Strain imaging is based on the principle that myocardial cells not only contract in a radial direction, but also in a circumferential and longitudinal direction, almost like the ringing of a towel.
Strain imaging uses speckle tracking to track two spots or speckles moving through multiple frames in systole. The ultrasound software checks to see if those speckles move closer together, signifying a meaningful deformation or keep the same distance, suggesting a lack of deformation or contraction of the muscle.
Because we want those two speckles to move closer together, strain is reported as a negative number. The more negative the number, the better the regional contraction. So for example, -20 is better than -10.