The reductive dimerization of thiazolidine-4(R)-carboxylic acid sodium salt (I) with Na in liquid ammonia gives N,N-ethylene-1,2-diylbis(cysteine) (II), which is esterified with ethanol and dry HCl to the corresponding diethyl ester (III). Finally, this compound is treated with sodium pertechnetate and a reducing agent such as stannous salts, NaBH4 or Na2S2O4.