Ultrasound Imaging How do you know it s SAFE? · Heating and cavitation Range Gate Width (pulse...
Transcript of Ultrasound Imaging How do you know it s SAFE? · Heating and cavitation Range Gate Width (pulse...
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European Committee for Medical Ultrasound Safety (ECMUS)
Ultrasound Imaging
How do you know it’s SAFE?
EFSUMB Safety Committee
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European Committee for Medical Ultrasound Safety (ECMUS)
Why are we concerned about safety?
• The range of clinical applications is becoming wider
• The number of patients undergoing ultrasound examinations is increasing
• New techniques with higher acoustic output levels are being introduced.
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European Committee for Medical Ultrasound Safety (ECMUS)
What are bio-effects?
The result of ultrasound
interacting with biological
molecules as it passes
through tissue
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European Committee for Medical Ultrasound Safety (ECMUS)
Safety
Is a bioeffect likely to cause a
problematic change (e.g. cell,
gene or DNA damage) ?
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European Committee for Medical Ultrasound Safety (ECMUS)
Mechanisms of Action
Heat (Ispta)
Cavitation (P-)
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European Committee for Medical Ultrasound Safety (ECMUS)
Attenuation =
Absorption +
Scatter
INCIDENT BEAM
IMAGE
Absorption 60-80%
HEAT
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European Committee for Medical Ultrasound Safety (ECMUS)
HEATING (Absorption)
Increases with: frequency, exposure duration,
pulse repetition frequency
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European Committee for Medical Ultrasound Safety (ECMUS)
2
Pulsed
Doppler Colour
Doppler
Harmonic
imaging B-mode
2D,3D
M-mode
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European Committee for Medical Ultrasound Safety (ECMUS)
ATL UM 9
(Linear-Array L 10-5)
Transducer Self Heating :
International limits for probe surface temperature due to self-heating : T < 41 °C (in a water bath 2 l / 10 min) IEC 601-2-5 T < 43 °C (coupled to skin) T < 50 °C (emitting into air) IEC 60601-2-37
© CKollmann, Euroson 2010
Temperature distribution due to probe self-heating for diagnostic devices (maximum) :
B-Mode Pulsed Doppler Colour Doppler
Ispta = 11 mW cm2, MI = 0,5 Ispta = 533 mW cm2, MI = 0,9 Ispta = 606 mW cm2, MI = 0,3
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European Committee for Medical Ultrasound Safety (ECMUS)
Biological consequences of heat depend on temperature rise
and duration.
Thermal Effects
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European Committee for Medical Ultrasound Safety (ECMUS)
Thermal Effects
Tissues containing a large component of actively dividing cells are most
sensitive to the effects of heat.
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European Committee for Medical Ultrasound Safety (ECMUS)
ACOUSTIC CAVITATION
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European Committee for Medical Ultrasound Safety (ECMUS)
Cavitation
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European Committee for Medical Ultrasound Safety (ECMUS)
• Formation/activity of gas filled bubbles in an ultrasound exposed medium
• At MHz frequencies bubble radius ~1 µm
• Stable cavitation – bubbles oscillate
• Inertial cavitation – bubbles expand too far then collapse very rapidly, releasing enough energy to damage tissue
Acoustic Cavitation
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European Committee for Medical Ultrasound Safety (ECMUS)
Inertial Cavitation
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European Committee for Medical Ultrasound Safety (ECMUS)
How does the risk of heating & cavitation change
with imaging conditions?
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European Committee for Medical Ultrasound Safety (ECMUS) All modes
Co
nta
ct tim
e
Ca
vitation
2
He
atin
g
Ca
vita
tio
n
2
He
atin
g
Ou
tpu
t P
ow
er
Fre
quency
Ca
vita
tion
He
atin
g
Increase Increasing importance Decreasing importance
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European Committee for Medical Ultrasound Safety (ECMUS) Imaging & M-mode
M-m
ode
Cavitation
2
Heating
Secto
r fo
rmat
Heati
ng
Deep
er/
mo
re
foc
al
zo
ne
s
2
He
atin
g
Na
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w S
ecto
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2
He
atin
g
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European Committee for Medical Ultrasound Safety (ECMUS)
2
Pulsed Doppler
Other Factors which may influence Heating and cavitation
Range Gate Width
(pulse length may vary with gate width)
Range Gate Depth
(power may increase with depth)
Doppler Velocity Range (pulse repetition frequency may increase)
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European Committee for Medical Ultrasound Safety (ECMUS)
2
Receiver Gain has NO effect on heating or cavitation
So …..
Maximise it!
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European Committee for Medical Ultrasound Safety (ECMUS)
A diagnostic exposure that produces a maximum in situ
temperature rise of no more than 1.5C above physiological levels
(37C) may be used clinically without reservation on thermal
grounds
WFUMB’98 - 01
Thermal Effects
WFUMB 1992
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European Committee for Medical Ultrasound Safety (ECMUS)
A diagnostic exposure that elevates embryonic and fetal in
situ temperature above 41oC (by 4oC) for 5 min should be
considered to be potentially hazardous.
WFUMB’98 - 01
Thermal Effects
WFUMB 1992
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European Committee for Medical Ultrasound Safety (ECMUS)
“On Screen” Labelling
Designed to provide safety related information
AIUM/NEMA Output Display Standard
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European Committee for Medical Ultrasound Safety (ECMUS)
The Thermal index (TI) is an on-screen guide to the user of the potential for tissue heating.
Estimate of the tissue temperature rise in ºC which might be possible under "reasonable worst-case conditions”
THERMAL INDEX (TI)
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European Committee for Medical Ultrasound Safety (ECMUS)
TI = Acoustic Power Output
(Acoustic Power to produce a 1oC rise)
THERMAL INDEX
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European Committee for Medical Ultrasound Safety (ECMUS)
Soft tissue index
TIS
Bone
TIB
TIC – bone near transducer
THERMAL INDEX
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European Committee for Medical Ultrasound Safety (ECMUS)
The Mechanical index (MI) is an on-screen guide of the
likelihood and magnitude of nonthermal effects.
MECHANICAL INDEX
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European Committee for Medical Ultrasound Safety (ECMUS)
MI = p/f P: in situ pressure
f: frequency
MECHANICAL INDEX (MI)
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European Committee for Medical Ultrasound Safety (ECMUS)
Power setting
High power low gain
Lower power high gain
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European Committee for Medical Ultrasound Safety (ECMUS)
Power setting
High power low gain
Lower power high gain
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European Committee for Medical Ultrasound Safety (ECMUS)
From scientific evidence of ultrasound-induced biological effects to date, there is
NO REASON to withhold diagnostic scanning during
pregnancy,
provided it is
1. medically indicated
2. is used prudently by fully trained operators.
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European Committee for Medical Ultrasound Safety (ECMUS)
Ultrasound exposure during pregnancy
With increasing mineralisation of fetal bones, the possibility of
heating sensitive tissues such as brain and spinal cord increases.
So …. extra vigilance is advised
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European Committee for Medical Ultrasound Safety (ECMUS)
3D imaging
No additional safety considerations
(particularly if there are significant pauses during scanning
to study or manipulate the reconstructed images)
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European Committee for Medical Ultrasound Safety (ECMUS)
4D imaging (real-time 3D)
Involves continuous exposure
Guard against prolonging examination times unduly to improve the recorded
image sequence beyond that necessary for diagnostic purposes.
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European Committee for Medical Ultrasound Safety (ECMUS)
Ultrasound Contrast Agents (UCAs)
• UCAs are not licensed for pregnancy
• Caution should be exercised when using in tissues for
which damage to microvasculature may be important (eg.
eye, brain, neonate)
•Exercise caution when using UCAs in patients with
severe coronary artery disease and pulmonary
hypertension.
Keep MI low, and avoid long exposure times
Refer to EFSUMB CEUS guidelines (2011)
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European Committee for Medical Ultrasound Safety (ECMUS)
http://www.bmus.org/policies-guides
0 0.5 1.0 1.5 2.0 2.5 3.0
0.7
RECOMMENDED
RANGE
PROVIDED
ADEQUATE IMAGES
CAN BE OBTAINED (especially in 1st trimester)
Unlimited time
Observe ALARA
< 60
mins
< 30
mins
< 15
mins
< 4
mins
< 1
min NOT
RECOMMENDED
for
OB
scanning
Recommended scanning time limits for these TIs
(observe ALARA)
THERMAL INDEX
OBSTETRIC SCANNING
Monitor TIS up to 10 weeks post-LMP, TIB thereafter.
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European Committee for Medical Ultrasound Safety (ECMUS)
Epidemiological safety studies Recent systematic reviews:
• Torloni MR. WHO systematic review of the literature
and meta-analysis.
UOG 2009;33:599-608
• Whitworth M. Ultrasound for fetal assessment in early
pregnancy.
Cochrane Database Syst Rev 2010
• Salvesen KÅ. Ultrasound in pregnancy and non-right
handedness: meta-analysis of randomized trials.
UOG 2011;38:267-71
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European Committee for Medical Ultrasound Safety (ECMUS)
Only one controversial issue Prenatal ultrasound is associated with
left-handedness Published studies
Year Journal 1. author Type of study
1993 BMJ Salvesen RCT
1998 Early Hum Dev Kieler RCT
2001 Epidemiology Kieler Cohort
2002 Epidemiology Kieler Cohort
2011 Ultrasound Obstet Gynecol Heikkilä RCT
2011 Ultrasound Obstet Gynecol Salvesen Meta-analysis
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European Committee for Medical Ultrasound Safety (ECMUS)
Meta-analysis Forrest plot from Salvesen UOG 2011;38:267-71
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European Committee for Medical Ultrasound Safety (ECMUS)
We should worry – why? • 5 epidemiological studies demonstrate an
increased risk of left-handedness of 15-30%
• No other epidemiological study on ultrasound
and handedness has been published
• Experimental studies indicate effects on the
brain in some animal models
• Modern scanners produce higher outputs
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European Committee for Medical Ultrasound Safety (ECMUS)
We should not worry – why? •Being left-handed is normal
(10-15% of population)
•A statistical association does
not imply a causal relationship
•The biological plausibility of this association is questionable
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European Committee for Medical Ultrasound Safety (ECMUS)
European Committee for Medical Ultrasound Safety
www.efsumb.org/ecmus
British Medical Ultrasound Society
www.bmus.org
World Federation for Ultrasound in Medicine & Biology
www.wfumb.org
Safety Statements