Carl-Inge Colombo Nilsen

Beamspace adaptive beamforming for ultrasound imaging

Applying the Capon adaptive beamformer in medical ultrasound imaging results in enhanced resolution by improving the interference-suppressing capabilities of the array. This improvement comes at the expense of an increased computational complexity. We have investigated the application of a beamspace adaptive beamformer for medical ultrasound imaging, which can be used to achieve reduced computational complexity with performance comparable to that of the Capon beamformer. The idea behind beamspace beamforming is that, instead of using the spatial statistics of the elements in the array to differentiate between signals and interference, we use the spatial statistics of a set of orthogonal beams, which are formed in different directions. This represents a shift from element space to beamspace. Because the majority of interference in medical ultrasound imaging is constrained to a limited spatial interval due to the focused transmit beam, this latter space can be reduced to a dimension that is lower than that of element space. We show, using simulations and experimental data, that this dimension can be selected as low as 3 while still achieving performance comparable to its element space counterpart.

Wiener beamforming and the coherence factor in ultrasound imaging

The coherence factor (CF) is used for aberration correction and sidelobe suppression in ultrasound imaging. Unfortunately, it suffers from artifacts when the SNR is low. We show how the CF can be interpreted as an implementation of the Wiener postfilter for a delay-and-sum beamformer. In addition, we show that a minimum-variance, distortionless-response beamformer followed by CF weighting can be interpreted as an implementation of the Wiener beamformer. These interpretations provide us with a theoretical framework for analyzing and improving CF-based methods. We use this theory to develop more robust implementations for both the Wiener postfilter and beamformer. The performance of these implementations is shown on simulated and real data.

Implementing capon beamforming on a GPU for real-time cardiac ultrasound imaging

Capon beamforming is associated with a high computational complexity, which limits its use as a real-time method in many applications. In this paper, we present an implementation of the Capon beamformer that exhibits realtime performance when applied in a typical cardiac ultrasound imaging setting. To achieve this performance, we make use of the parallel processing power found in modern graphics processing units (GPUs), combined with beamspace processing to reduce the computational complexity as the number of array elements increases. For a three-dimensional beamspace, we show that processing rates supporting real-time cardiac ultrasound imaging are possible, meaning that images can be processed faster than the image acquisition rate for a wide range of parameters. Image quality is investigated in an in vivo cardiac data set. These results show that Capon beamforming is feasible for cardiac ultrasound imaging, providing images with improved lateral resolution both in element-space and beamspace.

P2B-13 Speckle Statistics in Adaptive Beamforming

We have examined the statistics of the speckle patterns in images formed using delay-and-sum and minimum variance beamforming. We show how the estimate of the spatial covariance matrix used in the latter beam- former affects the speckle patterns. By using purely spatial averaging in the estimate the speckle statistics are quite different from delay-and-sum: Areas that are apparently homogeneous in delay-and-sum images appear as a collection of point scatterers in the minimum variance images, reducing the overall brightness of the speckle. We show that temporal averaging reduces this effect without compromising the improved spatial resolution offered by the minimum variance beamformer. Results from both simulations and experimental RF data are shown.

Digital beamforming using a GPU

In this paper we investigate the use of GPUs as digital beamformers. We specify a parallel implementation of a beamformer in time and frequency domain and measure its performance. We also give examples of the processing limits of NVIDIA Geforce 8800 GPU with respect to application parameters: number of sensors, sampling frequency, bandwidth, and number of simultaneous beams. The results are compared to those of algorithms similarly implemented on a Intel Xeon CPU. We find that the GPU is able to process a larger amount of information than the CPU, and that it can be used as a digital beamformer for arrays with a large number of elements sampled at high rates. Exact results are given for the abovementioned application parameters.

Robust ultrasonic indoor positioning using transmitter arrays

As time-delay based ultrasound positioning often is noise sensitive at long ranges, the goal of the research reported here is to achieve sub-room ultrasound positioning with other methods. By combining a portable ultrasound receiver which measures signal strength with a transmitter array that sends steered, coded beams inside a room, the tag can determine which beam it is located in and carry out fine-positioning. The concept is demonstrated in an experiment using a 40 kHz system with 4–7 transmitter elements.

Applying Thomson's multitaper approach to reduce speckle in medical ultrasound imaging

To reduce the variance of speckle in coherent imaging systems, one must average images with different speckle realizations. Traditionally, these images have been formed by observing the target region from slightly different angles (spatial compounding) or by varying the involved temporal frequencies (frequency compounding). In this paper, we investigate a third option based on Thomsons multitaper approach to power spectrum estimation. The tapers are applied spatially, as array weights. Our investigations, based on both recorded ultrasound data and simulations, verify that the multitaper approach can be used for speckle reduction at a rate comparable to that of the more traditional method of spatial compounding. Because of the spectral concentration of the tapers, an added benefit is reduced side lobe levels, which can result in steeper edges and better definition of cyst-like structures.

Coherent plane-wave compounding and minimum variance beamforming

Achieving increased frame rate without compromising the image quality is desirable in medical ultrasound imaging. Coherent plane-wave compounding has recently been suggested as an approach to achieve this. This work proposes to generate coherent compound plane-wave images using a minimum variance adaptive beamformer. Through simulations of point scatterers and cyst phantoms, a threefold increase in frame rate is shown.

An Optimized GPU Implementation of the MVDR Beamformer for Active Sonar Imaging

The minimum variance distortionless response (MVDR) beamformer has recently been proposed as an attractive alternative to conventional beamformers in active sonar imaging. Unfortunately, it is very computationally complex because a spatial covariance matrix must be estimated and inverted for each image pixel. This may discourage its unnecessary use in sonar systems which are continuously being pushed to ever higher imaging ranges and resolutions. In this study, we show that for active sonar systems up to 32 channels, the computation time can be significantly reduced by performing arithmetic optimizations, and by implementing the MVDR beamformer on a graphics processing unit (GPU). We point out important hardware limitations for these devices, and assess the design in terms of how efficiently it is able to use the GPUs resources. On a quad-core Intel Xeon system with a high-end Nvidia GPU, our GPU implementation renders more than a million pixels per second (1 MP/s). Compared to our initial central processing unit (CPU) implementation, the optimizations described herein led to a speedup of more than two orders of magnitude, or an expected five to ten times improvement had the CPU received similar optimization effort. This throughput enables real-time processing of sonar data, and makes the MVDR a viable alternative to conventional methods in practical systems.

Distortion-free delta-sigma beamforming

We have analyzed a compact, digital delay- and-sum beamformer using 1-bit delta-sigma A/D converters, where dynamic delays are applied directly to the delta-sigma modulated bitstream. It has previously been shown that the introduction of this time-variant operation into the time-invariant delta-sigma demodulator distorts the output signal, and several methods for minimizing this effect have been suggested. We have described the effect using standard system terminology. Based on this description, a new method for removing the distortion without any significant increase in system complexity is introduced. Its advantages, performance-wise, are supported by theoretical assessments and simulations.