A study of phased array transducer topology for superharmonic imaging

Since its introduction in the 90s, tissue 2nd harmonic imaginghas become the standard in medical ultrasound. Recently, superharmonic imaging(SHI) was introduced. It targets the combination of the 3rd till5th harmonics. SHI offers increased spatial resolution, lowersidelobes and less artifacts compared to 2nd harmonic imaging.However, a system for SHI has to deal with the lower energy content of thehigher harmonics. The broad bandwidth (6 dB 130%) required for SHI prompts foran unconventional phased array design. One of the solutions divides the transmitand receive parts into separate acoustic stacks. Such a design reduces thesurface area available for reception. Firstly, we investigate the blockwise andinterleaved distribution (topology) of the transmit and receive elements interms of beam characteristics. Secondly, we research the optimal ratio betweentransmit and receive elements to increase the area dedicated to receiving whileretaining a high quality beam. The latter was assessed using the grating lobe tomain beam ratio. The acoustic fields were computed using a combination ofnumerical methods. FIELD II was used to determine the locations and the peakpressure in the fundamental main and grating lobes. The pressure levels of theharmonics in the main and grating lobes were calculated using the INCS methodand Burger's equation, respectively. 3 cycle Gaussian apodized sine bursts wereused in transmission. The MI was 1.5 at the transmit frequency of 1.2 MHzoptimal 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 harmoniccomponents were calculated for the interleaved topologies only. The 1/2 till 1/7interleaved topologies provided enough dynamic range (40 dB) for SHI with 1/7maximizing the surface area for reception. This increases the SNR by 5 dB.

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