Coherence beamforming and its applications to the difficult-to-image patient

Poor quality ultrasound images and inadequate or suboptimal visualization of imaging targets is a common problem in individuals that are overweight or obese. Acoustic reverberation is an incoherent noise source that is a common factor in overweight and obese individuals and is a significant contributor to the poor image quality. Specifically, diffuse acoustic reverberation is problematic because it appears similar to common tissue texture in ultrasound images, thereby exacerbating the inadequate and suboptimal visualization. We describe the coherence imaging technique called the short-lag spatial coherence (SLSC) beamformer and its related imaging methods as potential solutions to the inadequate and suboptimal visualization problem. The SLSC beamformer detects the spatial similarity of the backscattered ultrasound waves, with a greater emphasis on the spatial similarity at closely-spaced positions. Because diffuse reverberation is spatially incoherent in the wavefield, noise can be differentiated from tissue and other desired imaging targets. Applications of the SLSC beamformer to in vivo imaging and adaptations of the technique to other imaging modalities, including flow imaging, molecular ultrasound imaging, and photoacoustic imaging are reviewed. Although computationally more intensive than conventional delay-and-sum beamforming, we describe several techniques for fast computation of coherence, which enable real-time imaging. The challenges and criticisms of spatial coherence beamforming are reviewed, including the loss in phase information and the nonlinear behavior of the technique.