Strain elastography

Strain elastography (also known as tissue strain elastography/static elastography/compression elastography) is a developing form of ultrasound that assesses tissues' macroscopic structure through the strain modulus. This is different from normal B-mode grayscale ultrasound which characterizes a tissue's elasticity but at a microscopic level.

Strain elastography relies on Young's modulus to detect strain in the axial dimension. The characteristics of an ultrasound beam through tissue before and after compression are compared. In some systems, the strain of tissues is measured in a semi-quantitative way, relying on Young's modulus, but not directly calculating it.

Applications of strain elastography are currently being developed for:

• breast ultrasound

• liver ultrasound

  • detection of small lesions

  • evaluation of diffuse liver disease

• prostate ultrasound

• thyroid nodule ultrasound

• musculoskeletal ultrasound

• obstetric ultrasound for preterm labor ²

There may also be some applications in echocardiography.

The technique is still being developed for clinical use. Currently, there are a number of different ways to perform strain elastography and continued improvement in differentiation between the lesion and background tissue is necessary for reliable clinical diagnosis.

Breast

Malignant breast lesion generally has higher stiffness and appears larger on elastography when compared to usual ultrasound B-mode images. There are three methods used in strain elastography of the breast ³.

First method is to use a 5-point color scale to determine the stiffness of a lesion. Color blue is rated as stiff while color red is rated as soft ³.

The second method is to calculate the strain ratio, which is comparing the strain of the surrounding fat tissue with the strain of the breast lesion. The cut-off point of strain ratio, however, is vendor dependent ³.

The third method is to calculate the E/B ratio, which is comparing the diameter of the lesion on elastography with the diameter of the same lesion on B-mode ultrasound. E/B ratio less than 1 is considered as benign. E/B ratio is more sensitive and specific than other two methods ³.