Oscillating gradients, imaging

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. 

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... 

A MFM can be operated in a two-pass way in much the same manner as the EFM. Like the EFM, the MFM can also be operated in two methods, with or without oscillation of the tip in the lift pass across the sample. In the former case, one can get variations in the magnetic stray fields on the surface directly. However, this method is not very sensitive to small stray fields. The second method is analogous to EFM and results in a magnetic field gradient image. 

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. 

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.

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