Advances in low-field NMR relaxometry (häftad, eng)

In the context of this work, three unilateral NMR sensors were designed, constructed and characterized. For the new sensors, the concept was chosen to vary the angle of the polarization of the magnets. This affects the size and the position of the sensitive slice. The MOUSEPAD, based on the modified design of the Profile NMR-MOUSE with 5 mm penetration depth, achieved a penetration depth of 17 mm.

The Mini-MOUSEPAD is based on the Mini-MOUSE and has an increased penetration depth of 2.5 mm. The penetration depth is a crucial parameter for investigations with unilateral NMR, since the object of interest can be arbitrary shaped and the relevant areas at higher depths might be difficult to access.

Therefore, these sensors might show a strategy for further miniaturization of state-of-the-art unilateral NMR sensors. The special magnet arrangement of both sensors results in a reduction of the magnetic field strenght (4.9MHz for the MOUSEPAD and 9.0 MHz for the Mini-MOUSEPAD) and much lower gradient strenght compared to prior state of the art sensors.

Consequently, the weaker diffusive attenuation leads to a decreased diffusion weighting. Furthermore, in combination with the extended lateral size of the sensitive area, it leads to an increased SNR, compared to the PM5 and Mini-MOUSE. Sensors with higher sensitivity benefit from shorter measurement times and a higher information density.

The third sensor is the Hexarray. It is a compact (110 x 95 x 30 mm³, 1.6 kg) unilateral sensor with two sensitive clices. It operates at 4.9 MHz at 4 mm above the sensor surface. If one coil is oerated while the other is in the waiting period and vice versa it is possible to perform measurements quasi-simultaneously without any mutual interactions.

The setup can be used to reduce the measurements time. If both spots measure a similar sample, this setup can be used to increase the SNR by adding up both decays. To exploit the full potential of these sensors, further work is necessary.