The time of acquisition for a conventional spin echo or gradient echo sequence is the product of the repetition time, phase encoding steps, and number of averages (TR x phase steps x NEX). For example, with a one second TR, 128 phase steps, and two averages we would get an acquisition time of about 1 x 128 x 2= 256 seconds or 4 minutes and 16 seconds. The actual time will be slightly longer. Fast spin echo and fast gradient echo sequences perform more than one phase encoding step per repetition time, resulting in reduce acquisition time. This is discussed further elsewhere.