US: QA and Safety

Question 1

What are the measured parameters in a basic ultrasound QA test?

Answer 1

Spatial properties - resolution in axial lateral and slice thickness.
Amplitude properties - penetration, noise, dynamic range, contrast resolution
Temporal properties - The ability to image rapidly moving targets.
Accuracy of measurement tools - callipers etc.

Question 2

What are the properties of an ultrasound phantom?

Answer 2

C = 1540m/s
Attenuation = 0.5-0.7dB/cm/MHz
Scattering Coeffieicnt Nonlinearity Parameter (B/A) = 4-12
Similar elasticity and thermal propoerites to skin.
Contains traget - wires cysts contrast agents
Self contianed housing

Question 3

What is the “break point” temperature for thermal heating in vivo?

Answer 3

43 degrees C

Question 4

Under the “break point” temperature what is the equation for the time taken for damage to occur?

Answer 4

t = 4^(43-T)
t is the time in minutes
T is the temperature of the tissue.

Question 5

Over the “break point” temperature what is the equation for the time taken for damage to occur?

Answer 5

t = 2^(43-T)
t is the time in minutes
T is the temperature of the tissue.

Question 6

What does the Thermal Index (TI) on an ultrasound scanner show?

Answer 6

The TI gives a rough estimate of the worst possible case of temperature rise.

Question 7

What does the Mechanical Index (MI) on an ultrasound scanner show?

Answer 7

The MI is a value related to the likelihood of non-thermal bioeffects like damage due to bubbles and gas bodies.

Question 8

How is the Thermal Index (TI) defined?

Answer 8

TI = W(0)/W(deg)
W(0) is the time-averaged acoustic power emitted by the transducer
W(deg) is the estimated power required to produce a 1deg temperature rise.

Question 9

What are the three types of Thermal Index (TI) and when are they used?

Answer 9

TIS - soft tissue (no bone nearby, 1st trimester)
TIB - bone at focus (2nd and 3rd trimesters)
TIC - cranial bone (scanning of non-fetal head)

Question 10

What does the Thermal Index (TI) depend on?

Answer 10

W(0) - the time-averaged acoustic power emitted by the transducer
I(ta) - the time-averaged intensity of the scanner.

Question 11

How is W(0) measured?

Answer 11

Using a radiation force balance.

F=W(0)/c

Question 12

How is I(ta) measured?

Answer 12

Axial I(ta) profiles can be obtained by hydrophone scanning.

Question 13

How is the Mechanical Index (MI) defined?

Answer 13

MI= p(-)/(f(c)^0.5)
p(-) is the peak rarefactional pressure (MPa) measured at the focus of the beam and derated at 0.3bB/cm/MHz
f(c) is the centre frequency (MHz)

Question 14

What is the threshold for internal cavitation of bubbles in water?

Answer 14

MI > 0.7.

Question 15

What is the limit set by the FDA for MI for ophthalmology?

Answer 15

MI = 0.23

Question 16

What is the limit set by the FDA for TI for ophthalmology?

Answer 16

TI = 1.0

Question 17

What is the limit set by the FDA for MI for non-ophthalmic ultrasound?

Answer 17

MI = 1.9

Question 18

What is the limit set by the FDA for TI for non-ophthalmic ultrasound?

Answer 18

TI = 6.0

Question 19

What is the equation for the initial heating pattern from an ultrasound transducer?

Answer 19

H = 2af(c)*I(ta)

I(ta) is the in situ (assumed as derated) time averaged intensity.

Question 20

What does the TI not take into account?

Answer 20

Self-heating of the transducer
Non-linear distortion
Long fluid path in the first trimester

Question 21

What controls won’t affect the TI and MI?

Answer 21

Monitor brightness
TGC
Tint
Maps
Auto-Optimise
Dynamic Range
Edge
2D gain
Speckle reduction

Question 22

What controls will affect the TI and MI?

Answer 22

Transmit power
Space-Time (res-speed)
Depth
Focus
Harmonics
Zoom (when live)
Frequency
Compound imaging
Freeze

Question 23

How is ALARA practised in ultrasound imaging?

Answer 23

Transmit power kept as low as reasonably achievable
Freeze control used to limit exposure duration
Clinical applications specialist to set up scanner presets