Image Resolution and Artefacts

Question 1

Why does ultrasound imaging work?

Answer 1

Some variation in the impedance of the medium so waves are reflected back to the transducer

The sound speed must not vary too much, or it would be impossible to calculate the depth of the reflections from their arrival times

Question 2

For artefact-free, perfect resolution B-mode imaging, what are the conditions?

Answer 2

1) the sound speed & absorption are known and constant (uniform)

2) all scatterers are weak (the incident wave is not perturbed)

3) only single scattering occurs (multiple scattering is negligible)

4) the array’s elevation focusing restricts beam to thin image plane

5) beamforming restricts beam to a thin line within the image plane

6) the transducer can emit & detect at all frequencies (infinite bandwidth)

7) the source pulse duration is infinitesimally short

8) there is no scattering from beyond the imaging depth

9) the data is noise-free.

Question 3

What is spatial resolution?

Answer 3

The ability to distinguish between two features located close together

(made up of axial, lateral and elevation)

Question 4

What is spatial resolution influenced by?

Answer 4

Choice of transducer

Question 5

What is lateral resolution and which condition is failed if this isn’t true?

Answer 5

Ability to distinguish two objects at same depth (beam width)

Failure of condition 5

Question 6

How can lateral spatial resolution be improved?

Answer 6

By multi-zone focusing and receive beamforming

Question 7

What is axial spatial resolution and which condition is failed if this isn’t true?

Answer 7

Ability to distinguish two objects at different depths (determined by emitted pulse width)

Failure of conditions 6 and 7

Question 8

How can reflected pulses be distinguished?

Answer 8

If their separation in time is greater than the FWHM pulse envelope

Question 9

How does the resolution in elevation direction fail condition 4?

Answer 9

Elevation focusing is weak: there is a trade-off between the tightness of focus and the depth over which the image slice is thin

Question 10

What is the typical element height in the elevation direction?

Answer 10

20 to 30 λ

Question 11

How do out-of-plane artefacts fail condition 4?

Answer 11

Finite slice thickness means reflections from objects out-of-plane can contribute to image

Question 12

How does a reverberation artefact fail condition 3?

Answer 12

Multiple reflections between strong specular scatterers

(cause perpendicular lines in image)

‘Comet-tail’, ‘ringdown’, ‘B-lines’ are also artefacts due to reverberation

Question 13

How does a mirror-image artefact fail condition 3?

Answer 13

Specular reflection from a large interface can result in object and its reflection appearing in the ultrasound image

Question 14

How does an enhancement artefact fail condition 1?

Answer 14

Enhancement beyond a fluid-filled (low attenuation) region causes time-gain-compensation to be wrong as attenuation is not uniform

Question 15

How do shadowing artefacts fail conditions 1 and 2?

Answer 15

Large strong scatterer/absorber prevents ultrasound from reaching the shadow zone

Question 16

How do refraction artefacts or aberrations fail condition 1?

Answer 16

Wave bending, due to changes in sound speed, can cause objects to appear in the wrong place or to be missed altogether

Question 17

How does grating and side lobe artefacts fail condition 5?

Answer 17

Echoes received from pulses in side lobes and grating lobes leads to
spurious images in the field-of-view

Question 18

How does range ambiguity artefact fail condition 8?

Answer 18

Scatterers from depths below the imaging depth scatter significantly and
appear within the image at shallower depths (seen by changing image depth)

Question 19

How does speckle fail conditions 4 to 7?

Answer 19

Tissue contains many scatterers smaller than the wavelength

The scatterers may have both higher and lower impedances than the average value of the background medium

The scatterers are essentially randomly distributed

Question 20

What happens when the reflected waves reach the transducer?

Answer 20

The recorded time signal will contain constructive and destructive interference between the scattered signals from different scatterers

Question 21

What does speckle pattern depend on?

Answer 21

Transducer and processing (e.g. frequency, bandwidth, aperture size etc)

Question 22

How can speckle and noise be reduced?

Answer 22

By averaging together several ultrasound images taken under different conditions -> compounding

Question 23

What is temporal compounding?

Answer 23

Several subsequent image frames are averaged

Question 24

What is spatial compounding?

Answer 24

Images from different scan directions are averaged

Question 25

What is frequency compounding?

Answer 25

Images at different frequencies are averaged

Question 26

What does compounding typically reduce?

Answer 26

Frame rate (temporal resolution)

Question 27

What aspect of transducer can be chosen?

Answer 27

Bandwidth can be chosen (depending on what part of body is being imaged)

deeper structures need smaller frequencies

Question 28

What needs to be compromised when picking a transducer frequency?

Answer 28

Comprise between image resolution and penetration depth:

The higher the frequency the greater the spatial resolution

Question 29

What is the pulse repetition frequency limited by?

Answer 29

The time required for pulse to travel to desired maximum depth Lmax and return to transducer

Question 30

What is frame rate affected by?

Answer 30

Reduced by image compounding

Increased by capturing multiple A-line simultaneously

Question 31

How is slice thickness determined?

Answer 31

Beam width in elevation direction (fixed)