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CT ARTIFACTS.pptx
- 1. CT ARTIFACTS DR. PradeepPatil Prof. Department of Radio-diagnosis, DY Patil medical college, hospital & research institute Kolhapur
- 2. Definition • The definitionof an image artifact is not as clearly defined as one might expect. • Theoretically, an image artifact can be defined as any discrepancy between the reconstructed values in an image and the true attenuation coefficients of the object
- 6. Physics based artifacts •beam hardening - cupping artifact / streak artifact / metal artifact • photon starvation • partial volume averaging • noise • aliasing • truncation artifact
- 7. BEAM HARDENING • X-rayphotons coming from a tube are made up of a full spectrum of X-rays energies (POLYCHROMATIC X-ray beam). • When an X-ray beam begins to penetrate a sample material the lower energy X-rays preferentially attenuate, resulting in an overall higher energy of the X-ray beam – “beam hardening”. • Beam hardening artefacts appear as dark bands/ streaks or cupping artefacts.
- 9. CUPPING ARTEFACT Beam passingthrough the middle part of the object is hardened more as compared to edge causing cupping artefact.
- 10. BEAM HARDENING ARTEFACTS Beam-hardeningartifacts. A- Postoperative CT image of the brain in an adult. B- Comparison of reductions in beam-hardening artifacts with use of special reconstruction techniques.
- 14. Solutions to beamhardening • Bow tie filter – to pre-harden the X-ray beam. • Beam hardening correction software. • Calibration correction.
- 19. Partial volume artifactoccurs when tissues of widely different absorption are encompassed on the same CT voxel producing a beam attenuation proportional to the average value of these tissues.
- 24. PARTIAL VOLUME ARTEFACT Liverscan showing partial-volume artifact. A, A 2.5-mm slice thickness shows good visibility of small liver lesions. B, A 10-mm slice thickness shows reduced lesion detectability with partial-volume averaging. (Images courtesy Dr. John Haaga.)
- 25. Noise • Noise incomputed tomography is an unwanted change in pixel values in an otherwise homogeneous image. • Noise in CT is measured via the signal to noise ratio (SNR) • Noise in a cross-sectional image will equal a decrease in the picture quality and inadvertently will hinder the contrast resolution.
- 28. Factors affecting noise •Slice thickness - The thicker the slice, the more photons available; and the more photons available, the better the SNR. However, this isn't without a trade-off because increasing the slice thickness will decrease the spatial resolution in the z-axis. • Reconstruction algorithm- Non-linear reconstruction algorithms can cause noise non-uniformity, meaning the intensity of noise varies across the image depending on regional structure. Uniform regions of the image will generally have lower noise levels than highly structured regions.
- 29. Aliasing artifact • Aliasingartifact, otherwise known as undersampling, in CT refers to an error in the accuracy of analog to digital converter (ADC) during image digitization. • Image digitization has three distinct steps: scanning, sampling, and quantization. • When sampling, the brightness of each pixel in the image is measured, and via a photomultiplier, creates an output analog signal that is then due to undergo quantization. • The more samples that are taken the more accurate the representation of the signal will be, hence if a lack of sampling has occurred the computer will process an inaccurate image resulting in an aliasing artifact.
- 33. Truncation artifact • apparentlyincreased curvilinear band of attenuation along the edge of the image. • This artifact is encountered when parts of the imaged body part remain outside the field of view (e.g. due to patient body habitus), which results in inaccurate measurement of attenuation along the edge of the image. The artifact can be reduced - if possible - by using an extended FOV reconstruction of the affected region.
- 34. Patient based artifacts •Motion artifact • Metal artifact
- 36. MOTION ARTEFACT Streak artifact(open arrow) caused by motion of bowel gas in a slow abdominal scan on a conventional single- slice CT scanner. Machine-caused streaks are also seen. Streak artifact (solid arrow) is caused by an unstable rotor in the x-ray tube, producing unstable focal spot motion during the scan.
- 44. Hardware based artifact •ring artifact • tube arcing • helical and multisection artifact - windmill artifact / conebeam artifact • multiplanar reconstruction artifact - zebra artifact / stair step artifact
- 53. CONE BEAM ARTEFACT Phantomstudy demonstrating cone-beam artifacts. A- Four-slice CT scanner study. The patient's ribs are positioned at a steep angle along the z-axis to demonstrate cone-beam artifact at pitch 1.25. B- Enhanced contribution of multislice CT cone-beam artifacts caused by divergence of the x-ray beam in the z-axis when the number of detector rings used to acquire a CT image is increased from four to eight.
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