Umberto Zanovello1
1Istituto Nazionale di Ricerca Metrologica (INRIM), Torino, Italy
Synopsis
Keywords: Transferable skills: Safety
Motivation: The strong MRI static magnetic field (SMF) may represent a safety concern in many circumstances.
Goal(s): To present conditions and causes that can lead to harmful effects for MR patients and operators. To provide an overview of how relevant guidelines cope with SMF safety in MRI.
Approach: The presentation covers different safety aspects related to the interaction with the MRI SMF.
Results: In the presentation, the effects of the interaction between ferromagnetic objects and the SMF are shown together with the effect of gradient- and movement-induced currents. The most relevant standards are introduced and solutions to measure the SMF are proposed.
Impact: The strong MRI static magnetic field represents a safety concern in many circumstances. An understanding of the conditions and causes leading to harmful events for patients and operators is of key importance to reduce the occurrence of such situations.
Introduction
The strong static magnetic field generated by an MRI scanner may pose a serious hazard if precautions are not undertaken. Infamous examples are the so called projectile or missile events, where extraneous ferromagnetic objects are strongly attracted by the scanner causing tremendous damage to any person standing in the object trajectory. Fortunately, these events are quite rare (about 9 % among 1548 adverse MRI-related events reported to the US Food and Drug Administration between 2008 and 2017 [1]) but can eventually lead to death, as only recently happened in 2023 in São Paulo, Brasil [2]. An understanding of the mechanisms regulating the phenomenon is therefore crucial to develop an even more cautious behavior aimed at further reducing the occurrence of such events.
MRI static magnetic field effects do not limit to the ferromagnetic interaction. For example, eddy-currents induced on conductive components due to movement through the fringe field of the MR scanner and gradient switching may cause torques and forces.
Finally, the static magnetic field can induce transient sensory disturbances on patients and staff including, for example, magnetophosphene, dizziness, vertigo and metallic taste.Magnetostatic Interaction
The behavior of an object inside a magnetic field can be defined as mainly diamagnetic, paramagnetic or ferromagnetic. Objects exhibiting a ferromagnetic behavior are subjected to torques and displacement forces when placed within an external magnetic field [4]. In particular, displacement forces are proportional to the local gradient of the external magnetic field and are responsible for the projectile effect when the ferromagnetic object approaches the strongly inhomogeneous fringe field of the MRI scanner.Induced Currents and Movement
Gradient field switching as well as movement of an object within an external magnetic field may result in eddy-currents induced in the conductive parts of the object. These eddy-currents are regulated by Faraday’s law of induction and, due to the presence of the external magnetic field, give rise to Lorentz forces resulting in potential vibrations and displacements [3]. Such an interaction is of special interest for patients carrying conductive implants. Despite the phenomenon seems to be of limited risk in this specific case, in principle it has to be accounted for.
Furthermore, being human tissues conductive, an electric field may also be induced in a patient or worker moving inside the scanner fringe field. This type of exposure potentially results in annoying symptoms such as vertigo, nausea, flickering visual sensations and neurocognitive effects [5]. Whereas these symptoms are generally harmless, they may for example impact the development of MRI-guided surgery procedures and represents therefore an indirect source of hazard.A Look at the Standards
There are a number of standards aiming at covering the different safety aspects related to the interaction of an external object or human body with a static magnetic field. The American Society for Testing and Materials (ASTM) covers aspects related to passive medical devices such as induced displacement force [6] and induced torque [7] in the MR environment. The ISO 10974 standard deals with the same aspect applied to active implantable medical devices (AIMDs) including malfunctions of the AIMD due to static magnetic field interactions gradient-induced vibrations [8].
Finally, human body exposure to a static magnetic field is regulated by ICNIRP [9,10], in general and by IEC [11], specifically for MRI. These guidelines propose magnetic field exposure limits both for uncontrolled and controlled operation modes by limiting the maximum dB/dt and induced electric field strength the patient and operator are exposed to.Static Magnetic Field Measurement
The most common commercial magnetic field probes that can be adopted for effective static field measurements above some millitesla are probably Hall-effect and NMR probes. The former outperform the latter in terms of sensor dimension, price and dynamic range. Thanks to their reduced size and working principle, Hall-effect sensors can be arranged in a triaxial fashion allowing for the simultaneous measurement of all three magnetic field vector components from few microtesla up to tens of tesla. However, when it comes to accuracy, NMR probes represent the best solution at the price of a higher cost, larger dimension, reduced dynamic range (from some tens of millitesla up to tens of tesla) and the possibility of measuring only the magnetic field amplitude [12].Acknowledgements
The author thanks his colleagues Dr. L. Zilberti and Dr. O. Bottauscio for the useful discussions and for providing precious material for preparing the lecture.The project (21NRM05 STASIS) has received funding from the European Partnership on Metrology, co-financed from the European Union's Horizon Europe Research and Innovation Programme and by the Participating States.References
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[6] ASTM F2052—21 Standard Test Method for Measurement of Magnetically Induced Displacement Force on Medical Devices in the Magnetic Resonance Environment.
[7] ASTM F2213—17 Standard Test Method for Measurement of Magnetically Induced Torque on Medical Devices in the Magnetic Resonance Environment
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[11] IEC 60601-2-33:2022 Particular requirements for the basic safety and essential performance of magnetic resonance equipment for medical diagnosis
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