Swept-3D Ultrasound Imaging of the Mouse Brain Using a Continuously Moving 1D-Array Part I

Volumetric 3D Doppler ultrasound imaging can be used to investigate large scale blood dynamics outside of the limited view that conventional 2D power Doppler images (PDIs) provide. To create 3D PDIs, 2D-matrix array transducers can be used to insonify a large volume for every transmission, however these matrices suffer from low sensitivity, high complexity, and high cost. More typically, a 1D-array transducer is used to scan a series of stationary 2D PDIs, after which a 3D volume is created by concatenating the 2D PDIs in post-processing, which results in long scan times due to the repeated measurements. Our objective was to achieve volumetric 3D Doppler ultrasound imaging with a high Doppler sensitivity, similar to that of a typical stationary recording using a 1D-array transducer, while being more affordable than using 2D-matrix arrays. We achieved this by mounting a 1D-array transducer to a high precision motorized linear stage and continuously translating over the mouse brain in a sweeping manner. For Part I of this paper we focused on creating the best vascular images by investigating how to best combine filtered beamformed ultrasound frames, that were not acquired at the same spatial locations, into PDIs. Part II focuses on the implications of sampling transient brain hemodynamics through functional ultrasound while continuously translating over the mouse brain. In Part I we show how the speed at which we sweep our 1D-array transducer effects the Doppler spectrum in a flow phantom. In-vivo recordings were performed on the mouse brain while varying the sweeping speed, showing how higher sweeping speeds negatively effect the PDI quality. A weighting vector is found to combine frames while continuously moving over the mouse brain, allowing us to create swept PDIs of similar sensitivity when compared to those obtained using a stationary 1D-array while allowing a significantly higher 3D Doppler volume rate and maintaining the benefits of having a low computational and monetary cost. We show that a vascular sub-volume of 6 mm can be scanned in 2.5 seconds, with a PDI reconstructed every 200 μm, outperforming classical staged recording methods.