Simulation studies of filtered spatial compounding (FSC) and filtered frequency compounding (FFC) in synthetic transmit aperture (STA) imaging

Speckle patterns are formed by constructive and destructive interference of backscattered waves from non-resolvable scatterers. Speckles can result in a low speckle signal-to-noise ratio (sSNR) in the ultrasound images of even a uniform sample. Speckles also reduce the contrast-to-noise-ratio (CNR) and the detectability of lesions, especially for low contrast lesions. Moreover, undesired signals arising from off-axis targets can result in sidelobes and clutters which lead to even lower lesion CNR. Typically, the speckle SNR can be increased by compounding, either spatial compounding (SC) or frequency compounding (FC). Here we propose methods to implement a 2-dimentional (2-D) aperture domain filter in the SC and FC processes, which are referred to as filtered spatial compounding (FSC) and filtered frequency compounding (FFC), for synthetic transmit aperture (STA) imaging. Both FSC and FFC can provide more homogeneous speckle patterns with improved speckle SNR and lesion CNR. The aperture domain filter reduces the interference effect of the off-axis signals to further enhance lesion CNR. Consequently, the target detectabilities (lesion-signal-to-noise ratio (lSNR)) in both FSC and FFC are increased significantly, up to around 3 times, compared to that in the standard delay-and-sum (DAS) method.