Iterative reconstruction algorithms on their Vendors now offer various implementations of Techniques have been proposed, and all major On the other hand, iterative methods useĪ statistical model of the noise to improve the That low photon counts result in a large PoissonĮrror. This assumesĪccurate projection data and ignores the fact Method on most scanners, the projection dataĪre filtered to sharpen edges and the filteredĭata are then backprojected. In FBP, which is the standard reconstruction The maximum attenuation has a larger effect on Photon counts approach zero, which means that This is because the noise increases rapidly as the Than on both sides in thoracic and cardiac CT. Similarly, large breasts should be constrained in the front of the thorax rather Should reduce noise relative to placing themĪt the sides. Volume, placing them on top of the abdomen The arms cannot be moved out of the scanned NoiseĬan also be reduced by moving the arms out of Reconstruction kernel (soft-tissue kernel instead There is a tradeoff between noiseĪnd resolution, so noise can also be reducedīy increasing the slice thickness, using a softer They also typically use bowtie filters,Ĭenter of the field of view compared with the Modern scanners can perform tubeĬurrent modulation, selectively increasing theĭose when acquiring a projection with highĪttenuation. Poisson noise can be decreased by increasing Tissue texture, and s 2 is the standard deviation Where s 1 is the standard deviation owing to the Heterogenous, then the standard deviation in It alsoĪssumes that the underlying tissue has perfectly Hounsfield units (HU) due to Poisson noise Ĭorresponding regions in two images acquired The right kidney in a patient with Hodgkin’s lymphoma.įor conventional f iltered backprojection The images show an enlarged retroperitoneal lymph node (arrow) and infiltration of Image obtained at 7.3-times higher dose has √7.3 = 2.7-times less noise (440 mA, 120 kVp, slice Same whether or not the abdomen or arms are partially outside the field of view. (A) Low-dose CT image obtained during a CT-guidedīiopsy shows extensive Poisson noise (60 mA, 120 kVp, slice thickness 5 mm). The detector is sufficient to fix this artifact,Īlthough occasionally the detector itself needsįigure 2: Effect of mA on Poisson noise. In this article, we illustrate commonlyĮncountered artifacts in clinical CT, how theyĬan obscure or simulate pathology and how theyĪ miscalibrated or defective detector elementĬreates a bright or dark ring centered on theĬenter of rotation. Those conditions are not met, then artifacts will Would be a perfect reflection of reality. X-rays, infinite detector resolution, perfectĭetectors, no motion and no scatter, CT images In an idealized situation, with high radiationĭose, and thus high photon counts, monochromatic Reconstruction ▪ metal artifact ▪ micro-CT ▪ noise ▪ pseudoenhancement ▪ ring artifact Keywordsīeam hardening ▪ dose reduction ▪ dual energy CT ▪ iterative Methods for reducing noise and out-of-field artifacts may enable ultra-high resolution limited field of view imaging of tumors and other structures. Dual- and multi-energy (photon counting) CT can reduce beam hardening and provide better tissue contrast. Metal artifacts can also be reduced using iterative reconstruction, resulting in a more accurate diagnosis. This enables lower radiation dose and higher resolution scans. Noise can be reduced using iterative reconstruction or by combining data from multiple scans. We review the cause and appearance of each type of artifact, correct some popular misconceptions and describe modern techniques for artifact reduction. There are many different types of CT artifacts, including noise, beam hardening, scatter, pseudoenhancement, motion, cone-beam, helical, ring and metal artifacts. Stanford, CA 94305, USA Corresponding Author: Dominik FleischmannĪrtifacts are commonly encountered in clinical CT and may obscure or simulate pathology. CT artifacts: causes and reduction techniques F Edward Boas & Dominik Fleischmann *
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