Bioeffect Flashcards
Science of identifying and measuring sound beam potential for biological effects
Dosimetry
Dosimetric Quantities:
- pressure
- power
- intensity
unit for pressure
Pascal
*force per unit area
unit for power
Watts
*rate at which work is performed
unit for intensity
W/cm2 or mW/cm2
*power per unit area
Ultrasound system output control will determine sound beam intensity, and the sonographer is directly responsible for the prudent use of ultrasound.
T or F ?
T
*prudent - acting with or showing care and thought for the future
Acoustic Exposure is determined by:
- sound beam intensity
- exposure time
To avoid over-exposure, only perform exam…
- when medically indicated
- with proper equipment settings
- while limiting exposure time
Current FDA regulatory limit SPTA =
720 mW/cm2
*SPTA: The Spatial Peak Temporal Average (SPTA) is the average intensity over a time and area
*SPTA: most common measurement for u/s bioeffects
For biosafety, intensity is described 2 ways:
- spatial: intensity related to space or distance
- Spatial Peak (max.) – area of beam with highest I
- Spatial Average – average beam I.
- Temporal: intensity over time
- Temporal Peak (max.) – highest I during “on” time
- Temporal Average – average I during “on” and “off” time
Three Types of Research:
- In vivo
- In vitro
- epidemiological
Vivo
within a living body
i.e. plants or animal
Vitro
outside the living body in an artificial environment.
i.e.
“in-glass” test tubes/petri dishes
Very high intensities can cause genetic damage and cell death
Epidemiological
long-term studies of people who have had u/s -mostly human fetuses (birth weight, anomalies, IQ, cancer, etc.). -No known effects when compared to fetuses without exposure.
What are the mechanical effects?
Radiation forces
streaming
acoustic cavitation
Cavitation occurs where there are existing gas bodies such as…
lungs, intestines, contrast agents
Mechanical Effects produces free radicals: 2 forms of cavitation….
- stable cavitation
- transient cavitation
What is mechanical index (MI)?
- Used to describe output in terms of possible cavitation
- Relates to likelihood of harmful bioeffects from cavitation
If MI is < 1
low risk of cavitation
If MI is > 1
risk of cavitation must be weighed against benefits of exam
*Related to Temporal Peak; thus, longer exposure time at high intensities will produce mechanical index > 1 and possible cavitation
What is cavitaion?
interaction of soundbeam with microscopic bubbles in tissue
Intensities ______ mW/cm2 will induce cavitation.
> 3300
AIUM accepted SPTA intensities range from ____mW/cm2 depending on application (grayscale, color Doppler, M-mode, pulsed Doppler)
1-290
What is stable cavitation?
- bubbles stay same size (they do not burst) but can still cause tissue damage
- bubbles grow and oscillate at lower MI levels
- Results in microstreaming in fluid surrounding cells and in cell shear stresses
What is transient cavitation?
- bubbles expand and collapse violently (i.e., they burst). Pressures are created in the tissue producing stress and cell death
- bubbles expand and collapse causing implosion at higher MI levels
- Results in shock waves and colossal temperatures.
Not as much is known about non-thermal (Mechanical) bioeffects compared to thermal issues
T or F ?
T
Cavitation is related to the ________ pressure
peak rarefactional
(the highest negative pressure)
◦ Bubbles are largest at peak rarefactional pressure
________ frequencies increase risk of cavitation
Lower
To minimize non-thermal bioeffects…
Decrease output power
Decrease dwell time
No adverse non-thermal bioeffects have been reported below ______ in tissues with existing gas bodies
0.4 MPa (≈ MI < 0.4)
In tissues without existing gas bodies, no adverse non-thermal bioeffects have been reported with _____
MI < 1.9
With ultrasound contrast, no adverse non-thermal bioeffects have been reported with _______
MI < 0.4
High MI from _____ pressure and _____ frequency
high
low
MI is proportional to _____ and related to ______
peak rarefactional pressure
Temporal Peak (TP)
FDA max MI =
1.9
Explain ALARA
As low as reasonably achievable
Minimize: power output, scan time, use of Doppler
Explain Thermal bioeffects
- Acoustic energy is converted to heat as it travels through the body due to attenuation
- Heat is deposited into tissues = temperature increase in tissues
What unit is used for TI?
- TI (Thermal Index) has no units
- It is a ratio of in-situ acoustic power in watts to the acoustic power required to raise tissue temperature by 1C.
Thermal index should be kept at what number?
Less than 1.0 during all examination
What is attenuation and what are the contributory factors?
- The loss of ultrasound energy as the wave propagates through tissues.
- Attenuation has two contributors: scattering and absorption: conversion of ultrasound energy to heat)
TIS
TI soft tissue
- Used when ultrasound path includes only soft tissue
- Obstetric scanning up to 10 weeks LNMOP (LNMP: last normal menstrual period)
- Assumes sound traveling through soft-tissue
- Use for obstetrics up to 10 weeks LMP
TIB
TI Bone
- Used when ultrasound path includes bone near focus
- Obstetric scanning from 10 weeks to term
TIC
TI Cranial bone
- Used when transducer next to skull (primarily neonatal)
Why is B-mode imaging considered to have less risk and exposure?
- Very short pulses/less frequency, therefore, the energy sent to the tissue is relatively modest
- Spatial peak temporal average intensity (ISPTA) is the lowest compared to other scan mode (B-mode: 34/M-mode: 106/Color Doppler: 290/Spectral Doppler: 1180)
- Note that even B mode cannot be considered safe
What is TI?
Thermal Index
- Used to describe ratio between output power and the amount of power it takes to raise tissue temperature by 1o C.
- Depends on TDR frequency, acoustical power, beam area, absorption, attenuation properties of tissue, thermal properties of tissue.
AIUM CONCLUSIONS ON CLINICAL SAFETY
No confirmed harmful bioeffects from diagnostic ultrasound.
Possible that bioeffects may be identified in the future.
Benefit of diagnostic ultrasound outweigh risks of exposure.
Use ultrasound prudently to benefit patients.
Never use ultrasound for entertainment purposes.
ALARA Principle – As Low As Reasonably Achievable
Thermal Effects
Acoustic energy is converted to _____ as it travels through the body, and A temperature increase of _____ appears to be safe (up to 50 hours)
heat
≤ 2.0 °C
Same as saying body temp < 39° C
Thermal Effects
Thermal effects not only dependent upon rate of heat _______ (how fast heat is put into the tissue), but also heat ______ (how fast heat is removed) by blood/flow etc
deposition
dissipation
Thermal Effects
_____ is deposited into tissues, and the temperature increase is related to: _____ & ________
Heat
◦ Volume of tissue being imaged
◦ Output power
Thermal Effects
What is scanned mode?
(B-mode, color Doppler) sweep
the beam so energy is distributed over large volume
Thermal Effects
What is unscanned mode?
(M-mode, spectral Doppler)
keeps the beam in one place, increasing dwell time over a smaller volume (thereby depositing more heat)
Thermal Effects
Thermal effects not only dependent upon rate of heat deposition, but also heat _________.
More perfusion = more chance for heat ____
dissipation
Thermal Effects
Bone
excellent heat absorber. Most likely to have a temperature elevation at a soft-tissue:bone interface
Assumes sound traveling through bone
Use at/after 10 weeks LMP
Thermal Effects
Fetal tissue
Fetal tissues at most risk for damage. Greatest concern is at soft- tissue:bone interfaces in fetus
Thermal Effects
The longer you spend on one area, the more energy you deposit into that tissue
Decrease your _______
dwell time
Thermal Effects
To minimize thermal bioeffects:
- Decrease output power
- Decrease dwell time
- Avoid bone when possible
Thermal Index (TI) predicts ________
maximum temperature increase
No confirmed bioeffects under _____ SPTA (unfocused beam)
100 mW/cm2
*note:
Spatial Peak: Measured at the beam’s center *SP tells us what the peak intensity is in the center of the beam (peak intensity is at the center of the beam)
Temporal Average: Measure 100% of the time *Temporal average (TA) includes the sending and receiving time
The order of worsening thermal effects (from not as bad to worse):
B-mode < Color Doppler < Spectral Doppler
Hydrophone (aka microprobe) measures:
- output intensity of the beam
- Intensity and pressure amplitude across the beam
- SPL, wavelength, period
In the last 60 years of US there are no known causes of biological damage or tissue injury at the power levels used in 2D medical diagnostic ultrasound BUT…benefit of performing the scan must ______________
outweigh the risks
*even if the risk is theoretical
ALARA
As Low As Reasonably Achievable
- Minimize scan time
- Minimize use of Doppler (spectral, color, power)
- Limit US for use as a diagnostic tool
Doppler at high power levels has been shown to cause bioeffects
T or F ?
T
What is the BEST way to correct the image below?
Increase overall gain
units for intensity
W/cm2
*Intensity = Power ÷ Area
unit for pressure amplitude
Pascal (Pa)
Amplitude
- represents the height of the wave
- The maximum ‘height’ of the vibration (cycle)
- Amplitude typically refers to pressure amplitude
- Units: Pascals (Pa)
- Amplitude decreases with depth
- Due to attenuation
If the pressure amplitude is increased by a factor of 2, then the intensity is increased by a factor of:
a. 1
b. 2
c. 3
d. 4
e. 6
d
If gain was 50dB and output power is reduced by one-half, the new gain is ___ dB
a. 37
b. 25
c. 100
d. 47
d
If we decrease the intensity of a sound beam by 1⁄2, it changed by -3dB
If we quadruple the intensity, we changed it by 6 dB
If intensity is 50 mW/cm2 and the power is reduced by half, what is the new intensity
- 25 W
- 25 mW/cm2
- 12.5 mW/cm2
- 47 mW/cm2
1
intensity & power are proportionally related (1:1)
At what depth has a 10 MHz transducer with an intensity of 100 mW/cm2 experienced 3 dB of attenuation ?
Key word: DEPTH • Ignore intensity
• HID=6/10 = 0.6cm
Intensity is proportional to the amplitude _______
Intensity is proportional to the amplitude squared
- If you double the amplitude, you quadruple the intensity
- If you halve the amplitude, you quarter the intensity
- Intensity is not uniform over spac and time with pulsed wave equipment
Intensity varies with space and time
WHERE was the energy measured?
Spatial Average and Spatial Peak
Intensity varies with space and time
Finds the energy distribution across the entire
beam and takes the average
Spatial Average
*The spatial average represents the intensity across the whole beam.
*The SA gives of the average of all the intensities in the beam; not just the center, but the outside edges as well
Intensity varies with space and time
Measured at the beam’s center
Spatial Peak
*SP tells us what the peak intensity is in the center of the beam (peak intensity is at the center of the beam)
*The sound intensity decreases from the center of the beam to the outer edges. (SP>SA)
*The spatial peak is measured at the beam’s center.
BUR
Beam uniformity ratio (How uniform is the beam?)
- Also called SP/SA factor
- BUR = SP/SA
- If the BUR = 1 then the edge intensity equals the center intensity
- The smaller the number, the more uniform the beam is (because the SA is closer to the SP)
- BUR is always 1 or greater
Intensity (TA and TP)
Measure 100% of the time
Temporal Average
Intensity (TA and TP)
Maximum intensity of the pulse
Temporal Peak
Intensity (TA and TP)
Average intensity only during sound transmission
Pulse Average
Intensity (TA and TP)
TA and PA are related by duty factor
- DF is the ratio of time that the sound is on
- Usually <1% for pulse-wave operation and
- 100% for continuous-wave operation
Therefore, TA = ????
TA=PAxDF
• If CW sound is used, then the DF = 1 and TA = PA
There are Six different intensities are used to describe the sound beam:
• SATA
• SPTA
• SAPA
• SPPA
• SATP
• SPTP
Which one of them is the weakest?
SATA
*SA: spatial average
*TA: temporal average
There are Six different intensities are used to describe the sound beam:
• SATA
• SPTA
• SAPA
• SPPA
• SATP
• SPTP
Which one of them is the strongest?
SPTP
*SP: spatial peak (beam center)
*TP: temporal peak
There are Six different intensities are used to describe the sound beam:
• SATA
• SPTA
• SAPA
• SPPA
• SATP
• SPTP
Which one of them is the most relevant with thermal bioeffect?
SPTA
*SP: spatial peak
*TA: temporal average (measures 100% of time)
Which factor will result in the highest TI?
- 5MHz, 6cm depth
- 3MHZ, 10cm depth
- 7MHz, 9cm depth
- 12MHz, 4cm depth
4
*higher f = more absorption = more thermal bioeffect
Which of the following intensity measurement is made during the PRP?
- SPPA
- SPTP
- SATP
- SPTA
4 (?)
What is defined as the action of an acoustic field within a fluid to generate bubbles?
cavitation
Which acoustic variable is defined as force divided by area?
pressure
*note: power/area = intensity
joules = kg*m2/s2 (amount of work done)
What does SPI stand for?
Sonography Principles and Instrumentation
