High-resolution depth profiling

Single-sided NMR sensors combine open magnets and surface rf coils to study arbitrary sized objects non-invasively [1]. The price paid to gain in access is the impossibility to generate homogeneous magnetic fields. Although it is in principle a limitation, the static gradient of the magnetic field can be exploited to get depth resolution into the material. During the last years several attempts have been made improve the spatial resolution by tailoring permanent magnet geometries, but a resolution hardly better than half a millimeter has been reported. Complicated magnet arrays are the result of optimization procedures where the field profile was improved by playing with the position and orientation of a large number of permanent block magnets [2,3]. The philosophy adopted in previous designs failed to request the magnet to generate planes of constant field in a large range of depths. Although this profile is convenient to select planar slices at different depths into the object by just electronically switching the tuning frequency, it forces the magnet and limits the final resolution.   

  In this work a magnet geometry optimized to define a plane of constant frequency at a single depth from the magnet surface is presented. Tuning the sensor to the resonance frequency defined at this plane selective excitation of a thin flat slice inside the object can be achieved. Repositioning the slice across the sample (See Figure 1) 1D profiles with a resolution better than 5 µm can be achieved [4].

 The maximum depth resolution that can be achieved is mainly determined by the uniformity of the static gradient. To measure the width of the point response function, a frontier glass-oil  was imaged. The derivate of this step image gives a direct measurement of the maximal resolution. This high resolution can be also appreciated in Fig. 2, where an image of a test sample consisting on two latex films 60 µm thick separated by a glass spacer of 160 µm is shown. Acquiring a  CPMG echo train, relaxation-times profiles can be obtained by calculating the ratio of the last echoes over the first ones.

References

1. G. Eidman, R  Savelsberg, P. Blümler, and B. Blümich, The NMR-MOUSE, a Mobile Universal    Surface Explorer, J. Magn. Reson. A 122 (1996) 104.
2. P. J. Prado, Single sided imaging sensor, Magn. Reson. Imaging, 21 (2003) 397-400 .
3. J. Perlo, F. Casanova, and B. Blümich, 3D imaging with a single-sided sensor: an open tomograph, J. Magn. Reson. 166 (2004) 228-235 .
4. j. Perlo, F. Casanova, and B. Blümich, Profiles with microscopic resolution by single-sided NMR, J. Magn. Reson. 176 (2005) 64–70.

< Back to Science