Demonstration of low-field NMR detection in static fields produced by unilateral magnets

Detection of NMR signals in low-fields produced by unilateral magnets would find large applications in portable medical diagnostics. However, use of low static magnetic fields for NMR detection remains a challenge due to low signal to noise ratio (SNR), lower splitting and lesser number of spin transitions between energy levels. Often techniques such as hyperpolarization, signal processing and pulse-sequencing are used to overcome such limitations. Besides these methods, it is also possible to tune the magnetic field strength and distribution around the magnets to achieve improved SNR. One way is to locate regions of uniform magnetic field outside hollow ring magnets. Typically referred to as saddle points, in these regions the magnetic fields are relatively invariant. Test samples can then be placed at these locations, and on application of precise pulsed magnetic fields, NMR signals under low uniform fields may be obtained. Therefore, in this work, finite element calculations are used to determine the location of saddle points and correspondingly detected NMR signals for chemical species such as ¹H are recorded.