MRI Bioeffects and MRI Bioeffects and SafetySafety

Dr. Peterson

History and BackgroundHistory and Background

Bloch (Stanford) & Purcell (Harvard) - 1946

Nobel Prize - 1952 - Discovery of Spectroscopy

Raymond Damadian – Rat Tumor – 1974– Cover of Science

Paul Lauterbur – Water image - 1973

Edinburgh Group/Peter Mansfield

New Diagnostic ModalityNew Diagnostic Modality

Compared to x-rays (1895!)

More Comparable to Ultrasound (1950’s)

Need to understand physics for safety

Basic Magnetic Field Basic Magnetic Field ConceptsConcepts

Units of Field Strength

Magnetic Domains

Static Fields (Main Magnet)

Time-Variant Fields (Gradient Magnets)

Units of Field StrengthUnits of Field Strength

The Gauss (a.k.a. The Oersted)– Measures the Earth's Field (~0.5 G)

The Tesla– Measures the MRI Field

The Relationship: 1T = 10,000 G– Also used: 1 mT = 10 G– Also used: 1 kG = 0.1 T

Magnetic DomainsMagnetic Domains

Example: Hemoglobin

– Iron atoms

The Static Field and the The Static Field and the EnvironmentEnvironment

Effects of the Environment on the Static Field

Effects of the Static Field on the Environment

Effects of the Environment Effects of the Environment on the Static Fieldon the Static Field

Why do we care?– Effects on the image

Shimming to Compensate

Effects of the Environment Effects of the Environment on the Static Fieldon the Static Field

Effects of the Static Field Effects of the Static Field on the Environmenton the Environment

Why do we care?– Credit cards, computer disks, CRTs.

Shielding to Protect

Effects of the Static Field Effects of the Static Field on the Environmenton the Environment

Bioeffects of Static Bioeffects of Static Magnetic FieldsMagnetic Fields

Cell FunctionsGrowth and healing of boneThrombolysisNerve FunctionCardiovascular EffectsTemperatureMagnetophosphenes

Cell FunctionsCell Functions

Rats had the permeability of the blood-brain barrier modified after 23 minutes at 0.15 T

Bone Growth and HealingBone Growth and Healing

There is much controversy on the effects of magnetic fields on the growth and healing of bone. No conclusions yet.

ThrombolysisThrombolysis

Indeterminate experiments on rats and guinea pigs. Can’t tell.

Nerve FunctionNerve Function

Rats showed strong avoidance behavior to magnetic fields.

Neuro function tests on humans before and after MR at fields up to 2T show that the bioelectric properties of neurons are not affected.

Fields of 4T resulted in nausea, vertigo and metallic taste, indicating neural involvement.

Cardiovascular EffectsCardiovascular Effects

Small increases in BP have been reported.

Also a slight leukopenia.

Magnetohydrodynamic Magnetohydrodynamic EffectEffect

Blood is a conductor flowing through the magnetic field.

This usually results in an increase in T- wave amplitude.

TemperatureTemperature

Reports go both ways; the best studies say static fields don’t affect body temperature.

Don’t confuse this with RF effects.

MagnetophosphenesMagnetophosphenes

Flashes of Light in the Visual Field

– None at 2T

– Reported at 4T

Static Field Bioeffects Static Field Bioeffects SummarySummary

Whole body: No effects at 2T Circulatory at 5T Extremities: No effects at 5T Discomfort; no adverse effects: 10T

Conclusion: Don’t go above 2T for head & trunk; 5T for extremities

Bioeffects of Time-Bioeffects of Time-Variant Fields Variant Fields

(Gradients)(Gradients)

MechanismNerve/Muscle StimulationNerves and EPIMagnetophosphenesAuditory ConsiderationsOther Biological Effects

Gradient Bioeffects MechanismGradient Bioeffects Mechanism

Changing magnetic field over time (dB/dt) induces voltage and current in conductors.

Example: Peak gradient of 1 G/cm, rise time = 500 sec 30 cm from the isocenter. This is 2,000 G/sec 30 cm, or 60,000 G/sec, or 6 T/sec.

The induced current is 1 A/cm2 for this gradient.

Biological effects usually occur beyond 3 A/cm2

Nerve/Muscle StimulationNerve/Muscle Stimulation

Example gradient induced a current of 1 A/cm2.

Need 15 to 100 A/cm2 for tetanic contractions of breathing muscles.

Need 0.2 to 1.0 A/cm2 to produce fibrillation.

Nerves and EPINerves and EPI

Twitching occurs beyond 60 T/sec

MagnetophosphenesMagnetophosphenes

None at 17 A/cm2 in 1.95 T systems.

Lots at 4 T and any gradient.

Auditory Considerations. Auditory Considerations.

Discussed later under Auditory Effects.

Other Biological Effects.Other Biological Effects.

Reduced effects of mannitol, fentanyl and morphine-induced analgesia.

No effect on amphibian embryogenesis, murine cell cytotoxicity, and litter number or growth rates of pregnant mice.

Gradient Magnet Gradient Magnet Bioeffects ConclusionsBioeffects Conclusions

No effects at 6 T/sec.

Nerve stimulation at 20 T/sec.

Don’t go above 20 T/sec.

Keep an eye on EPI in the future.

QuenchesQuenches

The Problem with the Field– Faraday’s Law

The Oxford Pig

Conclusions

The Problem with the FieldThe Problem with the Field

Faraday’s Law:

– When a conductor moves in a magnetic field, a voltage is induced in the conductor. It will also happen if the conductor is stationary and the magnetic field moves.

The Oxford PigThe Oxford Pig

A pig was placed in the bore.

1.6T magnet quenched at 1.76 T

Results: The pig was startled.

Quench ConclusionsQuench Conclusions

Quenches have happened several times with patients in the bore– Once it happened with the Chief Tech

in the bore (in Queensland, Australia)!

No deleterious effects have been reported, except for startle reflex.

Radiofrequency Radiofrequency BioeffectsBioeffects

RF Defined

Bioeffects

Temperature-sensitive organs

Quantifying Bioeffects

Limits

RF DefinedRF Defined

Non-ionizing electromagnetic radiation

Frequency range: DC to 3,000 GHz

Includes radar, TV, AM and FM radio

The Electromagnetic The Electromagnetic SpectrumSpectrum

BioeffectsBioeffects

Include visual, auditory, endocrine, cardio- vascular, immune system, reproductive, and developmental functions.

Simplified: It heats the tissues.

Cardiac output & blood flow increase

Sweat secretion & evaporation increase

Temperature-sensitive Temperature-sensitive organsorgans

The testes

The eyes

Limits: 10 W/kg for more than 10 minutes

Quantify bioeffects using:Quantify bioeffects using:

SAR: Specific Absorption Rate– Rate at which RF energy is coupled into

tissues.– Units: Watts per kilogram (W/kg)

SAR - Time Product– Units: W-min/kg or W-hr/kg

Temperature (°C)

FDA SAR LimitsFDA SAR Limits

FDA SAR LimitsFDA SAR Limits

Body average of 0.4 W/kg

Peak for any gram of tissue of 8.0 W/kg

Head average of 3.2 W/kg

FDA Temperature LimitsFDA Temperature Limits

A body core rise of less than 1 C No more than 38 C in the head No more than 39 C in the trunk No more than 40 C in the extremities

Exceptions: Patients with compromised heat regulation

FDA Temperature LimitsFDA Temperature Limits

PregnancyPregnancy

Bioeffects of the Static, Bioeffects of the Static, Gradient and RF FieldsGradient and RF Fields

Not enough is known for specific recommendations.

See the following study of Health Care Workers and it’s Conclusions.

Health Care WorkersHealth Care Workers

A survey of 2,000 female MR operators reported 1421 pregnancies; 280 were MR techs or nurses; 894 worked at another job, 54 were students, 193 were homemakers. There were no statistically significant differences between MR operator pregnancies and the group before it worked in MR. Spontaneous abortion rate, pre-term delivery rate, low birth weight, infertility and offspring gender were analyzed. Adjustments for maternal age, smoking, and alcohol use didn’t change the results.

Pregnancy ConclusionsPregnancy Conclusions

“MR imaging may be used in pregnant women if other non-ionizing forms of diagnostic imaging are inadequate or if the examination provides important information that would otherwise require exposure to ionizing radiation (e.g., fluoroscopy, CT, etc.). It is recommended that pregnant patients be informed that, to date, there has been no indication that the use of clinical MR during pregnancy has produced deleterious effects. However, as noted by the FDA, the safety of MR during pregnancy has not been proved.”

-SMRI Safety Committee

Bioeffects of Contrast Bioeffects of Contrast MediaMedia

Gadolinium is toxic but is caged by a chelate (DTPA).

The chelate may be dissolved by the kidneys, releasing the gadolinium.

Some IV contrast agents cross the placenta, are swallowed by the fetus, filtered and excreted by the kidneys as urine, swallowed; and the cycle then repeats many times. The end result of this cyclic process is not known.

CryogenicsCryogenics

Necessary for superconductivity– Niobium-titanium metal used for wires– Becomes superconducting at 10 kelvins

Liquid helium boils at 4.2 kelvinsLiquid nitrogen boils at 77 kelvinsThis classroom is at 295 kelvins

CryogensCryogens

Liquid Boils at:====== ================= Air 86 k -187° C -305° F

Nitrogen 77 k -196° C -321° F

Helium 4.2 k -269° C -452° F

CryogensCryogens

Helium is lighter than airNitrogen is heavier than airThey will drive the air out of the magnet

room if released into itBoth are incredibly cold and will burn

flesh on contact

Auditory EffectsAuditory Effects

Gradient magnet sounds– Protection or Distraction

Ear plugs/Headsets Noise cancellation devices

RF sounds– Clicks, buzzes, chirps or knocks– Seems to come from the back of the head– Usually masked by gradient noise

Cryogen release sounds (think hurricane!)

Laser Positioning Laser Positioning SystemsSystems

Possibility of eye damage

None has ever been reported

Most manufacturers now use incandescent lamps instead of lasers

Bibliography and LinksBibliography and Links

This information changes rapidly. See your handout for this class for the latest information.