Prilozi, Odd. biol. med. nauki, MANU, XXXIII, 1, c. 441–472 (2012)
Contributions, Sec. Biol. Med. Sci., MASA, XXXIII, 1, p. 441–472 (2012)
ISSN 0351–3254
UDC: 616-073.763.5
MAGNETIC RESONANCE IMAGING SAFETY: PRINCIPLES
AND GUIDELINES
Stikova E
Faculty of Medicine, Public Health Institute, Ss Cyril and Methodius
University, Skopje, R. Macedonia
A b s t r a c t: This paper provides an overview of the published literature
regarding magnetic resonance imaging (MRI) safety. It gives basic information about
the electromagnetic fields that are generated during the MRI diagnostic procedure, followed by a description of the biological effects of those fields on humans. It concludes
with a discussion of the safety issues related to MRI use in clinical practice, highlighting the existing MRI standards and guidelines.
Key words: magnetic resonance imaging, biological effects of electromagnetic radiation, MRI standards, safety guidelines.
Introduction
The 20th century saw an increase in the exposure of humans to electric
and magnetic fields, due to the growing electricity demands of modern society.
The widespread use of industrial equipment, communication devices and modern home appliances has resulted in increased levels of man-made electromagnetic radiation.
According to its frequency (measured in Hertz, Hz) and its ability to
ionize an atom or molecule in biological tissues, the electromagnetic (EM)
radiation is classified in two categories – ionizing and non-ionizing (Hansson
Mild, Alanko et al. 2009; Hartwig, Giovannetti et al. 2009).
The ionizing radiation refers to X-rays and gamma rays, which have the
ability to produce ionization of the biological tissues and cell damage. The non-
442
Stikova E ionizing part of the electromagnetic spectrum refers to static electromagnetic
fields, radiofrequency and optical waves. Figure 1 illustrates the EM spectrum
and the range of frequencies for different EM subclasses. (Hartwig, Giovannetti
et al. 2009)
Figure 1 – Electromagnetic spectrum and main application
of different electromagnetic fields
Medical diagnostic imaging is recognized as an important source of
human exposure to ionizing and non-ionizing electromagnetic radiation (X-ray,
CT, MRI, and ultrasound). While it is well established that ionizing radiation
(X-ray, CT) poses risks to human health mainly because of the ionizing of tissues, the available data on possible effects of non-ionizing radition on patient
and worker safety are not sufficient to draw firm conclusions (Hartwig, Giovannetti et al. 2009; International Commission on Non-Ionizing Radiation 2009).
In the last decades the use of non-ionizing radiation in medical imaging
has rapidly increased, mainly because of the more frequent use of magnetic
resonance imaging – MRI (Picano 2004). MRI is an imaging technique designed to visualize internal structures of the body using electromagnetic fields to
induce a resonance effect in hydrogen atoms. MRI provides a good contrast between the different soft tissues in the body, which makes it especially useful
in imaging the brain, muscles, and the heart. MRI procedures have been utilized
in clinical diagnostic practice for approximately 30 years. The use of MRI in the
clinical setting started in the 1980s. Since then, more than 100,000,000 diagnostic procedures have been completed worldwide, with relatively few major
incidents (Formica and Silvestri 2004).
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Magnetic resonance imaging safety: principles and guidelines
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Figures 2 and 3 show that the total number of MRI units in 27 OECD
countries has increased from 811 in 1990 to more than 17,000 two decades later
Source: OECD Health Data 2011 Statististics Figure 2 – Total number of MRI units in 27 OECD countries in the period 1980–2008
Source: OECD Health Data 2011 Statististics
Figure 3 – MRI units in 27 OECD countries (in % of total number)
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Stikova E with participation of US with 44% in total number of MRI units. The total number of MRI units varies from a few MRI units per one million populations in
Hungary, the Netherlands, Poland, Slovenia to more than 20 in Iceland, Italy
and the US.
The biggest number of MRI units is registred in Japan with 43 units/one
million population (Figure. 4).
Source: OECD Health Data 2011 Statististics
Figure 4 – Number of MRI units in 27 OECD countries per million population
(by country) Source: OECD Health Data 2011 Statististics
The number of examinations per 1000 population varies from 1.7/1000 in
Chile to 67.2/1000 in Turkey, 73.9/1000 in Luxemburg and 75.5/1000 in Iceland.,
reaching highest values of 91.2/1000 population in United States and 97.9/1000 in
Greece (OECD 2011). Magnetic Resonance Imaging is considered a safe technology due to its
ability to change the position of atoms without altering their structure, composition, or properties. With the growing number of magnetic resonance systems,
one concern related to the use of MRI is possible adverse biological effects, but
there are also other concerns related to the effects of magnetic fields on patients,
equipment and personnel (Dempsey, Condon et al. 2002; Coskun 2011; Boutin,
Briggs et al. 1994; Colletti 2004).Therefore, it is necessary to evaluate the possible risks and various biological effects caused by the MRI fields on human
Contributions, Sec. Biol. Med. Sci., XXXIII/1 (2012), 441–472
Magnetic resonance imaging safety: principles and guidelines
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health (Abart and Ganssen 1995). It is also necessary to develop guidelines and
protocols aimed at protecting the safety and health of staff and patients.
The MRI practices and safety guidelines target different focus groups:
MR physicists, research/academic radiologists, MR safety experts, electrical
engineers and/or manufactures, legal counsellors and others (Shellock and
Kanal 1991). There are also legislative issues that are country-specific, making
it difficult to reach a consensus about safe MR practices. In the absence of such
consensus, attempts should be made to observe the precautionary principle and
to avoid unnecessary examinations (Hartwig, Vanello et al. 2011).
This paper provides an overview of the published literature and the
main existing safety guidelines, with focus on possible biological effects and
safety issues relevant for the use of MRI as a medical diagnostic procedure. The
aim of the paper is to give basic information about the electromagnetic fields
that are generated during the MRI diagnostic procedure, their sources and physical characteristics, to briefly describe the potential biological effects and important safety issues related to MRI use in clinical practice, and to clarify the
necessity of maintenance of an appropriate MRI safety policy including application of safety standards (required or voluntary) and good practices.
To this effect, the paper is divided into three sections:
I. Basic information about MRI,
II. Biological effects of MRI, and
III. Safety issues in MRI
I. BASIC INFORMATION ABOUT MAGNETIC RESONANCE
IMAGING
The technology used for magnetic resonance (MR) procedures has evolved continuously during the past 40 years, yielding MR systems with stronger
static magnetic fields, faster and stronger gradient magnetic fields, and more
powerful radiofrequency transmission coils.
The following three magnetic field types are produced by the basic
components of an MRI system (Formica and Silvestri 2004; Hartwig, Giovannetti et al. 2009):
• High static magnetic field (B0) – it generates a net magnetization in
the human body and is responsible for alignment of nuclei;
• Gradient magnetic fields (dB/dt) – caused by cycling of power
through the gradient coils; they are used to localize aligned protons inside the
body allowing spatial reconstruction of tissue sections into images;
• Radio frequency magnetic fields (B1) (10 to 400 MHz) – responsible for exciting the patient's protons within the stable magnetic field and for
positioning of nuclei during the imaging process.
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Stikova E The static magnetic field is the main magnetic field in MRI that is used
to align the patient's protons in an organized and predictable fashion. It is
usually generated by a strong superconducting magnet and is represented by the
symbol B0. The static magnetic field is measured at the exact centre of the magnet at the intersection of the three orthogonal planes (X, Y, Z). This central
point is called the isocentre. The strength of the static field (Bo) is measured in
tesla (T), with one tesla equal to 10,000 gauss (G). Clinical imaging systems
today typically have static field strengths up to 3T. Spectroscopic systems,
usually used for research, are available with field strengths as high as 17.5T
(Hartwig, Giovannetti et al. 2009; International Commission on Non-Ionizing
Radiation 2009).
The very powerful magnet in the centre of the scanner is always on
even when the machine is not scanning, including all night, every weekend, and
even holidays. This means that a very strong static magnetic field is always present in the MRI environment.
The second type of magnetic field used in MRI is the gradient magnetic field. All MR imaging systems are equipped with a set of resistive wire
windings known as gradient coils. Gradient magnetic fields are created by the
cycling of power through the gradient coils and are called gradient or time
varying magnetic fields (TVM). The gradient magnetic fields are used to control
the selective excitation of the patient's protons. Time-varying magnetic fields
are superimposed on the static magnetic field to obtain spatial information in
MR imaging. These fields are of low magnitude compared with the static field.
There are 3 sets of gradient coils capable of producing a TVMF in all
three orthogonal directions (X, Y, and Z). For oblique imaging, gradient coils
are used in combination. Figure 5 is an example of the gradient coils creating a
varying magnetic field in the Z direction.
Source: Roth R.T.C. (1996)
Figure 5 – A representation of the magnet bore and the time-varying magnetic field
in the Z-direction
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Magnetic resonance imaging safety: principles and guidelines
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The third type of magnetic field used in MRI is the radiofrequency (RF)
magnetic field. During an MRI exam, the patient is exposed to RF radiation in
the range of 10 to 400 MHz (Shellock 2000; Formica and Silvestri 2004). The
source of this radiation is the radiofrequency coil that surrounds the patient
inside the magnet bore. Radio waves are used to excite the patient's protons
within the stable magnetic field.
II. BIOLOGICAL EFFECTS OF MRI
a) Biological effects of static magnetic fields
Interest in the biological effects of static magnetic fields has increased
with the invention of MRI. Several studies were carried out in order to understand the potential hazards associated with exposure to a strong static magnetic
field. The majority of these studies did not report positive results, thus postulating no adverse effects for human health (Formica and Silvestri 2004).
Static magnetic fields can interact with biological systems by exerting
forces on molecules and cells having diamagnetic susceptibility. They can also
affect enzyme kinetics and act on moving charges (including moving fluids).
The metabolic functions of human tissues require a large quantity of chemical
reactions, and it is therefore reasonable to suppose that a strong magnetic field
might alter the rates or the equilibrium conditions of these reactions (Formica
and Silvestri 2004).
With regard to short-term exposure, investigations include studies on
alteration in cell growth and morphology, cell reproduction and teratogenicity,
DNA structure, blood-brain barrier permeability, nerve activity, cognitive function and behaviour, haematologic parameters, circadian rhythms and other biological changes. The majority of these studies concludes that static electromagnetic fields produce no substantial harmful biological effects (Abart and Ganssen 1995; Schwenzer, Bantleon et al. 2007).
Only some sensory effects have been found associated with exposure to
a static magnetic field. There was a statistically significant (p < 0.05) finding for
sensations of nausea, vertigo, and metallic taste in subjects exposed to 1.5 and 4
T static magnetic fields, but no statistical significance was found for other
effects such as headache, hiccups, tinnitus, vomiting, and numbness. A higher
incidence of positive reports originated from those subjects exposed to a 4 T
field. However there was no evidence that these effects were at all harmful
(Schenck, Dumoulin et al. 1992; Formica and Silvestri 2004).
Many studies have been carried out on the in vitro effects of static
magnetic fields on cell growth, cell proliferation, cell cycle distribution pattern
and apoptotic cell death. The cells were not affected by an exposure up to four
days at field strengths up to 10T, while an exposure of 10–17T for 30–60 minuPrilozi, Odd. biol. med. nauki, XXXIII/1 (2012), 441–472
448
Stikova E tes can reduce number and size, cell organization and vitality in cultured mammalian cells.
These studies suggest that static magnetic fields might affect the process of cancer induction and/or progression by altering cellular responses to
some known carcinogens (chemicals, radiation). However, the findings available in the literature are often not comparable and in some cases also not reproducible making a definitive conclusion premature (Karpowicz, Gryz et al. 2011).
No changes in gene expression were found in exposed or sham-exposed
cells at the end of a two-hour exposure for any of the 498 tested protein genes.
The results suggest that MRI has no influence on protein-gene expression in
eugenic human lung cells (Schwenzer, Bantleon et al. 2007). The results of
other studies showed that the static magnetic field alone and MRI sequences at
3 Tesla have no effect on the induction of double–strand breaks (DSBs) in cells
(Schwenzer, Bantleon et al. 2007; Simi, Ballardin et al. 2008; Hartwig, Giovan‐
netti et al. 2009).
One phenomenon that has been observed is a magneto-hydrodynamic
effect (Shellock and Crues 2004). When a conductor moves through a magnetic field, a current is induced in the conductor. This phenomenon is present
during the blood flowing through the heart. The current induced in the blood
can produce the change in an elevation of the S-T segment on the patient's electrocardiogram (Figure 6). The degree of change is directly related to the strength
of the magnetic field. This change is present only when the patient is in the bore
of the magnet, and it is not associated with any serious bio-effects. The effects
are totally reversible by removing the patient from the magnet. However,
because an elevated S-T segment can be indicative of myocardial infarction,
ischaemia, or an electrolyte imbalance, it is imperative that any patient with
compromised cardiovascular function should be closely monitored with an
MRI-compatible pulse oximeter and/or blood pressure monitor (McKenna,
Laxpati et al. 2008).
Different studies, modelling and calculations of possible exposure dose
and time, suggest three groups of adverse effects of the static magnetic field to
the cardiovascular system as follows:
• minor changes in the rate of heart beat,
• induction of ectopic heart beats and
• increase in the likelihood of re-entrant arrhythmia (possibly leading
to ventricular fibrillation).
The first two are thought to have thresholds in excess of 8 T, while the
threshold values for increase in the likelihood of re-entrant arrhythmia is difficult to assess because of the modelling complexity.
Contributions, Sec. Biol. Med. Sci., XXXIII/1 (2012), 441–472
Magnetic resonance imaging safety: principles and guidelines
449
Source: Roth, R.T.C. (1996)
Figure 6 – Reversible increase in the amplitude of the T-wave noted
on electrocardiograms inside the magnet bore
In 2006, the World Health Organization stated that there is no evidence
on the short- and long-term adverse effects of MRI static MF on human health.
With regard to the long-term effects, there are interactions between tissues and
static magnetic fields that could theoretically lead to pathologic changes in
human subjects. There are no carefully controlled studies that demonstrate the
absolute safety of chronic exposure to powerful magnetic fields. Considering
the increased use of MR scanners with higher static MF values, there is aneed to
perform more studies to provide assurance about their safety (Franco, Perduri et
al. 2008).
b) Biological effects of gradient fields
During an MRI examination, the time-varying (gradient) magnetic
fields are often switched on and off. The biological response is based on the
opportunity of gradient fields to induce electrical currents in any conductor (so
called "eddy currents") and the ability of some biological tissues to act as a conductor. The human body has several excellent conductors: nerves, muscles, and
blood (So, Stuchly et al. 2004). The biological effects are related to the changes
in the magnetic field that induces the current (Stuart, Trakic et al. 2007). The
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Stikova E magnitude of the induced current depends on the strength and speed of the
gradient fields and the resistance of the conductor.
The most studied phenomenon that is produced by fast-varying gradient
fields is peripheral nerve stimulation (PNS). The time-varying magnetic fields
induce an electric field (current), which stimulates nerves and muscles, and this
could result in involuntary twitching, especially in the periphery of the bore
where the gradient variations are greatest. Direct stimulation of peripheral nerves could cause muscle contractions or sensations of tingling and itching,
depending on whether the stimulated nerve is a sensory or motor nerve (Abart,
Eberhardt et al. 1997). The peripheral nerve stimulation may cause discomfort
to the subjects, resulting in the subject moving in the scanner or possibly requesting the operator to stop the examination (So, Stuchly et al. 2004).
The stimulation threshold for nerves and muscles depends on the ratio
between rheobase and chronaxie.
The rheobase is the minimal electric current of infinite duration (practically, a few hundred milliseconds) that results in an action potential or the contraction of the muscle cell: 6–20 V/m for peripheral nerve and cardiac stimulation and 30–40 V/m for cortex. The calculated rheobase threshold for brain stimulation can be lower for patients with epilepsy, subjects with a family history
of seizures, users of anti-depressants, and neuroleptics, making them more vulnerable for MRI examination.
Chronaxie is the minimum time over which an electric current twice the
strength of the rheobase needs to be applied, in order to stimulate a muscle fibre
or nerve cell: 200–350 microseconds for peripheral nerve, 350–450 microseconds
for cortex.
Several studies in vivo have been performed to obtain gradient-induced
stimulation thresholds in animals and in humans (Bourland, Nyenhuis et al.
1999; Bencsik, Bowtell et al. 2002; Formica and Silvestri 2004). The mean
thresholds for peripheral stimulation in head and body are similar and the greatest frequency of reported peripheral stimulations occurs when the y-gradient is
used (Bourland, Nyenhuis et al. 1999; Den Boer, Bourland et al. 2002).
If the stimulation is in the cardiac muscle, it could theoretically disrupt
the normal cardiac cycle and lead to an arrhythmia. The cardiac stimulation could
cause ventricular fibrillation and it would be a very serious life-threatening condition (Glover 2009, Schaefer, Bourland et al. 2000).
Direct stimulation of the retina and/or optic nerve can produce a phenomenon known as magnetophosphenes and it occurs when retinal phosphenes are stimulated by induction from TVMF. Magnetic phosphenes can
cause a flashing sensation in the eyes that, together with disturbance of eyehand coordination, could affect the optimal working performances of the staff
executing delicate interventional MRI procedures.
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The time varying magnetic fields also produce acoustic noise, which
can reach up to 140 dB for some sequences (Shellock, Ziarati et al. 1998). Levels
greater than 100 dB may result in disturbance in microcirculation in the brain
and produce mechanical damage (McJury 1995; McJury and Shellock 2000).
In conclusion, MRI gradients can produce peripheral nerve stimulation,
and even cardiac stimulation in extreme cases. With the advent of the new
generation of MR systems characterized by higher static MF and faster gradient
fields, their effects on human health should be the object of further and properly
designed studies.
c) Biological effects of radiofrequency fields
The primary biological effects associated with exposure to RF radiation
are related to the thermogenic qualities of electromagnetic fields. The interactions of RF fields with biological tissues depend on the field frequency, type of
field (electric, magnetic, far-field, near-field) and the body size and shape. In the
case of MR systems, the spatial distribution of RF energy depends on the design
of the transmitter coils, the frequency, and the shape, size and tissue type of the
imaged object (Grandolfo, Polichetti et al. 1992; Abart and Ganssen 1995).
Fortunately, the human body has a very efficient thermoregulation system and can adequately respond to temperature increase by increasing the capillary flow. Several organs, such as eyes and testicles, cannot quickly respond to
heating, mainly because of their very poor capillary network. Because of this
they are under increased risk of thermal injury.
The parameter used for quantification of the effects is called the specific absorption rate – SAR (Gandhi and Chen 1992; Grandolfo, Polichetti et al.
1992). The SAR is the dose rate, defined as the rate at which RF energy is
imparted into a unit mass of the exposed biological body. SAR is measured in
watts per kilogram, and its maximum value should be less than 2 W/Kg for the
human head, according to the guidelines of the International Commission on
Non-Ionizing Radiation Protection – ICNIRP (Ahlbom, Bergqvist et al. 1998).
A whole-body averaged SAR of 6.0 W/kg can be physiologically tolerated by
individuals with normal thermoregulatory function. Further studies were conducted on volunteers exposed to whole-body average SARs ranging from
approximately 0.05 W/kg to 4.0 W/kg. These experiments document that body
temperature changes were always less than 0.6°C, though there were statistically significant increases in skin temperature, but without serious physiological
consequences (Shellock and Crues 1987; Shellock 2000; Shellock 2002).
During a MR scan the patient’s temperature is not easy to measure, so
SAR represents a convenient parameter to control any possible temperature
increases. Generally, the MRI scanner software allows monitoring of the SAR
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Stikova E for the whole body (Shellock and Crues 1987). If the SAR value exceeds the
standard limits, the software stops the scanning process.
Various underlying health conditions may affect an individual's ability
to tolerate a thermal challenge, including cardiovascular disease, hypertension,
diabetes, fever, old age, and obesity. In addition, medications such as diuretics,
β-blockers, calcium blockers, amphetamines, muscle relaxants, and sedatives
can also greatly alter thermoregulatory responses to a heat load (Shellock and
Crues 1987; Shellock 2002). In conclusion, interactions between RF and biological tissues during
MR procedures could be unsafe for patients (Formica and Silvestri 2004). Most of
the reported accidents are burns due to hot spots in the presence of conducting
materials close to the patient such as the leads of equipment for monitoring physiological parameters (heart rate, blood pressure, oxygen saturation and temperature). This kind of risk can be more serious in the case of internal biomedical
implants (aneurism clips, stents, etc.) especially for implants that have elongated
configurations and/or are electronically activated (neurostimulation systems, cardiac pacemakers) (Dempsey and Condon 2001; Kanal, Borgstede et al. 2004).
III. SAFETY ISSUES IN MRI
The technology used for magnetic resonance (MR) procedures has
evolved continuously, yielding MR systems with stronger static magnetic fields,
faster and stronger gradient magnetic fields, and more powerful radiofrequency
transmission coils. Most reported cases of MR-related injuries and the few fatalities that have occurred have apparently been the result of failure to follow
safety guidelines, or of use of inappropriate or outdated information related to
the safety aspects of biomedical implants and devices. To prevent accidents in
the MR environment, therefore, it is necessary to revise the information on biological effects and safety according to changes that have occurred in MR
technology and with regard to current guidelines for biomedical implants and
devices.
All staff must be aware of and trained to eliminate the MR hazards. It is
important to emphasize that many safety investigations have been carried out on
1.5T scanners, although in the last few years many centres install magnets of
3.0T and above. Because of this, further investigations will be regularly required to reassure staff and patients of their safety.
The MR safety policy and procedure should be reviewed and adjusted
to the newest scientific results and evidence-based information. This should
apply to all segments of the MRI environment – equipment, working regime
(e.g. adding faster or stronger gradient capabilities or higher RF duty cycle studies), hazard identification, measurement, workplace risk assessment and health
Contributions, Sec. Biol. Med. Sci., XXXIII/1 (2012), 441–472
Magnetic resonance imaging safety: principles and guidelines
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surveillance of exposed workers. The establishment of obligatory screening forms
for all patient, staff, family members and other people who enter in appropriate
MR’ zones is a very important part of safety practices. Finally, the appointment
of a well-trained and experienced MR safety officer for all MRI suites is essential for the development, implementation and maintenance of the appropriate
safety procedure.
Along with these people-oriented strategies of policies and training,
organizations also need to adopt the strategies of safety-oriented architectural
and interior design (International Commission on Non-Ionizing Radiation 2009).
Usually, MR equipment manufacturers provide design safety templates that
meet the minimum technical sitting requirements for the specific equipment.
However, appropriate procedures for design of patient/family waiting area,
interview areas, physical screening/changing areas, access controls, storage,
crash carts, induction, medical gas services, holding areas for patients after screening, infection control provisions, and interventional applications, are not
usually addressed in a typical vendor-provided safety design template.
a) Safety issues related to the static magnetic fields
Most safety issues concerning strong static magnetic fields are related
to the effects caused by the stray magnetic field (Baltisberger, Hediger et al.
2005). The portion of the stray static magnetic field that extends outside of the
bore of the magnet is called the fringe field (Kok, Raaymakers et al. 2009);
this field is measured in gauss (G) units. The fringe field should be less than or
equal to 5 G at the outside perimeter of the room (Iwamiya and Sinton 1996;
Kanal, Borgstede et al. 2002; Barbic and Scherer 2006).
The extent of the fringe field depends on:
• the main magnet field strength,
• the design of the magnet (open versus tunnel bore), and
• the shielding (active versus whole room shielding).
Source: Roth, R.T.C. (1996)
Figure 7 – Stray magnetic field – fringe field at the edges of the magnet bore
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Stikova E Projectiles
The strong static magnetic field can attract ferromagnetic metal objects,
which can act as projectiles. The strength of attraction is inversely proportional
to the square of the distance, which means that the force quadruples as the
object's distance from the bore is halved. A ferromagnetic object can reach speeds of up to 45 miles per hour on its way into a 1.5-T magnet. Such iron-containing metal objects can become powerful projectiles and can cause serious injury
or even death (Chaljub, Kramer et al. 2001).
All equipment, regardless of its function, should be tested with a strong
hand magnet of at least 3000 G before it is used in or around the MR scanner. If
the equipment is found safe after testing, it should be labelled as MRI-safe and
used only in the MRI suite (Gilk 2006; Gilk 2007; Gilk 2009; Gill and Shellock
2012).
Torque
Magnetically induced torque causes objects to rotate to align along the
magnet’s field lines. Torque is a twisting force on objects and is greatest in the
centre of the bore magnet, where it can cause ferromagnetic implants and foreign bodies to align with the magnetic field. The potential for an injury depends on the strength of the magnetic field, type and location of the implant,
physical dimensions and orientation of the implant, and the length of time the
implant has been in place (Becker, Norfray et al. 1988).
The two greatest potential complications of implant-related torque are
the eyes and brain. All patients with a history of metal-related eye injury and/or
sheet metal work or welding that do not have positive confirmation of the
absence of intra-ocular metal should be screened with conventional x-rays
before undergoing MRI. Research has indicated that metal fragments too small
to be seen on conventional radiographs (i.e. smaller than 0.1 × 0.1 × 0.1 mm)
are too small to damage the ocular tissue. Also, because intracranial aneurysm
clips can be ferromagnetic, implants in the brain should be approached with
caution. All patients with intracranial aneurysm clips should be excluded from
MRI unless the clip was tested before implantation. If the clip demonstrates no
attraction, it is probably MRI-compatible and the patient's operative notes
should be annotated accordingly (Shellock and Shellock 1999; Shellock and
Spinazzi 2008).
Other devicies that could produce implant-related torque effects are:
• Heart valves,
• Dental devices and materials (12 of 16 dental implants showed
measurable deflection to the magnetic field),
• Otological implants,
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• Ocular implants,
• Intra-ocular ferrous foreign bodies,
• Orthopaedic implants, materials and devices,
• Surgical clips and pins,
• Electrically, magnetically or mechanically activated or electrically
conductive implanted devices,
• Pacemakers – cardiac pacemakers are an absolute contraindication
for MRI. (Martin, Coman et al. 2004; Shinbane, Colletti et al. 2007; Shinbane,
Colletti et al. 2011).
Most forms of medical or biostimulation implants are generally considered contraindications for MRI scanning. These include pacemakers,vagus nerve
stimulators, implantable cardioverter-defibrillators, loop recorders, insulin pumps,
cochlear implants, deep brain stimulators and capsules retained from capsule
endoscopy (Teitelbaum, Bradley et al. 1988). Patients are therefore always
asked for complete information about all implants before entering the room for
an MRI scan. Several deaths have been reported in patients with pacemakers
who have undergone MRI scanning without appropriate precautions (Shinbane,
Colletti et al. 2011). To reduce such risks, implants are increasingly being developed to make them MR safe, and specialized protocols have been developed to
permit the safe scanning of selected implants and pacing devices. Cardiovascular stents are considered safe, as are titanium and its alloys (Teitelbaum,
Bradley et al. 1988; Teitelbaum, Yee et al. 1990).
Labelling
In 1997, the FDA's Center for Devices and Radiological Health
(CDRH) first proposed terms to be used to label MRI information for medical
devices. At that time devices were classified as MR safe or MR compatible
(Shellock, Woods et al. 2009). In 2005 the terminology was revised to adapt to
the increasing variety of available MR scanners and to make it clearer that MR
compatible devices are not necessarily MR safe. The current labelling categories are:
• MR Safe – an item that poses no known hazards in all MRI environments. "MR Safe" items are non-conducting, non-metallic, and non-magnetic. The "MR Safe" icon consists of the letters ‛MR’ in green in a white
square with a green border, or the letters ‛MR’ in white within a green square.
• MR Conditional – an item that has been demonstrated to pose no
known hazards in a specified MR environment with specified conditions of use.
The special conditions for use are determined by listing the acceptable static
magnetic field strength, spatial gradient magnetic field, slew rate (time rate of
change of the magnetic field), radio frequency (RF) fields, and specific absorption rate (SAR). The "MR Conditional" icon consists of the letters ‛MR’ in
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Stikova E black inside a yellow triangle with a black border. A device or implant would be
MR conditional if it contains magnetic, electrically conductive or RF-reactive
components that are safe for operations in proximity to the MRI, under the
defined conditions for safe operation (such as 'tested safe up to 1.5 Tesla' or
'safe in magnetic fields below 500 Gauss') (Greenberg, Weinreb et al. 2011).
• MR Unsafe – an item that is known to pose hazards in all MRI
environments. MR Unsafe items include magnetic items. The "MR Unsafe"
icon consists of the letters ‛MR’ in black on a white field inside a red circle with
a diagonal red band.
MR SAFE
MR CONDITIONAL
MR UNSAFE
Source: Kanal, E., A. J. Barkovich, et al. (2007)
Figure 8 – Safety labelling icons used in the MRI environment
Signs should be posted to deter possible entry into the scan room with
ferromagnetic objects and to prevent the occurrence of a possible dangerous
event (Shellock, Woods et al. 2009).
The very high strength of the magnetic field can also cause "missileeffect" accidents, where ferromagnetic objects are attracted to the centre of the
magnet, and there have been incidences of injury and death. To reduce the risks
of projectile accidents, ferromagnetic objects and devices are typically prohibited in proximity to the MRI scanner and patients undergoing MRI examinations
are required to remove all metallic objects and fill out pre-screening forms for
identification of risky issues (Sawyer-Glover and Shellock 2000).
Zoning of MR facilities and working positions
The MRI environment is the area within the 0.50 mT (5 Gauss) line of
an MR system, which includes the entire three-dimensional volume of space
surrounding the MR scanner. For cases where the 0.50 mT line is contained
within the Faraday shielded volume, the entire room is considered to be the MR
environment (Robitaille, Warner et al. 1999).
The MRI related space is divided in two categories:
Contributions, Sec. Biol. Med. Sci., XXXIII/1 (2012), 441–472 Magnetic resonance imaging safety: principles and guidelines
457
• A Controlled area is any area where the static magnetic field may
exceed 0.5 mT, or where gradient or RF fields may interfere with electronic
devices. This area should be controlled, by warning signs or some other means,
in order to exclude people who may have pacemakers, or implanted defibrillators or neurostimulators. Any areas adjacent to the MRI equipment to which
there is unrestricted access (corridors, adjacent rooms or outside areas) should
have fields less than 0.5 mT and be shielded from RF fields.
• A Restricted area is any area where the static magnetic fields may
exceed 3 mT, and it should be restricted to qualified MRI staff, or people under
the supervision of such staff. Qualified MRI staff are expected to be aware of
precautions which must be taken inside such areas and the possible hazards that
may arise. This would normally include MRI operating staff and maintenance
engineers, and possibly trained cleaning staff. Ferromagnetic materials, and
anything which has not been shown to be compatible with MRI equipment,
should not be brought into the restricted area (Shellock and Crues 2004).
The controlled and restricted areas of the MR site are conceptually divided into four zones (Kanal, Barkovich et al. 2007) and they are shown in Figure
9 below.
Zone I: This area is freely accessible to the general public and typically
is outside the MR environment itself. This is the place through which patients,
health care personnel, and other employees of the MR site access the MR environment.
Zone II: This area is the interface between the publicly accessible,
uncontrolled Zone I and the strictly controlled Zones III and IV. Typically, patients’ movement in Zone II is not free and should be supervised by MR personnel.
Zone III: This area is the region large enough to contain the 0.5 mT (5
Gauss) magnetic field contour and in which free access of unscreened non-MR
personnel is not allowed. The use of any ferromagnetic objects or equipment
can result in serious injury or death. All access to Zone III is strictly restricted,
and all entrances should be controlled and supervised by MR personnel. The
access to Zone III should be physically restricted by key locks, passkey locking
systems, or any other reliable, physically restricting method. Usually this zone
is called a controlled area. The access restrictions should be displayed at all
entrances in accordance with the IEC 60601-2-33 standard.
Zone IV: This area is the magnet room where the MR scanner is
located. This zone is laid out to contain the 3 mT (30 Gauss) magnetic field contour and the so-called inner controlled area. Zone IV, by definition, will always
be located within Zone III. Zone IV should also be demarcated and clearly marked because of the presence of very strong magnetic fields. Zone IV should be
clearly marked with a red light and lighted sign stating, "The Magnet is On".
The movement of staff in this area is restricted, and exposure assessment should
Prilozi, Odd. biol. med. nauki, XXXIII/1 (2012), 441–472
458
Stikova E be observed in the scope of the usually measured and proposed values of
magnetic strength near the bore of the magnet.
Source: Kanal, E., A. J. Barkovich, et al. (2007)
Figure 9 – Typical zone configuration in MRI facilities and strength
of the static magnetic field
The MRI activities of MRI staff level II in MRI room (zone IV) are performed in 4 typical areas (Stuart, Trakic et al. 2007) as folows:
• Area A – whole body exposure,
• Area B – exposure of hand and/or head if the worker is leaning
towards the patient,
• Area C – hand exposure,
• Area D – hand and/or whole body exposure when the worker
approaches very closely to the patient leaning against the magnet bore.
Figures 10 shows the typical position and working operation of the MR
staff in zone IV and the typical average strength of the static magnetic field. This
Figure 10 – Clasification of the zone 4 regarding the strength of the static field
Contributions, Sec. Biol. Med. Sci., XXXIII/1 (2012), 441–472
Magnetic resonance imaging safety: principles and guidelines
459
information would be very useful for estimation of occupational exposure, especially in a case of lack of nationally permitted total exposure and action levels
and personal dosimeters.
Classification of MR Personnel
The classification of the MR personnel is made using the criteria for
MRI environment zoning and in accordance with the risk of the general public,
patients and staff. The main purpose for this classification is to determine the
movement restriction and to prevent adverse health effects and dangerous
events. The main criteria for this classification are justification of the need for
entrance to the MRI environment based on education level, job activities and
safety training (Kanal, Barkovich et al. 2007).
Regarding these criteria, there are three categories of workers with permission to enter and move through the MRI environment zones:
• Level 1 MR Personnel
Individuals who have passed minimal safety educational efforts to
ensure their own safety while working in Zone III will be referred to as level 1
MR personnel (e.g. MRI department office staff and patient aides). They are
responsible for accompanying non-MRI personnel throughout zone III.
• Level 2 MR Personnel
Individuals who have been more extensively trained and educated in the
broader aspects of MR safety issues, including issues related to the potential for
thermal loading or burns and direct neuromuscular excitation from rapidly changing gradients, will be referred to as level 2 MR personnel (e.g., MRI technologists, radiologists, and radiology department nursing staff). They are permitted
to move through all zones and they are responsible for the admission of patient
and other non-MRI personnel to zone IV.
• Non-MR Personnel
Non-MR personnel are all patients, visitors, or facility staff who do not
meet the criteria of level 1 or level 2 MR personnel. All non-MR personnel wishing to enter Zone III must first pass an MR safety screening process using the
same screening form as patient, non-MRI and MRI personnel. Only MR personnel are authorized to perform an MR safety screen before permitting non-MR
personnel into Zone III (Kanal, Borgstede et al. 2002; Kanal, Borgstede et al.
2004; Kanal, Barkovich et al. 2007).
b) Safety issues related to the effects of MR time-varying fields
Acoustic noise
As current is passed through the gradient coils during image acquisition, a significant amount of acoustic noise is generated (Price, De Wilde et al.
Prilozi, Odd. biol. med. nauki, XXXIII/1 (2012), 441–472
460
Stikova E 2001). the noise results from Lorentz forces in the gradient coils and the noise
levels could be as high as 140 dB. As gradient amplitudes increase and rise
times decrease, the acoustic noise becomes greater.
MRI noise may cause the patient annoyance, anxiety, difficulties in verbal communication, distress, temporary hearing loss and even potential permanent hearing problems (Kanal, Borgstede et al. 2004; Kanal, Barkovich et al.
2007). Temporary hearing loss has been reported using conventional MRI sequences. Gradient noise may also interfere with patient communication or affect staff
and visitors (McJury, Blug et al. 1994; McJury, Stewart et al. 1997; McJury and
Shellock 2000). An acceptable and inexpensive means for the prevention of
hearing loss is the regular use of disposable earplugs. Earplugs should be made
available to anyone present in the imaging room during a scan, especially if
noise levels exceed 94 dBA. Commercially available earplugs can reduce the
noise levels associated with MRI by as much as 20 decibels (McJury, Stewart et
al. 1997). There are also commercially available noise cancellation systems or
anti-noise systems that reportedly can lower the noise levels by up to 70%
(Price, De Wilde et al. 2001).
Peripheral nerve stimulation (PNS)
All commercial MRI scanners have a limit on the maximum slew rate
(dB/dt) allowed, preventing the fast gradient switching that causes PNS. While
most MRI sequences are well below the slew rate (dB/dt) limit, sequences that
utilize echo-planar imaging (fMRI, diffusion) can sometimes result in peripheral
nerve stimulation and cause the subject discomfort. In such cases, the subject
should notify the operator, and the scan should be aborted.
c) Safety issues related to the effects of RF electromagnetic fields
Heating of implants and other medical devices
RFelectromagnetic waves can cause heating in metallic implants and
foreign bodies by inducing a current (Nitz, Brinker et al. 2005). Without a pathway to exit the body, the current builds up, and the implant gets warm. The degree of warming depends on the strength and rate of change of the magnetic field
and the conductivity of the implant. Military shrapnel, metal slivers, and wires
are all susceptible to heating. If these items are located in or around the eyes,
spine, heart, or brain, serious and irreversible damage can occur (Schaefer,
Bourland et al. 2000).
Heat stress
Pulses of radio-frequency (RF) energy heat the body. This heat can be
dissipated by the body, provided that the rate of heating is not excessive(Nitz,
Brinker et al. 2005). The human body has a very efficient thermoregulatory system and can handle reasonable changes in temperature. Even though the eyes
Contributions, Sec. Biol. Med. Sci., XXXIII/1 (2012), 441–472
Magnetic resonance imaging safety: principles and guidelines
461
and testicles have a poor capilary network and cannot quickly dissipate the heat
by increasing capillary flow, the risks are believed to be minimal. Some conditions (cardiovascular disease, hypertension, diabetes, fever, old age and obesity)
and some medications (diuretics, beta-blockers, calcium blockers, amphetamines, muscle relaxants and sedatives) may impair the normal cooling mechanisms. There are no adverse effects in the case of whole body exposure if the
rise in the core body temperature is less than 1oC for healthy adults and less
than 0.5oC for infants and persons with cardio-circulatory impairment (Shellock
2000).
Thermal burns
While not very common, thermal burns can occur, especially in patients
who are sedated during the scan. Tattoos and permanent cosmetics realized with
iron oxide or other metal-based pigments can cause reactions or adverse events,
including first and second-degree local burns. Certain types of underclothing
with antimicrobial silver microfibres (usually worn under braces to prevent
infection) have also resulted in local burns, so the best safety practice is to ask
the subjects to change into a hospital supplied gown. Also, during the scan the
subjects should be discouraged from crossing or forming loops with their arms,
to prevent induction of currents in the extremities.
d) Other MR related safety issues
Quench – cryogen safety
Currently, all superconducting magnets use liquid helium to maintain
the magnet coils at a temperature enabling superconductivity. The helium and
magnet coils are maintained in a vacuum. The temperature of liquid helium is
approximately -269°C or 4.17 degrees K, close to the liquid helium boiling temperature. Any disruption to the temperature or loss of the vacuum will cause the
liquid helium within the magnet to "boil off" causing an immediate and sudden
loss of the magnetic field. This event is referred to as a "quench". When this
occurs, the helium will expand and for every litre of liquid helium, 760 litres of
gas will be produced. This marked expansion in volume causes the gas to rush
out of the magnet and, under normal conditions, be vented outside the scan
room and building.
In the event of a quench, the patient should be removed from the scan
room as quickly as possible. Although not immediate, asphyxiation can occur
from breathing helium. Additionally, because of the extremely cold temperature
of helium gas, frostbite is also a concern. The quench produces a marked
increase in air pressure inside the scan room, which makes it difficult to open
the scan room’s exit door immediately. In the event of a large change in room
pressure, ruptured eardrums are also a possibility.
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462
Stikova E Every site should have a written policy establishing what to do in the
event of a quench. MR equipment (including the examination room and any
other areas where the coolant may be stored) should be installed with an appropriate exhaust system to vent gases to the outside in the event of a quench.
As a precaution in a case of quenching, an emergency evacuation procedure should be established and rehearsed. Oxygen monitors with an alarm audible
in the control room should be in the examination room and other areas where
coolants are stored. It is recommended that emergency quench buttons be installed both inside and outside the examination room.
Contrast agent safety
Several Gadolinium-based contrast agents have been introduced, and
more are in development. Adverse reactions have been relatively few and
mostly minor, and cautionary notes are included with the packaging. Contrast
agents should be administered with the same care and attention as any other
pharmaceutical product (Kanal, Broome et al. 2008).
Some agents have been shown to cross the placenta readily. When the
rate of clearance from the amniotic fluid is not known, a careful assessment of
the possible risks and benefits of contrast agents in pregnant women should be
made. Caution should be exercised before administering contrast agents to lactating mothers, as some agents are excreted in breast milk.
There are no well-defined policies for patients who are considered to be
at increased risk for having an adverse reaction to MR contrast agents. However, patients who have previously reacted to one MR contrast agent, all patients
with asthma or a history of allergic respiratory disorders are at higher risk of
adverse reaction and they should be premedicated with corticosteroids and antihistamines (Kanal, Barkovich et al. 2007).
Another contrast-based safety issue is the occurrence of a very rare
disease known as nephrogenic systemic fibrosis (NSF). The disease is seen only
in patients with severely impaired renal function. The first case was observed in
1997 and the disease was formally described in 2000 (Shellock, Parker et al.
2006; Kuo, Kanal et al. 2007). A few weeks or months prior to the onset of the
disease the patients had been administered gadolinium-based MR contrast
agents (GBMRCA). Literature data from Denmark suggest that 5% of patients
with severely impaired renal function treated with GBMRCA developed NSF.
The disease is characterized with increased tissue deposition of collagen (Boyd,
Zic et al. 2007), often resulting in thickening and tightening of the skin, predominantly of the distal extremities but also of the trunk, as well as fibrosisof
other parts of the body (heart, lungs, kidneys, periphery vasculaturesand skeletal
muscles) (Boyd, Zic et al. 2007). The disease is progressive and can be fulminant in approximately 5% of cases and can even be associated with a fatal
Contributions, Sec. Biol. Med. Sci., XXXIII/1 (2012), 441–472
Magnetic resonance imaging safety: principles and guidelines
463
outcome (Broome, Girguis et al. 2007). The disease predominantly affects middle-aged patients, with special predilection for patients with concurrent hepatic
disease (Grobner and Prischl 2008).
Based on the literature data, for patients with moderate to end-stage kidney disease, imaging methods other than MRI with a gadolinium-based contrast
agent should be recomended whenever possible (Kanal, Broome et al. 2008). If
these patients must receive a gadolinium-based contrast agent, prompt dialysis
following the MRI or MRA should be considered because the average excretory
rates of gadolinium is 78%, 96%, and 99% in the first to third haemodialysis
sessions. Special recommendation and guidelines are developed for patients
with kidney diseases (Grobner and Prischl 2007; Rosenkranz, Grobner et al.
2007).
The ACR Contrast Committee and the Subcommittee for MR Safety
recommends pre-screening of patients prior to the administration of Gadolinium-based MR Contrast Agents (GBMCA). It is recommended that prior to
elective Gadolinium-based MR Contrast Agent (GBMCA) administration, a
recent (e.g. last 6 weeks) Glomerular Filtration Rate (GFR) assessment be
reviewed for patients with a history of: renal disease (including solitary kidney,
renal transplant, renal tumour), age > 60, history of hypertension, history of diabetes, history of severe hepatic disease/liver transplant/pending liver transplant
(Kanal, Borgstede et al. 2004; Kanal, Barkovich et al. 2007; Kuo, Kanal et al.
2007).
Claustrophobia
Some patients may experience claustrophobia, even if they have no
personal or family history of the condition. This may be precipitated by the
restricted space inside the MR bore, anxiety about the scan and the possible
outcome, noise, and the duration of the examination (Kanal, Borgstede et al.
2002; Kanal, Borgstede et al. 2004; Kanal, Barkovich et al. 2007). Staff should
be aware that claustrophobic reactions may occur, and of the techniques which
may avert them.
Although the majority of these effects are transient, it is important to
have controllable air movement within the bore of the magnet to maintain a
comfort zone for patients (Spouse and Gedroyc 2000). This, along with good
patient contact and education, could help reduce claustrophobic reactions,
which clearly are a safety concern (Kanal, Borgstede et al. 2004).
Patient preparation and screening forms
Patient preparation involves many steps. The patient should be instructted to wear loose, comfortable clothing without zippers, snaps, or buttons (Shellock and Spinazzi 2008; International Commission on Non-ionizing Radiation
2009) For most examinations, patients can wear elastic-waisted, cotton sweat
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464
Stikova E pants and a plain T-shirt. Female patients should wear a sports bra without
hooks or wires; any other type of bra should be removed before the patient
enters the room. All accessories should be removed. Accessories include rings,
bracelets, watches, earrings, and body piercings. Face and eye make-up should
also be removed. If the patient arrives wearing clothes with zippers or other
metal, he/she should change into an appropriate gown (Shellock and Spinazzi
2008). The gowns should have loose-fitting sleeves so that the technologist can
check for watches and bracelets; loose sleeves are also an asset if the patient
requires an injection of contrast agent. The patient gowns should not have
pockets. It will be much more difficult for a patient to accidentally bring contraindicated materials into the room without pockets (Kanal, Borgstede et al. 2002;
Kanal, Borgstede et al. 2004; Kanal, Barkovich et al. 2007).
Occupational risk, standards and classification
Many organizations are involved in MRI safety and health regulation
and almost all of them are part of the World Health Organization’s framework.
The leading organization for producing and implementation of MRI safety standards and guidelines is the International Commission of Non-ionizing Radiation
Protection (ICNIRP). Usually their proposals and guidelines are incorporated in
EU Directives and standards produced by the European Committee for Electrotechnical Standardization (CENELEC), International Technical Commission
(IEC) and Institute of Electrical and Electronics Engineers (IEEE). How they
are connected between them and how they can support the preparation and
adoption of the national legislation is shown in the figure below.
Figure 11 – Organizations involved in EMF Regulation and Standardization
Contributions, Sec. Biol. Med. Sci., XXXIII/1 (2012), 441–472
Magnetic resonance imaging safety: principles and guidelines
465
The main document dealing with occupational exposure of workers in
Europe is the physical agents directive (PAD) 2004/40/EC. In addition to protecting workers occupationally exposed to non-ionizing radiation, the implementation of this Directive into EU legislation would have limited the use of MRI,
due to the restrictive exposure limits for MR staff. As a result of strong opposition from the MRI community, Directive 2004/40/EC was amended by Directive 2008/46/EC in April 2008, with the effect of postponing by four years the
deadline for transposition of Directive 2004/40/EC. On June 14, 2011 the European Commision adopted a proposal to derogate the medical and research use of
MRI from the exposure limit values. Workers using MRI will be protected
through existing regulations, which ensure its safe use. The established MR
safety standard IEC/EN 60601–2–33 defines criteria for minimizing physiological effects due to exposure to time-varying electromagnetic fields, thus aiming
to eliminate danger to patients and workers.
In the light of these recent proposals, many MR safety studies have
been conducted since 2004. A 2008 review summarized the studies on the
health effects of occupational exposure to static magnetic fields, and no firm
conclusions were drawn about these effects (Perrin and Morris 2008). However,
there are still concerns about workers repeatedly exposed to magnetic fields
exceeding regulatory limits with respect to modern MRI. There is a proposal for
a mandatory ambulatory magnetic field dosimeter capable of measuring the
fields in and around an MRI scanner in order to evaluate the regulatory guidelines and determine any underlying exposure risks. The numerical analysis shows
that at certain positions surrounding the MRI scanner edges, such fields may
induce current densities and electric fields that may exceed the relevant EU,
ICNIRP, and IEEE standards (Fuentes, Trakic et al. 2008; International Commission on Non-ionizing Radiation 2009). Usually, the time-weighted average magnetic field exposures in 1.5 T, 2 T, and 4 T were all within the regulatory limits
but the peak limits for the exposure of the head to static magnetic field measurements can be exceeded in some circumstances, especially when workers are
standing close to the gradient coils and when all three gradients are switched on
simultaneously (Gilk 2007; Fuentes, Trakic et al. 2008).
Conclusion
As the number of MRI scanners rises and the MRI technology advances, there is a growing need for understanding the biological effects of MRI and
for maintaining the safety of the patients and the MR personnel. While the
effects of exposure to MRI fields remain an active research area, clinical experience shows that MRI is a safe imaging modality, and that most of the risks are
preventable by obeying the safety guidelines listed above. Coordination betPrilozi, Odd. biol. med. nauki, XXXIII/1 (2012), 441–472
466
Stikova E ween local regulatory bodies and the organizations involved in EMF regulation
and standardization is essential for keeping the guidelines current and for ensuring that MRI remains a safe diagnostic procedure.
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Stikova E Резиме
БЕЗБЕДНОСТ ПРИ ИМИЏИНГ СО МАГНЕТНА РЕЗОНАНСА:
ПРИНЦИПИ И ПРЕПОРАКИ
Stikova E
Медицински факулtеt, Инсtиtуt за јавно здравје,
Универзиtеt Св. Кирил и Меtодиј, Скоpје, Р. Македонија
А п с т р а к т: Овој труд е преглед на постоечката литература за безбедност
при имиџинг со магнетна резонанса (ИМР). Трудот ги систематизира основните
информации за електромагнетните полиња што се создаваат при дијагностичката
процедура, а потоа дава опис на биолошките ефекти на овие полиња врз човекот.
Прегледот завршува со дискусија за безбедноста при употребата на ИМР во
клиничка практика, со посебен акцент на безбедносните процедури и стандарди.
Клучни зборови: имиџинг со магнетна резонанса, биолошки ефекти на електромагнетно зрачење, ИМР стандарди, безбедносна процедура.
Corresponding Author:
Prof. Elisaveta Stikova MD, PhD
University "Ss. Cyril and Methodius"
Faculty of Medicine
National Public Health Institute
Skopje, R. Macedonia
E-mail: elisaveta.stikova@fulbrightmail.org
Contributions, Sec. Biol. Med. Sci., XXXIII/1 (2012), 441–472