High resolution imaged laser speckle strain gauge for vascular applications

An imaged laser speckle strain gauge that yields strain rates directly is described for vascular applications. The strain gauge does not rely upon cross correlations between a reference image and subsequent strain-modulated images as most current speckle interferometric methods do. Instead, it relies upon a two-dimensional frequency transform of "stacked speckle histories" which are time series of one-dimensional views of the speckle patterns arranged into a spatio-temporal array such that space is along the abscissa and time is along the ordinate. The tilt of the stacked speckle histories is related to the time rate of speckle pattern shift. The strain gauge is sensitive only to in-plane strains. Strain rates of 30.1 +/- 3.2 and 24.83 +/- 2.1 mu epsilon/s were evaluated in vitro on a fresh human tibial artery and rat inferior vena cava, respectively. The total strains measured were 21.6 and 19.86 mu epsilon, respectively. This is at least one order of magnitude more sensitive than other current soft-tissue strain measurement techniques.