NMR sensor

The profiling method requires the sensitive slice to be shifted through the object. Figure 2.4.2 shows the mechanical lift used to move the sensor with respect to the sample. The object under study, for instance the lower surface of the arm in the picture, is positioned on top of a flat holder (A) and the NMR sensor is placed under it on a movable plate (B). The mechanical construction allows one to move the sensor up and down with a precision of 10 pm. The distance between the rf coil and the sensitive slice defines the maximum penetration depth into the sample (maximum field of view of the ID image). Depending on the application, the position of the rf coil with respect to the sensitive slice can be changed to maximize the sensitivity. 

Since it was proposed in the early 1980s [6, 7], spin-relaxation has been extensively used to determine the surface-to-volume ratio of porous materials [8-10]. Pore structure has been probed by the effect on the diffusion coefficient [11, 12] and the diffusion propagator [13,14], Self-diffusion coefficient measurements as a function of diffusion time provide surface-to-volume ratio information for the early times, and tortuosity for the long times. Recent techniques of two-dimensional NMR of relaxation and diffusion [15-21] have proven particularly interesting for several applications. The development of portable NMR sensors (e.g., NMR logging devices [22] and NMR-MOUSE [23]) and novel concepts for ex situ NMR [24, 25] demonstrate the potential to extend the NMR technology to a broad application of field material testing. 

The potential for developing NMR as an on-line sensor of the internal quality of fruits and vegetables is discussed. The literature on the NMR of horticultural products is first surveyed for potentially useful correlations between NMR characteristics and internal quality factors in commercially important harvested products. This is followed by discussions on the cellular origins of these correlations and of the technical problems to be overcome in the development of a commercial on-line NMR sensor. 

The motive for developing commercial NMR sensors of horticultural products hardly needs to be laboured. Consumers expect their fruit and vegetables to be of consistent high quality, at optimum ripeness, juiciness and texture and, of course, free from bruises, disease, infestation and other internal quality defects. Unfortunately for the fruit or vegetable grower, many of these quality factors are outside their control. Factors such as the weather, harvesting conditions and natural biological diversity mean that the quality of... 

Diffusive attenuation is not merely of academic interest because it will probably be an inherent cause of echo attenuation in on-line NMR sensors whenever the sample moves through gradients and inhomogeneities in the main sensor field. It is therefore important to understand their potential effect and possible use in assessing quality of horticultural products. 

J. C. Goswami, A. Sezginer, and B. Luong, On the design of NMR sensor for well-logging applications, IEEE Antennas Propag., 48(9), 1393-1402 (2000). 

Relaxation can be probed in inhomogeneous fields, so that the homogeneous polarization field B0 is not a necessity for successful applications of soft-matter imaging. Based on this fact, small portable NMR sensors can be built, which provide NMR data from a single volume element with the same specificity as the contrast in an NMR image [12]. Such... 

The NMR-MOUSE is a portable NMR sensor which works in highly inhomogeneous magnetic fields. Because of field inhomogeneity NMR spectroscopy of the chemical shift is not readily possible, but relaxation times and parameters of translational motion can be measured by echo techniques. These are the most important NMR parameters which are exploited for contrast in imaging. Unless fluids are investigated field inhomogeneities are essentially no obstacle for relaxation analysis [80], because molecular motion by translational diffusion is absent. 

Figure 7.22 The NMR-MOUSE (a) Schematic. The NMR sensor consists of an u-shaped permanent magnet with a solenoidal rf coil placed in the gap. (b) Photo of the NMR-MOUSE testing a tyre, (c) Example of a train of successive Hahn echoes generated according to Carr, Purcell, Meiboom and Gill (CPMG echo train) for carbon-black filled SBR measured by the NMR-MOUSE. The time constant of the echo-envelope defines T...

In unilateral NMR, a magnet and the RF communication antenna are placed on the object, which can be much larger than the magnet. With permanent magnets, NMR sensors as small as a computer mouse can be built and positioned on intact objects at different places to measure T2, which goes hand in hand with molecular mobility. 

R148 R. Gianferri and E. Brosio, Prospects for Nuclear Magnetic Resonance Future A Brief Note on Portable NMR Sensors , in Basic NMR in Foods Characterization, ed. E. Brosio, Research Signpost, Trivandrum, India, 2009, p. 137. 

Rata, D.G., Casanova, F, Perlo, J., Demco, D.E., and Bluemich, B. 2006. Self-diffusion measurements by a mobile single-sided NMR sensor with improved magnetic field gradient. J. Magn. Reson. 180 229-235. 

The NMR-MOUSE is simple mobile NMR sensor suitable for operation in an industrial environment. Its use for quality control is demonstrated in Figure 39 by the T2 statistics measin-ed at both sides of a conveyor belt with steel cords (193). The magnetic field Bo distortions from the steel cords were minimized by suitable orientation of the NMR-MOUSE with respect to the direction of the cords. 

Fast, flexible, and easy-to-implement target field algorithms was presented. For a single-sided handheld NMR sensor, the algorithm yields a 640 G field homogeneous to 16 100 ppm across a 1.9cm volume located... 

---