Single-sided NMR imaging

The application range of unilateral NMR has experienced considerable expansion during the last years [1,2]. The use of single-sided probes is not limited by the object size making NMR a truly non-invasive method suitable for near surface studies of arbitrarily large objects. To assess the degree of heterogeneity of samples imaging methods are required. In the last years special effort has been made to develop new imaging techniques to be used in the presence of the strongly inhomogeneous magnetic fields generated by open magnet geometries and the stray field of superconducting magnets [3-5]. Recently, two-dimensional images have been obtained implementing a single-point imaging method on a NMR-MOUSE, but the principal limitation of the method for 3D imaging was the long experimental time [4]. The imaging method implemented in highly inhomogeneous fields acquires the single echo generated by applying a Hahn-echo sequence. Although in many cases further echoes can be generated using a CPMG-like sequence, the use of them for signal improvement in imaging experiments was initially hindered because the space encoding could not be preserved.

Recently we have reported a fast imaging technique that takes advantage of the multi-echo acquisition in a Carr-Purcell-Meiboom-Gill (CPMG) sequence to improve the sensitivity of a 1D imaging experiment (12). The basic idea to speed up data acquisition relies on the fact that the echo time TE used for the Hahn echo sequence is usually much shorter than the T2 of the sample, and a train of echoes can be generated by applying a CPMG sequence. Asit was demonstrated, the extension from a single- to a multi-echo imaging experiment is not straightforward because the off-resonance excitation introduces significant distortions in the phase encoding defined by the gradient pulses. To overcome this distortion independent phase encoding was performed on each echo similar to the RARE method (14), applying two gradient pulses with opposite polarization before and after the echo formation.

In this work we propose a new multi-echo technique to produce a cross-section of a selected slice. The pulse sequence is divided into a first encoding period, where both gradient pulses are applied to achieve lateral resolution, followed by a detection period composed of a train of refocusing pulses for multi-echo acquisition. This technique has been implemented in an optimized U-shaped magnet design that provides a large volume where the static magnetic field defines planes of constant frequency parallel to the scanner surface. Under the strong gradient of the static field, hard RF pulses produce selective excitation of thin flat slices at different depths into the object selected by changing the excitation frequency.

To show the quality of the slice selective imaging technique an object with a 3D structure was made. A set of letters forming the word MOUSE was cut from a sheet of 2 mm thick rubber, and stacked one above the other without any spacer giving an object of 1 cm height along the depth direction. After calibrating both the maximum current needed for each gradient coil to keep the field of view (FoV) constant, and the dependence of the B1 field as a function of depth, five two-dimensional slices, each 1 mm thick, were acquired, one inside each letter. Figure 3 shows the set of images obtained with a spatial resolution of 1.5 mm. The acquisition time was 9 minutes for the complete set of images.

References

G. Eidman, R Savelsberg, P. Blümler, and B. Blümich, The NMR-MOUSE, a Mobile Universal Surface Explorer, J. Magn. Reson. A 122 (1996) 104.
A. Guthausen, G. Zimmer, P. Blümler, and B. Blümich, Analysis of Polymer Materials with the NMR-MOUSE, J. Magn. Reson. 130 (1998) 1.
P. J. Prado, B. Blümich, and U. Schmitz, One-Dimensional Imaging with a Palm-Size Probe, J. Magn. Reson. 144 (2000) 200.
F. Casanova, and B. Blümich, Two-dimensional imaging with a single sided probe, J. Magn. Reson. 163, 38-45 (2003).
P. J. Prado, Single sided imaging sensor, Magn. Reson. Imaging, 21 (2003) 397-400.
F. Casanova, J. Perlo, B. Blümich, and K. Kremer, Multi-echo imaging in highlyinhomogeneous magnetic fields, J. Magn. Reson. 166 (2004) 76-81 .
J. Perlo, F. Casanova, and B. Blümich, 3D imaging with a single-sided sensor: an open tomograph, J. Magn. Reson. 166 (2004) 228-235 .

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Drawing of the single-sided sensor built for this work. It shows the geometry of the U-shaped magnet, the gradient coil system and RF coil used for the open tomograph. This magnet geometry produces a static field with a main component along z.

New multi-echo sequence applied to sample the complete echo decay train. Both gradient pulses are applied simultaneously after the first RF pulse and determine the encoding echo time TEE of the first Hahn echo sequence. To maximize the number of echoes that can be generated during the detection period, the echo time is reduced after the formation of the first echo. The detection echo time TED is determined by the dead time of the probe.

a) Object made staking the letters of the word MOUSE cut from a rubber sheet 2 mm thick. The object was placed inside a holder with 1 mm walls. b) Expanded view of the object obtained by drawing the letters separate from each other to see the object structure. c) Images of each letter obtained applying the multi-echo imaging method and selecting a 1 mm thick slice in the center of each letter.