ELASTOGRAPHY

ELASTOGRAPHY

• Stiffness variations noted  between different tissue types is limited but there is significant stiffness variations between normal and pathological tissue.
• Most pathological changes are associated with changes in tissue stiffness.
•  Elastographic techniques stems from the fact that large differences in stiffness or modulus contrast exist between surrounding normal and pathological tissues that may otherwise possess similar image contrasts with conventional clinical imaging modalities

Some terms :

• Stress is defined as force per unit area.
• Shear stress has the same units as normal stress but represents a stress that acts parallel to the surface (cross section).
• Strain is the change in length per unit length.Computed as (Lf - L0) / L0 where Lf is the final length and L0 is the initial length.
• Strain Rate, specifies how quickly (or slowly) a material is being deformed or loaded, i.e. the amount of strain that occurs in a unit of time. 
• Young's Modulus is the constant of proportionality between stress and strain. 

• We can use physical phenomenon of STRAIN(Strain imaging),STRESS(Stress imaging),MODULUS(Modulus imaging) for imaging using ultrasound, MRI,Optical imaging tomography,x ray computed tomography.
• Mechanical stimuli is used for perturbation in strain imaging.Quasi-static compression is used in elastography as mechanical stimuli  and while low frequency vibration is used in sonoelasticity imaging as mechanical stimuli.

DEFINITION:

• Sonoelastography  enables evaluation of tissue elasticity and is based on differences in stiffness (hardness, compressibility,elasticity) of pathological changes and normal adjacent tissue.
• Elastographic imaging techniques are based on the hypothesis that soft tissues deform more than stiffer tissue, and these differences can be quantified in images of the tissue strain tensor or the Young’s modulus
• An imaging technique whereby local tissue strains are measured from differential ultrasonic  speckle    displacements. These displacements are generated by a weak, quasi-static stress field. The resultant axial-strain, lateral-strain, modulus or Poisson’s ratio images are all referred to as Elastograms.

APPLICATION:

• It has been found useful in many medical field and adopted readily by clinicians of different specialties.It gives more information than conventional ultrasound in evaluation of tumors, liver disease, skeletal muscles, rheumatoid nodules and other pathological changes

In-vivo Applications of Elastography

• Breast Imaging
• Prostate Imaging
• Thyroid Imaging
• Liver Imaging
• Treatment Monitoring
• Intravascular Strain Imaging
• Cardiac Elastography
• Deep Vein Thrombosis
• Kidney Transplant Monitoring

• In general, two criteria have to be satisfied for successful clinical elastographic imaging, namely the ability to apply a quasi-static deformation and the ability to ultrasonically image the tissue being deformed. 
• Superficial organs such as skin, breast, neck, thyroid, lymph nodes, deep vein thrombi are some of the best candidates for strain or modulus imaging . Approaches to deform organs located deeper in the body have been reported, that use intracavitary ultrasound transducers, intracavitary balloons for imaging the prostate gland ,saline infusion  for imaging the uterus  etc.Physiological stimuli have been utilized to image the heart, vasculature, liver, and the coronary and carotid arteries