Introduction

One of the most important characteristics of the main magnetic field (B0 field) is spatial homogeneity. It is very important to make a plot of internal magnetic field of the magnet (especially when it is newly installed) or to probe the magnetic field with full dynamics as occurring during actual MR procedures [1, 2]. Consequences of B0 field inhomogeneities are degradation of signal-to-noise ratio in MR measurements, broadening and distortions of the spectral line shapes within a spectroscopic voxel, signal dropouts via intravoxel dephasing effects, geometric distortions, image blurring, etc. [1, 2]. Sources of B0 inhomogeneities can be hardware induced (concomitant field changes, for example, caused by gradients that occur in addition to the imaging gradients due to intrinsic properties of the magnetic field) and inhomogeneities due to the object placed in the main magnetic field (susceptibility differences between the tissue and bone/air). Both type of inhomogeneities can be cancelled out or significantly reduced in a number of ways and the process is called shimming.
Based on measurements requirements, there are different techniques for B0 field measurements [3-5].

Methods

The measurement techniques depends on the required accuracy. For MRI and NMR purposes the required accuracy is very high ~ few ppm. There are different technologies for magnetometers, such as Faraday magneto-optic effect and Hall effect based, Josephson junction magnetometers, YIG filter magnetometers (based on electro spin resonance), optically pumped (Zeeman) magnetometers, etc. [3, 4]. Very often used sensors are inductive coil sensors and sensors based on Hall effect.
Inductive coil sensors are sensitive only to the flux that is perpendicular to their main axis [6]. The principle of operation of the inductive sensors is based on Faradey’s law of induction. Inductive sensors respond to a changing magnetic field which induces current in a coil of wire and produces voltage at its output. Hall effect sensor is composed of a thin strip of metal that has a current applied along it [3]. In the presence of a magnetic field, the electrons in the metal strip are deflected toward one edge, producing a voltage gradient across the short side of the strip (perpendicular to the feed current). The output voltage is directly proportional to the magnetic field strength through it.
The best technique for B0 field characterization during MRI experiment that satisfies requirements for sensitivity and temporal resolution is NMR magnetometry [5]. This technique is based on using of NMR probes [7]. Relying on the same physics as the MR experiments to be analyzed, NMR probes also exhibit the same high degree of sensitivity to magnetic field. Like an actual MR experiment, they are sensitive to field magnitude rather than individual field components. When used in a pulsed mode [5], their resonance signal can be sampled at very high frequency, with the signal phase reflecting the time integral of the magnetic field magnitude. Pulsed NMR magnetometry works by acquiring free induction decay (FID) signals from a small NMR-active sample inside a suitable probe [5]. For such measurements, to enable the observation of field dynamics in MR systems, the probes need to fulfill a set of performance requirements (sensitivity to phase accrual, probe signal lifetime, SNR of probe signals, as low as possible RF cross-talk, etc.).

Discussions and Conclusions

The most suitable method for the observation of the static magnetic field as well as field dynamics in MR systems is based on NMR probes [5, 7]. The NMR probe based measurement techniques provide high sensitivity, high temporal resolution, and tolerance to field inhomogeneity within a probe. NMR probes based on pulsed liquid-state NMR in mm-scale samples can solve shortcomings of the other presented magnetic field measurement techniques.

Acknowledgements

No acknowledgement found.