THEORETICAL SENSITIVITIES OF SWIFT AND THE IDEAL SEQUENCE (DELTA PULSE-ACQUIRE) FOR ULTRA-SHORT T2

1. THEORETICAL SENSITIVITIES OF SWIFT AND THE IDEAL SEQUENCE (DELTA PULSE-ACQUIRE) FOR ULTRA-SHORT T2 R. O’Connell, S. Moeller, C. Corum, D. Idiatullin, and M. Garwood Center for Magnetic Resonance Research, Department of Physics University of Minnesota, Minneapolis, MN, United States TR= 4.045 ms TR = 4.045 ms TR = 4.045 ms q q FID FID w1 w1 w1 Dw d-pulse sequence hard-pulse sequence SWIFT pulse sequence Instantaneous excitation followed by acquisition Finite duration excitation followed by acquisition Frequency swept pulse with simultaneous excitation and acquisition TA = 4 ms Tp < 45 ms TA = 4 ms Tp = 4 ms TA = 4 ms Ernst energy equation: Ernst angle equation: cos(qmax)= E1 E1= exp(-TR/T1) E2= exp(-TA/T2) M0: Magnetization at thermal equilibrium T1: longitudinal relaxation time T2: transverse relaxation time q: flip angle TR: repetition time TA: acquisition time NA: # of acquired points Nmax: Max # of points q = 900 Purpose: To compare the d-, hard-, and SWIFT pulse sequences for on- and off-resonance spins. Method: The NMR system was modeled by numerically solving the Bloch equations using the Runge-Kutta method. Then the results were compared to the Ernst equations. c a b d e f Figure 1: Flip angle (qmax) of maximum signal energy (S2max) for a given 1/T1,2=1/T1=1/T2 Figure 2: Relaxation rate constant (1/T1,2) atwhich maximum signal energy (S2max) occurs as function of flip angle (q). Figure 3: Energy (S2) contour plots for 90o excited a) d, b) hard, c) off-resonance hard, d) SWIFT, e) gapped SWIFT f) and off-resonance gapped SWIFT normalized to S2max of the d-pulse. Note in regions with T2 < T1 there is minimal difference between d-pulse-acquire, hard-pulse-acquire, and SWIFT. Gapping SWIFT (vs simultaneous RF transmission and reception) reduces S2 by a factor of 2 since acquisition duty cycle is reduced two-fold. Note the un-gapped SWIFT, d-pulse-acquire, and hard-pulse-acquire assume nearly 100% acquisition duty cycle. Also note that off-resonance excitation has little effect with SWIFT, while for hard-pulse-acquire the S2-dependence on T1 and T2 is affected. Experiment Conclusions SWIFT can be described by Ernst energy equations when TA is changed to TA/2 SWIFT has no adverse off-resonance effects Gapped SWIFT loses signal energy proportional to its duty cycle Hard pulses suffer signal loss due to relaxation and off-resonance effects A flip angle can be determined so that an object with given T1,T2 will have the greatest relative energy in the system, approximated by: Samples were prepared of SPIO with ~4 mM of Fe. The particles in each tube had different radii. Measurements were made using SWIFT with bandwidth = 125 kHz,Tp = 2 ms, and duty-cycle = 25%. on a 9.4 T magnet. These experiments confirm that the theoretical relationships (Ernst eqns) predict signal energy of SWIFT acquisitions. Figure 4: Normalized signal energy(S2/Sd2) vs relaxation rate constant (1/T1,2) at q = 200 Grants: This research was supported by NIH BTRC grant #P41-RR008079 References: [1] Idiyatullin et al., J. Magn. Reson., 181 (2006), p. 342-349 [2] Ernst et al. Oxford Sci. Pub., 1987, p. 153