Safety Aspects

Transcription

Safety Aspects

RUPRECHT-KARLSUNIVERSITY HEIDELBERG
Computer Assisted Clinical Medicine
Prof. Dr. Lothar Schad
12/10/2008 | Page 1
Master‘s Program in Medical Physics
Physics of Imaging Systems
Basic Principles of Magnetic Resonance Imaging VI
Chair in Computer Assisted Clinical Medicine
Faculty of Medicine Mannheim
University of Heidelberg
Theodor-Kutzer-Ufer 1-3
D-68167 Mannheim, Germany
Lothar.Schad@MedMa.Uni-Heidelberg.de
www.ma.uni-heidelberg.de/inst/cbtm/ckm/
12/10/2008 | Page 2
Safety Aspects
Safety Aspects
Seite 1
1
12/10/2008 | Page 3
MRI System with Superconducting Magnet
Siemens Magnetom
Symphony
• B = 1.5 Tesla
0
• Quantum-gradient:
30 mT/m
• slew rate: 125 T/m/s
• removable patient couch
• short magnet: 1.6 m
• relatively open bore
• 8 receiving channels
12/10/2008 | Page 4
radiofrequency RF
MRI Components: Physical Parameters
gradients Gxyz
shim coils
static field B0
transmitter
shim
receiver
350 MHz
control
panel
computer
350 MHz
image
processor
gradient
technical
component
Æ
static field B0
Æ
physical
parameter
M0
radiofreq. RF Æ
signal
gradients Gxyz Æ
image
Seite 2
2
12/10/2008 | Page 5
Operation Mode
normal operation mode
•
•
field exposition where a physiological stress of the patient can be excluded
routinely control of the patient is adequate
controlled operation mode 1. level
•
•
•
field exposition where a physiological stress of the patient is possible
medical patient control is required (i.e. monitoring of physiological
parameters)
MRI system has to notify higher operation mode which has to be confirmed
by the operator
controlled operation mode 2. level
•
•
•
•
field exposition with a significant risk for the patient
application possible for clinical testing
approval by the local ethic commission
MRI system should not allow an unintended switch
12/10/2008 | Page 6
Warnings
warnings:
• strong magnetic
field
• radiofrequency
interdictions:
• cardiac pacemaker
• open fire
• metallic implants
• watch, camera
• magnetic fire extinguisher
• metal: scissor, key
• magnetic disc
Seite 3
3
12/10/2008 | Page 7
Static Magnetic Field: B0 I
control area: 0.5 mT - line
source: Reiser and Semmler. “Magnetresonanztomographie” 2002
12/10/2008 | Page 8
Static Magnetic Field: B0 II
control area:
0.5 mT - line
Seite 4
4
12/10/2008 | Page 9
Static Magnetic Field: B0 = 3 Tesla
control area:
0.5 mT - line
12/10/2008 | Page 10
B0 - Danger
B0
•
missile effect
•
•
forces and torques to implants
•
•
•
stent
gunshot wound, rest of projectile
failure of electromagnetic devices
•
•
•
•
strong forces to ferromagnetic
materials
cardiac pacemaker
computer
controlling system
physiologic effects
•
vertigo when entering the magnet
(> 3 Tesla)
Seite 5
5
12/10/2008 | Page 11
B0 – Example I
12/10/2008 | Page 12
B0 – Example II
courtesy: Gross. Siemens Erlangen
Seite 6
6
12/10/2008 | Page 13
B0: Serious Accident of a Fireman
problem
•
•
•
no emergency plan of
institution
insufficient training of fire
department
neglecting of warnings
simulated image of accident, courtesy Olbricht. fire department Langenselbold
12/10/2008 | Page 14
B0: MR – Compatible Fire Extinguisher
Seite 7
7
12/10/2008 | Page 15
B0: Emergency Switch-Off Magnet
when ?
only in case of emergency
• if people have to be saved
• and if saving is not possible due
to magnetic field forces
•
how ?
inducing magnet quench
• press “magnet stop” switch
• all persons should be saved and
leave the magnet room quickly
• acute danger of suffocation
• protect magnet room
•
12/10/2008 | Page 16
B0: Danger from Cold Gases and Fluids
magnet as pressure tank
•
quench
•
•
•
•
evaporation of fluid helium
high pressure
quench pipeline to outdoor
cold gases can come into the
examination room
•
„burning“ at cold surfaces
displacement of oxygen
•
danger of suffocation
•
Seite 8
8
12/10/2008 | Page 17
B0: Magnet Stop and Emergency Stop
magnet stop
•
•
•
•
interrupts super-conduction,
magnet coil gets normal
conducting
current in magnet coil comes to
rest
coil wires get hot and helium
evaporates (quench)
magnetic field is switched off
emergency stop
•
•
•
•
all electric systems of the MR
system are switched off
magnetic field is still on
cooling system is off
magnet stop is still possible since
battery-operated
12/10/2008 | Page 18
B1: Gradient Field G(t)
G(t)
•
temporal variation of magnetic field
(dB/dt)
•
•
•
•
•
•
induction of eddy currents in the
object / body
eddy currents run through nerves
peripheral nerve stimulation
stimulation of myocardium ?
interaction with implants
extreme noise
•
ear protection
Seite 9
9
12/10/2008 | Page 19
B1: Critical Values
•
•
•
threshold of beginning of nerve
stimulation using switching gradients
(beginning of stomach prickling)
is determined in volunteer experiments
for each gradient system
complex function of
•
•
•
•
•
•
gradient amplitude
slew rate
pulse shape
pulse duration
number of pulses
dB/dt [T/s]
PNS - threshold
compliance of threshold is done by
software („SAFE model“)
stimulus duration [ms]
normal
1. level
2. level
80% PNS
100% PNS
>100% PNS
12/10/2008 | Page 20
RF: Radiofrequency
radiofrequency field
• is directly absorbed
• critical value:
specific absorption rate (SAR)
• can be resonant induced in
conductive structures
• creates offset currents in tissue
• massive heating
• flashover
• destroys not connected coils
Wagle and Smith. AJR 2000
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10
12/10/2008 | Page 21
RF: SAR Critical Values
normal
body part
1. level
2. level
whole body
part body
head
local (head, body)
local (extremities)
temperature increase
body stem
scales with the ratio of “exposed patient mass / total patient mass”
→ patient weight and age are relevant for safety !
12/10/2008 | Page 22
RF: Flashover
Seite 11
11
12/10/2008 | Page 23
RF: Burning from Induced Eddy Currents
ECG - system “Tesla plus” EFTES03D,
Mammendorfer Institute for Physics and Medicine
„MR compatible“ according to producer !
Kugel et al. Eur Radiol 2003
12/10/2008 | Page 24
B0 and RF: Implants
contra indication
•
•
•
•
•
cardiac pacemaker
neuro stimulators
(deep brain stimulators)
ferromagnetic implants
gun projectiles
paramagnetic stents up to 2 weeks
after implantation
information
•
•
http://www.mrisafety.com
Shellock FG.
Reference Manual for Magnetic
Resonance Safety, Implants, and
Devices: 2006 Edition.
Seite 12
12
12/10/2008 | Page 25
MR Safety Labeling
MR safe
•
an item which poses no known hazards in all MR environments
MR conditional
•
an item which has been demonstrated to pose no known hazards
in a specified MR environment with specified conditions of use.
Field conditions that define the specified MR environment include
•
•
•
•
•
•
field strength,
spatial gradient,
dB/dt (time rate of change of the magnetic field),
radio frequency (RF) fields, and
specific absorption rate (SAR).
additional conditions, including specific configurations of the item,
may be required
MR unsafe
•
an item which is known to pose hazards in all MR environments
ASTM International. Standard practice for marking medical devices and other items for safety in the magnetic resonance environment. F 2503-05
12/10/2008 | Page 26
Light Vizier
patient localizing and referencing
•
ask patient for closing eyes before
laser switch on
Seite 13
13
12/10/2008 | Page 27
Danger of Crushing
couch movement
•
•
•
watch position of arms and fingers
save connections, ECG-wires, and
alarm ball
push “couch stop” – patient couch can
be moved manually
12/10/2008 | Page 28
Other Safety Risks
lens effect
•
phantoms should not be placed
at direct sunlight
Seite 14
14
12/10/2008 | Page 29
MR Safety Aspects: Summary
static magnetic field
• missile effect
• influencing equipments
• physiological effects
gradients
• peripheral nerve stimulation
• noise
radiofrequency
• heating
• coils connected ?
cold gases / fluids
• suffocation
• “burning”
emergency procedures
• magnet stop
• emergency stop
• couch stop
patient information
• patient information sheet
• alarm ball
control area
• watch and take care about
• unknown people (i.e. service and cleaning
people, anesthesia doctors and staff, ...)
• unknown equipment and systems before
bringing into the magnet room
Seite 15
15

Safety Aspects

Transcription

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