Speckle reduction for ultrasonic imaging using frequency compounding and despeckling filters along with coded excitation and pulse compression

A method for improving the contrast-to-noise ratio (CNR) while maintaining the -6 dB axial resolution of ultrasonic B-mode images is proposed. The technique proposed is known as eREC-FC, which enhances a recently developed REC-FC technique. REC-FC is a combination of the coded excitation technique known as resolution enhancement compression (REC) and the speckle-reduction technique frequency compounding (FC). In REC-FC, image CNR is improved but at the expense of a reduction in axial resolution. However, by compounding various REC-FC images made from various subband widths, the tradeoff between axial resolution and CNR enhancement can be extended. Further improvements in CNR can be obtained by applying postprocessing despeckling filters to the eREC-FC B-mode images. The despeckling filters evaluated were the following: median, Lee, homogeneous mask area, geometric, and speckle-reducing anisotropic diffusion (SRAD). Simulations and experimental measurements were conducted with a single-element transducer (f/2.66) having a center frequency of 2.25MHz and a -3 dB bandwidth of 50%. In simulations and experiments, the eREC-FC technique resulted in the same axial resolution that would be typically observed with conventional excitation with a pulse. Moreover, increases in CNR of 348 were obtained in experiments when comparing eREC-FC with a Lee filter to conventional pulsing methods.