George David Associate Professor 08 Beam Measurements.

George DavidAssociate Professor

08 Beam Measurements08 Beam Measurements


IntensityIntensity

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

• for constant beam power, intensity increases with decreasing area


Significance of IntensitySignificance of Intensity

• safety

• bioeffect considerations


Intensity ComplicationIntensity Complication

• intensity changes across beam’s cross section

• water in a pipe does not all flow at same speed

IntensityIntensity

•Changes across beam’s cross section

• Non-uniformity makes it difficult to quantify intensity 60

50

52

50

48

Quantifying Intensity:Peak

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

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

Beam Uniformity Ratio (BUR)Beam Uniformity Ratio (BUR)

• 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

BURBUR=spatial peak / spatial average

Who Cares?Who Cares?• Spatial peak more indicative of very localized

effects (heating)

• Spatial average more indicative of regional effects (heating)

60

50

52

50

48

SP = 60SA = 52

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

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60

0

60

50

52

50

48 0

0

0

0

0

Pulsed Intensity Conventions


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

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PAIntensity

Pulse Average Intensity (PA)

Pulse Average Intensity (PA)

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

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60

0

60

50

52

50

48 0

0

0

0

0



• 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 EquationTemporal Average Equation

• Duty Factor: fraction of time sound is on

• DF = Pulse Duration / Pulse Repetition Period

TA = PA * Duty Factor

Temporal Average EquationTemporal Average Equation

• 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 soundduty factor < 1

TA < PA

Who Cares?Who Cares?• Temporal peak more indicative of instantaneous

effects (heating)

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

Complication: Non-constant pulsesComplication: Non-constant pulses• intensity does not remain

constant over duration of pulse

X


Non-constant Pulse ParametersNon-constant Pulse Parameters

• PA = pulse average» average intensity during production of sound

• TP = temporal peak highest intensity achieved during sound

production

TPPA

Combination IntensitiesCombination Intensities

Abbreviations Individual

» SA = spatial average» SP = spatial peak

» PA = pulse average» TA = temporal average» TP = temporal peak

Combinations

SATASAPASATPSPTASPPASPTP

The following abbreviations combine to form 6 spatial & pulse measurements

SPTP = 60SPTP = 60• SP: Only use highest measurement in set• TP: Only use measurements during sound production

60

50

52

50

48 0

0

0

0

0

0

0

0

0

00

0

0

0

0

0

0

0

0

0

SATP = 52SATP = 52• SA: Average all measurement in set• TP: Only use measurements during sound production

60

50

52

50

48 0

0

0

0

0

0

0

0

0

00

0

0

0

0

0

0

0

0

0

Average of 60, 50, 48, 50, & 52

SPTA = 12SPTA = 12• SP: Only use highest measurement in set• TA: Average measurements during sound & silence

60

50

52

50

48 0

0

0

0

0

0

0

0

0

00

0

0

0

0

0

0

0

0

0

Average of 60, 0, 0, 0, & 0

SATA = 10.4SATA = 10.4• SP: Average all measurement in set• TA: Average measurements during sound & silence

60

50

52

50

48 0

0

0

0

0

0

0

0

0

00

0

0

0

0

0

0

0

0

0

52 0 0

00

Average of 52, 0, 0, 0, & 0

Converting Intensities: Making the Math Easy

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 PhantomsUltrasound Phantoms

Gammex.com


Performance ParametersPerformance Parameters

• detail resolution

• contrast resolution

• penetration & dynamic range

• compensation (swept gain) operation

• range (depth or distance) accuracy

Tissue-equivalentPhantom ObjectsTissue-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 materialGammex.com


Doppler Test ObjectsDoppler Test Objects

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

Doppler Test ObjectsDoppler 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

Ultrasound Safety & Bioeffects


Sources of KnowledgeSources of Knowledge

• experimental observationscell suspensions & culturesplantsexperimental animals

• humans epidemiological studies • study of interaction mechanisms

heatingcavitation


CavitationCavitation

• Production & dynamics of bubbles in liquid medium

• can occur in propagating sound wave


PlantsPlants

• Plant composition: gas-filled channels between cell walls in stem leave root

• Useful models for cavitation studies


Static CavitationStatic 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


Transient CavitationTransient Cavitation

• Also called collapse cavitation

• bubble oscillations so large that bubble collapses

• pressure discontinuities produced (shock waves)


Transient CavitationTransient Cavitation

• results in localized extremely high temperatures

• can cause light emission in clear liquids significant destruction


Plant BioeffectsPlant 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


Heating Depends onHeating Depends on

• intensity heating increases with intensity

• sound frequency heating increases with frequency heating decreases at depth

• beam focusing

• tissue perfusion


Heating (cont.)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

Biological Consequences of Heating (cont.)

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

AnimalsAnimals• 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 SummaryUltrasound 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


Animal DataAnimal 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 Bioeffects

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 Studies25 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

hearingvisioncognitive functionbehaviorneurologic examinations


Prudent UsePrudent Use

• unrecognized but none-zero risk may exist

• animal studies show bioeffects at higher intensities than normally used clinically

• conservative approach should be used

Screening Ultrasound for Pregnancy

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


SafetySafety

• 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)


Statements to PatientsStatements to Patients

• no basis that clinical ultrasound produces any harmful effects

• unobserved effects could be occurring


Mechanical IndexMechanical 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).

Thermal IndexThermal 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 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.



• 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 08 Beam Measurements.

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