Ultrasound safety Flashcards
Main hazards of ultrasound
Thermal - tissue being heated due to absorption
Non-thermal - everything else
Non-thermal hazards
Acoustic cavitation
Gas-body effects
Radiation pressure
What is more important at higher and lower frequencies
Higher frequencies - thermal more important - proportional to f
Lower frequencies, acoustic cavitation more important - proportional to 1/f
Acoustic dose rat
Rate of absorption of energy per unit mass
Qm = (2 alpha_a I_ta)/ rho_0
for wave with time average intensity I_ta
Initial rate of temperature rise
dT/dt = Qm/C
C is tissues specific heat capacity
Radiation force
Fm = Qm/c_0
Maximum TI and MI
Opthamology: TI 1.0, MI 0.23, derated Ispta = 50
All others: TI 6.0, MI 1.9, derated Ispta = 720
derated Ispta is highest value of Ita at some point in field
What gives highest TI max values
PW doppler
What produces highest temperature changes
Stationary or non-scanned beams
Teratogen
Any agent that causes an abnormality following fetal exposure during pregnancy.
High enough temperature rise for too long can cause severe abnormalities
What temperature is okay for imaging
delta T < 1.5C may be used clinically without reservation
Fetal temperature harmful
> 4C for > 5 minutes considered potentially harmful
Core human body temperature and effect of heat above this
37 +- 0.8 C
Between 39-43 C time required to produce damage goes down by factor of four per degree
Thermal index
Non-dimensional indicator of worst case temperature rise. Correlates well with delta T - usually within a factor of 2
Types of TI
TIS for soft tissue, TIB for ‘bone at focus’ TIC for cranial bone (for non-fetal head).
Use TIB if near bone, TIS elsewhere
TIS in first weeks of pregnancy, TIB when bone has oscified.
TIS/TIB eq in scanned modes
TIS = TIB = (W_01 f_0)/210
W_01 is bounded square output power (mW)
f_0 is centre freq. (MHz)
Acoustic cavitation
Mechanical and chemical hazards - mechanical due to radial oscillation of vapour bubbles, chemcial due to bubble collapse and free radical creation.
When is cavitation a hazard
Only when using contrast agent
Gas-body effects
Mechanical only - due to complete reflection of ultrasound at air-tissue boundaries causing amplitude of wave to double, can damage lung capillaries.
Radiation pressure
Mechanical only - may cause stremaing and shear stress at beam edges. Visible in amniotic fluid but not considered a hazard.
Equation for MI
MI = p_r(0.3)/root fc
pr is derated peak rarefractional pressure
What happens to bubbles
Collapse until it becomes very small then rebounds. Shock wave given out - spherical acoustic wave given off. Temperature can be very high during collapse which can produce chemical hazard. Shock wave and rebounding cause mechanical damage.
Contrast agents
Made up of encapsulated microbubbles. Injected into blood and improve contrast between blood and tissue - scatter ultrasound strongly.
Need to use low MI as can cause bleeding.
Practicing ALARA
Use output power and gain control to keep TI and MI ALARA
Short scan times
Use freeze control
Specialist should set up scanner with lowest output values
Sonographer should know which controls affect TI/MI.
Take extra care with PW doppler and contrast.
Safe limits
Provided by BMUS, if TI < 1, unlimited time. TI<0.7 for obstetrics.
Don’t use MI > 0.7 for contrast
