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

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

7. Quantifying Intensity: Average
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?
beam on
off

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 50 52 48 60
beam on
off

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

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?
beam on
off

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

19. 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
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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 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

26. 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

27. 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

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 of pregnancy
European Federation of Societies for Ultrasound in Medicine and Biology
routine pregnancy scanning not contra-indicated

33. 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)

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