ANDRADE, F. D. de

COLNAGO, L. A.

MARCHI NETTO, A.

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rz" NUCLEARMAGNETICRESONANCE USERS MEETING3rdIBEROAMERICANNMRMEETINGMAY4" - OS••, 2009 - HOTEL DO FRADE, ANGRA DOS REIS,RJ,BRAZIL...... ,EXTENDEDABSTRACTSBOOKI .••••Nl JCLEAR MAGNETIC RESONANCE USERS MEETING I!3'· lBEROAMERlCAN NMR MEETING !EXTENDED ABSTRACTS BOOK ITr 89THEORETICAL ANAL YSIS OF CPMG DECAY MEASUREMENT WITH LOWREFOCUSING FLlP ANGLESFabiana Oiuk de Andrade'", Antonio Marchi Netto", Luiz Alberto Colnaqo?"lnstliute af Ghemistry af São Garfos - USP, "tnstitute af Physics af São Garfos - USP,cEMBRAPA Agriculturallnstrumentatian*fabianadiuk@iqsc.usp.brKeywords: CPMG; low refocusing flip angles; Bloch EquationThe Carr-Purcell-Meiboom-Gill (CPMG) sequence has used in many scientificand technological applications. Recently it has been used in on-line measurernents!".In this application, the CPMG pulse train has been applied for very long periods. In thiscase, 180° refocusing pulse can cause undesirable sample heating and equipmentoverload, mainly on probe and power amplifier, which can reduce their durability.Some measurements showed that 90° refocusing pulse can produce echoesand provide relaxation time T2 with accuracy. By using low refocusing f1ipangle (90°) itis possible to reduce the RMS power up to 75%. Thus, the purpose of this work wasthe theoretical study of refocusing pulse with angles lower than 180° and lhoinhomogeneous field effects of Boand B1 to measure T2.The theoretical signals were calculated by the Bloch equatíons+' and worocompared to experimental measurements. The experimenfàl CPMG data was oblainodfor deionizated water at 22 ± 1°C on SLK-100 Spin Lock spectrometer, model SL.IM.01with 0.23T permanent magnet. The sequence used n/2 and rt pulses of 5.8 I-lS and10.28 us, respectively; echo time of 9281-1sand 6000 echoes, acquisition time of 10.61-1sand four scan with repetition time of 15s. The parameters used in the simulations were:echo time of 928 us, relaxation time T1 of 2.74s and relaxation time T2 of 2.52s.The Bo inhomogeneous was simulated by assuming a Lorentzian distributionwith full width at half maximum (FWHM) of 250 and 10Hz to offset frequency discretevalues. To the B1 inhomogeneous was assuming a Gaussian distribution centered atthe refocusing flip angle (45°, 90°, 135° or 180~) ±10° with a full width of 5°.Figure 1 compares the simulated signals of CPMG with 90° refocusing pulse(CPMGgo) at FWHM of 250 Hz (A) and at FWHM of 10Hz (8). It shows thatinhomogeneity at FWHM of 250 Hz leads to a large oscillation in contrast with thesignal at 10Hz. T2 value is also affected in less homogeneous field (2.57s) than inmore homogeneous field (2.53s), which is eloser to the experimental value (2.52s).A) B)1.0T2 = 2.57 1.00.8 0.8:J -:J.e0.6.e0.6~ ~cn 'üjc: 0.4 c:Q) Q) 0.4C C0.2 0.20.0 0.0o 5 10 15 20 25 oT2 = 2.535 10 15 20 25Time (s) Time (s)Figure 1. Simulated CPMGgo signal, A) at FWHM of 250 Hz and B) at FWHM of 10Hz.!I AUREMN! MA Y 4~ . OS"', 2009, HOTEL DO FRADE,I ANGRA DOS REIS, RJ, BRAZILFigure 2A shows simulated CPMGgo signal for the same condition of figure 1A,considering 81 inhomogeneity and assuming a Gaussian distribution centered at 90±1 0°. In this Figure is demonstrated that 81 inhomogeneity does not affect T2comparing with the signal simulated without 81 inhomogeneity (Figure 1A) despite thereduction in signal oscillation.Figure 28 shows the experimental signal acquired frorn CPMGgo sequence,indicating also a higher T2 value than the measured by conventional CPMG180, in thesame magnet (FWHM of 250 Hz).A) B)1.0 1.0T2 = 2.570.8 0.8- -:J :J~ 0.6 ~ 0.6ç ç·üi ·üic 0.4 c 0.4Q) Q)c c0.2 0.20.0 0.0o 5 10 15 20 25 o 2T2 = 2.5910 12Time (s) Time (s)Figure 2. A) Simulated CPMGgo signal under effect of 81 inhomogeneity, 8)Experimental CPMGgo signal.Thoso results show that T2 value, with refocusing flip angles lower than 180°, isnlfuctuu by 80 inhomogeneity, but not by 81 inhomogeneity. Threugh this information itI posulblo dotermine a limit to use different f1ip angles depending on conditions of 80Iflho/l1og f"IêOUS field used, which it can reduce power by more than 75%.REFERENCE1. Prestes, R.A.; Colnago, L.A.; Forato, LA.; Vizzotto, L.; Novotny, E.H.; Carrilho E.Ana/ytica Chimica Acta. 2007, 596, 325-329.2. 8loch, F. Physical Review. 1946,70,460-474.FAPESP, EMBRAPA Agriculturallnstrumentation~+..194

Theoretical analysis of CPMG decay measurement with low refocusing flip angles.

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