Elastography is a noninvasive technique of imaging stiffness or elasticity of tissues by measuring movement or transformation of tissue in response to a small applied force Virtual palpation which can overcome the subjectivity flaw and provide objective as well as quantitative measure of tissue ID: 931778
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Slide1
ELASTOGRAPHY
- DR SHEFALI MESHRAM
Slide2Elastography
is a non-invasive technique of imaging stiffness or elasticity of tissues by measuring movement or transformation of tissue in response to a small applied force.
Virtual palpation which can overcome the subjectivity flaw and provide objective as well as quantitative measure of tissue stiffness.
Slide3PHYSICS
Stress
- force per unit area.
Compression
- acts perpendicular to the surface and causes shortening of the object.
Shear
- acts parallel to the surface and causes deformation.
Unit of stress is Newton/sq. m ( Pascal).
Stress can be applied exogenously ( transducer compression, vibrators or acoustic radiation).
Endogenous motion of vessels, cardiac or respiratory movements can be used, however, these are difficult to quantify.
Slide4Slide5Strain-
when subjected to stress an object tends to undergo deformation of its original shape and size, the amount of deformation is known as strain. It is
unitless
.
Elasticity-
it is the property of a material to return back to its original form after stress is removed.
Slide6Hooke’s law-
stress is proportional to strain within an object’s elastic limit.
Young’s modulus-
ratio of stress over strain.
Young’s modulus quantifies the tissue stiffness. Hard tissues have higher Young’s modulus than soft ones.
Slide7Slide8DEPICTION OF ELASTOGRAMS
These are usually viewed simultaneously with sonograms to identify the areas of abnormality.
This can be done either with Gray scale depiction or a semitransparent color overlay of
elastogram
over sonogram.
Slide9In gray scale
elastograms
, stiffer lesions are darker and appear to increase in size compared to sonograms.
Slide10Slide11On color overlay images, red to yellow depict stiff areas and blue and green depict soft areas.
Slide12Slide13DIFFERENT TECHNIQUES.
Elastography
techniques vary depending
u
pon:
The method used for tissue excitement ( mechanical or ultrasonic).
By the response of tissue to compression ( static or dynamic)
Slide14COMPRESSION ELASTOGRAPHY
A strain profile is calculated in a direction perpendicular to the tissue surface in response to externally applied force.
The radiofrequency pulses generated by a tissue in response to external compression are
analysed
.
Hard or stiff materials tend to move as a whole with all points displacing with the same amount on compression, which results in zero or small rate of change of displacement called as zero or no strain.
Slide15Softer tissues show larger change in rate of displacement giving large strain values.
The deformation measurement is mapped on
elastogram
on which stiffer areas are depicted as dark and more elastic areas are lighter.
Slide16LIMITATIONS
The tissue strain is dependant on the amount of compression applied. This makes it operator dependant.
It is a qualitative imaging of relative stiffness so the actual strain value cannot be compared with the follow up imaging.
However, it can used as
semiquantitative
method where strain ratio is calculated ( ratio of strain between the lesion and adjoining normal tissue)
Slide17ACOUSTIC RADIATION FORCE IMPULSE (ARFI)
Here, short duration acoustic forces known as pushing pulses are used to cause tissue displacements.
Pushing pulses can be applied by the ultrasound transducer array (2-7MHz) to a volume of 2 sq.mm for 1ms.
Peak displacement, time taken to reach peak displacement and recovery time are utilized to characterize tissue response.
Slide18ADVANTAGES
ARFI images are found to be more homogenous and have better contrast than surface displacement ( compression)
elastography
.
Deeper tissue, not accessible by superficial external compression can be evaluated.
Slide19DISADVANTAGE
Physiological ( respiration, pulsation) and transducer motion can degrade image quality as 1-3 ms is required.
Lower ultrasound frequencies and motion compensation techniques like premonitory physiological motion and subtracting this expected motion have been applied to overcome this limitation.
Tissues at a depth of > 10cm cannot be accurately assessed due to attenuation of radiation force at greater depths.
Slide20SHEAR WAVE ELASTOGRAPHY
It uses transient pulses to generate shear waves in the body.
The tissue’s elasticity is directly deduced by measuring the speed of wave
propogation
.
It is the only approach able to provide quantitative and local elastic information in real time.
Slide21It uses the acoustic radiation force induced by ultrasound beams to perturb underlying tissues. This pressure or acoustic wind pushes the tissue in the direction of propagation.
An elastic medium such as human tissue will react to this push by a restoring force. This force induces mechanical waves and more importantly shear waves which
propogate
transversely in the tissue.
Slide22Slide23APPLICATIONS
Breast imaging.
Prostate imaging.
Thyroid imaging.
Liver imaging.
Lymph nodes imaging.
Treatment monitoring.
Deep vein thrombosis.
Slide24BREAST IMAGING
As compared to gray scale, malignant lesions tend to be larger and more irregular on
elastography
likely secondary to stiff peripheral
desmoplastic
reaction.
Slide25Slide26Slide27Slide28COMPLEX CYST V/S SOLID LEIONS
Elastography
has the potential to differentiate complicated cysts from solid mass.
Shear wave propagation does not occur in cysts and therefore cysts should have
elastography
values of zero and will appear mostly black or homogenously blue on color overlay
elastogram
.
Slide29Slide30Slide31Lesion stiffness can be measured quantitatively with shear wave
elastography
.
Malignant lesion- 80-100kPa.
Fat- 7kPa.
Normal breast parenchyma- 30-50kPa.
Slide32PITFALLS
Slide33LIVER STIFFNESS
Assessed by ultrasound. More recently by MRI.
Evaluates velocity of propagation of a shock wave within liver tissue.
Normal liver is viscous and is not favorable to wave propagation.
Fibrosis increases hardness of the tissue, favors more rapid propagation.
Slide34LIVER STIFFNESS CUT OFFS IN CHRONIC LIVER DISEASE.
Slide35Slide36OTHER APPLICATIONS IN LIVER
Decreased stiffness post anti-viral treatment and increased stiffness in relapse.
Splenic
stiffness >9kPa correlates with portal hypertension.
Biopsy site from the stiffest region.
Much larger liver volume assessed than biopsy.
Slide37LYMPH NODES
Mainly to d/b benign and malignant nodes.
Slide38Slide39Slide40PITFALLS
Large lesions can be under assessed with portions of lesion lying out of the view.
Painful lesions may be under represented because of increased discomfort.
Technically challenging in organs like salivary glands and obese people.
Slide41Inspite
of few shortcomings, it is a big radiological find as an adjunct to the other modalities.
THANK YOU.