1. A method of generating a fused ultrasonic image, comprising: extracting an anatomical location from a stored ultrasonic image by at least: determining a high threshold value and a low threshold value; obtaining a weighting factor according to the high threshold value and the low threshold value, wherein (i) if the pixel value is larger than or equal to the high threshold value, the weighting factor is 1, (ii) if the pixel value is smaller than or equal to the low threshold value, the weighting factor is 0, and (iii) if the pixel value is between the high threshold value and the low threshold value, the weighting factor is between 0 and 1 and is positively correlated to the pixel value; and weighting a pixel value of the stored ultrasonic image using said weighting factor; and generating the fused ultrasonic image according to the anatomical location and a currently obtained ultrasonic image.