Topic 1 - safety and intro

Question 1

What information do you think should be given to pregnant women before undergoing an obstetric ultrasound?

Answer 1

• that the purpose of the test is the detection of fetal abnormalities
• the diagnostic capacity of the test
• the limitations of the test (not all congenital malformations can be detected; genetic abnormalities can only be suspected in a certain percentage of patients and the diagnosis of those always requires invasive testing)
• the concentration required to perform a detailed ultrasound examination
• if an abnormality is detected, further (invasive) investigations are often recommended and difficult decisions may need to be made

Question 2

What possible problems can you see when patients are largely unaware of the purpose of their scan?

Answer 2

o The patient‟s priority may be to have a nice picture or videotape of the baby or to find out what the gender is. Patients can become very disappointed when these expectations are not fulfilled.
o The patient may expect to chat with the sonographer during the entire ultrasound examination, asking questions, making remarks. Patients can become offended when asked to be quiet during the examination.
o The patient may assume that the ultrasound examination offers the guarantee that the baby is normal.
o The patient may not be aware that sometimes further invasive testing may be required, and may have declined the examination had she known that in advance.
o The patient is completely unprepared for the necessary further investigations and difficult decision making when an abnormality is diagnosed.

Question 3

What are the most important things you can do to help when an abnormality is diagnosed?

Answer 3

• Usually refer to sonologist
• if you deliver news only say things you are certain of
• if you are uncertain say so
• rapidly refer
• for miscarriage refer immediately to referrer or emergency department
• for serious abnormality involve a genetic counsellor
• for treatable abnormalities refer to a paediatric surgeon (this helps alleviate anxiety)

Question 4

What can you do to help during follow up of an abnormality?

Answer 4

• All health professionals should be informed about the situation. It is unacceptable to ask the patient why she is having another ultrasound or what they found previously.
• Always look at the ultrasound report of the previous scans. Information needs to be accessible and up-to-date.
• Understand practical difficulties (travel, childcare, expense, etc.).

Question 5

What is the mechanical index?

Answer 5

relative potential for ultrasound to induce an adverse bioeffect by a nonthermal mechanism (cavitation)

Question 6

What is the thermal index?

Answer 6

indicator of the relative potential for a tissue temperature rise

Question 7

What threshold should the MI and TI be kept under?

Answer 7

1

Question 8

What are some ways of keeping the MI and TI under threshold?

Answer 8

• Start the scan at low power output and increase power only if necessary (ALARA)
• It is preferable to use gain compensation over increasing output power.
• Be very cautious when using Doppler in the first trimester or in the immediate vicinity of bone.

Question 9

What are the three important parameters under end-user control?

Answer 9

o scanning/operating mode (including transducer choice)
o system setup and output control
o dwell time.

Question 10

Why is choice of scanning mode important?

Answer 10

• spatial peak temporal average intensity (ISPTA) increases from B-mode (average, 34 mW/cm2) to M-mode to color Doppler to spectral Doppler (average, 1180 mW/cm2)
• 1 /cm2 in Doppler mode but can reach 10 W/cm2

Question 11

Why does colour Doppler have higher intensities than b-mode?

Answer 11

• Color Doppler has higher intensities than B-mode - much lower than spectral
• Due to of mode of operation: sequences of pulses, scanned through the area of interest (“box”).

Question 12

Why does pulsed Doppler have higher intensity than b and colour mode?

Answer 12

• High pulse repetition frequencies are used in pulsed Doppler techniques means greater temporal average intensities and thus greater heating potential.
• • beam needs to be held in relatively constant position over the vessel of interest in spectral Doppler ultrasound, temporal average intensity may further increase.

Question 13

Why is transducer choice important in regards to ultrasound safety?

Answer 13

• transducer choice is important because it will determine frequency, penetration, resolution, and field of view.

Question 14

What are the controls that regulate output?

Answer 14

focal depth, usually with greatest power at deeper focus
- occasionally with highest power in the near field
- increasing frame rate
- limiting the field of view, as by high-resolution magnification or certain zooms.
- In Doppler mode
changing sample volume
velocity range (to optimize received signals) will change output

Question 15

What is dwell time?

Answer 15

Dwell time is the actual scanning time

• The examiner is in control of this
• Not taken into account in the calculation of the safety indices
• generally is not reported in clinical or experimental studies.
• It is directly correlated to examiner experience

Question 16

What is a bio-effect?

Answer 16

Bio-effect = a known effect of the interaction of ultrasound radiation and human tissue.

Question 17

What is a bio-hazard?

Answer 17

Bio-hazard = a known damaging consequence of a bio-effect, associated with some risk.

Question 18

What are the three thermal indices?

Answer 18

TIS - soft tissue index; only soft tissue is in the beam
TIB - bone index; bone in the focal zone
TIC - cranial bone index; scanning through superficial bone.

Question 19

What does a TI of one mean?

Answer 19

A temperature rise of 1 degree Celsius

Question 20

What rise in temp is thought to be safe?

Answer 20

<2

Question 21

What rise in temp is thought to be significant?

Answer 21

> 2.5

Question 22

Why does worst case heating occur later in pregnancy?

Answer 22

maximal ultrasound-induced temperature increase occurs in the fetal brain near bone, worst-case heating will occur later in pregnancy, when the ultrasound beam impinges on bone

Question 23

Which thermal indices are used in different stages of pregnancy?

Answer 23

thermal index for bone (TIB) late in pregnancy and thermal index for soft tissue (TIS) earlier.

Question 24

When is the fetus most susceptible to thermal effects?

Answer 24

Early first trimester

Question 25

What may be some hazards of thermal effects early in the first trimester?

Answer 25

• central nervous system (CNS) is most at risk
• neural tube (anencephaly, microencephaly) and the eyes (microphthalmia, cataract)
• Associated with CNS defects are functional and behavioral problems
• Other organ defects secondary to hyperthermia include:
  o defects of craniofacial development (e.g., clefts)
  o anomalies of the axial and appendicular skeleton
  o the body wall
  o teeth
  o heart

Question 26

Why might the fetus be at higher risk early in the first trimester?

Answer 26

minimal fetal perfusion may reduce heat dispersion

Question 27

If the MI is less than 1….

Answer 27

cavitation effects are thought to be minimal

Question 28

Why is it assumed that the risk from mechanical effect secondary to cavitation is minimal

Answer 28

Because gas bubbles are not present in fetal lung or bowel

Question 29

What is cavitation?

Answer 29

formation of gas bubbles from the fluctuations of the ultrasound pressure wave.

Question 30

What is stable cavitation?

Answer 30

ultrasound pressure wave interacts with gas bubbles already present and causes them to expand and contract. As they do they grow in size and they can resonate with the pressure wave of a certain frequency once they reach a certain size. These oscillating micro-bubbles can lead to large forces that can damage molecules, and also lead to micro-streaming.

Question 31

What is inertial cavitation?

Answer 31

transient cavitation
more serious than stable cavitation
gas bubbles grow rapidly and suddenly implode during a rarefaction cycle of the ultrasound pressure wave.
implosion causes a shock wave that can disrupt and damage molecules and even lead to the formation of highly reactive chemical radicals
Inertial cavitation can also cause haemorrhaging.

Question 32

What is the asum statement on safety for ultrasound?

Answer 32

ALARA
• Only perform a scan when medically indicated
• The examination should be undertaken in the minimum time with the lowest possible power consistent with a diagnostically meaningful image
• The sonographer should not rest the transducer on the patient when on, but not actually scanning
• Use the TI and MI (when displayed) as a guide to the dose being delivered (always aim to keep under 1)
• Be particularly careful about dose when using spectral Doppler (power high), and when scanning bone (thermal effects) or tissue containing micro-bubbles (mechanical effects).