Three-dimensional mapping of strain in ex vivo porcine cornea with an ultrasound elasticity microscope

High frequency strain mapping of a porcine cornea was produced by three-dimensional speckle tracking of a three-dimensional confocally merged ultrasonic data set. Previous two-dimensional elasticity imaging was limited by speckle moving in the non-imaged dimension. This study used an ultrasonic transducer (53 MHz center frequency, 31 MHz bandwidth, 1.67 f#) scanned in three dimensions. A fresh porcine eye globe was embedded in gelatin up to the cornea/sclera junction and pressurized to physiological pressure. A portion of the cornea was imaged with a single element transducer scanned in three directions. Three-dimensional volume sets were created from the data by confocally merging volume sets at several depths. In the vertical dimension, parallel to ultrasonic propagation, the transducer was moved by nearly half its depth of field. Overlaps between adjacent depths were correlated using phase-sensitive speckle tracking to determine precise shifts. Two-dimensional images over multiple depths of field were combined to form a single 2-D image over an entire vertical scan range. Multiple planes were then stacked in the remaining direction to form a 3-D volume set. Next, the cornea was deformed and then imaged again. Three-dimensional speckle tracking was used to create strain maps from the two volume sets. Vertical strain, horizontal strain, and transverse strain behave as expected and provide insight into mechanical properties of corneal tissue.