Computational and Experimental Modeling of Speckle for Diagnostic Ultrasound Imaging

Abstract

Speckle, named such after a similar effect appearing in laser optics, has been a recognized phenomenon in ultrasonic imaging since the late twentieth century. Speckle is best described as the noise that seems to obscure b-mode images obtained with ultrasonics. In the past, it has been theorized that ultrasonic speckle arises due to constructive and destructive interference at the wavefront scattered from the object of interest being imaged. This type of interference is thought to be caused by scatterers within a sample that are within the regime of Rayleigh scattering, approximately, which is when scattering particles are much smaller than the wavelength at use.

This project began by building upon a mathematical model based in impulse response theory and coherent interference theory, which first introduced by Foster et al. in the early eighties. We verified the legitimacy of this model by constructing a computational simulation using scattered and incident pressure field, building off FIELD II. We simulated ultrasonic scans in MATLAB using a single element 5 MHz transducer and a computational phantom containing seven point scatterers. The simulated data was then processed to produce a B-mode image, which we compared to our experimental data, taken using an agar phantom containing metal or glass beads ranging in size from 3.45mm to 0.5mm to simulate scatterers and a single element 5 MHz piezoelectric transducer.

Using agar phantoms containing a single row of seven scatterers of a size above the Rayleigh scattering limit, we successfully obtained b-mode images by taking RF data and processing through MATLAB. The scatterers were uniform in size and shape. We conducted pulse echo experiments on phantoms containing scatterers ranging from 3.45mm-2.85mm, 1.03mm-1.0mm and 0.75-0.5mm. We then compared these images to simulated b-mode images produced in MATLAB. With these preliminary results, we found that these simulated and experimental images show high agreement, particularly as the experimental scatterers decreased in size and approached the Rayleigh scattering limit.