Computer aided diagnostics of early stages of the breast cancer is one of the most challenging problems of the contemporary medical image processing. Computerized detection of the breast tumors from ultrasound images provides the way which helps the physicians to decide whether a certain solid tumor is benign or malignant. However, it is one of the most difficult types of images to assess. We propose a new method to improve the accuracy of the tumor detection based on phase portrait method combined with the multiresolution analysis. This approach is used as a pre-processing procedure followed by the generalized gradient vector flow field and detection by active contours (snakes). We analyze our approach with the synthetic images and a series of the real ultrasound breast tumor images and compare the results with the ground truth hand-drawn by the radiologists. Our numerical experiments show that the proposed method over performs the conventional generalized gradient vector field endowed with classical preprocessing such as the Gaussian smoothing, median filtering, etc.