Sparse Orthogonal Diverging Wave Imaging on a High-Frequency Phased Array

A new ultrafast encoding scheme for phased array imaging named sparse orthogonal diverging wave imaging (SODWI) is presented. Hadamard codes that encode the array elements are surfed on top of a diverging wave generated from the full aperture during transmit. A complete set of orthogonal Hadamard codes is equivalent to the number of elements on the imaging array. From within the full set of codes, a subset can be sparsely selected to increase frame rate and the tradeoff between frame rate and image quality has been investigated. The technique was implemented on a 64-channel custom beamforming platform with a 40 MHz phased array. When a complete set of 64 codes were used, SODWI exhibits a 3-dB improvement in SNR over traditional focused imaging. Each time the number of transmit codes is halved, the SODWI SNR is reduced by an average of 3-dB and the level of secondary lobes are suppressed by an average of 3.5-dB less. The FWHM of the directivity functions are increased to 136 ^θ , 160 ^θ , and 140 ^θ , from 136 ^θ , 56 ^θ , and 30 ^θ when 64, 32, and 16 Hadamard codes are used without diverging waves. For 8 and 4 Hadamard codes, the signal energy spans 120θ when surfed on a diverging source, whereas the pressure from a plane wave source drops to less than 10% before spanning 40 ^θ in both cases.