1. A method for detecting brachytherapy seeds and determining their location in tissue, comprising: imaging a brachytherapy seed with an ultrasonic transducer to obtain radio-frequency (rf) echo signals; digitizing the rf echo signals and converting them to zero-mean envelope-detected signals, s; performing a singular spectrum analysis on the envelope-detected signals in the computing unit to determine a P value for the spectral power of a predetermined segment of the echo signals; and combining information from a previously determined brachytherapy seed signature profile with the P value determination to locate the brachytherapy seed in tissue.