Optimally-weighted non-linear beamformer for conventional focused beam ultrasound imaging systems

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.

F-number optimization for synthetic aperture delay-multiply-and-sum reconstruction

The choices of transmit and receive f-numbers impact both ultrasound image contrast and spatial resolution. Although previous studies have evaluated the impact of receive f-number in delay-and-sum (DAS) plane wave imaging, there has not been a systematic study of f-numbers in DAS or delay-multiply-and-sum (DMAS) synthetic aperture (SA) imaging. In this study, we measured the impact on main lobe to side lobe energy ratio (MSER), generalized contrast-to-noise ratio (gCNR), and spatial resolution when varying receive and transmit f-numbers from 1 to 5 in 0.2 increments in DAS and DMAS reconstructed SA images. A wire target in a water tank and a standard imaging phantom were used to measure these metrics. From the water tank wire target images, higher MSER values were achieved with middle-range transmit f-numbers (2-4) and high receive f-numbers (>4) for both DAS and DMAS. From the phantom contrast target images, DAS produced images with high gCNR when using high transmit f-numbers (>4) and high receive f-numbers (>4). This came at the cost of reduced spatial resolution. DMAS produced images with high gCNR when using low transmit f-numbers (<3) and high receive f-numbers (>4). DMAS was not found to have as severe of a tradeoff in spatial resolution when seeking maximum gCNR. However, gCNR was typically lower for DMAS than DAS. For both DAS and DMAS, point target images had high spatial resolution when using low receive f-numbers (<2). Spatial resolution was typically higher for DMAS than DAS. Hanning apodization was found to produce similar trends as those found with rectangular apodization. These findings give insight on the behaviors of DAS and DMAS SA reconstruction algorithms and could be used to guide f-number selection.

Novel spatio-temporal non-linear beamformers for sparse synthetic aperture ultrasound imaging

The development of two modified non-linear beamformers, Spatio-Temporal Delay Multiply and Sum (ST-DMAS) and Spatio-Temporal Delay Euclidian-Weighted Multiply and Sum (ST-DewMAS) is reported in this paper. A sparse-transmit scheme (with only 8 transmits) on Synthetic Transmit Aperture technique (sparse STA) was chosen to evaluate the beamformers ability to generate the high-resolution Ultrasound image. These methods allow for obtaining superior-quality imaging at enhanced frame rates. The different beamformers of ST-DewMAS, ST-DMAS, Filtered Delay Multiply and Sum (F-DMAS), and Delay and Sum (DAS), were compared in terms of the Axial and Lateral Resolutions, AR and LR, respectively, Contrast-to-Noise Ratio (CNR), Contrast Ratio (CR), and Generalized CNR (GCNR). Experimental results demonstrate that the developed ST-DMAS and ST-DewMAS reconstruction on sparse STA technique resulted in better quality images compared to those obtained using DAS and F-DMAS. Specifically, the metrics of AR, LR CR, CNR, and GCNR showed improvements of more than 25% (for ST-DMAS) and 40 % (for ST-DewMAS) over those from DAS and F-DMAS beamformed images, respectively. Thus, the results demonstrate that the frame rate and image quality of an US system can both be enhanced by ST-DewMAS compared to the beamformers of F-DMAS and DAS.

Correlation-based modified delay-multiply-and-sum beamforming applied to medical ultrasound imaging

BACKGROUND AND OBJECTIVE

Recently, the Filtered Delay-Multiply-and-Sum (F-DMAS) beamformer was successfully applied to Ultrasound Imaging (UI), improving the image quality compared to the conventional data-independent Delay-and-Sum (DAS) beamformer. However, its reconstructed images lead to restricted resolution, contrast, and dark regions in the speckle. Various beamformers based on F-DMAS were proposed to mitigate these issues; some improved resolution and contrast at the expense of more dark regions; others reduced the dark points with lower contrast than the F-DMAS beamformer. This study aims to propose a novel beamformer, improving resolution and contrast while reducing dark points in the speckle.

METHODS

This study proposes a modified version of the F-DMAS beamformer, using two modifications to compensate for the aforesaid trade-off. Firstly, coupled signals' Correlation Coefficient (CC) was calculated and compared to a threshold value. The multiplications were applied only to the high-correlated (those whose CC is higher than the threshold value) signals. Secondly, a new Modified Coherence Factor (MCF) was applied to the high-correlated signals. Then, these two new beamformers were combined to reach a novel beamformer entitled "Modified DMAS (MDMAS)."

RESULTS

The performance of MDMAS was evaluated using simulating Point-Spread-Function, Cyst phantom, the experimental geabr dataset, and an in vivo dataset. Moreover, we evaluated the performance of the MDMAS beamformer quantitatively. Full-width-half-maximum (FWHM), contrast-ratio (CR), contrast-to-noise-ratio (CNR), speckle signal-to-noise-ratio (sSNR), and generalized-CNR (gCNR) were assessed.

CONCLUSIONS

This paper modified the conventional F-DMAS beamformer by adaptively multiplying signals. Then, CF was implemented on high correlated signals (MCF) and combined with the adaptive beamformer to compensate for the poor contrast. Results highlight that the MDMAS beamformer outperforms F-DMAS in terms of resolution and contrast without compromising the speckle from the dark region artifact.