helpsimple searchclear selectionselect all
Dictionary assisted search
All words
Any word/input
Exact phrase
in these
lexical topics:
  • Physics, Techniques and Procedures
  • Normal Anatomy
  • Musculoskeletal Imaging
  • Breast Imaging
  • Gastrointestinal Imaging
  • Urogenital Imaging
  • Chest Imaging
  • Cardiovascular Imaging
  • Neuroradiology
  • Head and Neck Imaging
  • Paediatric Imaging
 
 
 
Physics, Techniques and Procedures

Ring artefact

an artefact in a computed tomography (CT) image which appears as an increase or decrease in the image values along a circle, or portion of a circle, centred at the centre of rotation of the system.

In a CT raw data set, the distance of a ray from the centre of rotation is constant as the projection angle is rotated. In other words, all the measurements at a given position along a projection are at the same radial distance from the centre of rotation. As the projection angle is rotated, the point along the ray which is closest to the centre of rotation traces out a circle or ring. If that projection ray has an error that is constant as a function of angle, the errors will add most rapidly along the circle described above. This circular artefact, or portion of a circle if the error is present in a subset of the views, is called a ring artefact. In X-ray CT, ring artefacts are caused by a gain error at one position in many adjacent views. A third generation CT scanner is particularly vulnerable to ring artefacts because the same physical detector channel measures all the rays that form a ring. Thus, for example, drift in the gain of a detector channel since the previous calibration process will produce a ring artefact.

A ring artefact does not affect solely the pixels along the ring. A positive error in the projections will cause a positive ring along the defined circle, but also a negative ring just inside the circle. Because the circumference of the ring increases with ring diameter, for a fixed gain error the ring artefact intensity decreases with increasing radius. As a result, ring artefacts are more likely to be visible near the centre of rotation as opposed to the edges of the field of view (FOV).

If multiple adjacent detectors have similar errors, cancellation between the adjacent positive and negative ring components will diminish the intensity of the artefact (which will now appear as a band). As a result, ring artefacts are caused not so much by gain errors as by changes in the relative gain of neighbouring projection rays (Fig. 1).

Historically, the high sensitivity that the third generation CT design has to ring artefacts, led to the development of fourth generation CT scanners (see CT generation). However, manufacturers of third generation CT systems have developed stable enough components and precise enough calibration methods so that ring artefacts are absent or rare in a well-functioning system.

While ring artefacts are most often discussed with respect to X-ray CT, they can appear in other applications of projection reconstruction such as PET, SPECT and MRI. In projection reconstruction MRI, for example, if frequency encoding is used to localize the signal for each view, any additive signal at a fixed frequency, such as bleed-through of RF energy through the RF screen which is consistent in all views, will cause a ring artefact.

NP/HJS-04

To view high resolution images,
please register first.

Click  here to register.

Already registered? Enter your e-mail in the window below.
Re-register

Fig.1

CT of the brain with multiple, concentric ring artefacts caused by malfunctioning analogue-to-digital conversion of the signals from the central part of the detector array of a third generation unit.
Ring artefact, Fig.1