Variation in measured transverse relaxation in tissue resulting from spin locking with the CPMG sequence

It is shown both theoretically and experimentally that the Carr-Purcell-Meiboom-Gill (CPMG) sequence is equivalent to a spin-locking sequence in the limit of short interpulse spacing (tcp). The increase apparent in the CPMG relaxation time observed in biological tissue as tcp decreases is explained in terms of spin locking. At an effective locking field frequency f 1 = 1 2 t cp the transverse relaxation time ( T 2 † ) measured with the CPMG sequence is indistinguishable from the rotating-frame spin-lattice relaxation time (T1ϱ). The origin of the variation in measured relaxation times in tissue is discussed in terms of low-frequency dipolar interactions of water.