  High frame rate 3-D ultrasound imaging technology combined with
super-resolution processing method can visualize 3-D microvascular structures
by overcoming the diffraction limited resolution in every spatial direction.
However, 3-D super-resolution ultrasound imaging using a full 2-D array
requires a system with large number of independent channels, the design of
which might be impractical due to the high cost, complexity, and volume of data
produced.
  In this study, a 2-D sparse array was designed and fabricated with 512
elements chosen from a density-tapered 2-D spiral layout. High frame rate
volumetric imaging was performed using two synchronized ULA-OP 256 research
scanners. Volumetric images were constructed by coherently compounding 9-angle
plane waves acquired in 3 milliseconds at a pulse repetition frequency of 3000
Hz. To allow microbubbles sufficient time to move between consequent compounded
volumetric frames, a 7-millisecond delay was introduced after each volume
acquisition. This reduced the effective volume acquisition speed to 100 Hz and
the total acquired data size by 3.3-fold. Localization-based 3-D
super-resolution images of two touching sub-wavelength tubes were generated
from 6000 volumes acquired in 60 seconds. In conclusion, this work demonstrates
the feasibility of 3D super-resolution imaging and super-resolved velocity
mapping using a customized 2D sparse array transducer.
