Design of a Multi-Angle Multi-Depth Flow Phantom for Ultrasound Flow Calibration

Ultrasound flow phantoms are essential tools for the development of new flow imaging algorithms and for performing quality assurance in medical imaging. A comprehensive system calibration, from shallow to deep flow regions, is increasingly needed to test novel velocity imaging methods. This study describes the design, implementation, and testing of a novel multi-angle, multi-depth flow phantom. Such key features are achieved by offsetting the two flow channels from the surface and incorporating acoustic wedges into the design. Our experiments show that the phantom was effective in evaluating the accuracy of Doppler velocity measurements attainable by sonicating vessels located at different depths with a commercial linear array probe whose acoustic lens was focused on elevation at 20 mm. An experimental multigate spectral Doppler analysis highlights the following behavior: 1) although the F-number was kept constant, spectral broadening increased with depth; 2) mean velocity was increasingly underestimated at greater depths. Both effects were significantly reduced by repeating the experiments with a custom-modified version of the same probe, not including any elevation lens, i.e., characterized by an approximately constant elevation beamwidth over the depth range of interest. Using the novel multi-depth flow phantom, it was thus possible to demonstrate experimentally that spatial velocity gradients determine performance drops that are not compensated by dynamic focusing, and appropriate arrays must be used when large depth ranges have to be investigated.