PHYSICS OF ELASTOGRAPHY
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
‘VIRTUAL PALPATION’ which can
overcome the subjectivity flaw and provide
objective as well as quantitative measure of
Going back to school days….
Stress: It is defined as force per unit area.
Stress can due to: Compression-which acts
Perpendicular to the surface and causes
shortening of an object
: Shear stress which acts
parallel to the surface and causes
Strain: When subjected to stress an object
tends to undergo deformation of its original
size and shape; the amount of deformation is
known as strain. Unit less-expressed as change
in length per unit length of the object.
Elasticity: It is the property of the materials to
return back to its original form after stress is
The Basics of Human Tissue
Tissue stiffness is generally measured by a
physical quantity called Young’s modulus and
expressed in pressure units - Pascals or kilo
The Young’s modulus is defined simply as the
ratio between the applied stress and the
Young’s modulus, or elasticity E, quantifies
tissue stiffness. Hard tissues have a higher
Young’s modulus than soft ones.
TYPE OF SOFT TISSUE
YOUNG’S MODULUS –(E = s
HOW EXACTLY DOES THIS CRAP
Three step methodology:
1. Generate a low frequency vibration in tissue to induce
2. Image the tissue with the goal of analyzing the resulting
3. Deduce from this analysis a parameter related to tissue
If the Young’s modulus, or elasticity of the tissue, can be
determined directly from the analysis, the technique is
TYPES OF ELASTOGRAPHY
Elastography techniques are commonly
classified according to the type of
vibration applied to the tissue. There are
three classes of Elastography :
Shear wave based
Static elastography uses a uniform compression at
the surface of the body to cause deformation of
The compression is applied by the user and the
ultrasound scanner calculates and displays the
induced deformation in the imaging plane.
Young’s modulus cannot be reconstructed as the
stress within the tissues induced is unknown.
Static Elastography. Bmode (left) and elastogram (right).
On the elastogram, less deformed tissue appears darker
Dynamic elastography utilizes a continuous
Stationary waves induced in the body are analyzed
to deduce tissue elasticity.
Dynamic elastography is well suited for MR
systems as the vibration pattern is not time
dependent but must be assessed in a volume.
It is a quantitative approach but suffers from the
usual MR drawbacks: high cost, limited
availability, and lack of real time imaging.
SHEAR WAVE ELASTOGRAPHY
Shear wave based elastography makes use of
transient pulses to generate shear waves in
The tissue’s elasticity is directly deduced by
measuring the speed of wave propagation.
Shear wave based elastography is the only
approach able to provide quantitative and
local elastic information in real time
Shear Wave Elastography 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
propagate transversely in the tissue.
A shear wave induced by an ultrasound beam focused
in the center of the image
Compared to gray-scale
ultrasound, malignant lesions tend to be
larger and more irregular on elastography
likely secondary to stiff peripheral
When measuring lesion size on
elastography, the lesion should be measured
in the exact position on both the elastogram
and B-mode image.
Heterogeneous echo texture , irregular shape and stiff color
elastogram, which appears larger than the gray scale image.
The color scale is a measure of stiffness. In these images, red
indicates very stiff tissue, green/yellow indicates intermediate
stiffness and blue indicates low stiffness IDC
Benign lesions demonstrating : homogenoeus oval shape and very soft
elastogram, which also appears the same size on both gray-scale and shearwave elastography.. Clustered microcysts
COMPLEX CYST V/S SOLID
Elastography has the potential to
differentiate complicated cysts form
Shear-wave propagation does not occur
in cysts and therefore cysts should have
elastography values of zero and will
appear mostly black or homogeneously
blue on the color overlay elastogram
Large simple cyst which shows no elasticity within the lesion and
A bull’s eye artifact has also been described as a characteristic feature
present in benign breast cysts, where central fluid may appear bright with
a surrounding dark ring
Elastography has the potential to downgrade BI-RADS
4a lesions to BI-RADS 3, using qualitative shear-wave
elastography and color assessment of lesion
stiffness, oval shape and a maximum elasticity value of
less than 80 kPa without a significant loss in sensitivity.
Elastography may also be used to identify oval
circumscribed cancers detected on ultrasound and may
be used to upgrade a BI-RADS 3 lesion to BI-RADS 4.
Furthermore, elastography feature analysis also has the
potential to downgrade BI-RADS 3 lesion to BI-RADS 2
Oval circumscribed hypoechoic mass on gray-scale imaging, which has
benign ultrasound features. However, elastography demonstrates a
Lesion stiffness can also be measured
quantitatively with shear wave elastography.
Stiffness of malignant lesions is generally greater
than 80–100 kPa), while fat has relatively low
elasticity values near 7 kPa and breast
parenchyma have elasticity values ranging from
However, one must be careful when using kPa in
lesion evaluation, as some soft cancers may have
low kPA values between 20-80 kPa, similar to
On compression elastography,
hard tissue appears blue and soft tissue appears red to green.
Some cancers lack a significant
desmoplastic reaction and may be
soft, resulting in a false negative elastogram
With shear-wave elastography, some
cancers may have a mean stiffness of less
than 50 kPa .
Similarly, some benign lesions may appear
stiff including hyalinized fibroadenomas, fat
necrosis and fibrosis.
A heterogeneous mass with indistinct margins on grayscale ultrasound appears
stiff, heterogeneous, large and suspicious on shearwave elstography.
Biopsy demonstrated benign breast tissue with stromal fibrosis
Assessed by US & more recently by MRI
Evaluates velocity of propagation of a shock wave
within liver tissue (examines a physical parameter of
liver tissue which is related to its elasticity)
Normal liver is viscous
Not favorable to wave propagation
Fibrosis increases hardness of tissue
Favors more rapid propagation
Liver stiffness cut-offs in chronic liver
LSM 2.5 – 7 kPa → Mild or absent fibrosis is likely
LSM > 12.5 kPa → Cirrhosis is likely
LIMITATIONS OF US ELASTOGRAPHY
Acute liver injury
Narrow intercostal spaces
OTHER APPLICATIONS IN LIVER
Decreased stiffness post anti-viral treatment
and increased stiffness in relapse.
Splenic stiffness > 9kPa correlates with
To d/d between HCV and non HCV
infections in liver transplant recipients.
Biopsy site from the stiffest region.
Much larger liver volume assessed then
Mainly to d/d between benign and
malignant nodes esp. in axillary and
Score of metastatic nodes in axilla are >
Scores of metastatic nodes in neck > 2
Sensitivity of > 85 % but less specificity.
Elastography image on left shows pattern 1, absent or small hard
area. B-mode sonographic image on right shows score of 5, reactive.
Final diagnosis from clinical and serologic findings was reactive
Longitudinal sonogram of level 5 lymph node in 52-year-old man
with nasopharyngeal carcinoma. Elastography image on left shows
pattern 4, peripheral hard and central soft areas. B-mode
sonographic image on right shows score 7, metastatic
TO DIAGNOSE PRIMARY
TO GUIDE FOR CORE BIOPSY
TO SEE EXTRA CAPSULAR
LARGE LESIONS CAN BE UNDER ASSESSED WITH
PORTIONS OF LESION LYING OUT OF THE VIEW
PAINFULL LESIONS MAYBE UNDER REPRESENTED
BECAUSE OF INCREASED DISCOMFORT
TECHNICALLY CHALLENGING IN ORGANS LIKE
SALIVARY GLANDS AND OBEESE PEOPLE.
INSPITE OF THE FEW SHORT COMINGS, IT’S A
BIG RADIOLOGICAL FIND OF THIS CEENTURY AS
AN ADJUNCT TO THE OTHER MODALITIES