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08 Beam Measurements

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08 Beam Measurements

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    1. 08 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 Changes across beam’s cross section Non-uniformity makes it difficult to quantify intensity

    6. Quantifying Intensity: Peak spatial peak (SP) peak intensity across entire beam at a particular depth

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

    8. Beam Uniformity Ratio (BUR) Quantitative indication of beam uniformity BUR always >=1 peak always >= average BUR = 1: perfectly uniform beam Actual beam BUR > 1

    9. Who Cares? Spatial peak more indicative of very localized effects (heating) Spatial average more indicative of regional effects (heating)

    10. 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?

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

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

    13. Pulse Average Intensity (PA) PA = 60 since 60 is (peak) intensity during production of sound

    14. Pulsed Intensity Conventions Temporal average intensity (TA) beam intensity averaged over entire time interval sound periods and silence periods averaged

    15. Temporal Average Equation Duty Factor: fraction of time sound is on DF = Pulse Duration / Pulse Repetition Period

    16. 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

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

    18. Complication: Non-constant pulses intensity does not remain constant over duration of pulse

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

    20. Combination Intensities Abbreviations Individual SA = spatial average SP = spatial peak PA = pulse average TA = temporal average TP = temporal peak

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

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

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

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

    25. Converting Intensities: Making the Math Easy Change initials one pair at a time Ignore initials that do not change Use formulas below

    26. Ultrasound Phantoms

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

    28. 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

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

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

    31. Ultrasound Safety & Bioeffects

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

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

    34. Plants Plant composition: gas-filled channels between cell walls in stem leave root Useful models for cavitation studies

    35. 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

    36. Transient Cavitation Also called collapse cavitation bubble oscillations so large that bubble collapses pressure discontinuities produced (shock waves)

    37. Transient Cavitation results in localized extremely high temperatures can cause light emission in clear liquids significant destruction

    38. 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

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

    40. 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

    41. Biological Consequences of Heating (cont.) palate defects brain wave reduction microencephaly anencephaly spinal cord defects

    42. 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/cm2)

    43. 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

    44. 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

    45. 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

    46. 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

    47. 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

    48. 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

    49. 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)

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

    51. 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).

    52. 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

    53. 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.

    54. 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

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