NMR Microscopy by Strong Radiofrequency-Field Gradients with Spatial Resolution Better Than Five Micrometers

✍ Scribed by F. Humbert; B. Diter; D. Canet

• Publisher: Elsevier Science
• Year: 1996
• Tongue: English
• Weight: 412 KB
• Volume: 123
• Category: Article
• ISSN: 1064-1858
• DOI: 10.1006/jmra.1996.0245

✦ Synopsis

NMR imaging, as well as self-diffusion measurements suitable for microimaging or diffusion experiments. On the by NMR, is fundamentally based on the spatial variation other hand, a single-turn coil is in principle capable of deliv-(gradient) of one of the two magnetic fields used in the ering a uniform B 1 radiofrequency-field gradient in a region NMR experiment, namely the static field B 0 or the radiofreranging from 0.2 r to 0.9 r (r being the coil radius) (9, 10). quency field B 1 . The spatial dependency of the precession In this respect, Maffei et al.

(3) obtained images with a frequency [n 0 Å gB 0 (r)] or of the nutation frequency [n 1 resolution around 100 micrometers with a gradient amplitude Å gB 1 (r)] is involved in the former and the latter, respecof 2.5 G cm 01 . This resolution appears rather poor in comtively (r represents nuclear spin coordinates and g the gyroparison with the one achieved with B 0 gradients and, clearly, magnetic ratio). In imaging with conventional B 0 gradients, stronger B 1 gradients must be sought. A simple way of inspatial resolution of a few micrometers (1 to 5 mm) is in creasing the gradient is to reduce the coil radius. Taking principle reached by relying on proton resonance at high advantage of this feature, we managed to attain gradients up field and using gradient strengths of the order of a hundred to 20 G cm 01 which prove especially useful in the context gauss per centimeter. Nevertheless, susceptibility inhomogeof localized diffusion measurements (11). However, it must neity and molecular self-diffusion are prone to distort images be borne in mind that the smaller the radius, the smaller the and to deteriorate the spatial resolution (1). A way of overgradient uniformity region, which in addition gets closer to coming these problems is to use larger gradients (up to 600 the coil; hence a compromise must be found which depends G cm 01 ), involving highly sophisticated, miniaturized coil on the size of the objects to be imaged. Another way to assemblies (2).

further enhance the gradient is to increase the number of Here we shall be concerned with the second method of turns of a flat coil. spatial encoding (B 1 gradients) (3-6). This method is in-

The above considerations led us to design a probe includstrumentally much simpler and is nearly insensitive to susing two coils, one aimed at delivering the B 1 gradient, and ceptibility inhomogeneity (7). However, the resolution obthe other essentially for detecting the NMR signal (repretained with this method (3, 8) does not seem as good as sented in Fig. ). The major difficulty in this setup stems the one which has been achieved by conventional NMR from the necessity of adjusting the relative geometry of the microscopy. The achievable resolution limit depends on varitwo coils in order to obtain the optimum gradient uniformity. ous factors such as transverse relaxation time T 2 , molecular To reach this goal, a micrometric screw enables one, from self-diffusion, signal-to-noise ratio, and, of course, the the outside of the probe, to move the Helmholtz coil vertistrength and quality of the field gradient. By quality, we cally. mean that the gradient must be constant over a sufficient

In order to evaluate the global quality and the strength volume, or at least must vary regularly within this volume of the gradient over the entire sensitive volume, diffusion in a perfectly known way.

measurements were performed at 25ЊC for some usual model Using a toroid cavity detector which produces a B 1 field liquids through the sequence presented in Fig. , for which gradient inversely proportional to the distance from the centhe principle has been described elsewhere (12). As in claster axis of the torus, Woelk et al. (8) predicted a maximum sical measurements by B 0 gradients, analysis of the NMR resolution of 6.5 mm, close to the surface of the central signal evolution as a function of the square of the gradientconductor and indeed obtained one profile showing 20 mm pulse duration leads to the product D(g 1 ) 2 , where D is the resolution in a polymer sample. However, the absence of self-diffusion coefficient and g 1 the gradient strength. Results other examples would suggest that their probe is not very obtained for the model compounds (see Fig. ) reveal an excellent agreement with literature (for the ratios of diffusion coefficients) and lead to a g 1 value of 75 G cm 01 .