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Beam Measurements

Learn about the concepts of ultrasound intensity, including its measurement conventions, quantification methods, and implications for safety and bioeffects. Discover the challenges posed by non-uniform intensity and changing pulse parameters, and explore the performance parameters of ultrasound phantoms. Gain insights into ultrasound safety and bioeffects based on experimental observations and studies on heating and cavitation effects.

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Beam Measurements

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  1. Beam Measurements

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

  3. Significance of Intensity • safety • bioeffect considerations

  4. Intensity Complication • intensity changes across beam’s cross section • water in a pipe does not all flow at same speed

  5. Intensity 60 50 48 52 50 • Changes across beam’s cross section • Non-uniformity makes it difficult to quantify intensity

  6. Quantifying Intensity:Peak • Establish a measurement convention • peak value • spatial peak (SP) • peak intensity across entire beam at a particular depth Peak Peak

  7. Quantifying Intensity:Average • Establish a measurement convention • average • spatial average (SA) • average intensity across entire beam at a particular depth Average Average

  8. Pulsed Intensity beam on beam off beam on beam off beam on • 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?

  9. Pulsed Intensity 0 60 50 0 48 0 52 0 50 0 beam on beam off beam on beam off beam on • SP = 60 when beam is on • SP = 0 when beam is off • How do we define pulsed intensity in a single number? 60 0

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

  11. Pulsed Intensity Conventions beam on beam off beam on beam off beam on • 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?

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

  13. Who Cares? • Temporal peak more indicative of instantaneous effects (heating) • Temporal average more indicative of effects over time (heating)

  14. Complication: Non-constant pulses • intensity does not remain constant over duration of pulse X

  15. Non-constant Pulse Parameters • PA = pulse average • average intensity during production of sound • TP = temporal peak • highest intensity achieved during sound production TP PA

  16. 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

  17. Ultrasound Phantoms Gammex.com

  18. Performance Parameters • detail resolution • contrast resolution • penetration & dynamic range • compensation (swept gain) operation • range (depth or distance) accuracy

  19. 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

  20. Doppler Test Objects • String test objects • moving string used to calibrate flow speed • stronger echoes than blood • no flow profile

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

  22. Ultrasound Safety & Bioeffects

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

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

  25. 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

  26. Heating Depends on • intensity • heating increases with intensity • sound frequency • heating increases with frequency • heating decreases at depth • beam focusing • tissue perfusion

  27. 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

  28. 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

  29. 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

  30. 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

  31. 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

  32. 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

  33. 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)

  34. Statements to Patients • no basis that clinical ultrasound produces any harmful effects • unobserved effects could be occurring

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