Laser Speckle Strain Measurements in Soft Tissue

The importance of understanding the mechanical behavior of tissues is starting to be recognized in medical diagnostics. It is becoming clear that pathological tissues behave mechanically differently than do healthy tissues. However, traditional optical evaluation methods, particularly speckle-based methods, have not proven to be effective in assessing the mechanics of biological soft tissues. This is due, in part, to the rapid temporal decorrelation of speckle patterns and the depolarization of the scattered light, making fringe generation and interpretation extremely difficult. A laser-speckle strain sensor will be described that has been shown to be effective in high-resolution strain and strain rate measurements in fully hydrated biological hard and soft tissues. The sensor does not rely upon fringe generation as do most speckle-based methods, but instead relies upon high-resolution speckle tracking based on a maximum likelihood shift estimator coupled with a differential illumination arrangement that makes the sensor sensitive only to strains in the plane of the illumination and observation. Herein, we will focus on a variation of the strain sensor that can be used to quantify certain viscoelastic parameters of soft biological tissues. The ability of the sensor to differentiate between healthy and chemically altered tissues will also be discussed.