An overview of electromagnetic safety considerations associated with magnetic resonance imaging

External Fixation Devices Within the Magnetic Resonance Imaging Bore: A Safety and Radiologic Analysis

OBJECTIVES

To (1) report the thermal changes encountered at the pin/skin interface in a cadaver with a knee-spanning external fixator inside the magnetic resonance imaging (MRI) bore and (2) report on the quality of the MRI sequences collected.

METHODS

Three commonly used external fixation systems were placed on cadaveric lower extremities to simulate knee external fixation. Fiber optic thermal probes were placed at the pin/skin interface of a femoral and tibial pin. A control probe was embedded in the soft tissues of the thigh. Full knee MRI scans were performed using a 1.5-Tesla magnet. Real-time thermal data were collected. A clinically significant increase in temperature compared with the control was defined as 2°C. Two blinded radiologists evaluated the images for image quality and overall diagnostic utility using a standardized 5-point grading scale.

RESULTS

There were statistically significant differences in the temperature changes between the femoral/tibial pin sites and the control probe sites during each phase of the MRI scan. However, there was only one clinically significant difference in temperature change during a single sequence of one MRI scan of one of the external fixator devices. Overall image quality was graded as a 4 for each image set with 100% interobserver agreement (k = 1.0).

CONCLUSIONS

Despite significant differences in temperature changes between the pin sites and controls over multiple MRI sequences in commonly used external fixator devices, the differences in temperature change are likely not clinically relevant. Overall image quality and interpretability of the images were excellent.

Magnetic Resonance Imaging of Trauma Patients Treated With Contemporary External Fixation Devices: A Multicenter Case Series

OBJECTIVES

To report the safety and clinical outcomes of placing current magnetic resonance imaging (MRI) components inside and outside the MRI bore during MRIs.

DESIGN

Retrospective case series.

SETTING

Four trauma centers (3 Level I and 1 Level II), from January 2005 to January 2015.

PATIENTS

All patients who had MRIs with external fixators in place either inside or outside the MRI bore.

INTERVENTION

MRI of patients with external fixator in place.

MAIN OUTCOME MEASUREMENTS

Adverse events were defined as catastrophic pullout of the external fixator during the MRI, thermal injury to the skin, severe field distortions precluding the intended imaging, alterations of the magnetic field, or visible structural damage to the magnet casing.

RESULTS

Thirty-eight patients with 44 external fixators were identified who had MRI with the fixator inside or outside the MRI bore. Twelve patients with 13 external fixators had MRI with the external fixator inside the MRI bore. Twenty-seven patients with 32 external fixators had MRI with the external fixator outside the MRI bore. There were no adverse events.

CONCLUSIONS

Although no universal guidelines exist, there are circumstances in which obtaining MRIs of patients with external fixators can be safe. This is the first clinical series with the primary outcome of safety when placing modern external components both inside and outside an MRI bore during a scan.

LEVEL OF EVIDENCE

Prognostic Level IV. See Instructions for Authors for a complete description of levels of evidence.

L'uomo elettromagnetico e la tomografia a RM

In questi ultimi anni gli effetti biologici indotti dai tre campi energetici dell'imaging a RM sono passati in seconda linea rispetto agli effetti esercitati sui materali ferromagnetici e sui dispositivi elettronici sensibili presenti nel corpo del soggetto esaminato. Tuttavia, l'aumento dei valori di campo previsto in un prossimo futuro rinnova l'interesse sull'interazione magnetobiologica vera e propria.

Come base di partenza per comprendere meglio gli effetti biologici correlati con gli esami a RM, l'autore prende in esame le componenti elettriche, magnetiche ed elettromagnetiche dell' uomo elettromagnetico. Nella review di aggiornamento, vengono considerati gli effetti sul sistema nervoso centrale e periferico, sull'apparato cardiocircolatorio, sullo sviluppo embrionale, gli effetti da esposizione cronica e professionale ad alti campi, gli effetti su cellule in coltura e su sistemi enzimatici.

Un particolare sviluppo è riservato al significato biologico degli impulsi di gradiente temporale (dB/dt) e degli impulsi a radiofrequenza e viene anche proposta una ipotesi dell'autore relativa alla genesi di infrasuoni e ultrasuoni nei tessuti del paziente per effetto dei gradienti e, rispettivamente, degli impulsi a radiofrequenza.

I fatti esaminati portano a concludere che l'odierna, apparente innocuità dell'imaging a RM potrebbe non essere più tale con l'aumento del campo magnetico statico oltre i 2–4 T e con l'aumento conseguente dell'onda a RF, del SAR e dei gradienti.

Magnetic resonance safety

Magnetic resonance imaging (MRI) has a superior soft-tissue contrast compared to other radiological imaging modalities and its physiological and functional applications have led to a significant increase in MRI scans worldwide. A comprehensive MRI safety training to protect patients and other healthcare workers from potential bio-effects and risks of the magnetic fields in an MRI suite is therefore essential. The knowledge of the purpose of safety zones in an MRI suite as well as MRI appropriateness criteria is important for all healthcare professionals who will work in the MRI environment or refer patients for MRI scans. The purpose of this article is to give an overview of current magnetic resonance safety guidelines and discuss the safety risks of magnetic fields in an MRI suite including forces and torque of ferromagnetic objects, tissue heating, peripheral nerve stimulation, and hearing damages. MRI safety and compatibility of implanted devices, MRI scans during pregnancy, and the potential risks of MRI contrast agents will also be discussed, and a comprehensive MRI safety training to avoid fatal accidents in an MRI suite will be presented.

Safe use of subdermal needles for intraoperative monitoring with MRI

OBJECTIVE

The purpose of this study was to develop safe, site-specific procedures for placing and leaving subdermal needle leads for intraoperative monitoring (IOM) during intraoperative MRI procedures.

METHODS

The authors tested a variety of standard subdermal needle electrodes designed and FDA-approved for IOM in the conventional operating room. Testing was used to determine the conditions necessary to avoid thermal injury and significant image artifacts with minimal disruption of IOM and MRI procedures. Phantom testing was performed with a fiber optic (lead) temperature monitoring system and was followed by testing of leads placed in a healthy volunteer. The volunteer testing used electrode placements typical of standard IOM cases, together with radiofrequency (RF) coil placement and imaging sequences routinely employed for these case types. Lead length was investigated to assess heating effects for electrodes placed within the RF coil.

RESULTS

The authors found that conventional stainless steel (SS) and platinum/iridium (Pt/Ir) subdermal needles can be used safely without significant heating when placed outside the RF coil, and this accounts for the majority or entirety of electrode placements. When placed within the RF coil, Pt/Ir leads produced minimal image artifacts, while SS leads produced potentially significant artifacts. In phantom testing, significant heating was demonstrated in both SS and Pt/Ir leads placed within the RF coil, but only during high-resolution T2-weighted scanning. This problem was largely, but not completely, eliminated when leads were shortened to 25 cm. Human testing was unremarkable except for nonpainful heating detected in a few electrodes during thin-slice (1.5 mm) FLAIR scanning. Transient irritation (skin reddening along the needle tract) was noted at 2 of the electrodes with detectable heating.

CONCLUSIONS

The authors were satisfied with the safety of their site-specific procedures and have begun with off-label use (following institutional review board approval and obtaining patient informed consent) of tested monitoring leads in cases that combine IOM and MRI. The authors recommend that all facilities perform their own site-specific testing of monitoring leads before proceeding with their routine use.

Implementation of a comprehensive MR safety course for medical students

This review article proposes the design of an educational magnetic resonance (MR) safety course for instructing medical students about basic MR and patient-related safety. The MR safety course material can be implemented as a traditional didactic or interactive lecture in combination with hands-on safety demonstrations. The goal of the course is to ensure that medical students receive a basic understanding of MR principles and safety considerations. This course will prepare medical students for patient screening and safety consultations when ordering MR studies. A multiple-choice exam can be used to document the proficiency in MR safety of the medical students. The course can be used by various medical school programs and may help to ensure consistent quality of teaching materials and MR safety standards.

Origins of intraoperative MRI

Neurosurgical diagnosis and intervention has evolved through improved neuroimaging, allowing better visualization of anatomy and pathology. This article discusses the various systems that have been designed over the last decade to meet the requirements of neurosurgical patients and opines on the potential future developments in the technology and application of intraoperative MRI. Because the greatest amount of experience with intraoperative MRI comes from its use in brain tumor resection, this article focuses on the origins of intraoperative MRI in relation to this field.

Origins of intraoperative MRI

Neurosurgical diagnosis and intervention has evolved through improved neuroimaging, allowing better visualization of anatomy and pathology. This article discusses the various systems that have been designed over the last decade to meet the requirements of neurosurgical patients and opines on the potential future developments in the technology and application of intraoperative MRI. Because the greatest amount of experience with intraoperative MRI comes from its use in brain tumor resection, this article focuses on the origins of intraoperative MRI in relation to this field.

Testing the diagnostic value of electrical ear canal stimulation in cochlear implant candidates by functional magnetic resonance imaging

Prior to cochlear implant (CI) surgery in children, the integrity of the auditory pathway is sometimes assessed by electrical ear canal stimulation (ECS). However, the evaluation of reactions as auditory is subjective. To test the prognostic value of ECS, functional magnetic resonance imaging (fMRI) was performed during ECS vicariously in 18 adult CI candidates. Activation of the primary auditory cortex was detected in 9 of 16 cases when auditory sensations during ECS occurred, and tended to be more bilaterally distributed in CI candidates than in normal-hearing controls. ECS sensations only tended to correlate with fMRI activations. However, solely frequency discrimination during electrical stimulation predicted CI outcome, but neither other auditory sensations nor fMRI activations did so satisfactorily, which limits the diagnostic value of these measures. Instead, preoperative residual hearing (nonamplified and amplified) was a robust predictor for CI benefit.

A comprehensive analysis of MRI research risks: in support of full disclosure

Magnetic resonance imaging (MRI) procedures have been used for over 20 years. This modality is considered relatively safe and holds great promise. Yet, MRI has a number of risks. In order for MRI research to meet the Canadian standard of disclosure, the investigator must communicate and make note of all risks in their research protocols and consent forms. Those creating and reviewing research protocols and consent forms must take notice of the different circumstances under which MRI poses a risk. First, this paper will describe the current standard of disclosure in Canada for research participants. Second, the paper will provide a comprehensive synthesis of the known physical and psychological risks associated with MRI. Third, the paper will provide recommendations concerning areas for further investigation and risk reduction strategies. This information will thus equip researchers and research ethics boards (REBs) with the criteria needed for the composition of research protocols that meet the Canadian disclosure standard.

Advanced imaging: Magnetic resonance imaging in implant dentistry

For accurate and safe placement of dental implants, and planning of associated surgery, a full assessment of the surgical anatomy of the site is necessary. Thus, it is highly desirable to have tomographic, sectional information available, to permit the implant to be aligned correctly. In recent years, X-ray computed tomography (CT) has become accepted as the gold standard in assessment, but the exposure to ionising radiation can be substantial. Artefacts due to dental restorations can also be significant, and some doubts may exist over the accuracy of reformatted CT. Magnetic resonance imaging (MRI) entails no exposure to ionising radiation, and allows direct acquisition of tomographic information in any desired plane. Sequential studies may be safely performed, allowing us a valuable insight into bone graft behaviour. Other than in a small number of cases, MRI may be safely used for presurgical assessments. Artefacts are few and in most cases localised. The surgical confidence from the sectional information gained is a significant step forward in the safe placement of dental implants.

Thermal injuries associated with MRI

Most physicians are aware of the absolute contraindications to magnetic resonance imaging (MRI). However, less familiar is the potential for an MRI-induced thermal or electrical burn associated with electrical monitoring devices. Although detailed studies concerning the burn hazard in MRI have not been reported, it is widely believed that direct electromagnetic induction in looped cables associated with the patient is responsible for the excessive heating and it is on this theory that present guidelines are based. Recent reports have however indicated that other mechanisms may cause the heating of metal, either in or on the patient. This document reviews numerous reported burn injuries sustained during MRI and addresses the underlying heating mechanisms possibly causing these events.

Accessory equipment considerations with respect to MRI compatibility

The MR Section of The National Electrical Manufacturers Association (NEMA), in response to a request from the Food & Drug Administration (FDA), recently issued a position paper to address generic issues related to the compatibility of accessory equipment produced by third party equipment manufacturers or MR equipment users and intended to be used in conjunction with MR equipment or within the MR scanning room. The recommendations concern scanning accessories, such as radiofrequency (RF) coils, patient monitoring equipment and injectors, as well as patient comfort accessories and positioning devices. The following issues related to safety performance are discussed: (a) the interaction of the equipment with the MR scanner, (b) interactions of the MR scanner with the equipment, and (c) potential safety hazards for patients and staff that can be posed by accessory equipment in the MR scan environment. The recommendations are based on combined input from NEMA member companies who manufacture MR systems and MR accessories and are presented for consideration in the design of MR accessory products and incorporation of these concepts into testing plans to ensure MR compatibility of third party devices.

Blood-brain barrier permeability in rats is altered by exposure to magnetic fields associated with magnetic resonance imaging at 1.5 T

We have previously reported that exposure of rats to low-field (0.15 T) magnetic resonance imaging (MRI) increases blood-brain barrier (BBB) permeability. However, a number of investigators have failed to observe this effect when high-field MRI (1.5 T or higher) is used. Therefore, we investigated whether or not we would observe changes using our technique at these higher fields. Adult male Sprague-Dawley rats were anaesthetised and then exposed to a 22.5 min imaging or sham procedure. Immediately following exposure, rats were injected with 1 MBq of 153Gd-DTPA intracardially and then immediately re-exposed for an additional 22.5 min. The rats were killed 1h following the second MRI exposure, at which time the brain was resected and 3 ml of venous blood collected. The ratio of radioactivity per gram of brain to radioactivity per milliliter of blood, known as the brain-blood partition coefficient, was determined and used as a measure of BBB permeability. Groups of animals had different exposures. Group 1 (n = 9) was exposed to a clinically relevant MRI procedure. Group 2 (n = 20) was exposed to the same procedure except the rf specific absorption rate (SAR) was reduced to 25% and the animals were positioned 15 cm from imager centre to increase the time-varying magnetic field from 0.4 to 2.8 T/s. For the sham exposures (n = 21), the animals were placed in the imager with the static field ramped down to zero and exposed to a sound recording simulating a MRI examination.(ABSTRACT TRUNCATED AT 250 WORDS)