Beam Measurements

Intensity

intensity = power / beam cross sectional areabeam area changes with depth

for constant beam power, intensity increases with decreasing area

Significance of Intensity

safetybioeffect considerations

Intensity Complication

intensity changes across beam’s cross section

water in a pipe does not all flow at same speed

IntensityChanges across beam’s cross section

Non-uniformity makes it difficult to quantify intensity

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Quantifying Intensity:Peak

spatial peak (SP)spatial peak (SP) peak intensity across entire

beam at a particular depth

Peak

• Establish a measurement convention

• peak value

Peak

Quantifying Intensity:Average

spatial average (SA)spatial average (SA) average intensity across entire

beam at a particular depth

Average

• Establish a measurement convention

• average Average

Pulsed Intensity

Pulsed ultrasoundbeam on for small fraction of time

1/1000 typical duty factorwhen beam is off, intensity is zero

Challenge: quantifying intensity that is changing over time?

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Pulsed IntensitySP = 60 when beam is onSP = 0 when beam is offHow do we define pulsed intensity in a

single number?

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Pulsed Intensity Conventions

Pulse average intensity (PA)Pulse average intensity (PA)beam intensity averaged only during

sound generationignore silences

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PAIntensity

Pulsed Intensity Conventions

Temporal average intensity (TA)Temporal average intensity (TA)beam intensity averaged over entire time

interval sound periods and silence periods

averaged

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What is weighted average of intensities

here and here?

TAIntensity?

TA = PA * Duty Factor

Temporal Average Equation

Duty Factor: fraction of time sound is on

DF = Pulse Duration / Pulse Repetition Period

Who Cares?Temporal peak more indicative of instantaneous

effects (heating) Temporal average more indicative of effects

over time (heating)

Complication: Non-constant pulsesintensity does not remain constant over

duration of pulse

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Non-constant Pulse Parameters

PA = pulse average average intensity during production of

soundTP = temporal peak

highest intensity achieved during sound production

TPPA

Combination Intensities

AbbreviationsIndividual

SA = spatial averageSP = spatial peak

PA = pulse averageTA = temporal averageTP = temporal peak

Combinations

SATASAPASATPSPTASPPASPTP

The following abbreviations combine to form 6 spatial & pulse measurements

Ultrasound Phantoms

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Performance Parameters

detail resolutioncontrast resolutionpenetration & dynamic rangecompensation (swept gain)

operationrange (depth or distance)

accuracy

Tissue-equivalentPhantom Objects

echo-free regions of various diameters

thin nylon lines (.2 mm diameter) measuredetail resolutiondistance accuracy

cones or cylinderscontain material of various scattering

strengths compared to surrounding material

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Doppler Test ObjectsString test objects

moving string used to calibrate flow speed

stronger echoes than bloodno flow profile

Doppler Test ObjectsFlow phantoms (contain moving fluid)

closer to physiological conditionsflow profiles & speeds must be accurately

knownbubbles can present problemsexpensive

Ultrasound Safety & Bioeffects

Sources of Knowledgeexperimental observations

cell suspensions & culturesplantsexperimental animals

humans epidemiological studies

study of interaction mechanismsheatingcavitation

Cavitation

Production & dynamics of bubbles in liquid medium

can occur in propagating sound wave

Plant Bioeffects

irreversible effectscell death

reversible effectschromosomal abnormalitiesreduction in mitotic indexgrowth-rate reduction

continuous vs. pulsed effectsthreshold for some effects much higher

for pulsed ultrasound

Heating Depends on

intensityheating increases with intensity

sound frequencyheating increases with frequencyheating decreases at depth

beam focusingtissue perfusion

Heating (cont.)Significant temperature rise

>= 1oCAIUM Statement

thermal criterion is potential hazard1oC temperature rise acceptablefetus in situ temperature >= 41oC

considered hazardous hazard increases with time at elevated

temperature

Ultrasound Risk Summary

No known risks based onin vitro experimental studiesin vivo experimental studies

Thermal & mechanical mechanism do not appear to operate significantly at diagnostic intensities

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

Fetal Doppler Bioeffectshigh-output intensitiesstationary geometryfetus may be most sensitive to bioeffectsNo clinical bioeffects to fetus based upon

animal studiesmaximum measured output values

25 Yrs Epidemiology Studies

no evidence of any adverse effect from diagnostic ultrasound based upon

Apgar scoresgestational agehead circumferencebirth weight/lengthcongenital infection

at birth

hearingvisioncognitive functionbehaviorneurologic examinations

Screening Ultrasound for Pregnancy

National Institute of Health (NIH) Consensus panelnot recommended

Royal College of Obstetricians & Gynaecologistsroutine exams between weeks 16-18 of

pregnancyEuropean Federation of Societies for

Ultrasound in Medicine and Biologyroutine pregnancy scanning not contra-indicated

Safety

British Institute of Radiologyno reason to suspect existence of any

hazardWorld Health Organization (WHO)

benefits of ultrasound far outweigh any presumed risks

AIUMno confirmed clinical biological effectsbenefits of prudent use outweigh risks (if

any)

Statements to Patients

no basis that clinical ultrasound produces any harmful effects

unobserved effects could be occurring