mri safety overview - forces for mr... · mri safety overview - forces zachary w. friis, ph.d.,...

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?!”


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


In an accelerator the magnetic field bends and controls the size of the beam

Same effect on the electron bean in a color CRT


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


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


EXAMPLES OF MAGNETS

Permanent Magnet

Resistive Magnet

Superconducting Magnet


• Magnetically induced Force

• Objects tend to be pulled towards the region there the field is stronger


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


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


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.


ISOGAUSS PLOT OF 3.0T ACTIVELY SHIELDED MAGNET


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


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


The gravity force (weight) on the object is where ρm is the mass density and g = 9.8 m/s2

Z-axis

Fg

Fm


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!!!


Transport Stretchers: around 50 lb, force around 2,000 lb

Oxygen cylinder: around 15-20 lb, force around 600-800 lb


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


Office chairs: around 5-10 lb, force around 200-400 lb

Scissors: around 4 ounce (0.25 lb), force around 10 lb



ASTM F2052 standard test method for Magnetically Induced Force

Acceptance Criterion: Magnetically Induced force is less than object weight


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


• Magnetically induced Torque

• Objects magnetized preferentially along longest dimension

• Objects tend to align to the field


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


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


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


Magnetically Induced Dynamic Torque Also called Lenz Force


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.



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


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


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


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.


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


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.


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.


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.


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


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


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!


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


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


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)


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


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


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

Thank You

Shellock FG, Kanal E. Magnetic Resonance Bioeffects, Safety, and Patient Management. New York,

NY: Raven Press; 1994.

Shellock FG. MRI Bioeffects and Safety. In: Atlas S, ed. Magnetic Resonance Imaging of the Brain and Spine. Philadelphia, Penn: Lippincott-Raven; 1996

References