Spectral analysis of demodulated ultrasound returns: detection of scatterer periodicity and application to tissue...

Ultrasound returns from tissue display variations in amplitude on several spatial scales. Although large-scale variations result from factors such as attenuation, variations on smaller scales are caused by tissue characteristics such as variations in scatterer spacing and reflectance. These small scale variations cause a corresponding variation in the amplitude of the ultrasound return. A simple and direct method for detecting and quantifying periodicity in these variations in the presence of attenuation is described. The radiofrequency ultrasound return is first demodulated by full-wave rectification. The normalized power spectrum of the demodulated return then yields an index that we call the relative Fourier energy. Both computer simulations and in vitro experiments were performed in order to study how relative Fourier energy performed in discriminating between periodic and random scatterer distributions. Computer simulations demonstrated significant differences between the returns from periodic and random scatterer distributions. Ultrasound returns from aortic tissue yielded a relative Fourier energy index that was significantly different between normal vs. atherosclerotic tissue (normal: 0.868 +/- 0.076, mean +/- s.d., fibrofatty plaque: 0.705 +/- 0.109, p < 0.01 vs. normal, calcified plaque: 0.753 +/- 0.078, p < 0.01 vs. normal). In contrast, no difference was found in comparisons of overall reflectance.