Various methods are used in ultrasound beamforming to increase signal-to-noise ratio (SNR) and improve
spatial resolution. SNR is typically improved by exploiting coherence in the RF channel data, for example summing
channel data after applying focal delays in the delay-and-sum (DAS) beamformer, and summing channel data after
applying a per-channel matched filter for the spatial matched filter beamformer. Inverse filter methods are
capable of improving spatial resolution at the cost of SNR ,, or can trade resolution for SNR using a
regularization parameter, but in general are very computationally intensive due to the large RF data sets used. We
propose a post-processing method operating on post-summed but pre-envelope detected beamformed image data
that can improve the pixel SNR and spatial resolution of any beamformer with low computational cost. This is
achieved by forming a new pixel for each point in the image as a linear combination of the surrounding
beamformed pixels. The weights for each pixel are calculated in advance using a quadratically constrained least
squares method to reduce PSF energy outside the mainlobe and noise energy. Simulations indicate that this method
can increase cystic contrast by up to 20dB without any cost in SNR, and can increase pixel SNR can by up 16dB
without affecting contrast. Alternatively, simultaneous gains in contrast and SNR can be achieved. Experimental
results show smaller performance improvements yet validate the feasibility of this technique.
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  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].
Angular scatter offers a new source of tissue contrast and an opportunity for tissue characterization in ultrasound imaging. We have previously described the application of the translating apertures algorithm (TAA) to coherently acquire angular scatter data over a range of scattering angles. While this approach works well at the focus, it suffers from poor depth of field (DOF) due to a finite aperture size. Furthermore, application of the TAA with large focused apertures entails a tradeoff between spatial resolution and scattering angle resolution. While large multielement apertures improve spatial resolution, they encompass many permutations of transmit/receive element pairs. This results in the simultaneous interrogation of multiple scattering angles, limiting angular resolution. We propose a synthetic aperture imaging scheme that achieves both high spatial resolution and high angular resolution. In backscatter acquisition mode, we transmit successively from single transducer elements, while receiving on the same element. Other scattering angles are interrogated by successively transmitting and receiving on different single elements chosen with the appropriate spatial separation between them. Thus any given image is formed using only transmit/receive element pairs at a single separation. This synthetic aperture approach minimizes averaging across scattering angles, and yields excellent angular resolution. Likewise, synthetic aperture methods allow us to build large effective apertures to maintain a high spatial resolution. Synthetic dynamic focusing and dynamic apodization are applied to further improve spatial resolution and DOF. We present simulation results and experimental results obtained using a GE Logiq 700MR system modified to obtain synthetic aperture TAA data. Images of wire targets exhibit high DOF and spatial resolution. We also present a novel approach for combining angular scatter data to effectively reduce grating lobes. With this approach we have been able to push the grating lobes below -50 dB in simulation and effectively eliminate their presence in the experimental wire target images.
Spatial compounding has been used for years to reduce speckle in ultrasonic images and to resolve anatomical features hidden behind the grainy appearance of speckle. Adaptive imaging restores image contrast and resolution by compensating for beamforming errors caused by tissue-induced phase errors. Spatial compounding represents a form of incoherent imaging, whereas adaptive imaging attempts to maintain a coherent, diffraction-limited aperture in the presence of aberration.
Using a Siemens Antares scanner, we acquired single channel RF data on a commercially available 1-D probe. Individual channel RF data was acquired on a cyst phantom in the presence of a near field electronic phase screen. Simulated data was also acquired for both a 1-D and a custom built 8x96, 1.75-D probe (Tetrad Corp.). The data was compounded using a receive spatial compounding algorithm; a widely used algorithm because it takes advantage of parallel beamforming to avoid reductions in frame rate. Phase correction was also performed by using a least mean squares algorithm to estimate the arrival time errors.
We present simulation and experimental data comparing the performance of spatial compounding to phase correction in contrast and resolution tasks. We evaluate spatial compounding and phase correction, and combinations of the two methods, under varying aperture sizes, aperture overlaps, and aberrator strength to examine the optimum configuration and conditions in which spatial compounding will provide a similar or better result than adaptive imaging. We find that, in general, phase correction is hindered at high aberration strengths and spatial frequencies, whereas spatial compounding is helped by these aberrators.