Ultrasound Fields: Single Element Flashcards
What can the field from a circular piston transduce be divided into?
Near field and far field regions
(near field used for imaging)
How does pressure vary in near field regions?
Strong variation in pressure amplitude in radial and axial direction
How does pressure vary in far field region?
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
What is a pulsed wave made of?
Plane wave and edge wave
What happens in an continuous wave from a circular piston transducer?
Over lapping pattern due to constructive and destructive interference between direct plane wave and edge wave
(maxima are created)
What is the last axial maximum?
The boundary between near field and far field
How can the positions of the maxima be found?
when sin() = 1
Where is the start of the far field?
Where the maximum path length difference is ΔR = λ/2 which is the same position as the last axial maximum
What doubles the far field distance?
Doubling the frequency
What is the beam’s far field divergence angle?
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)
Where is most of the energy?
In the main lobe
(most directional with increasing frequency/aperture size)
What affects far field divergence and directivity?
Changing aperture size: increasing frequency extends the near field region and reduces far field divergence (beam becomes more directional but attenuation is higher)
What is the effect of small aperture?
Smaller beam width but shorter near field region
What is the effect of large aperture?
Larger beam width but longer near field region
What causes image artefacts and noise?
Side lobes mean energy is emitted into multiple directions
How are side lobe levels reduced?
Apodisation: By designing transducer to emit more strongly at the centre
But width of main lobe is increased
What do short tone burst achieve?
Side lobe reduction
Give smoother response and give size of near field region which is approximately the same
What is the focusing?
The process of introducing phase (time) delays to produce a curved wavefront using lenses (for high frequency PZT transducers)
How can focusing be achieved electronically?
By using an array of elements
What affects the focal shape, size and focal amplitude?
Frequency, size and shape of the source
What is the width and length of focal zone ?
Width at which maximum pressure amplitude has decreased by factor of two
Distance at which maximum pressure amplitude has decreased by 6dB
What is the focusing gain (G) ?
The pressure at focus is greater than pressure at surface by this factor
What does focusing increase with?
Frequency, aperture size and curvature
What does the acoustic principle of reciprocity mean?
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
