Ultrasound super resolution imaging of nanodroplets with a multi-frequency hemispherical phased array

Abstract

High resolution imaging of microvasculature is desirable for diagnostic and therapeutic applications in the brain. Here, we investigated the use of a 256-module sparse hemispherical transducer array to map the emissions of lipid-coated Decafluorobutane nanodroplets (∼210 ± 80 nm, 107–109 droplets/ml) flowing through tube phantoms (0.8 mm inner diameter). Each array module comprised 4 concentric cylindrical PZT-4 elements (55/306/612/1224 kHz). Droplets were vaporized at 55 kHz (0.10–0.18 MPa, 145 μs bursts every 2 s) and the resulting emissions were received on either the 306, 612 or 1224 kHz subarrays. Low-resolution 3-D images were formed using delay-and-sum passive beamforming, and super-resolved images were obtained via Gaussian fitting of the estimated point-spread-function to the low-resolution data. With super-resolution techniques, the mean lateral (axial) full-width-at-half-maximum image intensity was 35 ± 6 (67 ± 11), 16 ± 3 (32 ± 6), and 7 ± 1 (15 ± 2) μm from 160 (2970), 241 (2970), and 117 (4950) vaporization events (total frames), corresponding to ∼1/85 of normal resolution at 306, 612 and 1224 kHz, respectively. The mean positional uncertainties were ∼1/350 (lateral) and ∼1/180 (axial) of the receive wavelength in water. The pressure threshold for vaporization detection increased with increasing receive frequency. This study demonstrates the feasibility of mapping vaporized nanodroplets with passive beamforming and super-resolution imaging techniques.