Ultrasound

Question 1

Describe what happens when an alternating voltage is applied to a PZT transducer so that ultrasound is produced.

Answer 1

The electric field across a transducer causes a change in physical dimensions. If and alternating voltage is applied the transducer vibrates at the same frequency

Question 2

Under what conditions will maximum energy conversion into ultrasound occur?

Answer 2

Maximum energy transfer occurs when the frequency of the applied voltage is equal to the natural mechanical vibration frequency of the transducer causing resonance to occur.

Question 3

Describe the procedures which are used to obtain this type of scan.
(graph y-axis = signal strength and x-axis =time and there is two peaks

Answer 3

The ultrasound transmitter or receiver or transducer is placed in contact with skin. Transmission is improved by using a contact gel layer between transducer and skin, this allows the pulses of ultrasound to betransmitted into the body more effectively.

Question 4

Explain how the spacing of the pulses is interpreted.

graph y-axis = signal strength and x-axis =time and there is two peaks

Answer 4

Echoes reflected back to transducer appear as voltage peaks or pulses on c.r.o. The spacing of the peaks gives the time delay between transmitted pulse and echoes.

Question 5

Give two reasons why the amplitude of the reflected pulses varies

Answer 5

The amplitude of pulses is attenuated as it penetrates deeper (caused by absorption and dispersion). The amplitude of the echo pulses depends on the proportions of ultrasound which are reflected and transmitted at each surface or boundary (or depends on the acoustic impedences of the two materials at the interface)

Question 6

Explain how a piezoelectric crystal is caused to generate waves of ultrasound

Answer 6

Two faces of a thin slice of crystal (coated with a thin layer of silver) act as electrodes. The electrodes are connected to a high frequency (several MHz) source of e.m.f. As the applied e.m.f. alternates it applies an alternating (rapidly reversing direction) electric field across the slice of crystal between the electrodes. The crystal expands and contracts at the same frequency as the applied e.m.f. The vibrations of the faces of the crystal slice produce ultrasound pressure waves

Question 7

Explain why the pulse of ultrasound must be short.

Answer 7

The pulse needs to be short compared with the transit time. Pulses are used for timing echoes which give measurements of depth in the body therefore the pulse must be short enough to ensure the leading edge of the pulse returns well after the trailing edge departs

Question 8

Short voltage pulses applied to the piezoelectric crystal make it vibrate and emit short pulses only if the crystal assembly is modified. Explain the modification which is necessary.

Answer 8

Behind the crystal a vibration-absorbing backing material is attached. This damps the vibrations - stops the vibrations quickly - after the electrical signal is stopped, ensuring that the pulse is short

Question 9

Under what conditions is ultrasound reflected strongly at boundaries between two types of material?

Answer 9

When there is a large difference in acoustic impedance [or significant change in density or significant change in elasticity or texture of tissue]

Question 10

State two physical properties of the materials which determine the proportion of ultrasound which is reflected at a boundary.

Answer 10

Tissue density and tissue elasticity/texture.

Question 11

Explain what a coupling medium or gel is and why, and where, it is used.

Answer 11

Ultrasound is reflected back at boundaries with big difference in density. Therefore air pockets between the skin and transducer would prevent the ultrasound from entering the skin - most of it would be reflected. Gel (of similar density to body tissue) between transducer and skin prevents loss of signal due to boundary reflection Acoustically well -matched gel gives good transmission (with minimum reflection at skin boundary)

Question 12

Describe the practical process, including details of the use of the transducer and the adjustment of the oscilloscope, required to produce an A-scan.

Answer 12

The surface of the body is covered with an acoustically matched gel (gel with similar density to human tissue) to improve transmission from the ultrasound transducer to body. Short ultrasound pulses are sent into the body and the echoes received from boundaries between media of different densities are detected by the transducer. An oscilloscope is set up so that the sweep time is synchronised with the ultrasound pulse frequency

Question 13

Give two reasons why the height of the second pulse is smaller than that of the first pulse in an A-scan

Answer 13

Extra distance in tissue results in more signal absorption
Smaller fraction of signal reflected at second surface
The pulse will be more spread over time
The signal is diffracted

Question 14

Why is it necessary for the pulse to be short?

Answer 14

The probe is used as a generator and receiver

Question 15

Explain the process by which the transducer produces short pulses.

Answer 15

The electrodes connected across a (high frequency/alternating) EMF source. The crystal expands and contracts at the frequency of the changing EMF. The vibration of the faces produce ultrasound (pressure) waves. A backing material damps the oscillations of crystal to stop the crystal oscillating between end of transmitted pulse and start of received pulse

Question 16

State one advantage and one disadvantage of ultrasound compared with X-rays in medical imaging

Answer 16

advantage: e.g. non-ionising therefore not harmful to living cells or soft tissue
disadvantage: e.g. cannot penetrate bone or low resolution

Question 17

In an ultrasound A scan explain how the received signals are detected,

Answer 17

The probe acts as receiver and thereceived signal causes crystal to vibrate. The vibration of crystal produces and alternating pd

Question 18

state why it is essential to use short pulses of ultrasound.

Answer 18

The transmission must be completed before the reflected pulse can be recieved.

Question 19

Explain the purpose of the backing material.

Answer 19

It damps the oscillation of the crystal to zero quickly when the driving signal is removed

Question 20

State the main difference between the probe used in an A-scan and that used in a B-scan.

Answer 20

An A-scan has a single transducer, but a B-scan uses a multi-transducer probe

Question 21

State two processes which reduce the strength of the reflected signal received by the probe

Answer 21

you only get a partial reflection at boundary You also get attenuation as the signal passes through body

Question 22

Under what condition is ultrasound strongly reflected at the boundary between two media?

Answer 22

When the acoustic impedances/densities/speed of sound of the two materials are very different

Question 23

An A-scan is used to find information about the depth and size of organs within a patient’s body. Explain the basic physical principles behind the A-scan

Answer 23

The transmitter produces short pulses
At an internal boundary some reflected, rest transmitted to next boundary
The reflected pulse received by probe and signal sent to oscilloscope
The oscilloscope sweep is started when pulse is first transmitted

Question 24

An A-scan is used to find information about the depth and size of organs within a patient’s body. how the results are used to find the size of an organ

Answer 24

Time taken between pulses from front and back of organ (from oscilloscope)
distance = speed x time/2

Question 25

State the two physical properties of a material which determine its acoustic impedance

Answer 25

density of the material

speed of sound in the material

Question 26

Under what condition is ultrasound strongly reflected at a boundary between two types of material?

Answer 26

When there is a large difference in acoustic impedance

Question 27

State where a coupling medium or gel is used in an ultrasound scan and explain why it is necessary

Answer 27

The gel must be placed between the probe and the skin of the patient The gel has similar acoustic impedance to human tissue.
Without the gel being in position there would be trapped air between the probe and the skin. These two media result in a large difference in acoustic impedance. Therefore withair excluded there is maximum transmission of the yltrasound into the patient.

Question 28

Explain how and why ultrasound is used to obtain an image of an unborn foetus. You might consider the following points in your answer
the method of obtaining the image

Answer 28

Each transducer emits an ultrasound pulse in turn and receives the echoes from the interfaces directly in line with it.
Ultra sound is reflected at the interface between two materials of different acoustic impedances. The greater the differernce the greater the strnegth of the reflection.
Each echo displayed as a bright spot on screen - the bigger the aplitude of the echo the brighter the spot.
The y position (depth of the point) is determined by the time taken from transmission to the time the echo is received back.
The x position is determined by the position of the transducer
Images are produces at about 25 per second and therefore appear as a real time moving image on a computer screen.

Question 29

Explain how and why ultrasound is used to obtain an image of an unborn foetus. You might consider the following points in your answer
practical considerations for the scan

Answer 29

A probe has line of transducers (approx 100 in typical probe used to look at a foetus)
A high frequency ac pulse is applied to each tranducer in turn
Each transducer has piezoelectric crystal to generate the ultra sound wave
Use of gel between probe and skin to eliminate air is important otherwise the difference in impedance between air and tissue would mean the ultrasound would virtually all reflect - very little entering the body.
Transducer acts as receiver of the echo, changing the ultrasound energy into an electrical signal which is transmitted to a computer.

Question 30

Explain how and why ultrasound is used to obtain an image of an unborn foetus. You might consider the following points in your answer
Safety

Answer 30

No harmful side effects known
Does not use ionising radiation so is safe for the unborn child.
Is not invasive.

Question 31

Explain why the pulses of ultrasound used in medical imaging must be of short
duration.

Answer 31

The transducer to be damped (made to stop oscillating) before the echo returns to allow the transducer to act as a receiver.
(This time is very short) as distances travelled are short
Emitted pulse must cease before echo arrives so that there is no overlapping at the transducer/ no interference