The spin-echo effect was discovered by Erwin Hahn when he applied two successive 90° pulses separated by short time period, but detected a signal, the echo, when no pulse was applied. This phenomenon of spin echo was explained by Erwin Hahn in his 1950 paper, and further developed by Carr and Purcell who pointed out the advantages of using a 180° refocusing pulse for the second pulse. The pulse sequence may be better understood by breaking it down into the following steps: