George David Associate Professor 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

Intensity Changes across beam’s cross section Non-uniformity makes it difficult to quantify intensity

Quantifying Intensity: Peak  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) »average intensity across entire beam at a particular depth Average Establish a measurement convention average Average

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 BUR BUR=spatial peak / spatial average

Who Cares? Spatial peak more indicative of very localized effects (heating) Spatial average more indicative of regional effects (heating) SP = 60 SA = 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 on beam on beam off beam off

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

Pulsed Intensity Conventions Pulse average intensity (PA)Pulse average intensity (PA)  beam intensity averaged only during sound generation  ignore silences beam on beam on beam on beam off beam off PA Intensity

Pulse Average Intensity (PA) PA = 60 since 60 is (peak) intensity during production of sound beam on beam on beam on beam off beam off

Pulsed Intensity Conventions Temporal average intensity (TA)Temporal average intensity (TA)  beam intensity averaged over entire time interval  sound periods and silence periods averaged beam on beam on beam on beam off beam off What is weighted average of intensities here and here? TA Intensity?

TA = PA * Duty Factor Temporal Average Equation Duty Factor: fraction of time sound is on DF = Pulse Duration / Pulse Repetition Period

TA = PA * Duty Factor Temporal 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 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

George David Associate Professor Non-constant Pulse Parameters PA = pulse average »average intensity during production of sound TP = temporal peak  highest intensity achieved during sound production TP PA

Combination Intensities Abbreviations  Individual »SA = spatial average »SP = spatial peak »PA = pulse average »TA = temporal average »TP = temporal peak Combinations SATA SAPA SATP SPTA SPPA SPTP The following abbreviations combine to form 6 spatial & pulse measurements

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

SATP = 52 SA: Average all measurement in set TP: Only use measurements during sound production Average of 60, 50, 48, 50, & 52

SPTA = 12 SP: Only use highest measurement in set TA: Average measurements during sound & silence Average of 60, 0, 0, 0, & 0

SATA = 10.4 SP: Average all measurement in set TA: Average measurements during sound & silence Average of 52, 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

George David Associate Professor 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

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

George David Associate Professor Doppler Test Objects String test objects  moving string used to calibrate flow speed  stronger echoes than blood  no flow profile

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

George David Associate Professor 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  >= 1 o C AIUM Statement  thermal criterion is potential hazard  1 o C temperature rise acceptable  fetus in situ temperature >= 41 o C considered hazardous »hazard increases with time at elevated temperature

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 > 39 o C 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/cm 2 )

George David Associate Professor 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 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 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

George David Associate Professor 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

Screening Ultrasound for Pregnancy National Institute of Health (NIH) Consensus panel  not recommended Royal College of Obstetricians & Gynaecologists  routine exams between weeks of pregnancy European Federation of Societies for Ultrasound in Medicine and Biology  routine pregnancy scanning not contra-indicated

George David Associate Professor 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).

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

George David Associate Professor 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