Ultrasound

Question 1

At what frequency are the pulses of ultrasound waves produced by an ultrasound scanner?

Answer 1

1 to 15 MHz

Question 2

How does an ultrasound scanner work?

Answer 2

The scanner is placed on the surface of the patient’s skin
When an ultrasound wave meets a boundary between two different materials, some of it is reflected and some of it is transmitted. The reflected waves are detected by the ultrasound scanner and are used to generate an image

Question 3

What is acoustic impedance?

Answer 3

The amount of reflection an ultrasound wave experiences at a boundary depends on the difference in acoustic impedance between the materials

Question 4

What happens if two materials have a large difference in impedance?

Answer 4

Most of the energy is reflected

Question 5

What happens if the impedance of the two materials is the same?

Answer 5

There is no reflection

Question 6

What does it mean when ultrasound waves undergo attenuation when traveling through a material?

Answer 6

This is when the waves are absorbed and scattered, making them harder to detect. This means that the
intensity of the reflected wave will always be lower the further the boundary is from the transducer.

Question 7

What gives better resolution?

Answer 7

Higher-frequency waves give better resolution

Question 8

What needs to be used to image tissue deeper within the body?

Answer 8

The higher the frequency of the wave, the more it is attenuated, this means lower-frequency waves have to be used to image tissue within the body

Question 9

Relationship between acoustic impedance and attenutation

Answer 9

The larger the impedance, the greater the attenuation of the ultrasound moving through the material

Question 10

What do piezoelectric crystals produce?

Answer 10

A potential difference when they are deformed (squashed or stretched) - the rearrangement in structure displaces the centres of symmetry of their electric charged. This is called the Piezoelectric effect.

Question 11

Where is ultrasound produced and detected?

Answer 11

On a transducer

Question 12

How is the crystal deformed?

Answer 12

When you apply a pd across it.

Question 13

What happens if the pd is alternating?

Answer 13

The crystal vibrates at the same frequency

Question 14

What can the piezoelectric act as?

Answer 14

• A receiver of ultrasound, converting sound waves into alternating voltages
• A transmitter, converting alternating voltages into sound waves

Question 15

What crystals do ultrasound devices use?

Answer 15

Lead zicronate titanate (PZT) crystals

Question 16

Why do ultrasound devices use PZT crystals?

Answer 16

The thickness of the crystal is half the wavelength of the ultrasound. Ultrasound of this frequency will make the crystal resonate and produce a large signal.

Question 17

Why is the PZT heavily damped?

Answer 17

It is damped using the backing material to produce short pulses and increase the resolution of the device.

Question 18

Why do you need a coupling medium?

Answer 18

Soft tissue has a very different acoustic impedance from air, so almost all the ultrasound energy is reflected from the surface of the body if there is air between the transducer and the body. This displaces the air and has an impedance much closer to that of body tissue.

Question 19

What is the use of coupling media an example of?

Answer 19

Impedance matching

Question 20

What is used as a coupling medium?

Answer 20

Gel or oil smeared onto the skin

Question 21

What are the two types of ultrasound scan?

Answer 21

The A-scan and the B-scan

Question 22

What is an A-scan?

Answer 22

The A-scan sends a short pulse of ultrasound into the body simultaneously with an electron beam sweeping across a cathode ray oscilloscope screen. The scanner receives reflected ultrasound pulses that appear as vertical deflections on the CRO screen.

Question 23

What is time-gain compensation?

Answer 23

When weaker pulses, that have travelled further in the body and arrive later, are amplified more to avoid the loss of valuable data.

Question 24

What do the horizontal positions of the reflected pulses indicate?

Answer 24

The time the ‘echo’ took to return, and are used to work out distances between structures in the body.

Question 25

What is a B-scan?

Answer 25

In a B-scan, the electron beam sweeps down the screen rather than across. The amplitude of the reflected pulses is displayed as the brightness of the spot.

Question 26

What can transducers in a B-scan do?

Answer 26

You can use a linear array of transducers to produce a two-dimensional image.

Question 27

What are B-scans used for?

Answer 27

• To check if the heart is functioning properly

- Allows the doctor to look at an image of the heart in real time.

Question 28

Advantages of ultrasound

Answer 28

• No known hazards or side effects
• No exposure to ionising radiation
• Good for imaging soft tissues, since you obtain real time images
• Ultrasound devices are relatively cheap and portable
• Scan is a quick procedure
• Patient can move during the scan

Question 29

Disadvantages of ultrasound

Answer 29

• Ultrasound doesn’t penetrate bone - can’t be used to detect fractures or examine the brain
• Ultrasound cannot pass through air spaces in the body - can’t produce images from behind the lungs
• Resolution is poor, so you can’t see fine detail
• Ultrasound cannot give information about any solid masses found