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Beam Measurements

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* * * * * * * Statements to Patients no basis that clinical ultrasound produces any harmful effects unobserved effects could be occurring ... – PowerPoint PPT presentation

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Title: Beam Measurements


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 beams cross section
  • water in a pipe does not all flow at same speed

5
Intensity
  • Changes across beams cross section
  • Non-uniformity makes it difficult to quantify
    intensity

6
Quantifying IntensityPeak
  • Establish a measurement convention
  • peak value

Peak
  • spatial peak (SP)
  • peak intensity across entire beam at a particular
    depth

Peak
7
Quantifying IntensityAverage
  • Establish a measurement convention
  • average

Average
Average
  • spatial average (SA)
  • average intensity across entire beam at a
    particular depth

8
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?

9
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
0
10
Pulsed Intensity Conventions
  • Pulse average intensity (PA)
  • beam intensity averaged only during sound
    generation
  • ignore silences

PA Intensity
11
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?
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
  • gt 1oC
  • AIUM Statement
  • thermal criterion is potential hazard
  • 1oC temperature rise acceptable
  • fetus in situ temperature gt 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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