Magnets and Coils for Single-Sided NMR

Abstract

The concept of single-sided NMR was introduced with inside-out NMR in the 1980s by the oil industry breaking the myth that NMR experiments can only be performed in the homogeneous fields located inside the magnet [1]. Although nowadays there are already several different NMR well-logging tools, all of them are based on the same basic concept: to position the sample to be studied outside the sensor in the stray field of both the magnet and the rf coil. An unavoidable consequence is that the magnetic field is low and inhomogeneous, leading to extremely low SNR. For slightly inhomogeneous magnetic fields the sensitivity loss can be quantified simply by the frequency bandwidth, while for grossly inhomogeneous fields not only the bandwidth but also the maximum sensitive volume that can be excited must be taken into account. The sensitive volume size and with it the number of spins excited is determined by the gradient strength and by the excitation bandwidth of rf pulses. For example, in the presence of a gradient of 0.1 T/m rf pulses 100 μs long excite a slice 2.4 mm thick, while the same pulses in the presence of a 1 T/m excite only 0.24 mm.