08 Beam Measurements

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08 Beam Measurements
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Intensity

• intensity power / beam cross sectional area
• beam area changes with depth
• for constant beam power, intensity increases with
  decreasing area

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Significance of Intensity

• safety
• bioeffect considerations

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Intensity Complication

• intensity changes across beams cross section
• water in a pipe does not all flow at same speed

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Intensity

• Changes across beams cross section
• Non-uniformity makes it difficult to quantify
  intensity

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Quantifying IntensityPeak

• Establish a measurement convention
• peak value

Peak

• spatial peak (SP)
• peak intensity across entire beam at a particular
  depth

Peak
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Quantifying IntensityAverage

• Establish a measurement convention
• average

Average
Average

• spatial average (SA)
• average intensity across entire beam at a
  particular depth

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Beam Uniformity Ratio (BUR)
BURspatial peak / spatial average

• Quantitative indication of beam uniformity
• BUR always gt1
• peak always gt average
• BUR 1 perfectly uniform beam
• Actual beam
• BUR gt 1

Average
Peak
BUR Peak / Average
BUR SP / SA
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Who Cares?

• Spatial peak more indicative of very localized
  effects (heating)
• Spatial average more indicative of regional
  effects (heating)

SP 60 SA 52
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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?

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Pulsed Intensity

• SP 60 when beam is on
• SP 0 when beam is off
• How do we define pulsed intensity in a single
  number?

60
0
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Pulsed Intensity Conventions

• Pulse average intensity (PA)
• beam intensity averaged only during sound
  generation
• ignore silences

PA Intensity
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Pulse Average Intensity (PA)

• PA 60 since 60 is (peak) intensity during
  production of sound

60
0
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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?
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Temporal Average Equation
TA PA Duty Factor

• Duty Factor fraction of time sound is on
• DF Pulse Duration / Pulse Repetition Period

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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 lt 1
• TA lt PA

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Who Cares?

• Temporal peak more indicative of instantaneous
  effects (heating)
• Temporal average more indicative of effects over
  time (heating)

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Complication Non-constant pulses

• intensity does not remain constant over duration
  of pulse

X
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Non-constant Pulse Parameters

• PA pulse average
• average intensity during production of sound
• TP temporal peak
• highest intensity achieved during sound production

TP
PA
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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
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SPTP 60

• SP Only use highest measurement in set
• TP Only use measurements during sound production

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SATP 52

• SA Average all measurement in set
• TP Only use measurements during sound production

Average of 60, 50, 48, 50, 52
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SPTA 12

• SP Only use highest measurement in set
• TA Average measurements during sound silence

Average of 60, 0, 0, 0, 0
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SATA 10.4

• SP Average all measurement in set
• TA Average measurements during sound silence

0
52
0
Average of 52, 0, 0, 0, 0
0
0
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Converting Intensities Making the Math Easy

• Change initials one pair at a time
• Ignore initials that do not change
• Use formulas below

SA SP / BUR
TA PA X duty factor
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Ultrasound Phantoms
Gammex.com
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Performance Parameters

• detail resolution
• contrast resolution
• penetration dynamic range
• compensation (swept gain) operation
• range (depth or distance) accuracy

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Tissue-equivalentPhantom 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
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Doppler Test Objects

• String test objects
• moving string used to calibrate flow speed
• stronger echoes than blood
• no flow profile

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Doppler Test Objects

• Flow phantoms (contain moving fluid)
• closer to physiological conditions
• flow profiles speeds must be accurately known
• bubbles can present problems
• expensive

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Ultrasound Safety Bioeffects
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Sources of Knowledge

• experimental observations
• cell suspensions cultures
• plants
• experimental animals
• humans epidemiological studies
• study of interaction mechanisms
• heating
• cavitation

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Cavitation

• Production dynamics of bubbles in liquid medium
• can occur in propagating sound wave

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Plants

• Plant composition gas-filled channels between
  cell walls in
• stem
• leave
• root
• Useful models for cavitation studies

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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

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Transient Cavitation

• Also called collapse cavitation
• bubble oscillations so large that bubble
  collapses
• pressure discontinuities produced (shock waves)

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Transient Cavitation

• results in localized extremely high temperatures
• can cause
• light emission in clear liquids
• significant destruction

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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

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Heating Depends on

• intensity
• heating increases with intensity
• sound frequency
• heating increases with frequency
• heating decreases at depth
• beam focusing
• tissue perfusion

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Heating (cont.)

• Significant temperature rise
• gt 1oC
• AIUM Statement
• thermal criterion is potential hazard
• 1oC temperature rise acceptable
• fetus in situ temperature gt 41oC considered
  hazardous
• hazard increases with time at elevated temperature

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Biological Consequences of Heating (cont.)

• amyoplasia
• forefoot hypoplasia
• tibial fibular deformations
• abnormal tooth genesis

• palate defects
• brain wave reduction
• microencephaly
• anencephaly
• spinal cord defects

• above effects documented for tissue temp gt 39oC
• occurrence depends on temp exposure time

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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 gt 10W/cm2)

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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

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Animal Data

• risks for certain intensity-exposure 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

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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

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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

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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

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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

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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)

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Statements to Patients

• no basis that clinical ultrasound produces any
  harmful effects
• unobserved effects could be occurring

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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).

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Thermal Index

• Ratio of power used to power required to cause
  maximum temperature increase of 1C
• Thermal index of 1 indicates power causing
  temperature increase of 1C.
• Thermal index of 2 would be 2X that power
• Does not necessarily indicate temperature rise of
  2C
• Temperature rise depends on
• tissue type
• presence of bone

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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.

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Thermal Index

• Mechanical thermal indexes must be displayed if
  scanner capable of exceeding index of 1
• Displayed indices based on manufacturers
  experimental modeled data