Three-dimensional ultrasound imaging of the rotator cuff: spatial compounding and tendon thickness measurement.

Three-dimensional (3-D) volume reconstructions of the shoulder rotator cuff were generated from freehand ultrasound (US) scans acquired with a magnetic tracking system. Image stacks acquired with lateral overlap from multiple acoustic windows were spatially compounded to provide an extended representation of the rotator cuff tendons. A semiautomated technique was developed for measuring rotator cuff thickness from the 3-D compound volumes. Scans of phantoms and volunteer subjects were used to evaluate the accuracy and repeatability of the thickness measurements. For an in vitro phantom with known thickness, the mean difference between the true value and the automatic measurements was 0.05 +/- 0.28 mm. Thickness measurements made manually from 2-D images and automatically from 3-D volumes were different by 0.03 +/- 0.44 mm in vitro and -0.06 +/- 0.36 in vivo. Repeated thickness measurements in vivo differed by 0.06 +/- 0.36 mm. The 3-D measurement technique offers a promising method for evaluating rotator cuff tendons.