Abstract
To investigate the displacement forces and image artifacts associated with passive medical implants for recently-developed low-field (<100 mT) MRI systems, and to compare these with values from higher field strengths used for clinical diagnosis. Setups were constructed to measure displacement forces in a permanent magnet-based Halbach array used for in vivo MRI at 50 mT, and results compared with measurements at 7T. Image artifacts were assessed using turbo (fast) spin echo imaging sequences for four different passive medical implants: a septal occluder, iliac stent, pedicle screw and (ferromagnetic) endoscopic clip. Comparisons were made with artifacts produced at 1.5, 3 and 7T. Finally, specific absorption rate (SAR) simulations were performed to determine under what operating conditions the limits might be approached at low-field. Displacement forces at 50 mT on all but the ferromagnetic implant were between 1 and 10 mN. Image artifacts at 50 mT were much less than at clinical field strengths for all passive devices, and with the exception of the ferromagnetic clip. SAR simulations show that very long echo train (>128) turbo spin echo sequences can be run with short inter-pulse times (5-10ms) within SAR limits. This work presents the first evaluation of the effects of passive implants at field strengths less than 100 mT in terms of displacement forces, image artifacts and SAR. The results support previous claims that such systems can be used safely and usefully in challenging enviroments such as the intensive care unit.
Highlights
Magnetic resonance imaging (MRI) is a valuable, non-invasive and widely applicable imaging technique
Note that since the endoscopic clip is ferromagnetic, it naturally aligns along the B0 axis, and so there are no data for directions perpendicular to this
This study provides, to our knowledge, the first semi-quantitative overview of different safety aspects and analysis of image artifacts produced by various passive implants at low-field, in this case 50 mT, and compares the results to those obtained at conventional clinical field strengths (1.5 T and 3 T), as well as ultra-high field strength (7 T) for completeness
Summary
Magnetic resonance imaging (MRI) is a valuable, non-invasive and widely applicable imaging technique. Since MR imaging involves placing the patient in a strong static magnetic field, transmitting several kilowatts of power into the patient, and rapid switching of secondary magnetic fields (produced by the gradient coils), there are a number of potential safety issues involved in clinical scanning. If implanted devices contain ferromagnetic components, they can constitute a safety hazard due to rotational and translational forces on the implant when the patient enters the MRI through the strong spatial static magnetic field gradient. In order to allow safe MRI scanning, a substantial number of federal/international guidelines must be adhered to, including the magnetically-induced displacement force Fm and the specific absorption rate (SAR) [2]. If implanted medical devices are metallic, their interaction with the static magnetic field can cause image artifacts which substan tially reduce the diagnostic quality of the scan, altering the risk/benefit analysis for the patient
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