MRI hazards and saftey Radiology technologist

1. MR hazards & safety concern
Khorfakkan Hospital
RADIOLOGY
Daniel jayaprakash

2. MR Hazards & Safety
Safety concerns in MR arise from 3
major sources
•Static magnetic field (main magnet)
•Gradient magnetic field (gradient
coil)
•RF magnetic field (RF coil)

3. MR Hazards and Safety

4. MR Hazards & Safety
Units of Field Strength
• The Gauss
– Measures the Earth's Field (~0.5 G)
• The Tesla
– Measures the MRI Field
• The Relationship: 1T = 10,000 G

5. MR Hazards & Safety Concerns
Static magnetic field
Since the static magnetic field is constantly ON it
poses a major risk.
MR imaging uses magnetic field strengths of 0.35T
to 4T.
These risks are manifested in 3 ways:
a) Effects on physiology.
b) Effects on implanted devices.
c) Effects on metal objects.

6. Static Magnetic Field
Effects on physiology
a) Cell and nerve function-de oxygenated
sickled cells align against or
perpendicular to the field.
b) Unusual sensation including nausea,
vertigo and metallic taste associated with
altered nerve functions.

7. Static Magnetic field
c) Cardiovascular effects-elevation of T wave
elevation.T wave elevation normally found in
patients with ischaemia & myocardial infarction.
d) Temperature-static magnetic field do not cause
alteration in skin and body temperature.
e) Magnetophosphenes-due to excitation of optic
nerve or retina caused by eye or head
movements.

8. Static Magnetic Field
Effects on implanted device
Ferromagnetic devices will align and twist
(torque) and cause unanchored implants
to move within the body and cause injury
to tissues.
Metallic implants can cause artifacts and
result in misinterpretation of images.

9. Static Magnetic Effects
Metallic implant-intracranial vascular clip.
Aneurysm clips are highly ferromagnetic
causing deflection.
Clip motion may damage the vessel
resulting in hemorrhage , ischaemia or
death.
Implanting physicians should provide
patients details of implant or an alert card.

10. Static Magnetic Field
Effects on metallic implants.
Orthopedic implant show minimal
deflection and cause image artifacts and
not likely to be dislodged.
Large implants like hip prosthesis can cause
heating which can be significantly low.

11. SM Effects on Metallic Implants
Cochlear implants like 3M/House,3M/Vienna
device, Nucleus mini 20 channel are highly
contra integrated.
These devices are made up of high field strength
magnet, in conjunction with external magnet
align with a transmitter coil on patient’s head.
Wires in these devices can induce current causing
injury to patient or interfere with the mode of
operation.

12. Static Magnetic Field
Effects on metallic implants.
Intravascular coils , filters and stents
considered safe for imaging after few
weeks are unlikely to be dislodged.

13. Static Magnetic Field
Extracranials vascular clip
Poppen Blaylock carotid artery clamp is contra
indicated.
Vascular access ports – these are safe for
imaging
To provide long term administration of
chemotherapy agents ,antibodies and
analgesics

14. Static Magnetic Field
Heart valves showed less deflection and
safe for MR imaging.
Cardiac pace makers strictly
contraindicated.
Risk of movement of device, damage to
reed switch and change in the mode of
operation.Intection of current in lead wires
can cause fibrillation and thermal injury.

15. Static Magnetic field
Dental devices - These have minimal deflection
and safe for imaging. The amount of deflection
can cause image artifact or can be dislodged.
Ocular implants – The fatio eyelid spring, retinal
tack (martensitic stainless steel),troutman
magnetic ocular implant, unitec round eyelid
spring can cause severe injury like vitreous
hemorrhage and blindness. Plain x-ray is an
acceptable tool for unresolved FB and implant

16. MR hazards & safety
Halovest and device
MR can be performed only on
specific designed vest because
current induced in the devices can
heat and burn the surrounding
tissues ,so it is advisable to perform
MTC pulse sequence.

17. MR hazards & safety
Static magnetic effects on metal
objects
Projectile effect-ferro magnetic
objects have the potential to act as
missile flying into the bore. Small
objects are pulled with a velocity of
40MPH, causing serious risk to
patient and damage expensive
equipments

18. MR Hazards and Safety

19. MR hazards & safety
Gradient Magnetic field
These are magnetic field of the
order of 0.1 to 1 gauss/cm super
imposed over the main magnetic
field to cause predictable variation
along a predetermined plane. In
the absence of gradients all
protons in an imaging plain
precess in same phase. (eg) slice
select ,frequency and phase
encoding

20. MR hazards & safety
Gradient fields induce current in
tissues and cause power
deposition (1to3µ A)
Eg. Gradient applied
500µ sec at 1g/cm will be
2000g/sec /cm that means
2000g x30cms=60,000g/sec
60,000g =6T/scc

21. MR Hazards & Safety
GM- cardio muscular & vascular system
Since blood is a tissue it acts as a conductor
. A 1 T magnet induces a current of 1µ
A/cm² . Beyond 1µ A there is ventrcular
fibrillation . 15 to 100 µ A/cm produce
large tetanic contraction of skeletal
muscle(breathing) .

22. MR hazards & safety
Gradient magnetic fields
Cell and nerve functions
GM fields can stimulate or
impair impulses along nerve
path way .so normal function of
nerve cells and muscle fiber
may be affected. Beyond 6T
there can be painful peripheral
nerve stimulation, so the
patients should not be allowed
to clasp their hands & feet to
form a ring

23. MR Hazards & Safety
Gradient magnetic field - Bio effects
Magnetophosphenes – Rapid changing
gradients induce current which stimulate
optic nerve and retina . Persons
experience flashes of light within the eyes
None at 1.5T but at 4T .

24. MR Hazard & Safety
GM effects
Neural stimulation – The current induced
by the rapid changing magnetic field cause
direct peripheral muscle stimulation ,
especially in echo planar imaging which
uses gradients of 2.5G/cm . Uncontrolled
involuntary skeletal muscle contraction or
twitching has been reported in human.

25. MR Hazard and Safety
Gradient Field Induced Acoustic noise
Gradient coils are not coils of wire but broad
thick copper conducting bands.
Loud tapping, knocking or chirping sound are
heard when gradient coils expand and impact
against their mounting resulting in temporary or
permanent hearing loss, difficulty in verbal
communication. So all patients must be fitted
with noise attenuation equipment like earplugs,
headphones or anti noise system.

26. MR Hazards and Safety
Oscillating (RF) Magnetic Field
RF coils in the bore are the source of RF
during application of RF pulses in different
sequences. Range of frequency used is 1
to 100 MHz. This RF energy induces
current resulting in tissue heating, which
depends on patient size, body habitus,
tissue sensitivity, duration and type of RF.

27. MR Hazards and Safety
MRI RF Pulse Sequences
SR 90º…90º…90º
SE 90º…180º….90º…180º….90º…180º
IR 180º…90º…180º….180º…90º…180º
GRE αº…αº…αº
α is an RF pulse less than 90º.
EPI 90º…180º…

28. MR Hazards and Safety
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).

29. MR Hazards and Safety
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

30. MR Hazards and Safety
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

31. MR Hazards & Safety
• Fringe Magnetic Field (Stray Magnetic Field)
These are the components of the main
magnetic field that extend outside the main
magnet and its housing
Permanent Magnet (0.3T) – Fringe Field <1m
Resistive Magnet (0.2T) 2m
Super Conductive Magnet (0.5T to 4T) -10m
It is recommended that those person not
screened remain out of this field

32. MR Hazards & Safety
• Pregnant Patient
There are no known biological effects of
MR on fetuses . If the information to be
gained by MR would have required more
invasive test , MR is acceptable .
High risk pregnant patients can delay the
examination until the first trimester .

33. MR Hazards & Safety
Pregnant Employees
Pregnant employees can safely enter the scan
rooms , but should leave while RF and gradient
fields are employed .
It is recommended that the employees stay out of
the magnetic field during the first trimester .
However to leave an environment which is
hazardous may be advisable .
These are the guidelines from ISMRM .