ELASTOGRAPHY - DR SHEFALI MESHRAM

Elastography 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.

PHYSICS 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.

Strain- 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.

Hooke’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.

DEPICTION 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.

In gray scale elastograms, stiffer lesions are darker and appear to increase in size compared to sonograms.

On color overlay images, red to yellow depict stiff areas and blue and green depict soft areas.

DIFFERENT TECHNIQUES. Elastography techniques vary depending upon: The method used for tissue excitement ( mechanical or ultrasonic). By the response of tissue to compression ( static or dynamic)

COMPRESSION 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.

Softer 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.

LIMITATIONS 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)

ACOUSTIC 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.

ADVANTAGES 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.

DISADVANTAGE 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.

SHEAR 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.

It 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.

APPLICATIONS Breast imaging. Prostate imaging. Thyroid imaging. Liver imaging. Lymph nodes imaging. Treatment monitoring. Deep vein thrombosis.

BREAST IMAGING As compared to gray scale, malignant lesions tend to be larger and more irregular on elastography likely secondary to stiff peripheral desmoplastic reaction.

COMPLEX 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.

Lesion stiffness can be measured quantitatively with shear wave elastography. Malignant lesion- 80-100kPa. Fat- 7kPa. Normal breast parenchyma- 30-50kPa.

PITFALLS

LIVER 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.

LIVER STIFFNESS CUT OFFS IN CHRONIC LIVER DISEASE.

OTHER 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.

LYMPH NODES Mainly to d/b benign and malignant nodes.

PITFALLS 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.

Inspite of few shortcomings, it is a big radiological find as an adjunct to the other modalities. THANK YOU.