Chapter 17 - Using Ultrasound In Medicine

Question 1

Define transducer?

Answer 1

A general term used for any device that changes one form of energy into another.

Question 2

Define piezoelectric crystal?

Answer 2

A material that produces an induced e.m.f. when it is compressed. When a voltage is applied across it in one direction it drinks slightly. Reverse voltage, expands slightly.

Question 3

Explain how a piezoelectric transducer works to produce US?

Answer 3

An alternating voltage is applied across the crystal causing it to expand and shrink, acting as a vibrating source of ultrasound waves.

Question 4

Explain how the piezoelectric transducer can be used to detect US?

Answer 4

The piezoelectric effect works in reverse. Therefore the US waves cause varying stress on the crystal leads to a varying voltage.

Question 5

What are three features of an ultrasound transducer?

Answer 5

1) crystal made of polyvinylidene protected by a case
2) at the base there is an acoustic window to transmit US
3) damping material behind crystal to prevent crystal vibrating too much due to resonance

Question 6

Define acoustic impedance and give equation?

Answer 6

The product of density and speed of sound of the substance

Z=pc

Question 7

Explain what happens to an incident Ultrasound wave at a boundary?

Answer 7

The wave is partially refracted and partially reflected. We detect the reflected part of the wave

Question 8

What is the equation to show the ratio of reflected intensity to incident intensity?

Answer 8

Ir/Io = ((z2-z1)^2) / ((z2+z1)^2)

Question 9

When will a large fraction be reflected?

Answer 9

When there is a big change in the acoustic impedance

Question 10

Why do you hospitals use a gel when doing ultrasounds?

Answer 10

There is a big difference in acoustic impedance between skin and air, so the transducer is coupled to the skin using a gel who’s impedance matches that of the skin to prevent a big reflection at the air-skin boundary.

Question 11

Define impedance matching?

Answer 11

Reduction in intensity of reflected ultrasound at boundary between substances, achieved by two substances having similar acoustic impedances.

Question 12

What is ultrasound good for scanning and bad for scanning?

Answer 12

Good - tissues/bones

Bad - lungs (due to air-tissue boundary)

Question 13

What is ultrasound?

Answer 13

A sound wave with frequency above upper limit of human hearing (>20kHz)

Question 14

Explain what an A scan is?

Answer 14

• Pulse of US sent into body, reflected waves detected, displayed as a V/t graph
• Each partial reflection is shown as a peak on the graph

Question 15

Note for reading A-scan graphs?

Answer 15

The change in time often had to be divided by 2 since US wave goes there and back

Thickness = cΔt/2

Question 16

When is an A scan used?

Answer 16

Simple procedures like measuring thickness of eye lens

Question 17

Explain what a B-scan is and how it works?

Answer 17

It’s a detailed is image of a patient’s cross-section, using a number of sensors in different positions and at different angles to build up an image.

1) US transducer moves across area of interest
2) each pulse analysed to determine depth from surface (time) and nature of surface (amplitude of reflected wave).
3) 2-D image built up by positioning dots

Question 18

Disadvantage of B-scans?

Answer 18

Image can blur if organs of interest are moving

Question 19

Define Doppler effect?

Answer 19

The change in frequency and wavelength of a wave caused by the relative movement between source and detector.

Question 20

Explain the five steps for Doppler ultrasound?

Answer 20

1) US probe help against something moving (eg. Blood)
2) if blood moving away from source, each wave is reflected from slightly diff. position
3) result - reflected waves have greater wavelength, and since v is constant, f decreases
4) tf greater the speed of receding reflector, greater the decrease in freq.
5) speed of reflector can be deduced from change in freq of waves

Question 21

2 uses of Doppler ultrasound?

Answer 21

1) determining steady bloodflow

2) kidney transplant patients have it to see how well blood flows through their new kidney

Question 22

Why is it desirable to have a short wavelength for an US scan?

Answer 22

Shorter wavelength will give finer detail in the picture