1. A method for classifying ultrasound images, the method comprising: providing a plurality of ultrasound images, each ultrasound image having a corresponding RF spectrum; randomly selecting a first subset of frequency bins from the RF spectrum; training a first classifier to classify objects using the first subset of frequency bins; checking accuracy of the first classifier in classifying objects; randomly selecting a second subset of frequency bins from the RF spectrum; training a second classifier to classify objects using the second subset of frequency bins; weighing an input to the second classifier to give greater weight to objects that the first classifier misclassifies than to objects that the first classifier correctly classifies; combining the first and second classifiers into a combined classifier; classifying an object by applying the combined classifier to the object; and displaying a classified image based on the classification by the combined classifier.