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 if echo signals and converting them to zero-mean envelope-detected signals, s; performing a singular spectrum analysis on the envelope-detected signals in a computing unit to obtain a periodic reconstructed signal s r , wherein the analyzed envelope-detected signals are a time signal, s, composed of N samples, and that a sliding window of M samples is used to estimate a correlation matrix A where M N, wherein A is determined by: A i , j = 1 N - M ⁢ ∑ k = 1 N - M ⁢ ⁢ s ⁡ ( k ) ⁢ s ⁡ ( k +  i - j  ) ; determining a P value indicative of the likelihood of seed presence based on the spectral power of s r ; determining a maximum amplitude of s r ; and using the maximum amplitude of s r to locate a brachytherapy seed in tissue.