Fundamental studies on contrast resolution of ultrasound B-mode images

grantor: University of TorontoThe primary objective of this thesis is to study the factors that affect the contrast resolution of ultrasound B-mode images. A computer simulation model of B-mode image formation has been developed and validated. The model simulates beam profiles from linear, 1.5-D and curvilinear arrays. An initial study was performed to compare the accuracy and computational efficiency of different methods that have been used to calculate the field profile from linear arrays. It was concluded that the far-field approximation for the impulse response method yields the best trade-off between accuracy and efficiency. The model was used to investigate the effects of transducer design, and tissue parameters on the image characteristics, specifically, texture and contrast resolution. It was shown that the scatterer distribution, as well as the transducer geometry, determines the textural appearance and the contrast of an image. It was also demonstrated that post-processing schemes can significantly alter the image appearance by changing the mean signal levels and speckle statistics. The simulation model can be used to predict optimum values for the parameters used in post-processing algorithms. Different measures of contrast resolution were examined, and it was concluded that the contrast-to-speckle ratio is the most accurate measure in defining an imaging system's ability to detect low-contrast targets in background speckle noise. In order to illustrate the clinical application of the model, a simple, approximate model of a carotid artery wall with fatty plaque was developed and a preliminary study was performed to analyze the B-mode image of this geometry. The simulated image indicates that the wall region is more echogenic than the plaque region as is the case in clinical images. The image also reveals sharp reflection signals at the wall-plaque, and plaque-lumen interfaces. It was shown that the signal levels at these interfaces depend on the backscattering coefficients and the distributions of the scatterers in the neighbouring media.Ph.D