Ensuring Safety for Infants Undergoing Magnetic Resonance Imaging

Laura A. Stokowski, RN, MS

Adv Neonatal Care. 2005;5(1):14-27.

Biomedical Implant Hazards

A significant risk to some patients undergoing MRI is the way implanted or internal ferromagnetic material can react to the strong magnetic field and RF impulses. When implanted ferromagnetic material, such as a metallic aneurysm clip, is exposed to the magnetic field, it can be subjected to torque and translational forces strong enough to tear surrounding tissues.^[15] MR compatibility of these devices must be demonstrated by manufacturer's declaration (including conditions under which testing was conducted) or Food and Drug Administration (FDA) clearance.^[15] A list of biomedical implants and compatibility information is available online at www.MRIsafety.com .

The metallic cardiac occluders used in infants for patent ductus arteriosus ligation or ventricular septal defect occlusion can be made of weakly ferromagnetic material. They are considered safe 6 to 8 weeks following surgery, when enough tissue growth has occurred to cover the occluder and keep it in place.^[18] Certain cochlear implants used in infants have small internal magnets that experience substantial magnetic field interactions. These magnets must be removed by a brief surgical procedure before an MRI can be conducted.^[17] Safety and compatibility information about biomedical implants should be prominently documented in the patient's medical record.

Staff members accompanying infants to the MRI suite are also screened for biomedical implants or metallic foreign bodies that might create a hazard in proximity to the MR magnet^[7] (Fig 3). In addition, the MR system magnet can erase magnetically coded information on credit cards and passkeys and temporarily affect the function of analog watches.

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Comparing and Contrasting Neuroimaging Tests in Infants ^[4,5,6]
Neuroimaging Options	Primary Uses	Advantages	Disadvantages
Cranial Ultrasound	Screen for IVH in high-risk premature infants Monitor progression of GMH, IVH, PVL, ventriculomegaly Define cerebral anatomy Assess cerebral blood flow velocity	Mobile, portable Can be performed at any time; amenable to serial studies Bedside testing completed with minimal infant disturbance Full intensive care continues during examination Safe, harmless; no ionizing radiation	Limited accuracy in defining HIE, arterial infarcts Findings do not always correlate well with neurodevelopmental outcomes
Computed Tomography (CT)	Detect intracranial hemorrhage in infants with history of acute encephalopathy, significant birth trauma, and evidence of low hematocrit or coagulopathy Detect basal ganglia/thalamic lesions Detect calcifications	Often preferred over ultrasound in full-term infants at low risk for IVH and higher risk for other cranial processes Examination is quick Sedation not required Monitoring of patient possible during examination	Timing of the test impacts accuracy in detecting specific lesions Exposure to ionizing radiation Involves transport to radiology department
Magnetic Resonance Imaging (MRI)	Determine cause of early neonatal seizures, with or without signs of birth asphyxia Detect neonatal cerebral arterial infarction (stroke) Define pattern of tissue injury in neonatal encephalopathy Diagnose cerebellar malformations, superior ability to visualize the posterior fossa Diagnose CNS manifestations of metabolic disorders	Unparalleled sensitivity to changes in gray and white matter and ability to differentiate myelinated from unmyelinated white matter Does not use ionizing radiation; biologically harmless at current field strengths Excellent predictive value for outcomes related to neonatal encephalopathy^[6] Even more promising techniques on the horizon	Timing of the test impacts accuracy in detecting specific lesions Must transport infant to radiology department; challenge to continue provision of intensive care Difficult to monitor infant during examination Noisy Safety issues related to static magnetic field May require sedation of infant; sensitive to movement

Abbreviations: CNS, central nervous system; GMH, germinal matrix hemorrhage; IVH, intraventricular hemorrhage; PVL, periventricular leukomalacia; HIE, hypoxic-ischemic encephalopathy.

Magnetic Resonance Imaging Definition of Terms ^[9,10]
Diffusion-weighted imaging (DWI)	An imaging technique that analyzes the molecular motion of water in tissues such as the brain. Clinical applications include assessment of white matter pathology in preterm infants and term infants with neonatal encephalopathy.
Ferromagnetic	A substance, such as iron, that has a large, positive, magnetic susceptibility (ability to become magnetized when placed within a magnetic field).
5-Gauss line	Perimeter around an MR system within which the static magnetic fields are higher than 5 gauss. Five gauss and below are considered "safe" levels of static magnetic field exposure for the general public.
Flux	Invisible lines of force that extend around a magnetic material.
Fringe field	The region surrounding a magnet and exhibiting a magnetic field strength that is significantly higher than the earth's magnetic field. The size of the fringe field depends on the magnet type and field strength. The higher the field strength, the larger the fringe field. Fringe field is also called the stray field.
Gauss (G)	A unit of magnetic flux density. Gauss is 1 of 2 units used to measure magnetic field strength; the other and current preferred (SI) unit is the Tesla (10,000 Gauss = 1 Tesla).
Gradient magnetic fields	A magnetic field that changes in strength in a certain direction. Gradient magnetic fields are used in MR imaging with selective excitation to select a region for imaging and also to encode the location of MR signals received from area being imaged.
Magnetic resonance	Phenomenon resulting in the absorption and/or emission of electromagnetic energy by nuclei or electrons in a static magnetic field, after excitation by a radiofrequency magnetic field.
Magnetic resonance angiography (MRA)	Imaging of the flow of blood in the arteries and veins of the body. Clinical applications in infants include evaluation of arteriovenous malformations, vascular accidents (strokes), and assessment of carotid artery blood flow following extracorporeal membrane oxygenation (ECMO) therapy.
Magnetic resonance spectroscopy (MRS)	Utilizes the principle that nuclei in different chemical structures have different characteristic resonance patterns, or spectra. MRS is used to study brain biochemistry and metabolism. Clinical applications in the newborn include hypoxic-ischemic encephalopathy and inborn errors of metabolism. Often performed in conjunction with MRI.
Missile effect	Occurs when a ferromagnetic object rapidly and forcefully accelerates into the bore of a magnet. Also called the projectile effect.
MR-safe	The device, when used in the MR environment, has been demonstrated to present no additional risk to the patient, but may affect the quality of the diagnostic information.
MR-compatible	The device, when used in the MR environment, is MR safe and has been demonstrated to neither significantly affect the quality of the diagnostic information nor have its operations affected by the MR system.
Radiofrequency electromagnetic energy	Electromagnetic waves with a frequency band in the electromagnetic spectrum, or the same general range as that used for transmission of radio and television signals.
Radiofrequency pulse	Burst of RF energy delivered by the RF transmitter. These low-frequency pulses generate the signal that is measured during each scan. They also generate the heat associated with MRI (estimated by the specific absorption rate or SAR).
Room shielding	Magnetic shielding with high permeability in the walls, floor, and ceiling of the magnet room; can be complete or partial depending on the need to reduce the fringe field.
Rotational force	Twisting (torque) that occurs when a ferrous object attempts to align itself with the north-south orientation of the MRI magnet. Rotational force is greatest at the center of the magnetic field.
Quench	An unexpected loss of superconductivity caused by rapid increase in resisitivity of the magnet. Generates heat resulting in boiling off and rapid evaporation of cryogen (liquid helium). Can cause damage to the magnet and anoxic conditions in the atmosphere of the magnet room if not properly vented.
Specific absorption rate (SAR)	Energy deposited in tissues in the form of heat from the use of rapidly changing electromagnetic fields. The SAR is the amount of energy dissipated in the tissues, in watts per kg of tissue mass. Inhomogeneity of the RF field can lead to a local exposure where most of the power is absorbed by one body region. SAR limits (averages) are established by the FDA, usually per unit of time.
Static magnetic field	A component of the MR environment that is always present, even when the scanner is not imaging. This static magnetic field is typically between 0.2 and 2.0 tesla (T) measured in the center of the magnet bore. A 1.0 T magnet's static magnetic field would be roughly 20,000 times stronger than the earth's magnetic field.
Superconducting magnet	A type of magnet that has no electrical resistance when operated at temperatures near absolute zero.
Tesla	The preferred (standard international or SI) unit of magnetic flux density.
Time-varying magnetic fields (TVMFs)	Also called gradients, these are magnetic field changes introduced to spatially encode the MR signal during a scan. TVMFs can induce currents in conductive material lying within the rapidly changing magnetic field, such as muscles, nerves, and blood vessels of the human body, which are all conductive materials. TVMFs are also the source of loud noises within the scanner.
Translational force	The attraction that acts to draw an object linearly into the center of the magnet; it is the force that causes the projectile or missile effect.

Hazardous Magnetic Field Interactions
Magnetic Field Type	Hazard	Potential Adverse Effects
Static magnetic field	Translational force: powerful attraction of ferromagnetic object to intense magnetic field. Rotational force/torque: rotation of object to align with the magnetic field.	"Missile effect": acceleration of object into the bore of the magnet. Tearing of tissues, pain, dislodgement of some implants. Tearing of tissues, pain, dislodgement of some implants.
Radiofrequency electromagnetic fields	Heating due to absorbed RF energy. Electromagnetic interference.	Overheating, burns (thermal, electrical). Device malfunction; imaging artifact.
Gradient magnetic field	Induced currents in conductive tissues. Induced currents in electrical devices.	Nerve and muscle stimulation. Device malfunction/failure.

Adapted from Center for Devices and Radiological Health. A Primer on Medical Device Interactions With Magnetic Resonance Imaging Systems. U.S. Department of Health and Human Services, Food and Drug Administration, 1997.

Sidebar: How Does a Magnet Create an Image?

A basic understanding of the process of MR image acquisition will help caregivers take safe actions and avoid unsafe ones, when caring for infants undergoing MRI. When the infant is placed in the bore of the scanner, 2 things happen to the hydrogen atoms that comprise the fat and water of the body's tissues. The usually randomly oriented protons (A) contained in the nuclei of the hydrogen molecules become uniformly aligned either parallel or antiparallel to the powerful magnet field (B). In addition, the protons, which normally spin on their axes much like the earth spins on its axis, start spinning around the direction of the external magnetic field.^[11]

Next, the coils surrounding the infant's head or torso deliver short bursts of electromagnetic waves called radiofrequency (RF) pulses. This energy is absorbed by the protons, causing them to resonate. The applied energy deflects the protons off their alignment with the magnetic field and causes them all to briefly spin in phase with one another.^[11]

When the RF pulse is off, the superenergized protons recover their alignment and out-of-phase spin, a process called relaxation.^[11] During relaxation, protons release the stored energy, emitting detectable RF signals. Relaxation behavior is strongly dependent on the tissue type in which the nucleus is located (e.g. gray matter, white matter, cerebrospinal fluid). These differences in relaxation times are the source of tissue contrast in the MR image.^[2]

The signals that are returned do not have spatial information, so 3 magnetic field gradients are switched on and off with the RF pulses. Known as time varying magnetic field gradients, these are brief, low-strength currents applied to provide unique magnetic fields that help to localize signals to the region of interest. This encoding process allows control of the location and thickness of the slice of tissues that will be excited by magnetic resonance.¹²

The signals transmitted from the infant are transferred via the receiving coils to a computer, where they are translated into detailed 2- or 3-dimensional anatomic images of the tissues of interest. A great deal of data is obtained to generate the final MR image. The more data points collected, the more detailed the final image.^[11]

(Enlarge Image)

Randomly oriented spinning protons (A) aligning within a magnetic field (B) at the start of the MRI process. BMJ . 2002;324:35. Used with permission from the BMJ Publishing Group.

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Ensuring Safety for Infants Undergoing Magnetic Resonance Imaging

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Sidebar: How Does a Magnet Create an Image?

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