Ultrasound: QA and Safety Flashcards
What are the measured parameters in a basic ultrasound QA test?
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
What are the properties of an ultrasound phantom?
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
What is the “break point” temperature for thermal heating in vivo?
3 degrees C
Under the “break point” temperature what is the equation for the time taken for damage to occur?
t = 4^(43-T)
t is the time in minutes
T is the temperature of the tissue.
Over the “break point” temperature what is the equation for the time taken for damage to occur?
t = 2^(43-T)
t is the time in minutes
T is the temperature of the tissue.
What does the Thermal Index (TI) on an ultrasound scanner show?
The TI gives a rough estimate of the worst possible case of temperature rise.
What does the Mechanical Index (MI) on an ultrasound scanner show?
The MI is a value related to the likelihood of non-thermal bioeffects like damage due to bubbles and gas bodies.
How is the Thermal Index (TI) defined?
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.
What are the three types of Thermal Index (TI) and when are they used?
TIS - soft tissue (no bone nearby, 1st trimester)
TIB - bone at focus (2nd and 3rd trimesters)
TIC - cranial bone (scanning of non-fetal head)
What does the Thermal Index (TI) depend on?
W(0) - the time-averaged acoustic power emitted by the transducer
I(ta) - the time-averaged intensity of the scanner.
How is W(0) measured?
Using a radiation force balance.
F=W(0)/c
How is I(ta) measured?
Axial I(ta) profiles can be obtained by hydrophone scanning.
How is the Mechanical Index (MI) defined?
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)
What is the threshold for internal cavitation of bubbles in water?
MI > 0.7.
What is the limit set by the FDA for MI for ophthalmology?
MI = 0.23
What is the limit set by the FDA for TI for ophthalmology?
TI = 1.0
What is the limit set by the FDA for MI for non-ophthalmic ultrasound?
MI = 1.9
What is the limit set by the FDA for TI for non-ophthalmic ultrasound?
TI = 6.0
What is the equation for the initial heating pattern from an ultrasound transducer?
H=2af(c)*I(ta)
I(ta) is the in situ (assumed as derated) time averaged intensity
What does the TI not take into account?
Self-heating of the transducer
Non-linear distribution
Long fluid path in the first trimester
What controls won’t affect the TI and MI?
Monitor brightness TGC Tint Maps Auto-optimise Dynamic Range Edge 2D gain Speckle reduction
What controls will affect the TI and MI?
Transmit power Space-time (res-speed) Depth Focus Harmonics Zoom (when live) Frequency Compound imaging Freeze
How is ALARA practised in ultrasound imaging?
Transmit power kept as low as reasonably achievable
Freeze control used to limit exposure duration
Clinical applications specialist to set up scanner presets
How do the acoustic intensities used in medical imaging compare with those used in physiotherapy?
Slightly lower
How does the heating effect of an ultrasound beam vary in most soft tissues as the frequency is increased?
Increases roughly linearly with frequency
How does the acoustic pressure threshold for the onset of inertial cavitation in water vary as the frequency is increased?
Decreases roughly as the square root of the frequency
What is the unit of acoustic dose rate?
W/kg
Which of the following parameters is acoustic dose rate proportional to?
Time-average intensity and absorption coefficient
What is the expression for the initial rate of temperature rise in a tissue?
Acoustic dose rate divided by the specific heat capacity of the tissue
What upper limits on the acoustic output of non-ophthalmological ultrasound scanners does the US Food and Drug Administration specify?
MI = 1.9, TI = 6.0 and derated Ispta = 720 mW/cm2
Which mode of operation usually produces the highest values of TIB?
PW Doppler
According to WFUMB, what combination of temperature rise and exposure time is ‘potentially hazardous’ in antenatal ultrasound scanning?
Greater than 4 °C for 5 minutes or more
According to BMUS, what are the maximum recommended antenatal scanning times for TI values of 1.0, 2.0 and 3.0, respectively?
1 hour, 15 minutes and 1 minute
According to Francis Duck’s 2008 article, what levels of risk due to heating do antenatal, transcranial and cardiac scanning pose, respectively?
Low, medium and very low
According to Francis Duck’s 2008 article, what levels of risk due to cavitation do contrast-enhanced and conventional scanning pose, respectively?
High and none
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.
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
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)
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
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 )
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
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)
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
