Aperture design analysis for vector flow imaging

We present the results of a study exploring the upper velocity limit of vector flow estimation with Heterodyned Spatial Quadrature. Since this is highly dependent on apodization, we investigated a set of apodization windows designed to explore means of extending the practical velocity range. Heterodyned Spatial Quadrature (HSQ) is a recently described vector flow technique that has been shown to provide accurate flow estimates in both the axial and lateral directions. The complex PSF created by this technique induces a modulation in the received echo of a scatterer traversing the resolution volume at a frequency proportional to the scatterer lateral velocity. We measure the rate of phase change of this modulation to provide an estimate of the lateral flow velocity component. The technique is extendible to 3D vector flow estimation with a 2D array. We expect lateral tracking methods in general to be limited by a number of factors including the PSF beam width and the system amplitude sensitivity. Using a Siemens Elegra ultrasound scanner with a 7.5 MHz linear array, we simulated flow up to 405 μm per step in a tissue-mimicking phantom, corresponding to velocities up to 4.05 m/sec for a PRF of 10 kHz, at Doppler angles of 60° and 90°. We estimated lateral velocities to within 5% relative bias up to 315 cm/sec in an f/2 geometry at a 90° Doppler angle. The spatial quadrature receive aperture utilized a bi-lobed Blackman apodization with a width of 1/2D, where D is the full width of the array. Computer simulations of the system under similar conditions produced lateral velocity estimates up to 303 cm/sec. As expected, the maximum estimable velocity scales with focal depth. In simulations, the same aperture estimated flow velocities up to 672 cm/sec at a focal depth of 75 mm, representing an f/5 geometry.