Ring artifact

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Ring artifacts are concentrically arranged, exactly circular rings that appear alternately darker and lighter in an image generated by an imaging process . They simulate alternately more radiopaque and radiopaque annular areas. Ring artifacts belong to technical-systematic errors and must be differentiated from stochastic influences (noise).

General

Ring artifacts ( ring artifact or ring artefact ) play a role in medical and radiological diagnostics in computed tomography , sonography , micro-CT , PET and SPECT . Sometimes not complete rings can be seen, but only concentric circle segments. In the case of ring-shaped artifacts in ultrasound diagnostics, aura signs are also used. Exactly concentric, ring-shaped structures are unknown in humans, so that these concentric artifacts are usually easily recognizable. However, at best can inner wall and outer wall tangentially from the radiation Met large vessels or bronchi sometimes have similar ring artifact correlates. The center of the artifacts (= center of the concentric circles) coincides with the axis of rotation of the scanner. In principle, the effect is most clearly visible near the center of the rings.

Ring artifacts can be observed particularly in so-called third generation CT scanners (from 1974) and accelerated the introduction of fourth generation devices as well as further developments; the introduction of spiral CT further improved the image quality in this regard. An optimal calibration of the scanner ensures a largely error-free image.

In rare cases, ring artifacts can lead to incorrect findings and misdiagnoses. In any case, they impair an assessment and are therefore the subject of research and training.

causes

Ring artifacts are caused by various mechanisms, which have a gain error , as faulty or deviating from the standard sensitivity of individual sensors in common.

  • A defective individual sensor in a CT scanner automatically leads to a clearly recognizable ring artifact.
  • Failure to calibrate , which must take place at regular intervals, can lead to less clearly visible ring artifacts, usually of a complex nature. In this case there are signal errors ( gain errors or gain drifts as differences in sensitivity of individual sensors) and are not sufficiently compensated by calibration on a phantom (or air).
  • Changes in the linearity of individual sensors over time.
  • Changes in the sensitivity of individual sensors due to aging .
  • Temperature influences

A high sensitivity and the highest possible spatial resolution increase the probability of these technically caused artifacts.

Detection / exclusion of ring artifacts

In case of doubt, a different imaging method at the same time provides information (e.g. comparison CT-MRI). A ring artifact could be proven by repeating a CT examination while simultaneously moving the patient sideways. However, since a CT examination represents a very high radiation exposure for the patient , any short-term repetition of a CT examination is generally prohibited.

Ring artifacts can be observed over several layers, which is usually not the case with biological structures.

Clearly dented ring-shaped structures and several laterally offset ring-shaped structures without a common center point are not signs of ring artifacts.

literature

  • CL Tsai, HP Wang, WC Lien, CC Chen, TI Lai, WJ Chen: Ring down artefacts on abdominal sonography to predict pulmonary abnormalities in the emergency department. In: Emergency Medicine Journal, October 22, 2005, 10, ISSN  1472-0205 , pp. 747-748.

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