Beam Measurements

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

Significance of Intensity safety bioeffect considerations

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 60 50 52 48

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

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 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? 60 50 52 48 60 beam on off

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

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 off

Temporal Average Equation TA = PA * Duty Factor 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 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

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

Ultrasound Phantoms Gammex.com

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

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

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

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

Plant Bioeffects irreversible effects reversible 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 on intensity sound frequency beam focusing heating increases with intensity sound frequency heating increases with frequency heating decreases at depth beam focusing tissue perfusion

Heating (cont.) Significant temperature rise AIUM Statement >= 1oC thermal criterion is potential hazard 1oC temperature rise acceptable fetus in situ temperature >= 41oC considered hazardous hazard increases with time at elevated temperature

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

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

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

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 World Health Organization (WHO) 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 Patients no basis that clinical ultrasound produces any harmful effects unobserved effects could be occurring