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, âtextbookâ 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 âartifactsâ 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 measurements. (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 106 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 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.