CT artefacts


Causes of image artefacts can be grouped into a few categories:

  • Physics based
  • Patient properties
  • Scanner based
  • Helical and multislice artefacts

Physics based

Beam hardening

An x-ray beam has photons of different energies that vary around a mean 'beam energy'. As the beam passes through a dense area the lower energy photons are more likely to be absorbed and the higher energy photons are more likely to remain. This results in a higher mean beam energy. This focally increased mean beam energy is interpreted as being due to it passing through a less attenuating material relative to the surroundings and so a lower Hounsfield unit is assigned and the image will be represented as more black.

 

Conceptual representation of beam hardening artefact

Beam hardening artefact

 

This is particularly common in the posterior fossa on a CT head scan due to the dense petrous bones.

 

Beam hardening example

 

Cupping artefact

 

Cupping artefact

 

This beam hardening artefact also produces another type of artefact called the cupping artefact. The centre of an object is usually the thickest and, therefore, the beam will become harder in the centre than at the periphery and is assigned lower Hounsfield units. This can be corrected with a 'beam hardening correction' algorithm.

Solutions to beam hardening

  • Pre-patient filter: This absorbs the soft x-rays and minimises the beam hardening artefact
  • Bow-tie filter: Pre-harden the x-ray beam

Partial volume artefact

 

Partial voluming artefact

 

If a dense object only partially protrudes into a detector stream the attenuation is averaged with its surroundings and it will be assigned a lower Hounsfield unit. In the image above, the dense circle lies on a less dense background. The object fills detector stream 2 resulting in a very high attenuation (white). In detector stream 3 none of the dense object is imaged and so the attenuation is low (black). In detector stream 1 the object is only partially imaged and so the attenuation is an average between the dense object and the less dense background.

N.B. partial voluming will only ever reduce the apparent attenuation of an object, it will never increase the apparent attenuation.

 

Incomplete projection

 

Streak artefact

 

An object may protrude into the slice in one projection but not in the opposing projection, especially at the periphery of the image where the beam is more divergent. If this happens a variant of partial voluming artefact occurs in which the object appears streaked due to the inconsistencies produced during imaging.

These streak artefacts can be caused, for example, when a patient's arms are by their side and are imaged in some projections but not others.

Solution

  • Smaller slice thickness

Photon starvation

 

Photon starvation example

 

This is another cause of streak artefacts. In projections that have to travel through more material, e.g. across the shoulders, as the x-ray beam travels through more x-ray photons are absorbed and removed from the beam. This results in a smaller proportion of signal reaching the detector and, therefore, a larger proportion of noise. The streaks are due to the increased noise which is why they occur in the direction of the widest part of the object being scanned.

Solutions

  • Adaptive filtering: the regions in which the attenuation exceeds a specified level are smoothed before undergoing backprojection.

 

Adaptive filtering

 

  • mA modulation: the tube current (mA) can be varied with the gantry rotation. HIgher mA's (greater signal) are used for the more attenuating projections to reduce the effect of photon starvation. The mA required can either be calculated in advance from the scout view or during the scan from the feedback system of the detector.

 

Current modulation

 

 

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Patient properties

Metallic artefacts

 

Metal artefact example

 

The metal produces a beam-hardening and photon starvation artefact. This can also happen with other high attenuation materials such as IV contrast.

Patient motion

 

Motion artefact example

 

Motion artefact can be caused by:

  • Patient swallowing
  • Breathing
  • Pulsatility of heart and vessels
  • Patient moving

If a patient or structure moves as the gantry rotates the object will be detected as being in several positions and represented in the image as such.

Solutions

  • Scan parameters
    • Shorten scan time
    • Spiral scanning
    • ECG gating: this can be used prospectively to trigger image acquisition during a specific point on the ECG when heart motion is lowest, or retrospectively by reconstructing acquired data from specific ECG phases
  • Patient parameters
    • Breath hold
    • Ensure comfortable patient position
    • Tell patient to stay still and give clear instructions

Incomplete projections

If there are objects lying outside the field of view, especially high attenuation objects such as the arms, this will create streak artefacts within the imaged area as the arms will be detected in some projections and not others leading to inconsistencies in the data.


Scanner based

Ring artefact

If there is a faulty detector and the detectors do not have the same gain relative to each other (they are operating at different baselines) then as the gantry rotates around the patient this detector will outline a circle. On back-projection this will cause a ring artefact.

 

Ring artefact

 


Spiral and multislice scanning artefacts

Helical artefacts

In spiral scanning, as the gantry rotates it is also moving in the z-axis. This means that a row of detectors is moving in a spiral path. This can cause artefactual representation of structures that are changing in shape or position in the z-axis as they will be in different positions for different projections used in the reconstruction of the image. Nowadays this artefact is rare as scanners have a large number of detectors and pitch <1.

Worsened by:

  • Increasing pitch
  • Increased contrast between object and surrounding structures

Cone beam artefact

This is a particular artefact caused by multislice scanners. As the section scanned increases per rotation, a wider collimation is used. Because of this the x-ray beam becomes cone-shaped instead of fan-shaped and the area imaged by each detector as it rotates around the patient is a volume instead of a flat plane. The resulting artefact is similar to the partial volume artefact for off-centre objects. This is particularly pronounced at the edges of the image. With modern scanners cone beam reconstruction algorithms correct this artefact.

 

Comparison of fan beam and cone beam

 

Solution

  • Reconstruction algorithm minimises cone beam artefacts

Σ  Summary

1. Physics based

Beam hardening

  • Dense objects remove more lower energy photons from the x-ray beam leaving a higher average energy beam. A higher average energy of incident beam is interpreted as having passed through a structure that causes less attenuation of the beam and represented as such on the image (i.e. black bands)
  • Cupping: variation of beam hardening that occurs in spherical objects. Corrected with a beam hardening correction algorithm
  • Solutions:
    • Pre-patient filter to absorb soft x-rays
    • Bow-tie filter to equalise the attenuation across the patient profile

Partial volume artefact

  • If object is smaller than slice thickness its attenuation will be averaged within the slice resulting in a displayed lower attenuation of the object
  • Incomplete projection: variation of partial volume artefact in which an object is present in the x-ray beam in some projections and not others causing streak artefact
  • Solutions:
    • Thinner slices

2. Patient properties

Metallic artefact

  • Analogous to beam hardening artefact caused by high density structures such as metal or iodinated contrast.
  • Solutions:
    • Same as for beam hardening
    • Avoid metal

Movement artefact

  • Solutions:
    • Breath hold
    • ECG gating with prospective or retrospective image formation
    • Ensure patient in comfortable position
    • Tell patient to stay still and give clear instructions
    • Short scan time
    • Spiral scanning

3. Scanner based

  • Ring artefact: caused by a faulty detector element or incorrect relative gain setting

4. Spiral and multislice based

  • Helical artefacts: As gantry rotates it is moving in the z-axis. Any object that changes in position or size along the z-axis may be distorted as they will be in different positions for different projections
  • Cone beam artefact: Due to wider collimation, the beam has a volume and becomes cone-shaped. A similar artefact to partial voluming occurs for off-centre objects in the detector field. The artefact is worse for objects at the edges of the beam.

Next page: CT dose


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