A Novel 3-D Multiparametric Ultrasonic Phantom for Anatomy, Elasticity, Blood Flow and Tissue Orientation Imaging

The complexity of biological tissues calls for imaging techniques capable of measuring complementary tissue characteristics in 3-D. Multiparametric imaging is a fast-growing field, especially for applications such as cancer diagnosis. Developing multiparametric imaging sequences and the associated post-processing often requires finding trade-offs between the constraints of different techniques, imaging quality and imaging time. As a consequence, phantoms capable of capturing different tissue parameters in the same location are becoming crucial to testing and optimizing such sequences without a change in setup. Thanks to recently developed techniques, ultrasound imaging today enables the measurement of tissue rigidity, structure and blood flow, but concurrent imaging is still under development. The aim of this study was therefore to design and evaluate an ultrasound phantom combining these different aspects into the same field of view; to our knowledge the first in the field of ultrasound imaging.

A gel-based phantom was built with a rigid inclusion controlled by the proportion of agar. Blood flow was generated inside a looped wall-less vessel made by delicately removing a thin catheter after gel-setting. Tissue orientation was obtained by mixing magnetic particles in the phantom gel and controlling their orientation during setting with external magnets. The phantom was characterized in terms of tissue rigidity values measured with shear wave elastography, vessel diameter measured with ultrasensitive Doppler imaging and tissue orientation measured with backscatter tensor imaging.

Excellent image contrast was obtained for rigidity and flow imaging, and clearly oriented tissue structure was visible with backscatter tensor imaging. The tissue rigidity ratio between the 3-D inclusion and matrix was within 12% of the theoretical value, the vessel diameter measured in 3-D was within 3% of the true value and tissue scatterer orientation was validated visually against histological phantom slabs.

This phantom provides not only a framework for 3-D multiparametric ultrasound evaluation, but could also be used as a more clinically realistic alternative to some standard phantoms.