the sound pulse SENT TO THE BODY by the transducer.
When too bright from high output power, this will degrade lateral resolution - widening the beam.
Limitation of increasing the output power?
Degrades lateral resolution - makes beam wider.
What does increasing receiver gain do?
Increases brightness by adjusting the strength of the sound pulse AFTER IT RETURNS to the transducer - postprocessing.
What is Time Gain Compensation?
Progressively increases the amount of amplification applied with depth.
Make the top and bottom uniform - compensates for loss of echo strength caused by depth of the reflector.
Treats echoes differently depending on which depth they are returning from.
What are Harmonics?
US beam travels into the patient’s tissues, transmitted pulse is progressively distorted - occurs primarily in the central zone of the main beam where intensity is high.
Distorted pulse will give rise to distorted echoes and these have significant energy at harmonic frequencies.
Tissue harmonics are make using the second harmonic component of the echo signal with the fundamental frequency excluded. Undistorted echoes coming from lower intensity areas (fringes of the beam, side lobes, superficial tissues) are not seen.
Transmit at one frequency and receive at another - “second harmonic”
Possible b/c body tissues cause some distortion of the wave.
Wave has to travel some distance to become distorted enough to generate harmonics
Harmonics are NOT produced in the near field (haven’t traveled far enough)
Improves lateral resolution
Reduction in artifacts - specifically reverberation
Dealing with higher frequency - low some depth penetration.
Where are harmonics produced?
Farther down - not produced in the near field
What does harmonics improve?
Lateral resolution
What does harmonics reduce?
Reduced artifacts - specifically reverberation.
What is Compound Imaging?
Electronic steering of the US beams from the transducer image an object in multiple different directions - will sharpen the edges and cause loss of posterior shadowing (can make a cyst look solid).
Difference in a breast lesion in Normal US vs Harmonics vs Compound
Normal - Hypoechoic but not totally anechoic - Some shadowing, but not a ton - Blurry margins - Reverberation artifact
Harmonics - Looks more anechoic - Shadowing tends to be worse - Blurry margins stay - Reverberation artifact IS GONE
Compound - Hypoechoic but not totally anechoic - Shadowing is GONE - Margins are SHARP - Reverberation artifact is BETTER, but not gone.
Harmonics can make a solid thing look cystic (by turning it anechoic) and compound imaging can make a cystic thing look solid (by removing the posterior features).
What is Thermal Index?
Maximum temperature rise in tissue secondary to energy absorption.
Based on a homogeneous tissue model with certain instrument parameters.
What is Mechanical Index?
How likely it is that cavitation will occur considering peak rarefaction pressure and frequency.
This index is the indicator of mechanical bioeffects (streaming and cavitation).
Matters most with contrast enhanced US.
What safety index matters most in contrast enhanced US?
Mechanical Index
How likely it is that cavitation will occur considering peak rarefaction pressure and frequency.
This index is the indicator of mechanical bioeffects (streaming and cavitation).
What is cavitation?
Sonically generated activity in compressible bodies of gas and/or vapor.
Stable and Transient.
What is Stable Cavitation?
Micro bubbles are already present in the media. Expand and contract as the waves cycle (responding to pressure).
This occurs at low and intermediate US intensities (as used clinically)
The mechanical index is an estimate for producing cavitation.
What is Transient Cavitation?
“The violent one” - Bubble oscillations become so large that the bubbles collapse - results in shock waves rippling through tissue planes causing tissue damage.
Thermal induced damage is a _____ phenomenon?
Threshold phenomenon - get no tissue damage until a certain temperature is reached.
At what parameters is cavitation most likely to occur?
Low frequency and high pressure
Which deposits more heat, gray-scale or spectral Doppler?
Spectral Doppler
Per the NCRP, a risk-benefit decision is needed when the TI and MI exceed what?
TI exceeds 1
MI exceeds 0.5
When in pregnancy is heating an issue?
2nd and 3rd trimesters.
Temperature rises are most likely at bone surfaces and adjacent soft tissues - get increasing mineralization (2nd and 3rd trimester).
Different “thermal indexes” for soft tissue, bone, and cranium. Use “soft tissue index” in early gestation. “Bone index” after 10 weeks (2nd and 3rd trimester) when bone ossification is evident.
What are general recommendations for 1st trimester US?
Pulsed doppler (spectral, power, and doppler) should NOT be routinely used
M-mode US is recommended instead of spectral Doppler to document HR.
Keep the TI under 1.0 (some say 0.7)
Scanning maternal uterine arteries with doppler is probably ok (as long as the fetus is outside the beam).
What are the Thermal Index limits?
Below 0.7 - for OB imaging
Between 1.0-1.5 - US should not exceed 30 min
Between 2.5-3.0 - US should not exceed 1 min
Greater than 3.0 - US should not be used.
