Ultrasound: 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

3 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 distribution
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

Question 24

How do the acoustic intensities used in medical imaging compare with those used in physiotherapy?

Answer 24

Slightly lower

Question 25

How does the heating effect of an ultrasound beam vary in most soft tissues as the frequency is increased?

Answer 25

Increases roughly linearly with frequency

Question 26

How does the acoustic pressure threshold for the onset of inertial cavitation in water vary as the frequency is increased?

Answer 26

Decreases roughly as the square root of the frequency

Question 27

What is the unit of acoustic dose rate?

Answer 27

W/kg

Question 28

Which of the following parameters is acoustic dose rate proportional to?

Answer 28

Time-average intensity and absorption coefficient

Question 29

What is the expression for the initial rate of temperature rise in a tissue?

Answer 29

Acoustic dose rate divided by the specific heat capacity of the tissue

Question 30

What upper limits on the acoustic output of non-ophthalmological ultrasound scanners does the US Food and Drug Administration specify?

Answer 30

MI = 1.9, TI = 6.0 and derated Ispta = 720 mW/cm2

Question 31

Which mode of operation usually produces the highest values of TIB?

Answer 31

PW Doppler

Question 32

According to WFUMB, what combination of temperature rise and exposure time is ‘potentially hazardous’ in antenatal ultrasound scanning?

Answer 32

Greater than 4 °C for 5 minutes or more

Question 33

According to BMUS, what are the maximum recommended antenatal scanning times for TI values of 1.0, 2.0 and 3.0, respectively?

Answer 33

1 hour, 15 minutes and 1 minute

Question 34

According to Francis Duck’s 2008 article, what levels of risk due to heating do antenatal, transcranial and cardiac scanning pose, respectively?

Answer 34

Low, medium and very low

Question 35

According to Francis Duck’s 2008 article, what levels of risk due to cavitation do contrast-enhanced and conventional scanning pose, respectively?

Answer 35

High and none

Question 36

A hydrophone is used to characterise the ultrasonic field of a 2 MHz transducer in water. If a peak negative pressure of 5 MPa is measured at an axial distance of 200 mm, calculate the MI value.

Answer 36

0.88
The measured acoustic pressure must be derated using a = 0.3 dB cm−1 MHz−1

Hence, the derating factor is afbz = 0.3 × 2.01.0 × 20.0 = 12.0 dB (corresponding to an acoustic pressure ratio of 1/4 = 0.25)

Hence, MI = pr(0.3)/sqrt(fc) = 5.0 × 0.25 /sqrt (2.0) = 0.88

Question 37

A soft tissue has the following physical and acoustical properties –

– mass density, ρ0 = 1050 kg/m3
– specific heat capacity, C = 3500 J kg−1 °C−1
– bulk modulus, K = 2.5 GPa
– absorption coefficient, aa = 0.3 dB cm−1 MHz−1

Calculate the dose rate due to a 3 MHz beam with derated Ispta = 720 mW/cm2. (Answer in W/Kg)

Answer 37

140
The ‘true’ absorption coefficient is αa = 0.3 × 3.0 / 8.69 = 0.103 nepers cm−1

Hence, the dose rate, Qm = 2αaIspta/ρ0 = 2 × 10.3 × 7200 / 1050 = 140 W/kg

Question 38

A soft tissue has the following physical and acoustical properties –

– mass density, ρ0 = 1050 kg/m3
– specific heat capacity, C = 3500 J kg−1 °C−1
– bulk modulus, K = 2.5 GPa
– absorption coefficient, aa = 0.3 dB cm−1 MHz−1

Calculate the initial rate of temperature increase due to a 3 MHz beam with derated Ispta = 720 mW/cm2. (Answer in °C/s )

Answer 38

0.04
The dose rate, Qm = 2αaIspta/ρ0 = 2 × 10.3 × 7200 / 1050 = 140 W/kg

Then, dT/dt = Qm/C = 140/3500 = 0.040 °C/s

Question 39

A soft tissue has the following physical and acoustical properties –

– mass density, ρ0 = 1050 kg/m3
– specific heat capacity, C = 3500 J kg−1 °C−1
– bulk modulus, K = 2.5 GPa
– absorption coefficient, aa = 0.3 dB cm−1 MHz−1

Calculate the radiation force due to a 3 MHz beam with derated Ispta = 720 mW/cm2 (Answer in N/Kg)

Answer 39

0.0925
The ‘true’ absorption coefficient is αa = 0.3 × 3.0 / 8.69 = 0.103 nepers cm−1

Hence, the dose rate, Qm = 2αaIspta/ρ0 = 2 × 10.3 × 7200 / 1050 = 140 W/kg

Then, dT/dt = Qm/C = 140/3500 = 0.040 °C/s

Also, c0 = (K/ρ0)1/2 = (2.5 × 109 / 1050)1/2 = 1540 m/s

and Fm = Qm/c0 = 140/1540 = 0.091 N/kg