Ch 5b Elastography Flashcards
What is elastography?
A technique that gathers qualitative or quantitative information regarding the stiffness, hardness or softness of tissue
Do all u/s machines have the elastography setting?
No, is an advanced imaging feature on SOME machines
What is Young’s Modulus?
A measure of the stiffness (harness/softness) of a material
Unit for young’s modulus?
kPa (kilopascal)
Do soft materials result in larger or smaller strain?
Larger strain
(inversely proportionate b/c the more the material moves, the less stiff it must be)
Do hard/stiff materials result in larger or smaller strain?
Little strain
(inversely proportionate b/c the less the material moves, the stiffer + harder it must be)
Are soft materials known as small or large young’s modulus?
Small (with large strain)
Are hard/stiff materials known as small or large young’s modulus?
Large (with little strain)
Young’s modulus values in kPa for soft tissue range from what?
Approx 1 for fat to 300 for fibrotic liver
Formula for young’s modulus?
Force/strain
-When a material (such as tissue) is subjected to a stress that compresses or stretches the material, the ratio of applied stress to the resulting strain is called young’s modulus
Stress = force (per unit area) applied to material
Strain = the increase/decrease of the length of a segment of the material divided by its original length
Elastography is m/c used on what 2 structures?
-Liver
-Breast lesions
Give an example of a soft + hard/stiff material that represents young’s modulus?
Soft: think stress ball (is soft, small + high strain)
Hard/stiff: think bowling ball (is hard/stiff, large + little strain)
Hardness of tissue is a useful indicator of what?
Determines the normality or abnormality
Elastography provides an imaging means of what?
“palpation”
Can elastography be quantitative in its evaluation of tissue hardness?
Yes - qualitative or quantitative
List the 2 elastography methods?
-Static
-Dynamic
What do static methods involve?
Subjecting tissues to a manual force initiated by the sonographer physically pushing on the probe, with the instrument tracking the movement of tissues by spatial correlation
(image shows probe pushing down into the material compressing it)
Will softer tissues compress more or less than hard ones?
More
Static methods are used clinically for what 4 things?
-Cancer detection
-Characterization of small parts (breast, thyroid, prostate)
-Assess viability of myocardium
-Monitor therapies that alter tissue composition (such as ablation procedures)
Are static methods simple or hard to implement?
Simple
Are static methods qualitative or quantitative assessments?
Only qualitative!
Are static methods operator dependent?
Yes
Can young’s modulus be calculated using static methods?
No! B/c the stress/force is unknown as it is the sonographer’s arm pressure which we can not calculate
Define force + strain in simple terms?
Force (stress) = think pressure applied to the material
Strain = think the amount that the material gets compressed, the increase/decrease of the length of the material
What do dynamic methods use?
A short transient force OR a time varying repetitive force that is applied to tissue
The force with dynamic methods initially produces what kind of strain?
A compressional (transient or oscillatory) strain that travels with a high propagation speed, through a phenomenon called radiation force which can convert into shear waves (TRV waves)
What is radiation force?
Change in wave momentum due to scattering at an obstacle
(rate of momentum changes + averages over the wave period which = radiation force)
Where is radiation force most intense at?
At focal zone
(displacement occurs at the focal spot)
What type of force can convert into shear waves?
Radiation force
Another name for shear waves?
Transverse waves
Do shear waves travel at higher or slower speeds than compressional waves? What is the speed?
Much slower speeds (0.5-10m/s)
What is the speed of compressional waves?
1440-1640 m/s
What direction do shear waves travel in?
In directions perpendicular to the travel of compressional waves
(think shear + compression waves are opposites)
How can shear wave displacements be tracked?
By ultrafast means by using high FRs to determine their propagation speed
(imaging stiffness this way is called ARFI)
Imaging the stiffness in the way that we use high FRs to determine shear wave propagation speed is called what?
Acoustic radiation force impulse imaging (ARFI)
The shear wave propagation speed is quantitatively or qualitatively related to other material characteristics, such as young’s modulus?
Quantitatively
Do dynamic methods determine tissue hardness qualitatively or quantitatively?
Both!
Explain the variables in the following dynamic methods formula: Cs = [E/3Q]^1/2
Cs = propagation speed
E = young’s modulus
Q = density of propagating medium
(formula shows how young’s modulus relates to the shear wave propagation speed)
What units is shear wave in?
m/s
List the shear wave procedure/protocol?
-Fast for 4-6 hours
-Done in supine (or slight LLD as needed)
-Right arm above head
-Scan intercostally with enough pressure to maintain stable contact
-Take measurements in seg 7 or 8 of liver
-Place top of shear wave box 1-2cm below liver capsule with the middle of the shear wave box b/w 3-6cm for best results
-Avoid rib shadows
-Avoid vessels in shear wave box
-Obtain measurement on suspended breath hold, not inspiration
-Acquire 10 measurements
What is a good example of a shear wave image?
-Good contact at top of screen
-ROI placed in middle of image + away from shadowing
-Gains at/near default (do NOT adjust them)
-Uniform color fill in (in shear box)
-Homogeneous color pattern (in shear box)
What is the default GAIN set at on the machines?
LE9: 55
LS8: 85
If our shear wave box is pixelated + not homogeneous (no longer solid blue color), what is this due to?
Inadequate contact - measurements will now be inaccurate
Can our shear wave box be placed over vessels?
NO! Must avoid them
What is the distance our shear wave box must be from the capsule?
At least 1-2cm below capsule
(the capsule is a stiff region)
Can we increase our color gain to fill in our shear wave box if it is lacking?
NO! Must keep color gain at factory default, never increase!
(keep all gains at default - 55)
How does a higher BMI affect our shear wave signal?
-Difficult to image pt’s due to high BMI + a stiff liver will create a weaker/poorer shear wave signal, causing a lack of color fill in (in our shear wave box)
-NEVER increase color gains to fix this issue as it leads to inaccurate measurements
List 6 DOs for shear wave imaging?
-Ensure good probe contact with pt + optimize window to get best B-Mode quality before starting shear wave
-Place ROI in shadow free region
-Place ROI near center of image (laterally) if possible
-Place ROI in region free of vessels + away from liver capsule
-Take measurement when >50% of ROI has color fill in, with default gain (55 for LE9 + 70 for LS8)
-Take measurement on region with uniform color fill in
List 6 DON’Ts for shear wave imaging?
-Place ROI near B-Mode shadow artifacts
-Place ROI near extreme left/right edge of image
-Change ANY image parameters during shear wave acquisition
-Take measurement when <50% of ROI has color fill in with default gain
-Increase shear wave gain to force color fill in if there is inadequate fill in
-Take measurement on region with speckled non-uniform color
Give an example of a good shear wave measurement?
-Up to 12 measurements can be taken
-ROI at least 1cm in diameter
-Over 50% of ROI has color fill in
-Narrow range of measurements with IQR/median ratio less than 30%
Is it acceptable for the measurement ROI to extend outside of the color box?
Yes, this does not affect measurement
(do not overlap measurements tho, take from different spots)
List 4 calculation options for shear wave?
Mean: average value of all measurements
Standard Deviation: amount of variation in measurements
Median: measurements in order from smallest-largest, than middle # is taken
Interquartile Range (IQR): discards lowest + highest 25% of measurements, variability is given based on remaining values (similar to standard deviation). Must be <30%
List the 4 liver fibrosis staging with their metavir score + m/s?
1: Normal - Mild = F1= 1.35-1.66 m/s
2: Mild - Moderate = F2 = 1.66-1.77 m/s
3: Moderate - Severe = F3 = 1.77-1.99 m/s
4: Cirrhosis = F4 = >1.99 m/s
Is cardiac strain imaging a type of elastography?
Yes!
(static method)
When does the myocardium produce the strain force?
When contracting
Strain + strain rate are indicators of what when doing cardiac strain imaging?
Indicators of effectiveness of myocardial function + are useful diagnostic parameters
Strain + strain rate when doing cardiac strain imaging can be determined by what techniques?
2D speckle tracking techniques
(this tracks the movement of specific portions of myocardium)
Explain the variables in the shear modulus formula: G = pSWV^2
G = shear modulus
SWV = shear wave speed
p = tissue density
(is used to calculate shear wave speed)
Steps:
-Shear wave generation
-Measurement of shear wave propagation
-Estimate shear wave modulus
