Imaging with oscillating gradients

The power requirement for the large gradients can be reduced by making the gradient coils part of a tuned circuit. 

The gradient can be applied with sufficient strength to dominate the homogeneous dipolar broadening in soft solids. 

The rf pulses can be applied at a zero crossing of the sinusoid, so that the rf power requirement is simply that normally associated with solid-state NMR. 

The refocusing property of the oscillating gradient can be exploited for generation of gradient echoes. 

By admitting the second harmonic at three times the gradient modulation frequency, the advantages of schemes (b) and (c) are combined (Fig. 8.2.2(a)) [Codl]. Here the read gradient for frequency-encoding is modulated in amplitude according to 

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With oscillating gradients, echo times could be reduced from 2 ms to 128 xs on a solid-state imaging system. When echo times are as short as this, many polymers can be imaged by Fourier techniques adapted to oscillating gradients... 

Fig. 7. MR detection of ultrasonic waves oscillating at 515 kHz. (a) Phase image of a phantom without insonation. (b) Phase image with 40 W peak power insonation. In the NMR sequence 50,000 cycles of synchronized sine-shaped motion-sensitizing gradient were applied. Arrows indicate the null-gradient positions of the dedicated gradient coil system. Wavelength is around 2.9 mm and peak matter displacement is around 120 nm. From Ref. 30, reprinted by permission of Wiley-Liss, Inc., a subsidiary of John Wiley Sons, Inc.

Fig. 6.4.2 (a) Timing diagram for imaging with noise excitation and oscillating gradients. The... 

Imaging with multi-pulse line narrowing has been realized in different variants. These include time-invariant gradients, oscillating gradients, pulsed gradients, and CRAMPS. Each of these variants is addressed in the following. 

Most NC-AFMs use a frequency modulation (FM) teclmique where the cantilever is mounted on a piezo and serves as the resonant element in an oscillator circuit [101. 102]. The frequency of the oscillator output is instantaneously modulated by variations in the force gradient acting between the cantilever tip and the sample. This teclmique typically employs oscillation amplitudes in excess of 20 mn peak to peak. Associated with this teclmique, two different imaging methods are currently in use namely, fixed excitation and fixed amplitude. 

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