This paper presents a multi-point and modular magnetic field sensor system compatible with a 3T-MRI environment. The system features a three-axis magnetometer on a chip. This monolithic sensor is to our knowledge the only integrated sensor commercially available that provides full field vector information as well as sufficient dynamic range and acquisition rate for MRI-applications. We have validated experimentally our demonstrator through the measurement of static magnetic field and magnetic field gradients simultaneously acquired at nine locations within a MRI bore (Prisma, Siemens, Erlangen, Germany).
The system features a three-axis magnetometer on a chip (Metrolab Technology SA, Switzerland) [5]. This monolithic sensor is to our knowledge the only integrated sensor commercially available that provides full field vector information as well as sufficient dynamic range and acquisition rate for MRI-applications.
Measurements were performed on a 3T MRI system (Prisma, Siemens, Erlangen, Germany) (Fig. 1). Compared to previous work [6] our system provides a simultaneous multi-point acquisition, which enables faster measurements and the monitoring of non-reproducible effects. It is also important to note that the presented measurements were performed for the first time with a chip that combines on the same die the three-axis sensor element and the processing electronics, which includes a 16-bit analog to digital converter. This level of miniaturization strongly reduces electro-magnetic interferences. The sensor measures both the 3T static magnetic field and the gradients. With 16 effective number of bits, one can detect field changes of approximately 92 µT on top of a 3T base line. This is especially interesting when using the presented three-axis sensor for localization purposes. The static magnetic field allows estimating the sensor orientation, leaving only the sensor position left to be calculated (e.g. by using the unique relationship between the coordinate system of the MRI tunnel and the gradient information).
The presented sensor system does not replace a magnetic mapper based on NMR sensors, which provides ppm accuracy to perform tasks such as condition monitoring, maintenance and tuning of gradient systems or characterization of the static field distribution of MRI bores. Noticeable signal alteration in $$$^1H$$$ experiments can be induced by a field error of approximately $$$10^{-9} $$$ Ts, e.g. a field offset of 1µT during 1ms [7]. This is two orders of magnitude lower than the detection level we can achieve with 3D Hall effect sensors integrated in standard CMOS technology like the one we use in this experiment. However, our system allows measuring the full field vector information, which is not accessible through NMR measurements.
The next step is to assess the feasibility of using our system for the characterization of the gradient system. Errors introduced by the gradient amplifiers (such as offset, non-linearity, gain error and delays), and errors induced by eddy currents within the gradient coils, are of particular interest.
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