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Enhanced nonlinear magnetic resonance signals via square wave dipolar fields
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Image of FIG. 1.
FIG. 1.

(a) Standard iDQC-CRAZED pulse sequence. The first gradient pulse is applied to modulate the transverse magnetization. The mixing pulse transfers part of the modulation along the longitudinal axis to create the dipolar field. A second gradient pulse, twice as strong, coordinates with the first one to refocus only coherences. (b) ZEBRA sequence. A series of slice selective off-resonance inversion pulses are applied to the longitudinal magnetization to create the characteristic stripes pattern. The excitation pulse transfers this modulation onto the transverse plane. The mixing pulse transfers this modulation back onto the longitudinal axis to create a striped dipolar field. The evolution of the magnetization under the effect of the longitudinal dipolar field refocuses all the iMQC orders.

Image of FIG. 2.
FIG. 2.

Acquisition window from the striped experiment on a water sample. The acquisition window shows the echoes from double and zero-quantum coherences. In this case, a relatively long was used to separate the echoes in time. A spoiler gradient after the striping module and a pair of crush gradients right after the mixing pulse and before the acquisition window were used to avoid any radiation damping effects. A long TR of was used to avoid the refocusing of any stimulated echoes.

Image of FIG. 3.
FIG. 3.

A two dimensional spectrum using the sequence in Fig. 1(b) for a water and acetone solution on a Bruker Biospec MRI. The spectrum shows various iDQC and iZQC peaks between the two molecules. The characteristic iZQC and iDQC frequencies along the F1 dimension are marked, where and represent the frequency offsets of water and acetone, respectively.

Image of FIG. 4.
FIG. 4.

Comparison of the signal intensity generated with the ZEBRA sequence and the iDQC-CRAZED sequence. In both experiments, a short and an eight step phase cycle on the 90 , the mixing pulses , and the receiver were used to remove any single- or zero-quantum contaminations from the signal. The evolution time was stepped from .

Image of FIG. 5.
FIG. 5.

Comparison of the signal growth for optimized iZQC-CRAZED and ZEBRA sequences. A resonance offset of is assumed. The calculation is the same for short and variable interpulse phase ( in our case), or for variable and fixed . In the ZEBRA sequence, the signal growth is dependent. For and short values the growth is twice as fast as the best iZQC sequence (90-GT-45) and nearly twice as fast as the best iDQC-CRAZED sequence (90-GT-120-2GT). For large values of (readily accessible in samples with strong magnetization) the nonlinear dynamics is very complex and readily produces much larger signals than would be available in iZQC-CRAZED or iDQC CRAZED.

Image of FIG. 6.
FIG. 6.

Fourier transform of the signal from the ZEBRA sequence, for a sample without inhomogeneous broadening, for two different values of and two different values of . It is clear that the signal has different components for different values of . For small values of , most of the signal comes from the lower coherence orders, while for larger values of , higher orders become more important.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Enhanced nonlinear magnetic resonance signals via square wave dipolar fields