Visual and quantitative evaluation of selected image combination schemes in ultrasound spatial compound scanning

Multi-angle spatial compound images are normally generated by averaging the recorded single-angle images (SAIs). To exploit possible advantages associated with alternative combination schemes, this paper investigates both the effect of number of angles (N_θ) as well as operator (mean, median, mean-excluding-maximum (mem), root-mean-square (rms), geometric mean and maximum) on image quality (tissue delineation and artifacts), speckle signal-to-noise ratio (SNR_s) and contrast. The evaluation is based on in vitro SAI (±21° in steps of Δθ=7°) of formalin fixed porcine tissue containing adipose, connective and muscular tissue. Image quality increased with number of angles up to ±14° after which the improvements became debatable. The mem and median operators, which try to render the images more quantitatively correct by suppressing strong echoes from specular reflectors, provide some improvement in this regard. When combining the SAI with the mean operator, the SNR_s increases-in general-with N_θ. For N_θ=2, the SNR_s increases with Δθ as expected. When N_θ=7, the highest SNR_s is obtained for the mem, rms, and geometric mean operators, while the lowest SNR_s is obtained for the maximum operator. When comparing SNR_s for adipose and fibrous tissue, the level is close to 1.91 for adipose tissue but only 1.7 for fibrous tissue which contain relatively few organized scattering structures.