Improving image contrast using coded excitation for ultrasonic imaging

A method for improving the contrast resolution 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, the speckle is reduced and the contrast 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 contrast enhancement can be extended. Simulations and experimental measurements were conducted with a single-element transducer (f/2.66) having a center frequency of 2.25 MHz and a -3-dB bandwidth of 50%. Measurements of tissue-mimicking targets of various contrast levels and diameters were imaged. The image quality metrics used to compare eREC-FC to conventional pulsing (CP), REC, and REC-FC were contrast-to-noise, and histogram overlap. Maximum and minimum increases in CNR of 189% and 88% were observed when comparing eREC-FC to CP for the 12 mm in diameter +6 dB and -3dB targets, respectively. Image histrogram analysis between the target and background regions suggested that improved lesion detectability was observed by using eREC-FC as the overlap between the two histograms was minimized. With eREC-FC technique the axial resolution was preserved (i.e., same axial resolution that would be typically observed with conventional excitation with a pulse).