Curr Radiol Rep (2016)4:34 DOI 10.1007/s40134-016-0163-y
MRI SAFETY (M BOCK, SECTION EDITOR)
MR Safety Update 2015: Where Do the Risks Come From? Oliver Kraff1 • Mark E. Ladd1,2
Springer Science+Business Media New York 2016
Abstract Given the high technological complexity of magnetic resonance imaging (MRI) systems, there is particular concern for potential harm. Safety issues arise from the three main components of the MRI system: its main static magnetic field, which is always on in superconducting magnets, and the switched radiofrequency electromagnetic fields and gradient magnetic fields that are activated during imaging. This article reviews published literature mainly from 2015 with regard to these general safety considerations and their interaction with implanted medical devices. Furthermore, an update on regulatory issues will be provided along with an overview of miscellaneous other safety aspects that are related to an MRI exam but do not arise from the technology per se. Keywords MRI safety Static magnetic field RF exposure SAR Incident reports
This article is part of the Topical collection on MRI Safety. & Oliver Kraff
[email protected] 1
Erwin L. Hahn Institute for Magnetic Resonance Imaging, University Duisburg-Essen, Kokereiallee 7, Building C84, 45141 Essen, Germany
2
Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany
Introduction Magnetic resonance imaging (MRI) is a widely used imaging modality. Since its introduction into clinical settings in the mid-1980s, MRI has developed into one of the most flexible tools in diagnostic imaging, with an estimated 37.8 million MR procedures performed in the U.S. in 2015. The annual growth rate is approximately 4 % since 2011 [1]. Besides its importance for clinical diagnosis, MRI has always been a highly active field of medical and methodological research. The latter is predominantly signified by the continuous development of imaging technology, for example pulse sequences, gradient technology, components of the radiofrequency (RF) transmit and receive chains, and finally magnets. There are more than 30,000 installations worldwide (8465 in the U.S. in 2015 [1]) mostly operating at magnetic field strengths of 1.5 and 3 T. However, the currently installed-base of ultra-high field (UHF) MR systems for human imaging of 7–11.7 T has increased to more than 70 systems [2]. This rapid growth in the number of UHF sites has led to multiple pilot studies investigating clinical use from head to toe at 7 T, with demonstrated diagnostic benefits in the context of brain pathologies and degeneration of the musculoskeletal system [3, 4••, 5]. Although MRI is a non-invasive technology with a strong history of safe use, there are potential risks in the MR environment that need to be addressed. Safety issues arise from (i) the main static magnetic field (B0), which can lead to projectile forces and transient physiological effects, (ii) the RF electromagnetic field, which deposits power in human tissue described by the associated specific absorption rate (SAR), and (iii) the gradient magnetic fields, which produce acoustic noise and can lead to peripheral nerve stimulation. Before recent developments regarding these general safety considerations and their
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interaction with implanted medical devices are reviewed, an update on the regulatory status of MRI is provided in the following section.
Current Regulatory Status To date only MR systems with a maximum magnetic field strength of 3 T have received clearance for diagnostic, clinical imaging either in the US (approval needed by the Food and Drug Administration, FDA) or in Europe (regulated by the Medical Device Directive, Amendment M5— Directive 2007/47/EC) [6]. An accepted standard for ensuring basic safety in MRI is the International Electrotechnical Commission (IEC) standard 60601-2-33, which has been recently updated in June 2015 with edition 3.2 [7••]. This latest amendment increases the first-level controlled operating mode limit for the static magnetic field from 4 to 8 T, now matching the FDA declaration from 2003 that MRI up to 8 T constitutes a non-significant risk for adults, children, and infants of 1 month and older [8]. In addition to the FDA limits, the report from the International Commission on Non-Ionizing Radiation Protection (ICNIRP) from 2009 [9] as well as other peer-reviewed scientific literature indicating no serious health effects resulting from acute exposure to static magnetic fields up to 8 T was important discussion points leading to the amendment. Commercially, the current IEC standard now opens a door for MR vendors to bring 7 T MR systems of the next generation into clinical settings as recently announced by one vendor during the 23rd Annual Scientific Meeting of the International Society for Magnetic Resonance in Medicine (ISMRM) in May 2015 [10]. Furthermore, a non-compulsory option called the Fixed Parameter Option: Basic (FPO:B) has been added to the IEC standard. With this option, a set of operational limit values for the allowable RF field and gradient field outputs (peak and RMS) were defined for scanning patients with MR conditional implants at a specified B0. The addition ‘‘basic’’ denotes a specific implementation of FPO valid exclusively for 1.5 T MR systems. The option has been developed together with representatives from the MR system and medical implant vendors. In this context, the technical specification (TS) 10974 published in 2012 by the International Standards Organization (ISO) regarding the assessment of the safety of MRI for patients with an active implantable medical device should also be noted [11]. Although primarily intended for active implants, test methods described in the TS can also be adapted for passive implants instead of using the rather outdated F2182-11a American Standard for Testing and Materials (ASTM) [12].
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Projectile Forces From Static Magnetic Fields The attractive force with which a paramagnetic or (nonsaturated) ferromagnetic device is accelerated toward the MR system is proportional to the product of the system’s main magnetic field (B0) and its spatial magnetic field gradient (rB0). In addition, the device will be subjected to magnetically induced torque aligning the longest axis of the device with the direction of the main magnetic field. Projectile forces are still considered the number one risk factor in the MR environment, with multiple documented serious injuries and even a few fatalities in the past. Gu¨ttler et al. reported 137 documented incidents caused by ferromagnetic objects introduced into the MR environment between January 1992 and March 2015 [13]. Although these incidents make up only approximately 10 % of all documented incidents related to MRI (1739 in total) in that period, the injuries caused were far more severe compared to the others reported [13]. For the 10-year period between 2000 and 2009, a fivefold growth in reported MR adverse event frequency has been described in another study [14, 15]. Note that in this time period the number of MR installations also rapidly increased, so that the increase in reporting frequency does not necessarily reflect deteriorating safety vigilance at individual MR sites. Hence, still in 2015, multiple educational and review articles were published addressing safety aspects with regard to the main magnetic field. Gilk and Kanal discussed certain design aspects that could be taken into account during the planning process of a new MR suite to enhance safety, and which may also improve efficiency for both patients and healthcare workers [14]. Weidman et al. reviewed their safety standards and expressed positive experience with a novel ‘‘lights off’’ approach during patient screening [16]. To account for desensitization and alarm fatigue in combination with the use of a ferrous metal detector positioned in the entrance to the MR room (Zone IV according to the American College of Radiology (ACR) guidance document for Safe MR Practices: 2013 [17]), the lights within the MR room are dimmed if any ferromagnetic material is detected. In addition to the traditional auditory alarm the detector sounds when triggered, the lights-off measure establishes another layer in filtering risks and promoting safety according to the authors [16]. Gu¨ttler et al. reported a similarly positive experience in using a ferromagnetic detection system to complement screening procedures and safety training [13]. In case of implanted medical devices, knowledge of the strength and location of the maximum spatial magnetic field gradient is a prerequisite for defining whether a
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subject with a MR conditional implant may be exposed to the MR system or not. Although there has been progress by the device manufacturers in specifying device behavior in a magnetic field and some MR system manufacturers provide maps of the spatial magnetic field gradients, there remains significant confusion among the MR user community because of the lack of standardized terminology and reporting guidelines as discussed by Emanuel Kanal et al. on behalf of the ACR Subcommittee on MR Safety [18•]. In a research setting, an MR scan cannot be justified by a medical indication and a risk–benefit analysis, for example in the case of implanted medical devices. Therefore, Calamante et al. have published on behalf of the Safety Committee of the International Society for Magnetic Resonance in Medicine (ISMRM) an outline for the minimum level of safety and operational knowledge that a MR system operator should exhibit in order to perform imaging in human subjects in a research setting [19]. Specific risks of UHF MR imaging have been excellently reviewed by van Osch and Webb in 2014 [20] and will not be explicitly presented in this article.
Tissue Heating From RF Exposure Interaction of the RF fields with the human body is considered to be the primary safety risk during imaging, i.e., after the subject has been introduced into the main magnetic field. The RF exposure can lead to temperature increases in the human body due to deposition of the RF power in biological tissue via the induction of eddy currents. Furthermore, excessive local heating, especially in the presence of (implanted) conducting material, can lead to severe burns. Global and local power depositions are quantified in terms of the SAR, measured in W/kg. The SAR is proportional to the product of the electric conductivity and the square of the induced electric field. There is also an approximately quadratic dependence on the operating frequency (below 200 MHz, less steep above 200 MHz), introducing restrictions in imaging at UHF [21]. However, according to a study presented by Langman et al., the cumulative duration of high SAR exposure during clinical protocols typically remains relatively short at 3–9 min out of a 51-min protocol for cardiac (whole-body SAR: 3.5–4 W/kg) and 6-12 min out of a 32-min protocol for neurological exams (head SAR: 3.2 W/kg) [22]. Emerging transmit technologies developed for tackling transmit B1 inhomogeneities at UHF, i.e., parallel RF transmission with multi-channel transmit coils [23], have already been adopted at clinical field strength and introduce new challenges for safety supervision as these
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technologies are capable of generating electromagnetic field distributions optimized for each individual subject that are significantly different to those from traditional transmit body coils. In this context, fast numerical simulations are needed to account for different SAR distributions associated with the variable driving of the RF coil. Murbach et al. investigated the effect of RF shimming at 3 T with six anatomical human models from the Virtual Population using multiple imaging landmarks as well as different RF coils and thermoregulation models [24•]. While RF shimming can be considered safe for a broad patient population with normal thermoregulation, the full exploitation of RF shimming settings in the first-level controlled operating mode resulted in excessive thermal load in patients with impaired thermoregulation. When additional field elevations should be strictly avoided, e.g., in pregnant patients, the authors advised restricting the system to traditional circularly polarized (CP) transmission [24•]. Induced temperature increase is strongly influenced by thermoregulation and blood perfusion, and not directly related to SAR alone. Thermal stress and tissue damage result from a combination of the induced temperature increase and the duration of exposure. Consequently, new safety concepts that consider thermal dose are currently in discussion, such as the cumulative equivalent minutes at 43 C (CEM43) concept [25, 26]. On the other hand, it is still not possible to perform online, spatially resolved in vivo temperature monitoring, so that current safety guidelines still rely on SAR and supervision of transmitted RF power. An efficient novel approach for safety supervision has been published by Neufeld et al. based on a model that estimates CEM43 dose from SAR and pulse sequence information (exposure duration), as well as information about the subject’s initial temperature and his/ her thermoregulatory ability [27•]. While the general applicability of the model has been demonstrated, future work is needed to improve perfusion and thermoregulation models, which are still subject to substantial uncertainty. Furthermore, safety margins need to be defined to provide a well-accepted alternative to the standard SAR approach without introducing overly restrictive predictions. Patients with implanted medical devices made of metallic or conductive material present additional safety challenges as local hotspots can be created at the implanttissue interface leading to excessive temperature increases 10 C [28]. Elongated conductive implants with insulated leads having uninsulated electrodes at their termination, as is the case for cardiovascular implanted electronic devices (CIED) such as pacemakers and defibrillators, introduce a very high risk and are typically considered as an absolute contraindication for MRI. However, over the past decade, a better understanding of RF-induced tissue
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heating and technical improvements in the design of the devices have allowed an increasing number of patients with CIEDs to be safely imaged at 1.5 T systems. Two major articles were published on this topic in 2015 reviewing the specific precautions necessary when imaging patients both with conventional pacemaker systems as well as MRconditional devices as the new standard of care [29•, 30•]. Of course, the described risk of RF-induced heating also applies to passive implants. At UHF, many sites conservatively exclude all subjects with tattoos or passive metallic implants regardless of type or location. A comprehensive description of 7.5 years of experience in imaging carefully selected subjects with implants and/or tattoos at 7 T has recently been published by Noureddine et al. [31•]; the examinations were conducted only after collection of substantial safety information and evaluation of the detailed exposure scenario.
Acoustic Noise Switching of magnetic field gradients imposes Lorentz forces and vibrations that mechanically couple into the system structure and are the primary cause of acoustic noise during MR imaging. Patients typically experience elevated sound pressure levels of 80–119 dB(A) [32]. To avoid hearing impairment, proper protection is mandatory during scanning by applying headphones or ear plugs. Increased awareness is needed when imaging vulnerable populations such as children and the elderly, as well as in general when imaging is performed under anesthesia. Recent improvements in pulse sequence programming allow significant reductions in sound pressure levels without degradation in image quality. A threefold reduction in the acoustic noise of T1- and PD-weighted turbo spin echo sequences was achieved by Ott et al. by changing gradient ramps and shapes, improving phase-encoding timing, applying RF pulses to avoid the need to reverse the polarity of the slice-rewinding gradient, and utilizing an increased readout bandwidth [33]. Corcuera-Solano et al. reported an up to 28.5 dB reduction in sound pressure level for quiet T2 fast spin-echo and quiet T2 fluid attenuated inversion recovery (FLAIR) imaging [34]. In thoracic MRI, a respiratory-gated pointwise encoding time reduction and radial acquisition (PETRA) sequence was demonstrated to be an advantageous, silent alternative to conventional volumetric interpolated breath-hold (VIBE) imaging [35]. The open question of whether in utero exposure to MR imaging may lead to hearing impairment in newborns has been addressed in a study by Strizek et al. [36] The study showed no adverse effects on neonatal hearing function after exposure to routine imaging at 1.5 T in 751 neonates.
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Contrast Agents Paramagnetic (containing either gadolinium or manganese) and superparamagnetic iron oxide contrast agents are an integral part of many MRI exams in almost every body region. Adverse reactions to MRI contrast agents are usually mild and occur at a lower frequency (between 0.07 and 2.4 % [37]) than iodinated contrast agents used in computer tomography (CT). Hence, MRI contrast agents have long been recognized as combining a high diagnostic value with an excellent safety profile. However, in 2006, Grobner discussed a possible link between gadolinium-based MR contrast agents and a new disease, known as nephrogenic systemic fibrosis (NSF), alerting the medical community [38]. However, the occurrence was very low with approximately 40 confounded cases out of more than 40 million estimated administrations of contrast agents declared to be of high risk for developing NSF by the European Agency for the Evaluation of Medicinal Products (EMA) [39••]. Since late 2008, there have been no cases with clinical onset of NSF. The safety aspects in administering contrast agents for MRI and preventive measures to mitigate the risk of NSF have been excellently reviewed by Haneder et al. [39••]. Despite currently accepted preventive measures, it has recently been observed that gadolinium accumulates in some brain tissues after exposure to gadolinium-enhanced MRI, for example in the dentate nucleus; this accumulation has been verified in ex vivo tissue samples using mass spectroscopy and electron microscopy [40]. It has also been observed that gadolinium retention depends on the type of gadolinium-based contrast agent used, with a linear agent leading to more rapid accumulation than a macrocyclic agent used in one study [41]. The clinical relevance of these observations is still unclear, but it is likely that new guidelines for the use and application of gadolinium-based contrast agents will emerge in the near future.
Incident Reporting In a recent publication by Mansouri et al., incident reports related to MRI were collected at a large academic medical center in the U.S. over a period of more than 6 years [37]. The authors found 1290 reports out of 362,090 MRI exams, i.e., a rate of 0.36 %, and categorized them into different classifications. The most common reason for an MRI-related incident report was errors related to diagnostic test orders (31.5 %), followed by adverse drug reactions (19.1 %) and medication/safety of intravenous injection (14.3 %). Patient harm was identified in *1 out of every 1100 exams, with permanent or major patient harm in *1
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out of every 45,000 exams and death was found in *1 out of every 180,000 MRI exams. Overall, MRI-related incident reports in this study were an order of magnitude lower than hospital adverse event rates reported in the literature [37].
Conclusion Given the high technological complexity of MRI systems, with the inherent danger of strong magnetic fields and the deposition of RF power in tissue, high alertness and continuous training of staff are indispensable for maintaining a safe diagnostic environment for both patients and healthcare workers. Furthermore, due to the isolation of the patient during the rather long examination times compared to other modalities (CT, plain X-ray), special attention to the patient is of paramount importance, beginning with the ordering of the examination, continuing on during screening of the patient for possible contraindications, and persisting throughout the examination itself. Compliance with Ethics Guidelines Conflict of Interest Oliver Kraff and Mark E. Ladd each declare no potential conflicts of interest. Human and Animal Rights and Informed Consent This article does not contain any studies with human or animal subjects performed by any of the authors.
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