Ultrasound speckle reduction using coded excitation, frequency compounding, and postprocessing despeckling filters

A technique for improvement of ultrasonic B-mode imaging that uses coded excitation, pulse compression, and frequency compounding was developed. A coded excitation and pulse compression technique known as resolution enhancement compression (REC) was used to enhance the bandwidth of an imaging system by a factor of two. This bandwidth was subdivided into smaller subbands through the speckle-reducing technique known as frequency compounding (REC-FC). Frequency compounded images were generated using various subband widths and then averaged to reduce speckle and to improve contrast while preserving spatial resolution, known as enhanced REC-FC (eREC-FC). In this study, further improvements in contrast and reduction in speckle were obtained by applying post-processing despeckling filters. The following post-processing despeckling filters were explored and analyzed in regard to contrast improvement, speckle reduction, and image feature preservation: median, Lee, homogeneous mask area, geometric, and speckle reducing anisotropic diffusion (SRAD). To quantify the performance of each filter, contrast-to-noise ratio was used. Data from thirty simulated phantoms and experimental data from a tissue-mimicking phantom were generated and filtered. Results demonstrated that post-processing despeckling filters coupled with the eREC-FC technique could improve the image by up to 563%, in terms of the contrast-to-noise ratio, when compared to conventional ultrasonic imaging.