Image Resolution and Artefacts Flashcards
Why does ultrasound imaging work?
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
For artefact-free, perfect resolution B-mode imaging, what are the conditions?
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.
What is spatial resolution?
The ability to distinguish between two features located close together
(made up of axial, lateral and elevation)
What is spatial resolution influenced by?
Choice of transducer
What is lateral resolution and which condition is failed if this isn’t true?
Ability to distinguish two objects at same depth (beam width)
Failure of condition 5
How can lateral spatial resolution be improved?
By multi-zone focusing and receive beamforming
What is axial spatial resolution and which condition is failed if this isn’t true?
Ability to distinguish two objects at different depths (determined by emitted pulse width)
Failure of conditions 6 and 7
How can reflected pulses be distinguished?
If their separation in time is greater than the FWHM pulse envelope
How does the resolution in elevation direction fail condition 4?
Elevation focusing is weak: there is a trade-off between the tightness of focus and the depth over which the image slice is thin
What is the typical element height in the elevation direction?
20 to 30 λ
How do out-of-plane artefacts fail condition 4?
Finite slice thickness means reflections from objects out-of-plane can contribute to image
How does a reverberation artefact fail condition 3?
Multiple reflections between strong specular scatterers
(cause perpendicular lines in image)
‘Comet-tail’, ‘ringdown’, ‘B-lines’ are also artefacts due to reverberation
How does a mirror-image artefact fail condition 3?
Specular reflection from a large interface can result in object and its reflection appearing in the ultrasound image
How does an enhancement artefact fail condition 1?
Enhancement beyond a fluid-filled (low attenuation) region causes time-gain-compensation to be wrong as attenuation is not uniform
How do shadowing artefacts fail conditions 1 and 2?
Large strong scatterer/absorber prevents ultrasound from reaching the shadow zone
How do refraction artefacts or aberrations fail condition 1?
Wave bending, due to changes in sound speed, can cause objects to appear in the wrong place or to be missed altogether
How does grating and side lobe artefacts fail condition 5?
Echoes received from pulses in side lobes and grating lobes leads to
spurious images in the field-of-view
How does range ambiguity artefact fail condition 8?
Scatterers from depths below the imaging depth scatter significantly and
appear within the image at shallower depths (seen by changing image depth)
How does speckle fail conditions 4 to 7?
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
What happens when the reflected waves reach the transducer?
The recorded time signal will contain constructive and destructive interference between the scattered signals from different scatterers
What does speckle pattern depend on?
Transducer and processing (e.g. frequency, bandwidth, aperture size etc)
How can speckle and noise be reduced?
By averaging together several ultrasound images taken under different conditions -> compounding
What is temporal compounding?
Several subsequent image frames are averaged
What is spatial compounding?
Images from different scan directions are averaged
What is frequency compounding?
Images at different frequencies are averaged
What does compounding typically reduce?
Frame rate (temporal resolution)
What aspect of transducer can be chosen?
Bandwidth can be chosen (depending on what part of body is being imaged)
deeper structures need smaller frequencies
What needs to be compromised when picking a transducer frequency?
Comprise between image resolution and penetration depth:
The higher the frequency the greater the spatial resolution
What is the pulse repetition frequency limited by?
The time required for pulse to travel to desired maximum depth Lmax and return to transducer
What is frame rate affected by?
Reduced by image compounding
Increased by capturing multiple A-line simultaneously
How is slice thickness determined?
Beam width in elevation direction (fixed)
