Use of Carr–Purcell pulse sequence with low refocusing flip angle to measure T 1 and T 2 in a single experiment Fabiana Diuk de Andrade a , Antonio Marchi Netto b , Luiz Alberto Colnago c,⇑ a Instituto de Química de São Carlos, Universidade de São Paulo, Av. Trabalhador São-carlense 400, São Carlos, SP 13560-970, Brazil b Instituto de Física de São Carlos, Universidade de São Paulo, Av. Trabalhador São-carlense 400, São Carlos, SP 13560-970, Brazil c Embrapa Instrumentação, Rua XV de Novembro 1452, São Carlos, SP 13560-970, Brazil article info Article history: Received 20 June 2011 Revised 24 October 2011 Available online 17 November 2011 Keywords: Carr–Purcell CWFP SSFP Relaxation time Time domain NMR abstract The Carr–Purcell pulse sequence, with low refocusing flip angle, produces echoes midway between refo- cusing pulses that decay to a minimum value dependent on T  2 . When the refocusing flip angle was p/2 (CP 90 ) and s > T  2 , the signal after the minimum value, increased to reach a steady-state free precession regime (SSFP), composed of a free induction decay signal after each pulse and an echo, before the next pulse. When s < T  2 , the signal increased from the minimum value to the steady-state regime with a time constant (T / )=2T 1 T 2 /(T 1 + T 2 ) , identical to the time constant observed in the SSFP sequence, known as the continuous wave free precession (CWFP). The steady-state amplitude obtained with M CP90 = M 0 T 2 / (T 1 + T 2 ) was identical to CWFP. Therefore, this sequence was named CP-CWFP because it is a Carr–Purcell sequence that produces results similar to the CWFP. However, CP-CWFP is a better sequence for measur- ing the longitudinal and transverse relaxation times in single scan, when the sample exhibits T 1  T 2 . Therefore, this sequence can be a useful method in time domain NMR and can be widely used in the agri- culture, food and petrochemical industries because those samples tend to have similar relaxation times in low magnetic fields. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction The Carr–Purcell pulse sequence (CP) is based on the spin-echo sequence introduced by Hahn in 1950 to measure the transverse relaxation time, T 2 [1]. The Hahn sequence uses two p/2 pulses separated by a time interval (s), where T  2  s < T 2 . Hahn had ob- served that this sequence was sensitive to molecular diffusion through the inhomogeneous magnetic field. To solve the effect of diffusion in the T 2 measurement, Carr and Purcell [2] introduced a pulse sequence that used a p/2 excitation pulse, followed by a train of p refocusing pulses using the same phase of the excitation pulse. The time interval between the refocusing pulses was dou- bled compared to time between the excitation and the first refo- cusing pulse. This sequence produced echoes with a maximum amplitude midway through the refocusing pulses. Each echo was dephased by 180° from the preceding echo. This sequence used a small s value between the refocusing pulses, which minimized the effect of diffusion in the echoes’ signals. However, Meiboom and Gill [3] observed that the adjustment of p refocusing pulses was critical and this problem was more pronounced for samples with long T 2 (liquid) values and using an inhomogeneous magnet. Thus, it was necessary to use short values for s and a large number of refocusing pulses to eliminate the effects of diffusion. A small deviation from the exact value of p was cumulative; thus, the error of the refocusing pulses increased with the number of pulses and introduced error in the T 2 measurements as a consequence. To solve this problem, Meiboom and Gill [3] proposed a modification to the CP sequence by introducing a 90° phase shift between the excitation and the refocusing pulses. This improvement made the sequence very robust and insensitive to error in the refocusing pulse. Therefore, the sequence proposed by Carr–Purcell and im- proved by Meiboom and Gill, known today as CPMG, is the stan- dard method to measure T 2 . This sequence is so robust that we have recently shown that it is possible to measure T 2 using very low refocusing flip angles, as low as p/4 [4]. The major advantage in using CPMG with low refocusing flip angles occurs when it is necessary to reduce the applied power, as in online measurement [4] or in fast imaging techniques, to reduce the power deposition in the samples. In this paper, the effect of low refocusing flip angles in the CP pulse sequence is shown. The use of p/2 refocusing pulses pro- duced signal similar to continuous wave free precession (CWFP), a special condition of the steady state free precession (SSFP) regime [6]. The sequence was named CP-CWFP because it is a CP sequence with p/2 pulses and produced results similar to CWFP. Therefore CP-CWFP sequence can have similar CWFP applications, e.g. be 1090-7807/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jmr.2011.11.004 ⇑ Corresponding author. Fax: +55 16 21072902. E-mail addresses: fabianadiuk@yahoo.com.br (F.D. de Andrade), nettomarchi@ gmail.com (A. Marchi Netto), colnago@cnpdia.embrapa.br (L.A. Colnago). Journal of Magnetic Resonance 214 (2012) 184–188 Contents lists available at SciVerse ScienceDirect Journal of Magnetic Resonance journal homepage: www.elsevier.com/locate/jmr