Carr-Purcell sequences with composite pulses

We present novel Carr-Purcell-like sequences using composite pulses that ex hibit improved performance in strongly inhomogeneous fields. The sequences are designed to retain the intrinsic error correction of the standard Carr- Purcell-Meiboom-Gill (CPMG) sequence. This is achieved by matching the exci tation pulse with the refocusing cycle such that the initial transverse mag netization lies along the axis (n) over cap (B) characterizing the overall rotation of the refocusing cycle. Such sequences are suitable for relaxatio n measurements. It is shown that in sufficiently inhomogeneous fields, the echo amplitudes have an initial transient modulation that is limited to the first few echoes and then decay with the intrinsic relaxation time of the sample. We show different examples of such sequences that are constructed f rom simple composite pulses. Sequences of the form 90(0)degrees-(90(90 degr ees-theta /2)-theta (180-theta /2)-(90 degrees)(90-theta /2))(n) with theta approximate to 90 degrees and 270 degrees genera0 ate signal over a bandwi dth larger than that of the conventional CPMG sequence, resulting in an imp roved signal-to-noise ratio in inhomogeneous fields. The new sequence 127(x ,y)degrees-(127(x)degrees -127(-x)degrees)(n) only excites signal off-reson ance with a spectrum that is bimodal, peaking at Deltaw(0) = +/- w(1). Depe nding on the phase and exact timing to the first pulse, symmetric or antisy mmetric excitation is obtained. We also demonstrate several new sequences w ith improved dependence on the RIF field strength. The sequence (22.5(67,5) degrees -90 degrees-(22.5))- depends on B-1, allowing coarse B-1 imaging in a one-dimensional experiment. (C) 2001 Academic Press.