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Beam Measurements

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.

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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. Decades of 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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