Ultrasound safety

Question 1

Main hazards of ultrasound

Answer 1

Thermal - tissue being heated due to absorption
Non-thermal - everything else

Question 2

Non-thermal hazards

Answer 2

Acoustic cavitation
Gas-body effects
Radiation pressure

Question 3

What is more important at higher and lower frequencies

Answer 3

Higher frequencies - thermal more important - proportional to f
Lower frequencies, acoustic cavitation more important - proportional to 1/f

Question 4

Acoustic dose rat

Answer 4

Rate of absorption of energy per unit mass
Qm = (2 alpha_a I_ta)/ rho_0
for wave with time average intensity I_ta

Question 5

Initial rate of temperature rise

Answer 5

dT/dt = Qm/C
C is tissues specific heat capacity

Question 6

Radiation force

Answer 6

Fm = Qm/c_0

Question 7

Maximum TI and MI

Answer 7

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

Question 8

What gives highest TI max values

Answer 8

PW doppler

Question 9

What produces highest temperature changes

Answer 9

Stationary or non-scanned beams

Question 10

Teratogen

Answer 10

Any agent that causes an abnormality following fetal exposure during pregnancy.
High enough temperature rise for too long can cause severe abnormalities

Question 11

What temperature is okay for imaging

Answer 11

delta T < 1.5C may be used clinically without reservation

Question 12

Fetal temperature harmful

Answer 12

> 4C for > 5 minutes considered potentially harmful

Question 13

Core human body temperature and effect of heat above this

Answer 13

37 +- 0.8 C
Between 39-43 C time required to produce damage goes down by factor of four per degree

Question 14

Thermal index

Answer 14

Non-dimensional indicator of worst case temperature rise. Correlates well with delta T - usually within a factor of 2

Question 15

Types of TI

Answer 15

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.

Question 16

TIS/TIB eq in scanned modes

Answer 16

TIS = TIB = (W_01 f_0)/210

W_01 is bounded square output power (mW)
f_0 is centre freq. (MHz)

Question 17

Acoustic cavitation

Answer 17

Mechanical and chemical hazards - mechanical due to radial oscillation of vapour bubbles, chemcial due to bubble collapse and free radical creation.

Question 18

When is cavitation a hazard

Answer 18

Only when using contrast agent

Question 19

Gas-body effects

Answer 19

Mechanical only - due to complete reflection of ultrasound at air-tissue boundaries causing amplitude of wave to double, can damage lung capillaries.

Question 20

Radiation pressure

Answer 20

Mechanical only - may cause stremaing and shear stress at beam edges. Visible in amniotic fluid but not considered a hazard.

Question 21

Equation for MI

Answer 21

MI = p_r(0.3)/root fc
pr is derated peak rarefractional pressure

Question 22

What happens to bubbles

Answer 22

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.

Question 23

Contrast agents

Answer 23

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.

Question 24

Practicing ALARA

Answer 24

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.

Question 25

Safe limits

Answer 25

Provided by BMUS, if TI < 1, unlimited time. TI<0.7 for obstetrics.
Don’t use MI > 0.7 for contrast