MRI Bioeffects and Safety

MRI Bioeffects and
Safety
Dr.
Peterson
History 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 Modality

Compared
to x-rays (1895!)
More
Comparable to Ultrasound
(1950s)
Need
to understand physics for safety
Basic Magnetic Field

Concepts
Units
of Field Strength
Magnetic
Static
Domains
Fields (Main Magnet)
Time-Variant
Fields (Gradient Magnets)
Units 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 Domains
Example:
Hemoglobin
Iron atoms
The Static Field and the

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

Environment
Effects of the Environment

on the Static Field
Why
do we care?
Effects on the image
Shimming
to Compensate
Effects of the Environment

on the Static Field
Effects of the Static Field

on the Environment
Why
do we care?
Credit cards, computer disks, CRTs.
Shielding
to Protect
Effects of the Static Field

on the Environment
Bioeffects of Static
Magnetic Fields
Cell
Functions
Growth and healing of bone
Thrombolysis
Nerve Function
Cardiovascular Effects
Temperature
Magnetophosphenes
Cell Functions
Rats
had the permeability of the

blood-brain barrier modified after
23 minutes at 0.15 T
Bone Growth and Healing

There
is much controversy on the

effects of magnetic fields on the
growth and healing of bone. No
conclusions yet.
Thrombolysis
Indeterminate
experiments on rats
and guinea pigs. Cant tell.
Nerve 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 Effects
Small
increases in BP have been

reported.
Also
a slight leukopenia.
Magnetohydrodynamic
Effect
Blood
is a conductor flowing
through the magnetic field.
This
usually results in an increase

in T- wave amplitude.
Temperature
Reports
go both ways; the best

studies say static fields dont affect
body temperature.
Dont
confuse this with RF effects.
Magnetophosphenes
Flashes
of Light in the Visual Field
None at 2T
Reported at 4T
Static Field Bioeffects

Summary
Whole
body: No effects at 2T
Circulatory at 5T
Extremities: No effects at 5T
Discomfort; no adverse effects: 10T
Conclusion:
Dont go above 2T for

head & trunk; 5T for extremities
Bioeffects of TimeVariant Fields

(Gradients)
Mechanism
Nerve/Muscle
Stimulation
Nerves and EPI
Magnetophosphenes
Auditory Considerations
Other Biological Effects
Gradient 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.
induced current is 1 A/cm2 for this
gradient.
The
Biological
A/cm2
effects usually occur beyond 3
Nerve/Muscle Stimulation
Example
gradient induced a
current of 1 A/cm2.
15 to 100 A/cm2 for tetanic
contractions of breathing muscles.
Need
0.2 to 1.0 A/cm2 to produce

fibrillation.
Need
Nerves and EPI

Twitching
occurs beyond 60 T/sec
Magnetophosphenes
at 17 A/cm2 in 1.95 T
systems.
None
Lots
at 4 T and any gradient.
Auditory Considerations.
Discussed
Effects.
later under Auditory
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
Bioeffects Conclusions
No
effects at 6 T/sec.
Nerve
Dont
Keep
stimulation at 20 T/sec.
go above 20 T/sec.
an eye on EPI in the future.
Quenches
The
Problem with the Field
Faradays Law
The
Oxford Pig
Conclusions
The Problem with the Field

Faradays
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 Pig

A
pig was placed in the bore.
1.6T
magnet quenched at 1.76 T
Results:
The pig was startled.
Quench 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
Bioeffects
RF
Defined
Bioeffects
Temperature-sensitive
Quantifying
Limits
Bioeffects
organs
RF Defined
Non-ionizing
electromagnetic
radiation
Frequency
Includes
range: DC to 3,000 GHz
radar, TV, AM and FM radio
The Electromagnetic
Spectrum
Bioeffects
Include
visual, auditory, endocrine, cardiovascular, immune system, reproductive, and

developmental functions.
Simplified:
Cardiac
It heats the tissues.
output & blood flow increase
Sweat secretion & evaporation increase
Temperature-sensitive
organs
The
testes
The
eyes
Limits:
minutes
10 W/kg for more than 10
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 Limits
FDA 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 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 Limits
Pregnancy
Bioeffects of the Static,

Gradient and RF Fields
Not
enough is known for specific

recommendations.
See
the following study of Health

Care Workers and its Conclusions.
Health 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 didnt change the
results.
Pregnancy Conclusions
MR imaging may be used in pregnant women if other nonionizing 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
Media
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.
Cryogenics
Necessary
for superconductivity
Niobium-titanium metal used for wires

Becomes superconducting at 10 kelvins
Liquid
helium boils at 4.2 kelvins

Liquid nitrogen boils at 77 kelvins
This classroom is at 295 kelvins
Cryogens
Liquid
======
Boils at:
=================
Air
86 k
-187 C
-305 F
Nitrogen
77 k
-196 C
-321 F
4.2 k
-269 C
-452 F
Helium
Cryogens
Helium
is lighter than air

Nitrogen is heavier than air
They will drive the air out of the magnet
room if released into it
Both are incredibly cold and will burn
flesh on contact
Auditory 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
Systems
Possibility
None
Most
of eye damage
has ever been reported
manufacturers now use

incandescent lamps instead of lasers
Bibliography and Links

This
information changes rapidly. See your

handout for this class for the latest
information.

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MRI Bioeffects and

History and Background

(Stanford) & Purcell (Harvard) - 1946

Prize - 1952 - Discovery of

Damadian Rat Tumor 1974

Lauterbur Water image - 1973

New Diagnostic Modality

to understand physics for safety

Basic Magnetic Field

Fields (Main Magnet)

Fields (Gradient Magnets)

Units of Field Strength

Gauss (a.k.a. The Oersted)

Measures the Earth's Field (~0.5 G)

Measures the MRI Field

The Static Field and the

of the Environment on the Static

of the Static Field on the

Effects of the Environment

Effects on the image

Effects of the Environment

Effects of the Static Field

Credit cards, computer disks, CRTs.

Effects of the Static Field

had the permeability of the

Bone Growth and Healing

is much controversy on the

showed strong avoidance behavior to

increases in BP have been

usually results in an increase

go both ways; the best

confuse this with RF effects.

of Light in the Visual Field

Static Field Bioeffects

Dont go above 2T for

Bioeffects of TimeVariant Fields

Gradient Bioeffects Mechanism

magnetic field over time (dB/dt)

Peak gradient of 1 G/cm, rise time =

effects usually occur beyond 3

0.2 to 1.0 A/cm2 to produce

Nerves and EPI

occurs beyond 60 T/sec

at 4 T and any gradient.

later under Auditory

Other Biological Effects.

effects of mannitol, fentanyl

an eye on EPI in the future.

Problem with the Field

The Problem with the Field

When a conductor moves in a

magnetic field, a voltage is induced in

The Oxford Pig

pig was placed in the bore.

magnet quenched at 1.76 T

The pig was startled.

have happened several

in the bore (in Queensland, Australia)!

deleterious effects have been