Fluxgate for MRI
MRI (Magnetic Resonance Imaging) scanners are widely used in medical imaging and are commonly found in hospitals.
They rely on magnetic fields to image the patient’s body, but are sensitive to external magnetic field disturbances which can affect the quality of the image. MRI manufacturers therefore specify a maximum magnetic field profile that the ambient field distortions must be kept below.
In the event the field does not fit in the prescribed profile, there are mitigation actions which can be taken, including in the worst-case scenario active cancellation.
Finally, due to the strong magnetic field used in an MRI, safety of the patient within the MRI must be ensured by preventing any magnetic material from entering the MRI suite where it could be sucked in by the magnetic field and injure the patient.
We will review here the most suitable equipment to perform these different tasks.
MRI Site surveys
To ensure best images, it is essential that the ambient field around an MRI is kept within the manufacturer’s specifications.
In order to check the ambient field variations (both in DC/near DC and AC), a site survey needs to be carried out at the site where the MRI will be located. In order to be representative, the environment has to be within the same conditions as it will be during the MRI’s operation. That is, any nearby machinery, including elevators will need to be operating, and the same will be true for any vehicles that can move in the vicinity of the MRI.
Because both magnetic field and vibration can affect image quality, vibration measurements are also carried out on the finished floor where the MRI will be installed.
The Spectramag-6 was specifically designed for this type of work. Its ability to run a magnetometer together with accelerometers allow for joint recording of the magnetic field as well as vibration. The Mag-13MS1000 is the best suited magnetometer, as its 1 mT range allows for both pre-installation surveys and post-installation 5 Gauss line mapping, should this be required.
Surveys are carried out at DC to monitor for local transients (elevators, vehicles, etc.) and at AC to monitor the amplitude of the mains magnetic field or any other field generated by machinery. The use of survey parameters allows the user to program the maximum field excursion allowed by the manufacturer in both the AC and DC domains, and will trigger an alarm should these levels be exceeded.
The accelerometer inputs permit the connection of ICP accelerometers (up to 3) to record the vibrations.
In both cases (magnetic and vibrations), care should be taken during the survey to avoid anyone moving in the vicinity of the sensing elements, so as to not introduce artificially high distortion which wouldn’t be representative of normal operation.
The same technique can also be used for surveys of electron microscopes. An electron microscope uses a column of electrons to image a sample. External magnetic field variations can lead to some oscillation of the electron beam and reduce the image quality. Ensuring that the instrument is installed in a quiet magnetic environment will help maintain performance.
Active cancellation for MRI
In the event the results from the site survey show excursions beyond the tolerated fields, there are some mitigation measures available. Passive shielding uses ferromagnetic materials to distort the magnetic field lines away from the area being protected.
In extreme cases, this is not sufficient, and an active cancellation system is required. In this situation a fluxgate sensor (such as the Mag-13MS1000 or similar) is placed within the room. The sensor’s output is used to monitor the ambient field, with the sensor’s output fed into the control system of a 3-axis coil system around the MRI room. When an external disturbance is detected, the sensor output changes and this alters the current to the coils to counteract the disturbance. Commercial systems such as those from ETS Lindgren uses the Mag-03 for their cancellation system.
The same cancellation system can be used around electron microscopes to maintain a stable field around the electron beam column. Here the coils can be a lot smaller and typically just encompass the upper part of the instrument which contains the electron beam.
MRI Suite Access control
Once an MRI is running, the DC field generated is on the order of several Tesla. Any ferromagnetic object entering the room could get sucked into the bore of the magnet and injure the patient. ByMagnetometers placed at the entrance of the MRI suite can detect ferromagnetic materials crossing the threshold of the room and trigger an alarm warning personnel that they are carrying risky materials.
A range of single and three-axis sensors offer low-cost options for OEM integration into a portal system.
Single-axis sensor options include the Mag670 (packaged) or the Mag646 and Mag710 (unpackaged). If a three-axis option is required, the Mag690, Mag612U and Mag619U are all suitable options. Single-axis options have the benefit in this application that, when properly oriented, they are more sensitive to ferromagnetic material passing through the doorway than to material moving parallel to it.
