3 Powerful Magnetic Fields Static

MRI Safety

RF

Wm. Faulkner, B.S.,R.T.(R)(MR)(CT), FSMRT Gradient

3 Powerful Magnetic Fields

3 Powerful Magnetic Fields

Magnetize the Tissues

Excite the Protons / Detect Signals

Static

Static

RF

RF

Gradient

Gradient

3 Powerful Magnetic Fields

Static Magnetic Field

Localize the Signals Static

Vertical Field

RF

Gradient

Horizontal Field

Superconductivity The electrical resistivity of a metallic conductor decreases gradually as temperature is lowered. In ordinary conductors, such as copper or silver, this decrease is limited by impurities and other defects. Even near absolute zero, a real sample of a normal conductor shows some resistance. In a superconductor, the resistance drops abruptly to zero when the material is cooled below its critical temperature. An electric current flowing through a loop of superconducting wire can persist indefinitely with no power source

Field Strength Gauss

Tesla

1 Tesla = 10,00 Gauss

The earth’s magnetic field is approximately! 0.5 Gauss

Powerful Persistent Magnetic Field Small bar magnet is approximately! 100 Gauss

1.5 Tesla MRI system is ! 15,000 Gauss! 30,000 x the earth’s magnetic field

Powerful Persistent Magnetic Field 3.0 Tesla MRI system is ! 30,000 Gauss! 60,000 x the earth’s magnetic field

July 2001

JMRI 2013, Pg 3

Radio Frequency

JMRI 2013, Pg 3

RF Burns 3rd Degree from Pulse Ox

www.simplyphysics.com

Initial Injury All too common injury

Few days later

Always use proper insulating pads

Street Clothes Can ! Be Dangerous

Skin-to-Skin Contact

www.lululemon.com

Blouse catches fire during 1st sequence of a shoulder exam. 
 3rd degree burns to forearm. - FDA MAUDE database

Radio Frequency B1

Radio Frequency B1

Heating

Heating

Tissue

Device

Tissue

Device

Power In

Cooling

Cooling

Power In Heat

If input is greater than output, heating occurs

Normal Mode: No physiologic stress is expected 1st Level Mode: Physiologic stress can occur

Currently clinical systems cannot exceed! 4 W/kg whole body

From a GE system

www.mrisafety.com

Cooling Influenced By

“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 including diuretics, beta-blockers, calcium blockers, amphetamines, and sedatives can alter thermoregulatory responses to a heat load. Importantly, certain medications have a synergistic effect with RF radiation with respect to tissue heating. The environmental conditions (i.e., ambient temperature, relative humidity, and airflow) that exist in the MR system will also affect tissue temperature changes associated with RF energyinduced heating.”

Bore Temperature! Ambient Temperature! Relative Humidity! Air Flow Rate! Perspiration! Blood Flow

Radio Frequency B1

www.mrisafety.com “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 including diuretics, beta-blockers, calcium blockers, amphetamines, and sedatives can alter thermoregulatory responses to a heat load. Importantly, certain medications have a synergistic effect with RF radiation with respect to tissue heating. The environmental conditions (i.e., ambient temperature, relative humidity, and airflow) that exist in the MR system will also affect tissue temperature changes associated with RF energyinduced heating.”

Radio Frequency B1

Heating Tissue

Device

Heating Tissue

Device

Field Strength, Frequency, Wavelength

Cervical Fixation Device at 3.0 T Skull Pin 3-Tesla 32.00

30.00

Temperature (˚C)

28.00

Scalp burn from halo

26.00

24.00

22.00

20.00

18.00

0

143

286

429

571

714

857

1000

Time (sec)

wavelength @ 3 T is approx 2.4 meters! 20 - 25 cm “worse case”

For elongated or closed loop implant or device implant, heating can differ! significantly at different field strengths

Courtesy Frank Shellock, Ph.D.

MRI-Related Lead Heating: Pacing Lead, No IPG

Courtesy Frank Shellock, Ph.D.

T/R Head Coil vs. Transmit Body/Receive-only Head Coil

1.5 (1.4-W/kg) vs. 3-T (3-W/kg)

Temperature (˚C)

50.00

1.5 T

T/R

42.00

34.00

26.00

18.00

0

350

700

1050

1400

3.0 T

Time (sec)

For certain lead lengths, less heating at 3-T/128MHz vs. 1.5-T/64MHz 
 due to differences in resonant wavelength. Courtesy Frank Shellock, Ph.D.

Two Serious Injuries @ 1.0 T Philips Receive-only

GE

Guidelines not followed

Transmit/Receive

1.5 T ONLY! T/R Head coil ONLY! Head SAR 0.1 W/kg Courtesy Frank Shellock, Ph.D.

Time-Varying Magnetic Fields Gradient fields used for imaging

Time-Varying Magnetic Fields Current induction in conductive loops

Used for spatial encoding

Faraday’s Law of Induction Changing field Strong magnetic fields that switch rapidly during imaging

Time-Varying Magnetic Fields

time

Time-Varying Magnetic Fields

Current induction in conductive loops

Eddy Current Induction

Size and shape Conductive properties Position / Location Gradient slew rate

Peripheral Nerve Stimulation

Vibration Device / component damage

Labeling ASTM Standard F2503* Defines Three Terms:

“Decisions based on published MR safety and compatibility claims should recognize that all such claims apply only to specifically tested conditions, such as static magnetic field strengths, static gradient magnetic field strengths and spatial distributions, and the strengths and rates of change of gradient and radiofrequency (RF) magnetic fields.”

MR Safe

MR Unsafe

MR Conditional

*ASTM

standard F2503: Standard Practice for Marking Medical Devices and Other Items for Safety in the! Magnetic Resonance Environment

MR Safe An item that poses no known hazards in all MR environments

MR Unsafe An item that is known to pose hazards in all MR environments

Gadolinium MR Conditional An item that has been demonstrated to pose no known hazards in a specified MR environment with specified conditions of use

Non-Ionic

chelate, any of a class of coordination or complex compounds consisting of a central metal atom attached to a large molecule, called a ligand Non-Ionic

Gd+3

http://www.britannica.com/EBchecked/topic/108427/chelate

Published 2001

NSF

Sadowski, E. A. et al. Radiology 2007;0:2431062144

“...recently emerged among patient with renal disease” “...recent emergence of this condition” “...suggested a possible infections and/or toxic agent”

Broome DR et al. AJR:188, Feb 2007 586-92

Non-Ionic

Non-Ionic

Published data: frequency of NSF not equal among all agents 
 (Radiology 2008, 10.1148/radiol.2483072093)

Fundam Clin Pharmacol. 2006 Dec;20(6):563-76

Stability

ACR Grouping

No Unconfounded Cases of NSF ProHance Dotarem MultiHance

Contraindicated with eGFR < 30 Magnevist Omniscan OptiMARK

Understand the risks Provide the best outcome while minimizing risks

William Faulkner & Associates, LLC www.t2star.com