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
