Chapter 7 Image Artifacts: Appearances, Causes, and Corrections 7.1 What Is an Image Artifact?
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In  medical CT, Which algorithm has better motion suppression capability:

the half-scan or the underscan? Justify your answer. Q 7-22 from below attachment. Information regarding question is provided in same attachment.

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Chapter 7 Image Artifacts: Appearances, Causes, and Corrections 7.1 What Is an Image Artifact? Chapter 3 provided a representative flow diagram for the image generation process and stated that the preprocessing and postprocessing steps used to overcome nonideal data collection often outweigh, in terms of the number of operations, textbook tomographic reconstruction. In other words, a significant portion of the computation associated with CT image generation is related to the reduction or elimination of image artifacts. This chapter provides a broad overview of the causes of some artifacts and presents solutions to avoid or correct them whenever appropriate. The definition of 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. Although this definition is broad enough to cover nearly all types of nonideal images, it has little practical value since nearly every image produced by a CT scanner contains an artifact by this definition. In fact, most pixels in a CT image are artifacts in some shape or form. In practice, we have to limit our discussion to the discrepancies that are clinically significant or relevant as judged by the radiologists. We want to examine only the discrepancies that impact the radiologists performance. Compared to conventional radiography, CT systems are inherently more prone to artifacts. Recall the discussion in Chapter 3 that explained how a CT image is generated with a larger number of projections (about 1000). In a typical CT system, each projection contains roughly 1000 separate measure- ments. (In the case of a multislice CT scanner, which will be discussed in Chapter 10, the number of measurements in a single projection can easily be quadrupled.) As a result, nearly 10 6 independent readings or measurements are used to form an image. Because the nature of the backprojection process is to map a point in a projection to a straight line in an image, an error in the projection reading is no longer localized, as is the case for conventional 245 Downloaded From: http://ebooks.spiedigitallibrary.org/ on 09/16/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx
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radiography. Since inaccuracies in the measurements usually manifest themselves as errors in the reconstructed images, the probability of producing an image artifact is much higher for CT. Some errors or artifacts are merely an annoyance to radiologists, but others may cause misdiagnosis. 1 3 Image artifacts are caused by many things: the nature of the physics, suboptimal system design, limitations of current and new technologies, patient characteristics, and suboptimal or inappropriate use of the scanner. Methods used to combat CT image artifacts can be divided into two major classes: artifact correction and artifact avoidance. Artifact correction or reduction comes mainly from either academic institutions or industries. Many research papers and articles can be found in various journals and conference proceedings, and an even larger portion of the research results can be found in patents. A few summaries and overviews on the subject can be found in several books. 1 , 2 , 4 8 However, many of the artifact-combating methods are considered proprietary or trade secrets and are not available in the public domain. Artifact avoidance relies mainly on the combined efforts of CT manufacturers and operators. The best clinical protocols should be recommended by CT designers to optimize image quality. On the other hand, CT operators need to be properly trained to use the scanners in an optimal fashion. This chapter starts with a general definition of image artifacts and a brief explanation of the appearance of various artifacts. A clear artifact definition will lay the foundation for subsequent discussions and help to avoid potential confusion, while the brief explanations of artifact appearance can be linked to error patterns in the sinogram. These error patterns lead naturally to the physical sources of error. In particular, we want to link artifacts to major components in a CT system. For the purpose of this discussion, the overall system design is considered to be one of the major components (e.g., artifacts caused by inadequate projection or view sampling). Artifacts related to other components are divided into four major sections: x-ray source, x-ray detector, patient, and operator. Before discussing specific types of artifacts, we want to emphasize the fact that artifacts discussed in this section are general artifacts associated with different data-collection modes. In clinical CT, a patient can be scanned in either step-and-shoot mode or helical mode. In the step-and-shoot mode (also called axial mode), the patient remains stationary while the gantry rotates about the patient to collect a complete set of data. Once the data collection is completed, the patient is indexed to the next location for another scan. This is in contrast to the helical (or spiral) mode in which a patient is translated at a constant speed during the data acquisition (Chapter 9). This chapter covers artifacts that occur in either the step-and-shoot or the helical mode. It does not discuss artifacts that are specific to the helical scan mode or multislice CT (Chapter 10) for two reasons. First, it is difficult to understand the nature and 246 Chapter 7 Downloaded From: http://ebooks.spiedigitallibrary.org/ on 09/16/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx
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