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PHYS853: Radiation Protection in Medicine > Non-Ionising Radiation: Ultrasound Safety > Flashcards

Non-Ionising Radiation: Ultrasound Safety Flashcards

(66 cards)

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1
Q

Give 5 clinical applications of ultrasound

A
  • Diagnostic imaging
  • Physiotherapy
  • Wound healing
  • Ultrasonic scalpels
  • High intensity focused ultrasound surgery (HIFU)
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2
Q

What is ultrasound?

A

A mechanical wave idential to sound, except above the range of human hearing (> 20 kHz).

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3
Q

What is the frequency range of diagnostic ultrasound?

A

1 - 20 MHz

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4
Q

What are the 2 types of diagnostic ultrasound?

A
  • Pulsed
  • Continuous wave (CW) doppler
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5
Q

Whare are the two parts of an ultrasound wave?

A

Compressions (pressure above ambient)
Rarefactions (pressure below ambient)

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6
Q

Define acoustic impedance

A

The constant of proportionality between a waves pressure and the particle velocity.

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7
Q

Give the equation for acoustic impedance

A

z = acoustic impedance
p = pressure
v = particle velocity

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8
Q

Give the equation for acoustic impedance in terms of the speed of sound

A

z = acoustic impedance
ρ = density
c = speed of sound

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9
Q

Define intensity

A

Energy flux (how much energy is flowing through a unit area per unit time).

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10
Q

Give the equation for intensity

A

I = intensity
p = pressure
z = acoustic impedance

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11
Q

Define attenuation

A

The exponential loss of pressure amplitude with depth.

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12
Q

What are the causes of attenuation?

A
  • Absorption
  • Scatter
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13
Q

What is the main cause of attentuation at diagnostic frequencies?

A

Absorption

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14
Q

How are attuation and frequency related?

A

Higher frequency = more attenuation = less penetration

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15
Q

How are acoustic pressure and intensity normally measured?

A

Using a hydrophone in water (where there is almost no attenuation)

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16
Q

How are realistic values for pressure in situ calculated?

A

Pressure measurements recorded using a hydrophone in water are ‘derated’ by the amount that would have been absorbed by tissue.

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17
Q

What is the typical attenuation of tissue?

A

0.5 - 1.0 dB/cm/MHz

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18
Q

What tissue attenuation value is typically chosen when calculating safety indices?

A

0.3 dB/cm/MHz

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19
Q

Give the equation for derated pressure

A
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20
Q

Why are ultrasound waves focused?

A

To improve lateral resolution and increase intensity.

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21
Q

Define acoustic power

A

A measurement of the rate at which energy is emitted by the ultrasound transducer in watts.

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22
Q

What is the typical acoustic power of diagnostic ultrasound?

A

< 1 mW to ~100 mW

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23
Q

What is the typical acoustic power of therapeutic ultrasound?

A

~ 0.5-15 W

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24
Q

What is the typical acoustic power of HIFU?

A

~ 3-30 W

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25
Why does ultrasound increase tissue temperature?
Because 99.99% of the energy emitted is absorbed by the tissue rather than being reflected back to the transducer.
26
What is spatial-peak-time-averaged intensity?
The location of greatest intensity, taking into account attenuation, focusing, and scan type.
27
Where is the spatial-peak-time-averaged intensity for unscanned beams?
In the focal zone
28
Give 3 examples of unscanned beams
- Pulsed - Doppler - M mode
29
Where is the spatial-peak-time-averaged intensity for scanned beams?
Much closer to the transducer as pulses will overlap
30
Give an example of a scanned beam
- B mode
31
How does the spatial-peak-time-averaged intensity create an image?
It is time averaged over all pulsed to create an image
32
What are the 3 types of ultrasound intensity?
- Spatial-peak-time-averaged intensity - Temporal peak intensity - Pulse averaged intensity
33
How do the thermal effects of ultrasound vary with intensity?
Focal regions of intensity experience faster temperature rise. These regions conduct heat away and reach equilibrium more quickly than areas of uniform intensity.
34
Different tissues _______ ultrasound to different extents.
Absorb
35
True or false: Bone strongly absorbs ultrasound so it heats up very quickly.
True
36
What is transducer heating?
The heating of ultrasound probes that absorb lots of energy due to an impedance mismatch between the transducer crystals and tissue.
37
Are patients impacted by transducer heating?
Yes, transducer heating can transfer energy into the patient. It significantly contributes to tissue heating.
38
Why can thermal effects be dangerous in pregnancy?
Because developing tissues in an embryo are particularly susceptible to damage by heating.
39
What are the ultrasound guidelines for in situ temperature rise?
- Temperature rise of no more than 1.5ºC above physiological levels (37ºC) can be used clinically without reservation - An elevated embryonic/foetal temperature rise of 3ºC should be considered potentially hazardous
40
What is the thermal index (TI)?
A rough estimate of the increase in temperature that an ultrasound exposure is causing. A TI of 2 equates to an approximate temperature rise of 2º under 'reasonable worst-ase conditions'.
41
What does thermal index depend on?
A combination of: - Spatial-peak-time-averaged intensity - Absorption coefficient - Frequency - Output power - Scanning type - Tissue model being used
42
What are the 3 tissue models used to determine thermal index?
TIS = soft tissue TIB = bone at focus TIC = scanning through the cranium (or bone at surface)
43
Give the equation for thermal index
TI = thermal index
44
Describe the change in reccomended foetal ultrasound exposure time based on thermal index
45
What are ultrasound contrast agents made of?
'Microbubbles' contraining a gas encapsulated in a fat or protein shell. They are small, allowing them to travel through even small blood vessels.
46
What is the purpose of US contrast agent?
To improve ultrasound sensitivity by resonating at diagnostic frequencies (which creates larger reflections).
47
Describe the change in US contrast agent shape when exposed to increasing pressure
48
What are the potential safety hazards of ultrasound contrast agent?
- Embolism - Allergic reaction - Toxicity - Acoustic cavitation
49
Define acoustic cavitation
The process of forming new gas bubbles or modifying existing gas bubbles in a liquid through ultrasound exposure.
50
Define stable cavitation
The process where ultrasound contrast bubbles oscilate in size but remain stable at low pressure amplitudes.
51
Define inertial cavitation
The process where ultrasound contrast bubbles grow in a process of rectified diffusion until they come too big and implode, causing damage to nearby cells. This occurs at high pressure amplitudes.
52
What are the hazards of cavitation?
- Damaging cells due to the sheer force of the bubble collapse - The generation of free radials due to the high, localised pressures and temperatures
53
What is the mechanical index (MI)?
A measure of the potential for cavitation (as cavitation only occurs above an acoustic pressure threshold).
54
Under what mechanical index threshold is cavitation impossible?
MI < 0.7
55
Where is cavitation most likely?
When either microbubble contrast agents are used or in the presence of gas bodies (e.g. intestines or lungs).
56
Give the equation for mechanical index
57
Which ultrasound mode is most likely to cause heating? Why?
Doppler mode is more likely to cause significant heating than B-mode and M-mode as it has a higher spatial-peak-time-averaged intensity and acoustic power.
58
Which ultrasound mode is most likely to cause cavitation?
All modes have the potential to cause cavitation
59
What is sonoportation?
A pressure change due to ultrasound pulses that causes the cell membrane to open up slightly, allowing drugs to be much more easily absorbed.
60
What is acoustic force?
A radiation force exerted on tissue depending on the amount of energy absorbed. When liquids are present this can cause fluids to flow (fluid streaming).
61
How can acoustic force be used clinically?
To measure the elasticity of tissue in ultrasound electrography.
62
What is epidemiology?
The study of patterns of occurence of disease in human populations and of the factors influencing these patterns.
63
True or false: epidemiological studies have found a link between ultrasound and birth weight, childhood malignancies, hearing, visual acuity, cognitive function or behaviour, speech development, and dyslexia.
FALSE
64
What safety features are required to control the mechanical index and thermal index of an ultrasound scanner?
If a scanner can produce an MI or TI greater than 1, then it must be displayed when either is ≥ 0.4
65
What safety features are required to control the exposure limits of an ultrasound scanner?
Manufactuers must supply information on acoustic output for all scanners to ensure they don't exceed the allowable levels.
66
What are the 6 BMUS guidelines on ultrasound?
1) Medical ultrasound imaging should only be used for medical diagnosis 2) Ultrasound equipment should only be used by people who are fully trained in its safe and proper operation 3) Examination times should be kept as short as is necessary to produce a useful diagnostic result 4) Output levels should be kept sa low as reasonably achievable whilst producing a useful diagnostic result 5) Operators should aim to stay within the BMUS recommended scan times 6) Scans in pregnancy should not be carried out for the sole purpose of producing souvenir videos/photographs

PHYS853: Radiation Protection in Medicine (19 decks)

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