computed tomography equipment techniques 13.pdf

Middle Technical University (MTU) College of Health and Medical Techniques -Baghdad Radiological Techniques Department Computed Tomography EquipmentsTechniques Second stage/ 2nd coarse Title: CT image quality Image artifacts Types , causes & correction techniques Name of the instructor: Lec. Dr. Lam...

Middle Technical University (MTU) College of Health and Medical Techniques -Baghdad Radiological Techniques Department Computed Tomography EquipmentsTechniques Second stage/ 2nd coarse Title: CT image quality Image artifacts Types , causes & correction techniques Name of the instructor: Lec. Dr. Lamyaa Fadhil Abdul Hussein Target population: Students of second class 93 CT image quality Image artifacts Artifacts may be defined as any structure that is seen on an image but is not representative of the actual anatomy. Artifacts can degrade image quality, affect the perceptibility of detail, or even lead to misdiagnosis. They can cause serious problems for the radiologist who has to provide a diagnosis from images obtained by the CT scanner. Therefore it is mandatory that the technologist understand the nature of artifacts in CT. In general, an artifact CT image artifact is defined as discrepancy between the reconstructed CT numbers in the image and the true. Because CT numbers represent gray shades in the image, incorrect measurements will produce incorrect CT numbers that do not represent the attenuation coefficients of the object. These errors result in various artifacts that affect the appearance of the CT image. Most types of CT artifacts fall into 3 categories: Streak artifacts Shading artifacts Ring artifacts Streak artifacts may appear as intense straight lines across an image, which may be caused by improper sampling of the data, motion, metal, beam hardening, noise, spiral/ helical scanning, and mechanical failure or imperfections. These discrepancies are enhanced by the convolution process and manifested into lines during the back-projection, as shown in Figure (1,A). 94 Fig. (1): Different appearances of artifacts. A, Streak. B, Ring. C, shading. Streak artifacts may occur in all scanners. Although arising for many reasons, most are due to inconsistent or bad detector measurements. Factors causing inconsistencies include motion (anatomy in different locations during different parts of the scan), insufficient x-ray intensity (leading to high random errors), and malfunctions (tube arcing or system misalignment). An inconsistency due to partial-volume effects is illustrated in Figure (2). During a 360 axial scan, the same ray (or nearly the same ray) is sampled twice, but with x- rays traveling in opposite directions. Because of beam divergence, however, the cone-shaped x-ray beam samples slightly different volumes in each direction. A small structure, such as the edge of a bone, may partially extend into the volume so as to attenuate the beam traveling in one direction (say, downward when the tube is above) but may be missed when the beam is coming from the opposite direction (upward, when the tube is underneath). The two measurements of the same ray path are thus inconsistent and will lead to an image streak. 95 Figure (2): Partial-volume streaks are caused by opposing x-ray beams, which nominally pass through the same voxels but actually sample slightly different cone-shaped tissue volumes as a result of beam divergence. Small structure, such as piece of bone, is detected by beam from one direction but is missed by opposing beam. Resulting inconsistency leads to streak artifact. Shading artifacts often appear near objects of high densities and can be caused by beam hardening, partial volume averaging, spiral/helical scanning, scatter radiation, offfocal radiation, and incomplete projections, as shown in Figure 2, C. The most common type of shading artifact is beam- hardening effects. Beamhardening artifacts are actually present on all CT images to some extent and are due to imperfect beam-hardening correction. They appear as non- uniformities in the CT numbers of a uniform material, such as CT numbers that are lower at the center of a uniform phantom than at the periphery. Such nonuniformities are generally quite 96 small (< 5 HU) and not apparent unless one is viewing a scan of a uniform phantom with a very narrow window. Occasionally, however, a larger amount of hardening occurs when the scan is passing through thick regions of bone orthrough contrast medium. In that case, regions of hypointensity (i.e., CT numbers that are lower than expected) may appear downstream along the paths of rays that have been overly hardened. Scatter can also cause shading artifacts, although these are uncommon in most modern scanners. Ring or partial ring (arc) artifacts are produced when the projection readings of a single channel or a group of channels consistently deviate from the truth. They can be the result of defective detector cells or DAS channels, deficiencies in system calibration, or a sub- optimal image-generation process. This is predominately a thirdgeneration CT scanner phenomenon. Because a detector channel reading is always mapped to a straight line that is at a fixed distance to the iso- center of the system, any such inaccuracies in measurements from a particular detector occurring during a scan (or part of a scan) are backprojected along the ray paths measured by that detector. These inaccuracies contribute only slightly to pixels over most of the image (because several hundred backprojected values contribute to each pixel) but tend to reinforce along a ring of radius d, where several such rays intersect.a defective reading forms a ring pattern during the back-projection process, as illustrated in Figure 2, B. Ring artifacts are usually readily recognizable by software ring- correction algorithms and thus can be removed from the image. Small-radius rings (i.e., near the center of rotation) or arcs of small angular extent may not be recognized as artifacts and thus wind up in the image. In practice, third- generation scanners are sensitive to detector inaccuracies, which, without corrective algorithms, would be visible on most or all CT images. 97 Causes of artifacts CT artifacts are common and can occur for various reasons. Knowledge of these artifacts is important because they can mimic pathology (e.g. partial volume artifact) or can degrade image quality to non-diagnostic levels. CT artifacts can be classified according to the underlying cause of the artifact. Patient-based artifacts motion artifact transient interruption of contrast clothing artifact Physics-based artifacts beam hardening cupping artifact streak and dark bands metal artifact / high-density foreign material artifact partial volume averaging photon starvation aliasing truncation artifact Hardware-based artifacts ring artifact tube arcing out of field artifact air bubble artifact helical and multichannel artifact windmill artifact cone beam effect 98