STRAFI method

The free induction decay of the excited slice contains spatial information, which can be revealed by Fourier transformation as in most other conventional NMR techniques. If the sample is a thin film with a thickness less than the width of the excited slice, the STRAFI image obtained in one spatial dimension is the image across the whole film. Larger samples, however, must be scanned to get the whole ID projection. One way is to sweep the frequency of the pulse, but the older STRAFI method is to move the sample through the field. The first possibility is analogous to the method of frequency sweep in continuous-wave NMR spectroscopy. Field sweep in STRAFI-MRI has also been tried. 

STRAFI methods rely on sample translation and so the precision and reproducibility of the mechanical arrangement also limits the resolution, here to approximately 10 m. STRAFI methods are also set up to provide isotropic voxels in 3D-imaging, although much work has been done with ID-STRAFI measurements where the question has been carefully tailored so they can be answered by studying profiles of the sample. 

Magnetic resonance imaging techniques using F would seem to offer potential scope for studying solid fluoropolymer systems, and some work has been published [25]. It is suggested [26, 27] that the stray-field method (STRAFI) may be particularly suited to such studies, but as of 1997 this remains unrealised. 

STRAFI-MRI is a slice-selective method since even very short pulses excite only a narrow portion of the sample Samoilenko s sensitive slice . 

It seems clear that STRAFI-MRI will become the method of choice for the imaging of solids, and samples consisting of solids and liquids. Any type of nucleus may be used, including quadrupolar nuclides, in any type of solid. All types of materials may be addressed. The method will be seen to best advantage when simple ID profiles are sufficient for the particular study. Its worth has been shown already for water in cements (Figure 7) and concrete and other building materials, and for organic liquids... 

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