An open tomograph to image volume regions near the surface of large objects is described. The central achievement in getting such a tomograph to work is the design of a fast two-dimensional pure phase encoding imaging method to produce a cross-sectional image in the presence of highly inhomogeneous fields. The method takes advantage of the multi-echo acquisition in a Carr–Purcell–Meiboom–Gill (CPMG)-like sequence to significantly reduce the experimental time to obtain a 2D image or to spatially resolve relaxation times across the sensitive volume in a single imaging experiment. Depending on T2 the imaging time can be reduced by a factor of up to two orders of magnitude compared to the one needed by the single-echo imaging technique. The complete echo train decay has been also used to produce T2 contrast in the images and to spatially resolve the T2 distribution of an inhomogeneous object, showing that variations of structural properties like the cross-link density of rubber samples can be distinguished by this method. The sequence has been implemented on a single-sided sensor equipped with an optimized magnet geometry and a suitable gradient coil system that provides two perpendicular pulsed gradient fields. The static magnetic field defines flat planes of constant frequency parallel to the surface of the scanner that can be selected by retuning the probe frequency to achieve slice selection into the object. Combining the slice selection obtained under the presence of the static gradient of the open magnet with the two perpendicular pulsed gradient fields, 3D spatial resolution is obtained.
Perlo, F. Casanova, B. Blümich, 3D imaging with a single-sided sensor: an open tomograph, J. Magn. Reson. 166 (2004) 228-235. https://doi.org/10.1016/j.jmr.2003.10.018