The key idea is that a Doppler flow phantom is designed to simulate controlled blood flow so you can systematically evaluate different Doppler measurements. The phantom has a tube with moving fluid inside a tissue-mimicking medium, allowing you to adjust flow rates, angles, and depths to mimic real vessels. Range-gate accuracy is tested because you can place the sample gate at known depths along the flow path and verify that the system reports velocities at the correct location. Spectral Doppler velocity accuracy is tested by comparing the measured velocities from the spectral Doppler tracing to the known, preset flow speeds in the phantom. Color Doppler penetration is tested by imaging through the tissue-mimicking material at various depths and angles to see how far color flow can be reliably displayed before it becomes attenuated or lost. Since a Doppler flow phantom enables evaluation of all these aspects—range-gate positioning, velocity accuracy in spectral Doppler, and color Doppler visibility at depth—you can test all of the above with it. That’s why the best choice is all of the above.