Ultrasound Fields: Single Element

Question 1

What can the field from a circular piston transduce be divided into?

Answer 1

Near field and far field regions

(near field used for imaging)

Question 2

How does pressure vary in near field regions?

Answer 2

Strong variation in pressure amplitude in radial and axial direction

Question 3

How does pressure vary in far field region?

Answer 3

Smooth decay of pressure amplitude in radial and axial directions (phase difference can be ignored)

Path length difference between the plane waves and edge waves is small and waves are almost in phase

Question 4

What is a pulsed wave made of?

Answer 4

Plane wave and edge wave

Question 5

What happens in an continuous wave from a circular piston transducer?

Answer 5

Over lapping pattern due to constructive and destructive interference between direct plane wave and edge wave

(maxima are created)

Question 6

What is the last axial maximum?

Answer 6

The boundary between near field and far field

Question 7

How can the positions of the maxima be found?

Answer 7

when sin() = 1

Question 8

Where is the start of the far field?

Answer 8

Where the maximum path length difference is ΔR = λ/2 which is the same position as the last axial maximum

Question 9

What doubles the far field distance?

Answer 9

Doubling the frequency

Question 10

What is the beam’s far field divergence angle?

Answer 10

The angle at which the pressure first falls to zero in far field (found in side lobes)

(first occurs when J_1(x) = 0 when x = 3.83)

Question 11

Where is most of the energy?

Answer 11

In the main lobe

(most directional with increasing frequency/aperture size)

Question 12

What affects far field divergence and directivity?

Answer 12

Changing aperture size: increasing frequency extends the near field region and reduces far field divergence (beam becomes more directional but attenuation is higher)

Question 13

What is the effect of small aperture?

Answer 13

Smaller beam width but shorter near field region

Question 14

What is the effect of large aperture?

Answer 14

Larger beam width but longer field region

Question 15

What causes image artefacts and noise?

Answer 15

Side lobes mean energy is emitted into multiple directions

Question 16

How are side lobe levels reduced?

Answer 16

Apodisation: By designing transducer to emit more strongly at the centre

But width of main lobe is increased

Question 17

What do short tone burst achieve?

Answer 17

Side lobe reduction

Give smoother response and give size of near field region which is approximately the same

Question 18

What is the focusing?

Answer 18

The process of introducing phase (time) delays to produce a curved wavefront using lenses (for high frequency PZT transducers)

Question 19

How can focusing be achieved electronically?

Answer 19

By using an array of elements

Question 20

What affects the focal shape, size and focal amplitude?

Answer 20

Frequency, size and shape of the source

Question 21

What is the width and length of focal zone ?

Answer 21

Width at which maximum pressure amplitude has decreased by factor of two

Distance at which maximum pressure amplitude has decreased by 6dB

Question 22

What is the focusing gain (G) ?

Answer 22

The pressure at focus is greater than pressure at surface by this factor

Question 23

What does focusing increase with?

Answer 23

Frequency, aperture size and curvature

Question 24

What does the acoustic principle of reciprocity mean?

Answer 24

No need for new analysis of receive beam (can treat it like transmit beam)

Points in the transmit beam with the highest pressure output are the same points in the receive beam that will give the greatest electrical output

Question 25

What is the receive zone?

Answer 25

Focal region of receive beam is the region over which the transducer can detect a point source with high sensitivity