Optimization of frequency dependent receive apodization

2007 IEEE Ultrasonics Symposium Proceedings

2007

The delay-and-sum (DAS) beamformer, utilized in current state of the art ultrasound imaging systems, applies time delays on each receive channel to focus the returning echoes and weights the focused RF data on each receive channel prior to beamsummation. Apodization functions like the Hamming and Nuttall windows reduce the sidelobe levels the DAS beamformer's point spread function (PSF).As a result, apodization greatly impacts the contrast and resolution of the final output image. We propose a beamformer architecture for broadband imaging systems that uses unique finite impulse response (FIR) filters on each receive channel. The filter weights are constructed to maximize the contrast resolution of the system's spatial response. We present simulation results showing that FIR filters of modest tap lengths (3-7) can yield marked improvement in image contrast and point resolution. This increase in contrast resolution comes at the expense of a decrease in beamformer sensitivity. We investigate the effects of magnitude and phase aberration on the FIR beamformer by simulating near field thin phase screen aberrators.Specifically we examine the performance of the FIR beamformer in the presence of magnitude aberration characterized by an a priori root-meansquare (RMS) strength of 5 dB and a full-width at half-maximum (FWHM) correlation length of 2.1 mm as well as in the presence of phase aberration characterized by an a priori RMS strength of 28 ns and a FWHM correlation length of 3.6 mm. Contrast resolution results show that the aberrated FIR beamformer outperforms the unaberrated DAS beamformer by almost 10 dB. We believe that our array pattern synthesis technique and beamformer architecture have the potential to significantly improve the contrast resolution performance of broadband imaging systems.

Section: We Present Simulation Results Showing That Fir Filters Of Momentioning

confidence: 99%

2C-6 Optimal Contrast Resolution Beamforming

2007 IEEE Ultrasonics Symposium Proceedings

2007

“…Recently, significant effort has been made to apply sophisticated algorithms to the reconstruction of medical ultrasound images. The methods range from using adaptive beamforming algorithms [10, 33], deconvolution techniques [34–37], constrained and optimized receive apodization [2, 17, 38], to spatially matched filtered [3] beamformers. Other groups have investigated using statistical unbiased estimators [3, 11, 39] for ultrasound image reconstruction.…”

Section: Discussionmentioning

confidence: 99%

Robust finite impulse response beamforming applied to medical ultrasound

IEEE Trans. Ultrason., Ferroelect., Freq. Contr.

2009

We previously described a beamformer architecture that replaces the single apodization weights on each receive channel with channel-unique finite impulse response (FIR) filters. The filter weights are designed to optimize the contrast resolution performance of the imaging system. While the FIR beamformer offers significant gains in contrast resolution, the beamformer suffers from low sensitivity and its performance rapidly degrades in the presence of noise. In this paper a new method is presented to improve the robustness of the FIR beamformer to electronic noise as well as variation or uncertainty in the array response. A method is also described, which controls the sidelobe levels of the FIR beamformer's spatial response by applying an arbitrary weighting function in the filter design algorithm.The robust FIR beamformer is analyzed using a generalized cystic resolution metric that quantifies a beamformer's clinical imaging performance as a function of cyst size and channel input signal-tonoise ratio (SNR). Fundamental performance limits are compared between two robust FIR beamformers (the dynamic focus FIR (DF-FIR) beamformer and the group focus FIR (GF-FIR) beamformer), the conventional delay-and-sum (DAS) beamformer, and the spatial matched filter (SMF) beamformer. Results from this study show that the new DF-and GF-FIR beamformers are more robust to electronic noise compared to the optimal contrast resolution FIR beamformer. Furthermore, the added robustness only comes with a slight loss in cystic resolution. Results from the generalized cystic resolution metric show that a 9-tap robust FIR beamformer outperforms the SMF and DAS beamformer until receive channel input SNR drops below −5 dB; whereas, the 9-tap optimal contrast resolution beamformer's performance deteriorates around 50 dB SNR. The effects of moderate phase aberrations, characterized by an a priori root-mean-square strength of 28 ns and an a priori full-width at half-maximum correlation length of 3.6 mm, are investigated on the robust FIR beamformers.Full sets of robust FIR beamformer filter weights are constructed using an in silico model of the Ultrasonix Sonix RP scanner and the L14-5/38mm probe. Using the derived weights a series of simulated point target and anechoic cyst B-mode images are generated to further investigate the potential increases in contrast resolution when using the robust FIR beamformers. Under the investigated conditions, the 7-tap optimal contrast resolution beamformer and the 7-tap robust beamformer with added SNR constraint increase lesion detectability by 247% and 137% compared to the conventional DAS beamformer, respectively. Finally experimental phantom and in vivo images are produced using this novel receive architecture. The simulated and experimental images clearly show a reduction in clutter and an increase in contrast resolution compared to the conventionally beamformed images. This novel receive beamformer can be applied to any conventional ultrasound system where the system response is reasonably wel...

“…Schwann et al suggested a similar type of architecture using linear phase FIR filters and also discussed the calculation of frequency dependent optimal receive apodization profiles. Their simulation results showed contrast improvements, however, their multiple objective formulation required iterative procedures and made it difficult to determine an "optimal" apodization profile [11]. Our formulation on the other hand requires no iterations and produces optimal apodization profiles in a least squares sense that maximize cystic resolution.…”

Section: Discussuionmentioning

confidence: 97%

Receive channel FIR filters for enhanced contrast in medical ultrasound

Medical Imaging 2007: Ultrasonic Imaging and Signal Processing

2007

Aperture weighting functions are critical design parameters in the development of ultrasound systems because beam characteristics determine the contrast and point resolution of the final image. In previous work by our group, we developed a general apodization design method that optimizes a broadband imaging system's contrast resolution performance [1, 2]. In that algorithm we used constrained least squares (CLS) techniques and a linear algebra formulation of the system point spread function (PSF) as a function of the scalar aperture weights. In this work we replace the receive aperture weights with individual channel finite impulse response (FIR) filters to produce PSFs with narrower mainlobe widths and lower sidelobe levels compared to PSFs produced with conventional apodization functions. Our approach minimizes the energy of the PSF outside a defined boundary while imposing a quadratic constraint on the energy of the PSF inside the boundary. We present simulation results showing that FIR filters of modest tap lengths (3-7) can yield marked improvement in image contrast and point resolution. Specifically we show results that 7-tap FIR filters can reduce sidelobe and grating lobe energy by 30dB and improve cystic contrast [3] by as much as 20dB compared to conventional apodization profiles.We also show experimental results where multi-tap FIR filters decrease sidelobe energy in the resulting 2D PSF and maintain a narrow mainlobe. Our algorithm has the potential to significantly improve ultrasound beamforming in any application where the system response is well characterized. Furthermore, this algorithm can be used to increase contrast and resolution in novel receive only beamforming systems [4, 5].