Ch 15 Safety + Bioeffects Flashcards
What studies on bioeffects of u/s have been done?
Plant studies: used primarily for understanding cavitational effects in living tissue
Animal studies: allowed determination of conditions under which thermal + non-thermal bioeffects occur
Human studies (epidemiology): studies long-term effects on fetus’s or people with a history of previous sonograms (shows no known risks)
In the low MHz frequency range, were there confirmed adverse effects?
Nope
SPTA intensities should be below what?
Unfocused beam: <100 mW/cm^2
Focused beam: <1 W/cm^2
(temp increases to <1.5 degrees C)
What should peak rarefactional pressure be?
<0.4 Mpa (MI < 0.4) in tissues with natural gas bodies + with contrast enhanced diagnostic u/s
What should the MI be for tissue with gas + tissue w/o gas?
With gas: < 0.4
W/o gas: < 1.9
List 2 mechanisms by which u/s could produce biologic effects?
-Heating: thermal
-Mechanical: non thermal
Explain heating with u/s?
-U/s produces a temp rise as it propagates through tissues
-The temp rise depends on several factors
(factors: intensity, frequency, amount of beam focusing, tissue perfusion, time spent scanning, type of tissue scanned + scanned vs non-scanned modality)
When would tissue damage occur from heating?
If critical time temperature values were exceeded
(if temp rises over 1.5 degrees C = now considered significant)
The temperature should not rise above how many degrees C?
1.5
Approx. 80 known biological effects are due to what?
Hyperthermia
An excessive temperature increase can result in what?
Toxic effects
No significant adverse biological effects have been observed with a temperature of what?
< 1.5 degrees C (and in exposures up to 50 hours)
Are adult, fetal or neonatal tissues more tolerant of heat?
Adult
What are thermal indices (TI)?
-Provides a real time display of the probability that an u/s could induce thermal injury
-It is a guide for the sonographer regarding the magnitude of the temperature increase
(TI of 1: means approx temp rise of 1 degree C after a long exposure)
Differentiate TIS, TIB + TIC thermal index presets on the u/s machines?
TIS:
-for soft tissue
-suitable for 1st trimester
TIB:
-for bone
-suitable for 2nd/3rd trimester
TIC:
-for cranial bone
-suitable for neonatal, pediatric + adult scanning
List the TI that will not present a bioeffect + the TI that will cause a bioeffect to the embryo?
TI < 1.5 = no bioeffect
TI > 4 for 5 mins = bioeffect
List the TI recommendations for pre-natal exams?
TI < 0.5:
-most preferred setting
-suitable for 1st trimester
-used for extended scanning time
TI > 0.5-1:
-scan time must be less than 30 min
TI > 2.5:
-scan time must be less than 1 min
List the TI recommendations for post-natal exams?
TI < 2 = used for extended scanning time
TI 2-6 = scan time must be less than 30 min
TI > 6 = scan time must be less than 1 min
Mechanical mechanisms of interaction include what 3 things?
-Radiation force (think force by sound beam)
-Streaming (think stress on cells)
-Cavitation (think bubbles)
What are the 2 types of cavitation?
Stable:
-bubbles already present in the medium expand + contract during each cycle in response to the applied pressure
-bubbles expand under low pressure + contract under high pressure
-cavitation in tissues occurs under CW + high intensity conditions
Transient (worse form):
-bubbles are so large that they collapse + produce shock waves
-occurs in extremely high temps
-potential for significant destructive effects
Bubble related effects depend on what?
Peak rarefactional pressure
(rarefaction = regions of low pressure)
Cavitation can occur in tissues containing what?
-Gas bubbles with sufficient amplitude + frequency conditions
(contrast agents introduce an additional consideration regarding cavitation effects)
Is TI or MI formulated to assist users in evaluating the likelihood of cavitation?
MI
What is a mechanical index (MI)?
It assists users in evaluating the likelihood of cavitation related adverse biologic effects
Biologic effects have been confirmed with an MI above what?
MI > 0.4 with a gas body contrast
List the MI recommendations for pre-natal exams?
MI < 0.4 = if gas bodies are present
(if no gas bodies the MI can be increased, but should still remain low for bioeffects not related to gas bodies)
List the MI recommendations for post-natal exams?
MI < 0.4 = if gas bodies are present
MI < 1.9 = if no gas bodies
(MI should be much lower for neonatal studies than adult studies)
Explain the intensity in regards to time with mechanical + thermal bioeffects?
Mechanical: high intensity for short period time
Thermal: low intensity for long period time
What is the mechanical bioeffect + what is the safe limit to prevent it?
Bioeffect: cavitation
(when the sound beam reaches highest intensity in a physical space over a short period of time)
Safe limit to prevent cavitation: <190 W/cm^2
What is the thermal bioeffect + what is the safe limit to prevent it?
Bioeffect: heating
(when the sound beam reaches highest intensity in a physical space over a long period of time)
Safe limit to prevent cavitation: <720 W/cm^2
Differentiate the thermal bioeffect risks with scanned vs non-scanned modalities?
Scanned:
-2D B-mode, CD, power doppler, 3D imaging
-lower risk of bioeffects
(b/c energy is distributed line by line over a wide area, allowing time for heat to dissipate)
Non-scanned:
-CW, PW, M-mode
-higher risk for bioeffects
(b/c energy is constantly transmitted in same direction, decreasing time for heat to dissipate)
List 5 factors that affect the amount of heat produced during an u/s?
-Beam intensity (power + spatial distribution)
-PD + PRP
-Absorption rate of tissues
-Ability of the tissue to conduct heat away
-Scanning time
Put 2D B-mode, CD, CW, PW + M-mode in order from the highest thermal risk to lowest?
- PW
- CW
- CD
- M-mode
- 2D B-mode
Put 2D B-mode, CD, CW, PW + M-mode in order from the highest mechanical risk for cavitation to the lowest?
- 2D B-mode
- M-mode
- CD
- PW
- CW
What is the ALARA principle?
As low as reasonably achievable!!!
It means:
-exposure time + acoustic output should be kept at a min
-used only when medically indicated
