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MRI Safety Overview - ForcesZachary W. Friis, Ph.D., DABR
EM Fields in MRI
Three types of EM Fields in MRI:
The static magnetic field created by the main magnet assembly (~ B0 at the isocenter) Time-Varying or Gradient magnetic fields created by the gradient coils
Radio-Frequency magnetic fields created by the RF transmitters
Static Magnetic Field B0
Main magnet to align patient’s protons along longitudinal
magnetization
Ranges from Ultra Low Field (B0 < 0.2T) to High Field (1.0T < B0
< 3.0T) to Ultra High Field (B0 > 7.0T)
Most magnets are superconducting (others being resistive
and permanent)
Liquid Helium (~ 1700 Liters) refrigerated in a highly
evacuated environment ~ 4.2K
Active/Passive shielding
“Is 60,000 times the strength of Earth’s magnetic field high enough?!”
Static Magnetic Field B0
The magnetic field gives a force proportional to electrical charge
and velocity force perpendicular to the direction of motion. force perpendicular to the direction of motion → bending of trajectory
Static Magnetic Field B0
In an accelerator the magnetic field bends and controls the size of the beam
Same effect on the electron bean in a color CRT
Static Magnetic Field B0
HOW TO DETECT A MAGNETIC FIELD
In theory: the field B will exert a pull F=BIL on a wire of length L carrying an electrical current I
In practice:
use one of the many handheld teslameter (Gauss meter) available on the market
Static Magnetic Field B0
HOW TO MEASURE A MAGNETIC FIELD
Earth field
0.5G 50 uT
Permanent magnet (typical)
100 ~ 1000 G 10 ~ 100 mT
1 Tesla = 10,000 Gauss
MRI / Accelerator electromagnet (typical)
1 ~ 100 kG 0.1 ~ 10 T
Static Magnetic Field B0
EXAMPLES OF MAGNETS
Permanent Magnet
Resistive Magnet
Superconducting Magnet
Static Magnetic Field B0
• Magnetically induced Force
• Objects tend to be pulled towards the region there the field is stronger
Static Magnetic Field B0
CALCULATION OF MAGNETIC FORCE
Assume the object has an overall magnetic dipole moment p. The magnetic force Fm on the object is approximated by
Fm = ( p ·∇ ) B In Cartesian coordinates, the magnetic force is then expressed as
Static Magnetic Field B0
Consider now the force on a object only along the z-axis, the force then is:
Consider an object of volume V and saturation magnetization Ms, the force is then
Z-axis
where µ0 = 4π x 10-7 H/m is the permeability of free space
Static Magnetic Field B0
Translational force is proportional to the spatial gradient:
Don’t confuse the term “spatial gradient” with “time – varying gradient (dB/dt)” “Spatial gradient” magnetic field refers to the rate the static magnetic field strength changes over distance, using the unit of T/m or gauss/m.
Static Magnetic Field B0
ISOGAUSS PLOT OF 3.0T ACTIVELY SHIELDED MAGNET
Static Magnetic Field B0
MEASURED TRANSLATIONAL FORCE VERSUS DISTANCE ALONG THE AXIS OF BOAR OF A 1.5 T MR SYSTEM
Positive distance is away from the center of the bore and a distance of 0 corresponds to the edge of the bore
Static Magnetic Field B0
MEASURED TRANSLATIONAL FORCE VERSUS DISTANCE ALONG THE AXIS OF BOAR OF A 1.5 T MR SYSTEM
Positive distance is away from the center of the bore and a distance of 0 corresponds to the edge of the bore
Static Magnetic Field B0
The gravity force (weight) on the object is where ρm is the mass density and g = 9.8 m/s2
Z-axis
Fg
Fm
Static Magnetic Field B0
Now we can calculate the ratio of magnetic force to gravity force:
For an iron object with Ms ≈ 2.2 T,, ρm = 8,900 kg/m3 and a field gradient of 2 T/m, the ratio can go as high as 40 times the gravity force!!!
Static Magnetic Field B0
Transport Stretchers: around 50 lb, force around 2,000 lb
Oxygen cylinder: around 15-20 lb, force around 600-800 lb
Static Magnetic Field B0
FATAL ACCIDENT
• Michael Colombini was in the bore of a GE Signa • An anesthesiologist was in the scan room with the
patient. The built-in oxygen delivery system failed • A nurse heard the anesthesiologist calling for
oxygen, the nurse delivered a STEEL oxygen cylinder to the anesthesiologist thinking it was safe (ALUMINUM)
• Within 3 to 6 feet of the magnet, the STEEL cylinder flew into the bore, fatally injuring the patient
Static Magnetic Field B0
Office chairs: around 5-10 lb, force around 200-400 lb
Scissors: around 4 ounce (0.25 lb), force around 10 lb
Static Magnetic Field B0
Static Magnetic Field B0
ASTM F2052 standard test method for Magnetically Induced Force
Acceptance Criterion: Magnetically Induced force is less than object weight
Static Magnetic Field B0
ASTM International (formerly American Society for Testing and Material)
ASTM task group F04.15.11 on MR Safety and Compatibility of Materials and Medical Devices 5 Standards addressing the principal issues that produce safety concerns for implants and other devices in the MR environment
a) ASTM F2052-06 for Measurement of Magnetically Induced Displacement Force on Medical
Devices in the MR Environment b) ASTM F2119-01 for Evaluation of MR Image Artifacts from Passive Implants c) ASTM F2182-02a for Measurement of Measurement of Radio Frequency Induced Heating Near
Passive Implants During MRI d) ASTM F2213-06 for Measurement of Magnetically Induced Torque on Medical Devices in the MR
Environment e) ASTM F2503-05 Standard Practice for Marking Medical Devices and Other Items for Safety in
the Magnetic Resonance Environment
Static Magnetic Field B0
• Magnetically induced Torque
• Objects magnetized preferentially along longest dimension
• Objects tend to align to the field
Static Magnetic Field B0
MAGNETIC TORQUE
Geometry for evaluation of the torque on a soft ferromagnetic object. θ is the device angle relative to the x-axis and α is the direction of the magnetization relative to the normal.
Torque (L) on a object in magnetic field
L ∝ m×B0 => L ∝ V·B0 2·sin θ
Static Magnetic Field B0
Maximum static torque will be experienced by the device at the isocenter of the magnet where the static magnetic field is homogeneous and maximum
L max∝ V·Ms·B0
Representative example of magnetically induced torque exerted on a simple rod device placed in a homogeneous static magnetic field B0. The device is aligned relative to B0 dependent on geometry, the magnetic saturation of the material and orientation to B0: the resulting forces FT/2 act directly on the surrounding environment
Static Magnetic Field B0
ASTM F2213 – Test Method for Torque
Acceptance Criterion: Torque less than worst torque case due to gravity, defined as (Weight·Length)
Tors iona l spring
D e v i c e Holder
Turning Knob
Static Magnetic Field B0
Magnetically Induced Dynamic Torque Also called Lenz Force
Static Magnetic Field B0
The same object introduced parallel to the field lines will have no effect.
Any object made of conductive material introduced into a high flux field will induce a current (Eddy current) if the object is moving perpendicular to the magnetic field line. This new induced current will create a secondary magnetic field which will oppose the original field.
Static Magnetic Field B0
Static Magnetic Field B0
While not as apparent as translational forces, induced magnetic fields can cause patients discomfort or anxiety due to reactive forces on MRI-safe medical implants.
All pacemakers and implantable cardioverter or defibrillators should be considered contraindicated under any circumstance.
Magnetically Induced Dynamic Torque
Static Magnetic Field B0
Biological Effects
• Interaction with electrolyte flows = blood slowed down (est. 7% @ 5T)
• Magnetic forces on certain tissues (red cells alignment, etc …)
• Electron spin effects in some reactions
Biological Effects of B0
Dizziness
Biological Effects of B0
Magneto-Hydrodynamic Effect: When a conductor moves through B (or a stationary conductor is
exposed to a gradient magnetic field), E is induced in the conductor.
The degree of change is directly related to the strength of B.
The blood flowing through the heart creates this effect. The
current induced in the blood can be seen as an elevation of the S-T
segment on the patient’s electrocardiogram.
At higher B, this elevation is so pronounced that it can trigger the
acquisition process in a cardiac gated study.
The effect is completely reversible and it is not
associated with any serious bio-effects.
An elevated S-T segment can be indicative of
myocardial infarction, ischemia or an electrolyte imbalance.
Hence any patient with compromised cardiovascular
function be closely monitored with an MRI-compatible
pulse oximeter and/or blood pressure monitor
Biological Effects of B0
MAGNETO-PHOSPHENES
• If a patient moves his head while in a static magnetic field (> 2.0T), he may experience visual sensations best described as flashes of light.
• These visual sensations are referred to as magneto-phosphenes. They are not harmful and result from direct excitation of the optic nerve by currents induced by magnetic field.
Biological Effects of B0
There is no conclusive evidence for an irreversible or hazardous bioeffects related to an acute, short-term exposure of humans to static magnetic fields up to 2T. Extremely high ( >10 T) fields may have immediate/long term harmful effects, but … Clinical trials for 12 T full-body MRI are on going ( 17-21 T for animals)
Biological Concerns
Biological Effects of B0
FDA Guidelines (7/2003)
• FDA deems magnetic resonance diagnostic devices significant risk when used under any of the operating conditions described below:
Source: http://www.fda.gov/MedicalDevices/DeviceRegulationandGuidance/GuidanceDocuments/ucm072686.htm
Summary Effects of B0
Fringe field effects (missile effects) are
predominant
Magnetic torque effects on metal clips
etc.
Lenz Effects
Magnetophosphene - At very high B0
(i.e. > 1.5 T) mild sensory effects may be
experienced associated with movement in
the field
Dizziness
Gradient Magnetic Field
Generate gradients in main magnetic field, B0
Created by the cycling of power through the gradient coils and measured in mT/m
Control the selective excitation of the patient’s protons
Typical amplitude 20 - 100 mT/m and slew rate around 100 - 200 mT/m/msec
Higher the gradient amplitude, thinner the slice thickness possible ~ 0.7 mm
Shorter rise time (faster gradients) gives shorter echo spacing and hence better resolution &
more slices/TR
FDA Limit:
dB/dt < 6.0 T/s
h"p://www.magnet.fsu.edu/educa3on/tutorials/magnetacademy/mri/fullar3cle.html
Gradient Coils
Hence 3 dimensional MRI images!!!
RF EM Field B1
RF coils create oscillatory secondary magnetic field, B1,
to rotate patient’s proton yielding transverse magnetization
Same principles as those in a microwave oven only at a
significantly lower power
Perpendicular to B0 to accomplish excitation and
resonance
Could be T/R, R only or T only
Could be Volume or Surface
Composed of inductive and capacitive elements and
hence have to be tuned
Both gradients and RF are examples of time varying magnetic fields and both induce electrical currents in tissue. So why are their associated bioeffects so different?
“The Machine’s done something weird to Mr. Hendrickson”
Lin JC: Advances in electromagnetic fields in living systems Volume III. Kluwer, Academic; 2001.
Electric Properties of Biological Tissues
Biological tissues are conductive dielectrics
Dielectric permittivity (defines polarizability) and
conductivity are strongly non-linear functions of
frequency
Low frequency α dispersion is associated with ionic diffusion processes at the site of cellular
membrane
β dispersion in the hundreds of kHz region is due mainly to the polarization of cellular
membranes
γ dispersion in the GHz region is due to the polarization of water molecules
Electric Properties of Biological Tissues
Applied electric field gives rise to dipole moment distribution in atoms or molecules
Secondary fields are set up and thus net electric field is different
Applied electric field gives rise to electron drift
And drift results in current density (J) in the direction of electric field (E)
Hence conductivity, σ = J/E
Magnetic Properties of Biological Tissues
Majority of human tissues are diamagnetic or
weakly paramagnetic
Permeability (χ) of cells and tissues are
equivalent to the permeability of free space (χ0).
Much less critical to the analysis of EM
interactions
Lorentz force equation: F = q (E + v x B)
In intercellular flowing charges (such as enzymes), F will result in a
change in velocity and a resulting alteration in intended biological function
Moving electrons in DNA helices will begin to experience forces which
may repel them from each other and bend or even break the chain resulting
in increased DNA multiplication
Biological Effects of TVM Gradient Field
Faraday’s Law:
Switching of the gradients (dB/dt) induces
electric current in conducting tissues
EMF = -NΦ/dt = - N(BA)/dt
The magnitude of an induced current depends on
the strength and speed of the gradient fields and the
resistance of the conductor.
The human body has several excellent
conductors: nerves, muscles, and blood.
Variety of factors including the fundamental field frequency, the maximum flux density, the
average flux density, the presence of harmonic frequencies, the waveform characteristics of the
signal, the polarity of the signal, the current distribution in the body, the electrical properties,
and the sensitivity of the cell membrane.
Biological Effects of TVM Gradient Field
Peripheral Nerve Stimulation (PNS):
EPI - Rapidly changing magnetic fields associated with switching of gradients could cause
PNS
Greatest when oblique slices are used since slew rate could be greater by summing the
contributions from two or three sets of gradient coils
For coronal or sagittal EPI, where the possible current loops in the torso are the greatest
when the read gradient is in the H/F
Peripheral nerve stimulation sites were typically found at bony prominences. Since bone is less
conductive than the surrounding tissue, it may increase current densities in narrow regions of
tissue between the bone and the skin, resulting in lower nerve stimulation thresholds than
expected.
Biological Effects of TVM Gradient Field
Stimulation sites
x-gradients: the bridge of the nose, left side of the thorax, iliac crest, left thigh, buttocks, and the
lower back.
y-gradients: the scapula, upper arms, shoulder, right side of the thorax, iliac crest, hip, hands,
and upper back.
z-gradients: the scapula, thorax, xyphoid, abdomen, iliac crest, and upper and lower back.
Biological Effects of TVM Gradient Field
Stimulation of motor nerves and skeletal muscle could be disconcerting to the patient
Density of current required to elicit this response in healthy skeletal muscle is between 5 to 20
times higher than that produced by routine clinical scanners
At sufficient exposure levels, peripheral nerve stimulation is perceptible as “tingling” or
“tapping” sensations.
At gradient magnetic field exposure levels from 50% to 100% above perception thresholds,
patients may become uncomfortable or experience pain
Stimulation of Cardiac muscle disrupt the normal cardiac cycle and lead to an arrythmia.
Cardiac stimulation requires 80x PNS threshold
Respiratory stimulation ~ 3x PNS threshold
Biological Effects of TVM Gradient Field
Acoustic Noise:
Rapid alterations of currents within the gradient coils in the
presence of the strong B0, produce significant (Lorentz) forces
that act upon the gradient coils.
Movement of the coils against their mountings cause the
noise
Range ~ 84 - 103 dB
Actual level of the noise depends on the slice thickness,
FOV and scan timing (TR and TE) of the exam
Acquisitions with high resolution, thinner slices, and smaller FOVs are noisier due to the increased
slope of the gradient
Biological Effects of TVM Gradient FieldAcoustic Noise:
“Worst-case” pulse sequences that apply multiple gradients simultaneously e.g., three
dimensional, fast gradient echo sequences are among the loudest sequences ~ 103 - 113 dB
EPI have extremely fast gradient switching times and high gradient amplitudes ~ 115 dB
EPI on 3.0 T scanners ~ 126 - 131 dB
Human ear is a highly sensitive wide-band receiver
(20 Hz - 20000 Hz)
Problems associated include annoyance, verbal
communication difficulties, heightened anxiety
Recovery from the effects of noise occurs in a
relatively short period of time. However, if the noise
insult is particularly severe, full recovery can take up to
several weeks.
Presence and size of the patient may also affect the
level of acoustic noise
Biological Effects of TVM Gradient Field
Alteration of gradient output (rise time or amplitude) by modifying MR imaging parameters
causes the acoustic noise to vary
In addition to dependence on imaging parameters, acoustic noise is dependent on the MR
system hardware, construction and surrounding environment
Acoustic Noise - Permissible LimitsIn general, acoustic noise levels recorded in the MR environment have been below the
maximum limits permitted by the Occupational Safety and Health Administration of the U.S.
US FDA Guidelines:
Peak unweighted level 140 dB
A-weighted RMS level 99 dBA with hearing protection
UK Department of Health:
Recommends hearing protection for staff
exposed to an average of 85 dB over an 8-hour
day
Acoustic Noise - Control Techniques
Passive Noise Control:
Simplest and least expensive means ~ disposable earplugs or
headphones
Earplugs can abate noise by 10 to 30 dB
Hamper verbal communication with patients during scanning
One size fits all
Offer non-uniform noise attenuation over the hearing range, where high frequency may be
well attenuated, attenuation is often poor at low frequencies
Acoustic Noise - Control Techniques
Active Noise Control:
Controlling noise from a particular source by introducing
“anti-phase noise” to interfere destructively with the noise
source
Active-Passive combo:
Active system built into a headphone
An average noise reduction ~ 14 dB
43% of patients that were scanned without hearing protection experienced temporary
hearing loss.
Bioeffects of RF EM Field B1
Thermogenic Effects:
Primary effect of RF radiation is heating due to resistive losses
Safety standards are based on the requirements that the tissue heating no more than 1 oC is
tolerable
Amount of energy deposited in tissues is expressed as the SAR (in W/Kg)
The SAR is a complex function of numerous variables including the frequency (i.e., B0),
the type of RF pulse used (e.g., 90° vs. 180° pulse), TR, the type of transmit RF coil used, the
volume of tissue contained within the transmit RF coil, the configuration of the anatomical
region exposed etc.
Skin burns are provoked by the induced current in a conducting loop
Presence of metal enhances the impact!
Bioeffects of RF EM Field B1
US FDA limits:
RF exposure should be limited to producing less than 1 oC core body temperature rise
4 W/Kg Whole Body for 15 minutes
3 W/Kg averaged over the Head for 10 minutes
8 W/Kg in any gram of tissue in the Head/Torso for 15 minutes
12 W/Kg in any gram of tissue in the Extremities for 15 minutes
Shellock et al. indicated that an MR procedure performed at a whole body averaged SAR of 6.0 W/Kg can be physiologically tolerated by an individual with normal thermoregulatory function
MR System Operating Modes: The operating modes for MR systems as defined by the International Electrotechnical Commission
(IEC) are, as follows:
Normal Operating Mode - Mode of operation of the MR equipment in which none of the outputs
have a value that may cause physiological stress to patients.
First Level Controlled Operating Mode - Mode of operation of the MR equipment in which one or more outputs reach a value that may cause physiological stress to patients, which needs to be
controlled by medical supervision.
Second Level Controlled Operating Mode - Mode of operation of the MR equipment in which
one or more outputs reach a value that may produce significant risk for patients, for which explicit
ethical approval is required.
Operating Mode
SAR (W/Kg)
Whole BodyPartial Body Localized Region
Any Head Head Trunk Extremities
Normal 2 2 - 10 3.2 10 10 20
First Level Controlled 4 4 - 10 3.2 10 10 20
Second Level Controlled > 4 > 4 > 3.2 > 10 > 10 > 20
Short-term SAR SAR limits over any 10-s period should not exceed 3 times the stated SAR average
Operating Mode Core Temperature Rise (oC)Spatially Localized Temperature Limits (oC)
Head Torso ExtremitiesNormal 0.7 38 39 40
First Level Controlled 1 38 39 40
Second Level Controlled > 1 38 39 40
Bioeffects of RF EM Field B1
SAR Limit:
RF Temperature Limit:
Thermophysiologic Responses:
Depend on multiple physiologic, physical and environmental factors including -
Duration of exposure
Rate at which energy is deposited
Response of patient’s thermoregulatory system
Presence of underlying health condition
Ambient conditions within the MR system
Bioeffects of RF EM Field B1
Thermoregulatory System: Human body loses heat by convection, conduction, radiation and evaporation
If the effectors are not capable of totally dissipating the heat load, accumulation of heat
occurs
Various health conditions such as cardiovascular disease, hypertension, diabetes, fever etc.
Investigations demonstrated that changes in body temperature were relatively minor (i.e. <
0.6 oC)
Bioeffects of RF EM Field B1
Increases with square of Larmor Frequency and hence square of B0
Increases with the square of flip angle
Increases with the patient size
Increases with the number of RF pulses in a given time
SAR = σE2
2ρ=σπ 2r2ν 2α 2B1
2D2ρ
Bioeffects of RF EM Field B1
Factors to reduce the SAR:
Using quadrature rather than linear coils for transmission
Avoiding the use of body coil for certain exams, when using Head, Knee T/R coils etc.
Increasing TR
Using fewer slices
Reducing the ETL (or TF) in fast (or Turbo) Spin Echo sequences
Reducing the refocussing pulse flip angle, especially in FSE sequences
SAR = σE2
2ρ=σπ 2r2ν 2α 2B1
2D2ρ
Bioeffects of RF EM Field B1
MRI and PregnancyPregnant patients:
Present belief is that MRI has not produced any negative effects in pregnant humans
However, FDA has not established that MRI during pregnancy is safe
Potential risks include birth defects, developmental abnormalities, low birth weight and
spontaneous abortion
If the illness or injury is not immediately life-threatening, the examination should be
postponed until the end of the first trimester
Referring physician must
make the final decision!
MRI and Pregnancy
The overall decision to utilize an MR procedure in a pregnant patient
involves answering a series of important questions including, the
following:
Is sonography satisfactory for diagnosis?
Is the MR procedure appropriate to address the clinical question?
Is obstetrical intervention prior to the MR procedure a possibility
i.e. is termination of pregnancy a consideration? Is early delivery a
consideration?
MRI and PregnancyWith regard to the use of MR procedures in pregnant patients, this
diagnostic technique should not be withheld for the following cases:
Patients with active brain or spine signs and symptoms requiring
imaging.
Patients with cancer requiring imaging.
Patients with chest, abdomen, and pelvic signs and symptoms of
active disease when sonography is non-diagnostic.
In specific cases of suspected fetal anomaly or complex fetal
disorder.
MRI and Pregnancy
Pregnant Technologists or other healthcare workers: Though there are no obvious risks, it is a good policy to minimize exposure to any magnetic
fields during the first trimester
Importantly, technologists and healthcare workers should not remain within the MR system
room or magnet bore during the actual operation of the scanner
The Joint Commission recommends that healthcare organizations take the following steps:
Restrict access to all MRI sites by creating safe zones as recommended by the ACR
Use trained screening staff to perform double-checks of patients for items such as metal objects,
implanted or other devices, drug-delivery patches and tattoos
Ensure that the MRI technologist has the patient's complete and accurate medical history to
ensure that the patient can be scanned safely
Have a specially trained staff member accompany any patients, visitors and staff into the MRI
suite at all times
Annually educate all medical and ancillary staff who may accompany patients into the MRI suite
about the risk of accidents
Take precautions to prevent patient burns during scanning
Provide all MRI patients with ear plugs to diminish the loud "knocking" noise emanating from
the equipment
Never run a cardio-pulmonary arrest code or resuscitate a patient in the MRI room.
Reducing the Risk of MRI Injuries
Patient Preparation:
Patient should be instructed to wear loose, comfortable clothing without zippers, snaps or
buttons
Clothing decorated with decals and sequins should be avoided
All accessories should be removed (include rings bracelets, watches, earrings and body
piercings)
Face and eye make-up should be removed
Patient gowns should not have pockets
Patient Anxiety:
Explain the procedure before the patient enters the scan room
The anxiety level could be decreased by allowing an appropriately screened family member
to remain in the room with the patient during the exam
Reducing the Risk of MRI Injuries
Summary MRI scanners are becoming stronger and faster, so the potential for MRI-related injuries is
becoming greater
New implants are being developed almost daily. This means that there are new safety concerns
for MRI almost daily
A working knowledge of MRI safety is not a luxury; It is a fundamental and integral part of
being an MRI personnel
Establish MRI safety guidelines and follow them closely
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