Validation of high-frequency ultrasound measurements of tissue layer thickness

Abstract

High-frequency ultrasound imaging enables nondestructive measurement of layer thickness in tissue specimens. These measurements are valuable for mechanical testing of soft biomaterials. This paper demonstrates a method for assessing the accuracy of high-resolution ultrasonic thickness estimates. Three-dimensional images of six porcine aortic valve cusps were acquired in vitro using a 40 MHz ultrasound system with 40×80×80 μm3 spatial resolution. The cusps were then frozen in liquid nitrogen, sectioned into 10-μm slices, and micrographs of one slice from each specimen were acquired at 4× magnification. The two-dimensional micrographs were registered to the three-dimensional ultrasound images using a cross-correlation method. The boundaries of the fibrosa, spongiosa, and ventricularis layers were segmented in both sets of images using an active contour model. The average thicknesses of the tissue layers in the registered images were estimated and the absolute differences of the optical and ultrasonic estimates were computed. The absolute differences were 55.8±22.6 μm (mean ± standard deviation), 23.5±14.3 μm, and 22.7±17.2 μm for the fibrosa, spongiosa, and ventricularis, respectively. The measurement differences are comparable to the axial resolution of the ultrasound system and are not significant as determined by t-tests (p>0.30 for each layer).