Elastography

Physicians have been palpating lumps and bumps for centuries.  One of the features of a mass assessed with palpation is the stiffness/hardness of the mass.  In general, stiff, hard masses are more worrisome for malignancy than softer, more deformable masses.  Likewise, a stiff, hard liver is more concerning for cirrhosis than a soft, malleable liver.

Elastography is an ultrasound application that can also assess the stiffness of a lesion.  Some people have called it 'echopalpation'.

There are two basic types of elastography:

• strain elastography

• shear wave elastography

Strain elastography is a qualitative technique in which the tissue is compressed manually by the sonographer using the transducer. This deforms the tissue. The system tracks how much the tissue compresses or shifts under this pressure, comparing ultrasound frames from before and during the compression. The resulting tissue deformation (called strain) is then assessed by the system's computer. The stiffness of the lesion can be displayed in a grayscale overlay or displayed in color according to a color map. A typical map may indicate stiff lesions as blue and less stiff lesions as red. The image produced is called an elastogram.

Here's a short video from GE describing the strain elastography process.  

Shear wave elastography is a quantitative technique in which an acoustic "push pulse" is produced by the transducer. This localized force generates transverse shear waves that ripple outward through the surrounding tissue. Stiffer lesions produce faster shear waves. To capture these waves in real-time, modern 2D systems utilize plane-wave ultrasound. Instead of standard line-by-line scanning, plane-wave ultrasound flashes the entire field of view thousands of times per second (ultrafast imaging), successfully tracking the shear wave's rapid propagation. Using a region of interest, the system tracks this speed and calculates a precise quantitative value for tissue stiffness. This value is displayed either as shear wave speed in meters per second (m/s) or converted via physics formulas into Young’s Modulus, measured in kilopascals (kPa).

Here is an excellent video demonstrating the use of shear wave elastography in the evaluation of the stiffness of a variety of lesions.

More detailed information can also be found in my two videos on recent advances.