A study of phased array transducer topology for superharmonic imaging

Since its introduction in the 90s, tissue 2nd harmonic imaging has become the standard in medical ultrasound. Recently, superharmonic imaging (SHI) was introduced. It targets the combination of the 3rd till 5th harmonics. SHI offers increased spatial resolution, lower sidelobes and less artifacts compared to 2nd harmonic imaging. However, a system for SHI has to deal with the lower energy content of the higher harmonics. The broad bandwidth (−6 dB > 130%) required for SHI prompts for an unconventional phased array design. One of the solutions divides the transmit and receive parts into separate acoustic stacks. Such a design reduces the surface area available for reception. Firstly, we investigate the blockwise and interleaved distribution (topology) of the transmit and receive elements in terms of beam characteristics. Secondly, we research the optimal ratio between transmit and receive elements to increase the area dedicated to receiving while retaining a high quality beam. The latter was assessed using the grating lobe to main beam ratio. The acoustic fields were computed using a combination of numerical methods. FIELD II was used to determine the locations and the peak pressure in the fundamental main and grating lobes. The pressure levels of the harmonics in the main and grating lobes were calculated using the INCS method and Burger's equation, respectively. 3 cycle Gaussian apodized sine bursts were used in transmission. The MI was 1.5 at the transmit frequency of 1.2 MHz — optimal for cardiac SHI. The interleaved topology produces the best defined beam (straight and with low sidelobe levels) compared to the blockwise topologies. Consequently, the main to grating lobe ratios for the different harmonic components were calculated for the interleaved topologies only. The 1/2 till 1/7 interleaved topologies provided enough dynamic range (40 dB) for SHI with 1/7 maximizing the surface area for reception. This increases the SNR by 5 dB.