08 Beam Measurements
George David
Associate Professor
Intensity
• intensity = power / beam cross
sectional area
beam area changes with depth
• for constant beam power,
intensity increases with
decreasing area
George David
Associate Professor
Significance of Intensity
• safety
• bioeffect considerations
George David
Associate Professor
Intensity Complication
• intensity changes across beam’s
cross section
• water in a pipe does not all flow
at same speed
George David
Associate Professor
Intensity
• Changes across
beam’s cross section
• Non-uniformity makes it
difficult to quantify intensity
50
48
60
50
52
Quantifying Intensity:
Peak
• Establish a measurement convention
• peak value
Peak
spatial peak (SP)
» peak intensity across entire
beam at a particular depth
Peak
Quantifying Intensity:
Average
• Establish a measurement convention
Average
• average
Average
spatial average (SA)
» average intensity across entire
beam at a particular depth
Beam Uniformity Ratio (BUR)
BUR=spatial peak / spatial average
• Quantitative
indication of beam
uniformity
• BUR always >=1
peak always >= average
• BUR = 1: perfectly
uniform beam
• Actual beam
BUR > 1
Average
Peak
BUR = Peak / Average
BUR = SP / SA
Who Cares?
• Spatial peak more indicative of very localized
effects (heating)
• Spatial average more indicative of regional
effects (heating)
50
48
60
50
SP = 60
SA = 52
52
Pulsed Intensity
• Pulsed ultrasound
beam on for small fraction of time
» 1/1000 typical duty factor
when beam is off, intensity is zero
• Challenge: quantifying intensity
that is changing over time?
beam
on
beam
off
beam
on
beam
off
beam
on
Pulsed Intensity
• SP = 60 when beam is on
• SP = 0 when beam is off
• How do we define pulsed intensity in
a single number?
50
48
60
0
52
50
0
0
0
0
60
0
beam
on
beam
off
beam
on
beam
off
beam
on
Pulsed Intensity
Conventions
• Pulse average intensity (PA)
beam intensity averaged only during sound
generation
ignore silences
PA
Intensity
beam
on
beam
off
beam
on
beam
off
beam
on
Pulse Average Intensity
(PA)
• PA = 60 since 60 is (peak) intensity
during production of sound
50
48
60
0
52
50
0
0
0
0
60
0
beam
on
beam
off
beam
on
beam
off
beam
on
Pulsed Intensity
Conventions
• Temporal average intensity (TA)
beam intensity averaged over entire time interval
sound periods and silence periods averaged
What is weighted
average of intensities
here and here?
TA
Intensity?
beam
on
beam
off
beam
on
beam
off
beam
on
Temporal Average Equation
TA = PA * Duty Factor
• Duty Factor: fraction of time sound is
on
• DF = Pulse Duration / Pulse Repetition
Period
Temporal Average Equation
TA = PA * Duty Factor
• Duty Factor: fraction of time sound is
on
• for continuous sound
 duty factor = 1
 TA = PA
• if all else remains constant
 as duty factor increases, TA increases
 as PA increases, TA increases
•for pulsed sound
duty factor < 1
TA < PA
Who Cares?
• Temporal peak more indicative of instantaneous
effects (heating)
• Temporal average more indicative of effects
over time (heating)
Complication: Non-constant pulses
• intensity does not remain
constant over duration of pulse
X
Non-constant Pulse Parameters
• PA = pulse average
» average intensity during production of sound
• TP = temporal peak
highest intensity achieved during sound
production
TP
PA
George David
Associate Professor
Combination Intensities
The following abbreviations combine to form 6
spatial & pulse measurements
Abbreviations
Individual
» SA = spatial average
» SP = spatial peak
» PA = pulse average
» TA = temporal average
» TP = temporal peak
Combinations
SATA
SAPA
SATP
SPTA
SPPA
SPTP
SPTP = 60
• SP: Only use highest measurement in
set
• TP: Only use measurements during
sound production
50
48
60
0
0
52
0
0
50
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
SATP = 52
• SA: Average all measurement in set
• TP: Only use measurements during
sound production
50
48
60
0
0
52
0
0
50
0
0
0
Average of 60, 50,
48, 50, & 52
0
0
0
0
0
0
0
0
0
0
0
0
0
SPTA = 12
• SP: Only use highest measurement in
set
• TA: Average measurements during sound
& silence
50
48
60
0
0
52
0
0
50
0
0
0
Average of 60, 0,
0, 0, & 0
0
0
0
0
0
0
0
0
0
0
0
0
0
SATA = 10.4
• SP: Average all measurement in set
• TA: Average measurements during sound
& silence
52
50
48
60
0
0
52
0
0
0
0
50
0
0
0
Average of 52, 0,
0, 0, & 0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Converting Intensities:
Making the Math Easy
• Change initials one pair at a time
• Ignore initials that do not change
• Use formulas below
TA = PA X duty factor
SA = SP / BUR
Ultrasound Phantoms
Gammex.com
George David
Associate Professor
Performance Parameters
• detail resolution
• contrast resolution
• penetration & dynamic
range
• compensation (swept gain)
operation
• range (depth or distance)
accuracy
George David
Associate Professor
Tissue-equivalent
Phantom Objects
• echo-free regions of various
diameters
• thin nylon lines (.2 mm diameter)
measure
detail resolution
distance accuracy
• cones or cylinders
contain material of various scattering
strengths compared to surrounding material
Gammex.com
Doppler Test Objects
• String test objects
moving string used to calibrate flow speed
stronger echoes than blood
no flow profile
George David
Associate Professor
Doppler Test Objects
• Flow phantoms (contain moving
fluid)
closer to physiological conditions
flow profiles & speeds must be accurately known
bubbles can present problems
expensive
Ultrasound Safety &
Bioeffects
Sources of Knowledge
• experimental observations
cell suspensions & cultures
plants
experimental animals
• humans epidemiological
studies
• study of interaction
mechanisms
heating
cavitation
George David
Associate Professor
Cavitation
• Production & dynamics of
bubbles in liquid medium
• can occur in propagating sound
wave
George David
Associate Professor
Plants
• Plant composition: gas-filled
channels between cell walls in
stem
leave
root
• Useful models for cavitation
studies
George David
Associate Professor
Static Cavitation
• bubble diameter oscillates with
passing pressure waves
• streaming of surrounding liquid
can occur
shear stress on suspended cells or
intracellular organelles
• occurs with continuous wave
high-intensity sound
George David
Associate Professor
Transient Cavitation
• Also called collapse cavitation
• bubble oscillations so large that
bubble collapses
• pressure discontinuities produced
(shock waves)
George David
Associate Professor
Transient Cavitation
• results in localized extremely high
temperatures
• can cause
light emission in clear liquids
significant destruction
George David
Associate Professor
Plant Bioeffects
• irreversible effects
cell death
• reversible effects
chromosomal abnormalities
reduction in mitotic index
growth-rate reduction
• continuous vs. pulsed effects
threshold for some effects much higher for
pulsed ultrasound
George David
Associate Professor
Heating Depends on
• intensity
heating increases with intensity
• sound frequency
heating increases with frequency
heating decreases at depth
• beam focusing
• tissue perfusion
George David
Associate Professor
Heating (cont.)
• Significant temperature rise
>= 1oC
• AIUM Statement
thermal criterion is potential hazard
1oC temperature rise acceptable
fetus in situ temperature >= 41oC considered
hazardous
» hazard increases with time at elevated temperature
George David
Associate Professor
Biological Consequences of
Heating (cont.)
• palate defects
• brain wave reduction
• microencephaly
• anencephaly
• spinal cord defects
•amyoplasia
•forefoot hypoplasia
•tibial & fibular
deformations
•abnormal tooth
genesis
•above effects documented for tissue temp > 39oC
•occurrence depends on temp & exposure time
Animals
• Most studies done on mice / rats
• damage reported
fetal weight reduction
postpartum fetal mortality
fetal abnormalities
tissue lesions
hind limb paralysis
blood flow statis
wound repair enhancement
tumor regression
focal lesion production (intensity > 10W/cm2)
Ultrasound Risk Summary
• No known risks based on
in vitro experimental studies
in vivo experimental studies
• Thermal & mechanical
mechanism do not appear to
operate significantly at
diagnostic intensities
George David
Associate Professor
Animal Data
• risks for certain intensityexposure time regions
• physical & biological differences
between animal studies &
human clinical use make it
difficult to apply experimentally
proven risks
• warrants conservative approach
to use of medical ultrasound
George David
Associate Professor
Fetal Doppler
Bioeffects
• high-output intensities
• stationary geometry
• fetus may be most sensitive to
bioeffects
• No clinical bioeffects to fetus
based upon
animal studies
maximum measured output values
•
25 Yrs Epidemiology Studies
• no evidence of any adverse
effect from diagnostic ultrasound
based upon
Apgar scores
gestational age
head circumference
birth weight/length
congenital infection
at birth
hearing
vision
cognitive function
behavior
neurologic examinations
George David
Associate Professor
Prudent Use
• unrecognized but none-zero risk
may exist
• animal studies show bioeffects
at higher intensities than
normally used clinically
• conservative approach should
be used
George David
Associate Professor
Screening Ultrasound
for Pregnancy
• National Institute of Health (NIH)
Consensus panel
not recommended
• Royal College of Obstetricians &
Gynaecologists
routine exams between weeks 16-18 of pregnancy
• European Federation of Societies for
Ultrasound in Medicine and Biology
routine pregnancy scanning not contra-indicated
Safety
• British Institute of Radiology
no reason to suspect existence of any hazard
• World Health Organization
(WHO)
benefits of ultrasound far outweigh any
presumed risks
• AIUM
no confirmed clinical biological effects
benefits of prudent use outweigh risks (if any)
George David
Associate Professor
Statements to Patients
• no basis that clinical ultrasound
produces any harmful effects
• unobserved effects could be
occurring
George David
Associate Professor
Mechanical Index
• Estimate of maximum amplitude of
pressure pulse in tissue
• Gives indication of relative risk of
mechanical effects (streaming and
cavitation)
• FDA regulations allow a mechanical
index of up to 1.9 to be used for all
applications except ophthalmic
(maximum 0.23).
George David
Associate Professor
Thermal Index
• Ratio of power used to power required
to cause maximum temperature
increase of 1°C
• Thermal index of 1 indicates power
causing temperature increase of 1°C.
• Thermal index of 2 would be 2X that
power
Does not necessarily indicate temperature rise of 2°C
Temperature rise depends on
» tissue type
» presence of bone
Thermal Index
• Thermal index subdivisions
TIS: thermal index for soft tissue;
TIB: thermal index with bone at/near the
focus;
TIC: thermal index with bone at the surface
(e.g. cranial examination).
• For fetal scanning
highest temperature increase expected to
occur at bone
TIB gives ‘worst case’ conditions.
George David
Associate Professor
Thermal Index
• Mechanical & thermal indexes
must be displayed if scanner
capable of exceeding index of 1
• Displayed indices based on
manufacturer’s experimental &
modeled data
George David
Associate Professor