Basic procedure for encoding/decoding spin transition frequencies. The MQ transition frequencies are first spatially encoded using a MQ-DANTE excitation scheme, which consists of N repetitions of the following: evolution under a MQ Hamiltonian, with , followed by free evolution for a time τ during which an encoding gradient, , is applied for a time τ g . In this case, MQ coherences are generated at certain spatial locations, in Eq. (4) , that depend upon the MQ transition frequency, ω jk . After the excitation period, the -magnetization is imaged by applying a pulse and acquiring in the presence of a readout gradient, G readout. The transition frequencies for the MQ coherences that were excited during the frequency encoding period are then determined from the locations where “dips” in the -magnetization are observed.
Application of the pulse sequence in Fig. 1 using a DANTE excitation block [N = 30, , , and τ g = 400 μs] on a water/acetone mixture in D2O acquired on a 500 MHz Bruker Avance spectrometer. The -acquire spectra, with [Fig. 2(b) ] and without [Fig. 2(a) ] a readout gradient [G readout = 0.12 G/cm] applied during acquisition are shown for reference. In Fig. 2(c) , τ = 1.54 ms , G enc. = 1 G/cm, and both acetone and water were excited up to three DANTE harmonics at the same spatial locations within the sample. In Fig. 2(d) , τ = 1.54 ms, and G enc. = 1.5 G/cm, which resulted in the excitation of up to five DANTE harmonics. In Fig. 2(e) , G enc. = 1 G/cm, and τ was changed to τ = 0.95 ms . As a result, the “dips” in the acetone magnetization were no longer centered about ν cs, acetone (unlike the water “dips,” which were still centered about ). In Fig. 2(e) , the same conditions as in Fig. 2(c) were used, but with the RF transmitter shifted by νoff = −200 Hz. This resulted in both the acetone and water resonances being shifted and excited in different parts of the sample.
Experimental results (black spectra) after application of the pulse sequence in Fig. 1 using both (left) a DANTE excitation [ , N = 30, ] and (right) a 2Q-DANTE excitation [ in Eq. (5) , with N = 100, Hz] on a trans-chloroacrylic acid sample in a 3 mm solvent matched Shigemi tube taken on a 300 MHz Bruker Avance spectrometer using two different encoding gradients, G enc. = 1.04 G/cm (top) and G enc. = 1.56 G/cm (bottom). In all cases, τ g = 0.5 ms, τ = 0.8 ms, G readout = 0.059 G/cm, and . The corresponding -acquire spectra (red) is shown for reference, along with numerical simulations (green, scaled to be comparable in size) of both the DANTE and MQ-DANTE excitation sequences. The upfield spike (*) in the observed spectra (black) was an artifact from using a Shigemi tube during acquisition in the presence of a gradient. With the choice of τ, the “dips” in the DANTE spectrum are shifted relative to the 1Q spectrum. For the 2Q-DANTE excitation, ν2Q = 0 Hz, which resulted in “dips” located directly above the 1Q transition frequencies (higher 2Q-DANTE harmonics were also observed).
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