In the last 30 years, the contrast-to-noise ratio (CNR) has been used to estimate the contrast and lesion detectability in ultrasound images. Recent studies have shown that the CNR cannot be used with modern beamformers, as dynamic range alterations can produce arbitrarily high CNR values with no real effect on the probability of lesion detection. We generalize the definition of CNR based on the overlap area between two probability density functions. This generalized CNR (gCNR) is robust against dynamic range alterations; it can be applied to all kind of images, units, or scales; it provides a quantitative measure for contrast; and it has a simple statistical interpretation, i.e., the success rate that can be expected from an ideal observer at the task of separating pixels. We test gCNR on several state-of-the-art imaging algorithms and, in addition, on a trivial compression of the dynamic range. We observe that CNR varies greatly between the state-of-the-art methods, with improvements larger than 100%. We observe that trivial compression leads to a CNR improvement of over 200%. The proposed index, however, yields the same value for compressed and uncompressed images. The tested methods showed mismatched performance in terms of lesion detectability, with variations in gCNR ranging from -0.08 to +0.29. This new metric fixes a methodological flaw in the way we study contrast and allows us to assess the relevance of new imaging algorithms.

One of the most promising techniques for limiting complexity for real-time 3-D ultrasound systems is to use sparse 2-D layouts. For a given number of channels, optimization of performance is desirable to ensure high quality volume images. To find optimal layouts, several approaches have been followed with varying success. The most promising designs proposed are Vernier arrays, but also these suffer from high peaks in the sidelobe region compared with a dense array. In this work, we propose new methods based on the principles of suppression of grating lobes to form symmetric and non-symmetric regular sparse periodic and radially periodic designs. The proposed methods extend the concept of sparse periodic layouts by exploiting either an increased number of symmetry axes or radial symmetry. We also introduce two new strategies to form designs with nonoverlapping elements. The performance of the new layouts range from the performance of Vernier arrays to almost that of dense arrays. Our designs have simplicity in construction, flexibility in the number of active elements, and the possibility of trade off sidelobe peaks against sidelobe energy.

Many adaptive beamformers claim to produce images with increased contrast, a feature that could enable a better detection of lesions and anatomical structures. Contrast is often quantified using the contrast ratio (CR), and the contrast to noise ratio (CNR). The estimation of CR and CNR can be affected by dynamic range alterations (DRA), such as those produced by a trivial gray-level transformation. Thus, we can form the hypothesis that contrast improvements from adaptive beamformers can, partly, be due to dynamic range alterations. In this paper we confirm this hypothesis. We show evidence on the influence of DRA on the estimation of CR and CNR, and on the fact that several methods in the state-of-the-art do alter the dynamic range. To study this phenomenon, we propose a dynamic range test (DRT) to estimate the degree of DRA and we apply it to 7 beamforming methods. We show that CR improvements correlate with DRT with R2-adj=0.88 in simulated data and R2-adj=0.98 in experiments. We also show that DRA may lead to increased CNR values, under some circumstances. These results suggest that claims on lesion detectability, based on CR and CNR values, should be revised.

The filtered delay multiply and sum (F-DMAS) beamformer has recently been presented in the context of medical ultrasound image formation. This nonlinear beamformer produces images with improved contrast resolution and noise rejection when compared with the delay and sum (DAS) beamformer. In an attempt to better understand the origin of the improved image quality, this paper shows a theoretical study of the image amplitude statistics backed up by numerical simulations. The results show that the difference in image amplitude using the DAS or F-DMAS beamformers can be partly explained by the way signal coherence influences both beamformers. When using the F-DMAS compared with the DAS beamformer, the image amplitude is shown to be more dependent on the signal coherence. Experimental ultrasound images of a phantom confirm our findings.

To be able to describe more precisely the behavior of a real-time 3-D ultrasound system with either a dense array or various sparse designs, experimental data from a 2-D fully connected array prototype with 50/spl times/50 elements have been collected. The data have been processed off line to form synthetic aperture 3-D volume images. Simulated and experimental results are compared and show good correlation. The performance of the best sparse designs, all thinned to more than 50%, offer performances comparable to a dense array.

Sparse arrays offer a means for reducing the cost and complexity of beamforming systems. Most of the work in the literature has focused on sparse linear arrays with isotropic transducer elements, which simplifies analysis greatly. In this paper, we will focus on multibeam cylindrical arrays using highly directive elements for use in fishery applications, which requires a direction- ally independent imaging performance in the azimuth direction as well as beam steerability in the elevational direction. To populate such an array, we suggest a low periodicity in the azimuth direction of the array, which ensures a (near) directionally independent imaging performance in this direction. At the same time it reduces the complexity of the problem so that a suggested iterative method can find the optimal layout under the given constraints, within reasonable time. The optimality of the constrained solution is verified using a stochastic optimization procedure, with a "loosened" periodicity constraint. Simulations then show that the proposed layout, having low periodicity in the azimuth direction, has a reduced peak sidelobe level compared to the fully sampled array. All of the layouts have been required to support beam steering from -30deg to 0deg in elevation and in all 360deg in azimuth, without deterioration in performance.