Lecture 4

Question 1

1. What wave type is used for ultrasound?
2. Frequencies used in ultrasound?
3. How long are the pulses that are emitted?

Answer 1

1. High frequency sound waves.
2. 2-18 MHz. - Typically 5-10 MHz
3. ~1msec.

Question 2

1. What does an ultrasound need?
2. What is the wave velocity dept on?

Answer 2

1. Needs a material to travel through.
2. depends on the material it is travelling through.

Question 3

How does the wave velocity differ between materials?
Why can bone not be seen on ultrasound?
Why is ultrasound gel needed between the probe and the patient?
What gives the useful image on ultrasound?

Answer 3

Sound waves travel faster through bone than soft tissue and faster through soft tissue than air.

Bone absorbs most of sound waves.

Ultrasound does not travel well through air.

Reflection of the sound waves.

Question 4

Name the effect for the production of ultrasound.

How are the sound waves produced from an ultrasound?

Answer 4

Piezoelectric effect

Electrical voltage applied to a disc within transducer. Disc expands or contracts in proportion to the voltage, giving rise to a sound wave.

Question 5

What % time is spent producing ultrasound?
What % time is spent receiving ultrasound?

Answer 5

1%
99%

Question 5

How does receiving the signal work?

Answer 5

Sound returns from the tissues to the transducer.
Pressure of sound wave distorts disc.
Generates voltage proportional to pressure.
Voltage ‘processed’ by machine.

Question 6

How many wavelengths is a pulse typically?
How long in mm is the pulse?
What is the machine doing in the pauses between pulses?

Answer 6

3 wavelengths.

1.5mm

Waiting for echoes back from the tissues.

Question 7

How do you calculate acoustic impedance?

What happens when sound crosses a boundary between tissues of different acoustic impedance?

Answer 7

Density of tissue x speed of sound in tissue.

Some is reflected back. Some travels deeper into tissue to be reflected back at a deeper surface. Proportion reflected depends on the difference in acoustic impedance.
– Acoustic impedance relatively little at soft tissue boundaries e.g. fat/kidney interface.
– Acoustic impedance larger at soft tissues/bine interface. - Bone surface appears v bright (all sound either absorbed or reflected).

Question 8

2 types of reflection on ultrasound.
Explain both.

Answer 8

Specular and non-specular.

Specular = large smooth surface, quite perpendicular to probe, giving a strong reflection. e.g. liver/lung surface, small intestinal wall.

Non-specular = Beam hits small structures e.g. within the liver, re-radiated in all directions, giving weak echoes, gives texture to the organs.

Question 9

What is A mode? (display)
What is B mode? (display)
What is M mode? (display)

Answer 9

Amplitude (not commonly used now) (clinical use for ophthalmology – precise measurements).
Brightness (commonly used) – moving ultrasound.
Motion (only clinical use in cardiology)

**underlying process of sending sound and ‘listening’ for echo is the same with all modes.

Question 10

B mode.

Answer 10

Transducer converts returning sound echoes to voltage.
Brightness depends on amplitude (size) of signal.
Position depends on time for the signal to return – Velocity = distance/time.
See movement in ‘real-time’

Question 11

As images are taken in slices, what must be done to get a good idea of what is going on in the patient?

Answer 11

Take scans of organs in >1 plane.

Question 12

How is M mode used?

Answer 12

Start in B mode to position a single line.
Then flip to M mode.
Movement of points along the line are followed.
Image displayed as position vs time.
Continually updated, giving trace of movement.
Can be combined with ECG trace to assess movement during phases of cardiac cycle.

Question 13

The ultrasound examination

Answer 13

Can be done conscious or under light sedation.
Relatively quick.
Non-invasive
Safe

Question 14

What should be avoided when choosing area of body prep for scanning?
How should you prepare area of the body for scanning?

Answer 14

Intervening bone or (where possible) gas.

Clip the hair on the area (hair traps air).
Clean the skin – surgical spirit effective but may damage transducer.
Apply liberal quantities of acoustic gel.
Place transducer over area of interest.

Question 15

Transducers – construction

Answer 15

Now electronic (initially mechanical).
Phased arrays – beam electronically steered.
Linear arrays – Multiple crystals sequentially fired to build image.
Microconvex/convex arrays – Elements arranged in a curve.
Important to know advantages and limitations in order to obtain the best image possible.

Question 16

Considerations for transducers

Answer 16

Type – e.g. phased/linear etc.
‘Footprint’ – Contact area transducer makes with the animal.
Frequency

Question 17

1. Advantages of phased array/microconvex?
2. Disadvantage of phased/microconvex?
3. Advantages of linear?
4. Disadvantages of linear?

Answer 17

1. Easy to manipulate.
   Small contact area (footprint). – Easier to fit under ribs/between ribs etc.
   Wide field at depth.
2. Structures that are more superficial are more difficult to interpret.
3. Large field of view near the skin – good for superficial structures.
4. Large contact area so less easy to manipulate into smaller nooks.

Question 18

1. Velocity = __________ x ______________

Answer 18

1. Frequency x wavelength

Question 19

Relationship between frequency and wavelength?
Relationship between wavelength and resolution?

Answer 19

As frequency increases, wavelength decreases.
Smaller wavelength means better resolution (no overlap).

Question 20

Relationship between sound attenuation and frequency.

Trade-off of using higher frequency?

Answer 20

Proportional.

Sound does not penetrate so far into the body. So deeper structures won’t be imaged.

Question 21

1. At what frequency range is a transducer classed as higher frequency?
   Advantage?
   Disadvantage?
2. At what frequency is a transducer classed as lower frequency?
   Advantage?
   Disadvantage?

Answer 21

1. At 7.5 - 18+ MHz
   ++ Good resolution.
   – – Can’t image deeper structures.
2. At 2.5 - 5 MHz.
   ++ Can image deeper structures
   – – Poorer resolution.

Question 22

1. Define hyperechoic/echogenic.
2. Define hypoechoic.
3. Define anechoic/echolucent.

Answer 22

1. Lots of echoes – bright on image.
2. Less change in density. Not many echoes generated – darker/greyish on image. e.g liver.
3. No change in density, no echoes generated – Black on image. e.g. urine in bladder.

Question 23

Common ultrasound artefacts

Answer 23

Acoustic enhancement
Acoustic shadowing
Reverberation
Mirror image

Question 24

1. Where is acoustic enhancement seen?
2. Explain why it is seen.
3. How can this artefact be beneficial?

Answer 24

1. Seen distal/deep to fluid-filled structures e.g. bladder and cysts.
2. Low attenuating structures like the bladder will have sound waves pass through them with no change in density so by the time it reaches the other side of the low attenuating structure, there are more sound waves left in the animal, causing the returning echoes deep to the structure to be stronger, so increased echogenicity distal to the fluid-filled areas.
3. Useful to differentiate cysts from hypoechoic solid masses.

Question 25

1. Where is acoustic shadowing normally seen?
2. Explain why it is seen.
3. Describe what would be seen below bone – soft tissue.

Answer 25

1. Below a structure that is highly attenuating.
2. The highly attenuating structure that the sound waves are passing through has either absorbed or reflected all the sound waves and there are no sound waves left deep/distal to the structure to be able to produce any echoes, so there is a hypoechoic/anechoic area below the structure.
3. A clean shadow where most of the remaining sound is absorbed. Calculi behave like bone.
4. ‘dirty shadow’ due to reflection/reverberation.

Question 26

Explain reverberation.

Answer 26

Seen deep to gas.
Spurious echoes due to internal reflectors in path of sound e.g. gas interfaces.
Sound ‘bounces’ between interfaces.
Gives multiple echoes (series of lines) back to transducer from one pulse – only the first echo is correctly positioned.
‘Comet tail’ arises from 2 closely spaced reflective surfaces – line of regular bright echoes deep to structure.

Question 27

Explain mirror image artefact.

Answer 27

Occurs at highly reflective interfaces, typically the diaphragm: lung interface.
Sound waves reflected from the diaphragm: lung interface return to the liver, and are reflected back from the liver parenchyma to the diaphragm, and are then reflected back to the transducer.
For each of the subsequent echoes, the distance and therefore time to the transducer is increased, producing a mirror image of the liver deep to the diaphragm.

Question 28

Advantages of ultrasound.

Answer 28

Safe
Quick
Non-invasive
Useful if fluid present
Real-time information
– peristalsis, cardiac function etc.

Question 29

Disadvantages of ultrasound.

Answer 29

Clipping usually required
Some experience needed to interpret
– real-time procedure
Gas and fat hinder interpretation
Many findings are non-specific
– Tissue sampling needed.