Diffraction-based tissue characterization with ultrasound

Abstract

Although ultrasound has been used in medical imaging because of its nonionizing nature and ability to depict soft tissue structures, it also promises to provide noninvasive means of tissue characterization by signal amplitude quantitation, frequency dependent absorption measurement, and application of diffraction concepts. Diffraction-based methods provide ways of determining scatterer distribution within a sample volume on a scale corresponding to the wavelength. The underlying concept is wave interference resulting in selective reinforcement or cancellation of signals depending on wavelength, scatterer spacing, and geometry as in Bragg diffraction. By sweeping frequency or varying scattering angle, data analogous to x-ray diffraction is obtained for inference of scatterer size, or spacing from Fourier analysis. Scattered ultrasound intensity from regular arrays of cylinders has been measured as a function of frequency. Data reduction yields spacings that correspond well with actual values. Measurements of scattering as a function of angle from small particles have produced results which are in qualitative agreement with theoretical predictions. Preliminary in-vitro studies of scattering from human liver indicate the importance of off-axis scattering for tissue characterization. The studies show the promise of the approach but indicate that more work is required to develop diffraction concepts for ultrasonic characterization of tissue.