Study on a Strategy to Improve the Image Quality and Imaging Depth for Novel Synthetic Aperture Schemes: An Experimental Investigation.

Imaging depth remains a restriction for Synthetic Aperture (SA) approaches, even though SA techniques have been shown to overcome some of the drawbacks of Conventional Focused Beamforming (CFB) technique. This limitation is attributed to lesser energy per transmit in SA techniques compared to that of CFB technique. Therefore, in this paper, a systematic investigation is done to evaluate the improvement in imaging depth and image quality of B-mode ultrasound images in the case of SA technique using PZT transducer by boosting the input voltage to the transducer, while measuring the acoustic exposure parameters recommended in international standards. Note-while acoustic exposure parameters like Mechanical and Thermal index (MI and TI, respectively) are well-studied and reported for CFB techniques, the same is not the case with novel SA techniques. For analysis, the image quality metrics of Contrast Ratio (CR), Contrast to Noise Ratio (CNR) and generalized Contrast to Noise Ratio (gCNR) are employed for the SA techniques of Synthetic Transmit Aperture (STA) and Diverging Beam Synthetic Aperture Technique (DB-SAT). The results show that, when the input voltage is increased from 45 to 70 V in PZT transducer, the following improvements in image quality parameters were observed: CR by 15.6% and 21.25%, CNR by 21.39% and 36.23% and gCNR by 14.1% and 21.5% for STA and DB-SAT techniques, respectively. Results also suggest that the considerable imaging depth improvement is achieved for the incremental input voltage till 60 V, with the highest of 7.5% depth improvement at 60 V, above which the tradeoff between the gain in imaging depth and acoustic safety index becomes critical to consider. Recently, transducers having lead magnesium nitrate lead titanate (PMN-PT) piezoelectric single crystals have shown to produce ultrasound images with higher image quality at greater depths, irrespective of the transmit schemes. The same concept of relatively higher output transmitting energy by cost expensive PMN-PT single crystal transducers for the same applied voltage, compared to the traditional lead zirconate titanate (PZT) transducers, explains the enhanced imaging depth. This study is probably the first one that reports the comparison between PMN-PT and PZT transducer, not only in terms of imaging depth but also reporting them in the context of acoustic exposure safety limits in terms of MI and TI. The findings of this study may help system designers to minimize the trade-off between cost and image quality.