Imaging magnetic field gradients

Figure Bl.14.7. Chemical shift imaging sequence [23], Bothx- andj -dimensions are phase encoded. Since line-broadening due to acquiring the echo in the presence of a magnetic field gradient is avoided, chemical shift infonnation is retained in tire echo.

Figure 7.21 One-dimensional NMR imaging. When a magnetic field gradient is applied across a sample, it gives a spectrum that is a profile of the sample in the direction of the gradient.

Figure 7.22 The principle of creating a two-dimensional NMR image. A number of profiles of the sample are obtained in different orientations in the presence of magnetic field gradients pointing in different directions (designated by arrows). The x -gradient yields an x -profile, and a /gradient generates a y -profile. A combination of these profiles produces a two-dimensional image.

Fig. 2.9.7 Hahn spin-echo rf pulse sequence combined with bipolar magnetic field gradient pulses for hydrodynamic-dispersion mapping experiments. The lower left box indicates field-gradient pulses for the attenuation of spin coherences by incoherent displacements while phase shifts due to coherent displacements on the time scale of the experiment are compensated. The box on the right-hand side represents the usual gradient pulses for ordinary two-dimensional imaging. The latter is equivalent to the sequence shown in Figure 2.9.2(a).

The SPRITE [16] technique was developed to reduce the dangerous mechanical vibrations and noise produced during an SPI experiment. In addition to the reduced noise and mechanical vibration, the experiment is much faster than SPI and the image intensity is still easily understood. However, one experimental shortcoming is a magnetic field gradient duty cycle which is even more demanding than an SPI experiment. 

Fig. 3.4.2 Schematic description of the three-dimensional SPRITE imaging technique. Gz, Gx and Gy are the phase encode magnetic field gradients and are amplitude cycled. A single data point is acquired at a fixed encoding time tp after the rf excitation pulse from the free induction decay (FID). TR is the time between rf pulses. Notice that Gx is ramped (+GZ max to -GXt max) and one /c-space point is acquired for each value of the magnetic field gradient. Gy and Gz are on during the Gx magnetic field gradient ramp and turned off at the end.

Another issue of concern is the amplitude of magnetic field gradients required for solids imaging. For spectra up to a few kHz wide, standard frequency encoding schemes can be used to obtain reasonable spatial resolution. This will not work for broader lines, but it is well known that phase encoding is much more tolerant to line broadening and can be used for spectral widths in excess of 100 kHz with no major loss in spatial resolution. 

ESR spectroscopy can be transformed into an imaging method, ESRI, for samples containing unpaired electron spins, if the spectra are measured in the presence of magnetic field gradients. In an ESRI experiment the microwave power is absorbed by the unpaired electrons located at point x when the resonance condition, Equation (10), is fulfilled. 

Figure 6 Simulated ESR spectra of nitroxides (a) in the absence of magnetic field gradient (b) ID image for a homogenous radical distribution (in the presence of a gradient) and (c) ID image for radicals present only in thin layers near both surfaces of the plaque (in the presence of a gradient).

Up to this point, water mobility values obtained are average values for an entire sample. However, if magnetic field gradients in the x, y, and z directions are incorporated into a pulsed NMR experimental setup, the spatial distribution aspects of water mobility (7), T2, and D) can also be measured via the use of magnetic resonance imaging (MRI) techniques. 

---