DK1212_C011

DK1212_C011 - 11 Image Enhancement Image enhancement is...

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495 11 Image Enhancement Image enhancement is defined as processing that takes an image as its input and provides, as the output, another image, hopefully better in a sense. What the improvement exactly means depends on the intended use of the resulting image; the result is mostly evaluated based on the subjective visual impression, taking into account the purpose of the image, or the evaluation may be derived from the results of sub- sequent analysis applied to the enhanced image, e.g., in diagnostics. The enhancement may also be interpreted as ad hoc compen- sation of imperfections in the imaging process providing the image; however, on the difference from image restoration, the imaging properties, distortion, or noise parameters usually need not be identified for enhancement, or only roughly. From this viewpoint, some operations, like contrast transforms or sharpening, found generally useful for a particular modality or a certain type of imaged objects, may be done in batch mode automatically for all images of a group. Nevertheless, the general approach to image enhancement relies on individual trial-and-error attempts, the results of which are mostly evaluated immediately by visual inspection; when the output image is not satisfactory, a modified operation will be tried. This is particularly useful in diagnostic evaluation because the visual properties of the © 2006 by Taylor & Francis Group, LLC
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496 Jan inspected image, such as contrast, sharpness, noise smoothing, or geo- metrical magnification, directly influencing the detectability of impor- tant features, may be easily adapted to a concrete image. It is usually done interactively by the inspecting medical staff (e.g., a radiologist) on a local workstation, even when the original image is fetched from a hospital information system. The main purpose of the enhancement concerns: Contrast (and possibly color) transforms Sharpening of images, thus enhancing detail resolvability Suppression of noise and other disturbing image components Geometrical transforms, namely, zooming of partial image areas, and removal of distortion due to imperfection of the imaging systems The contrast (and color) transforms are typically simple non- linear point operations, though possibly based on nontrivial algo- rithms of calculating the brightness transfer functions. Sharpening and noise smoothing are conflicting operations and should therefore be considered simultaneously. They are, as far as enhancement concerns, realized mostly by local (mask) operators, linear and non- linear. Both point and local operations may be space invariant, or they may depend on regional or local properties of the image in an area surrounding the processed pixel; in this case, it is called adap- tive enhancement. Geometrical modifications rely on the transform and interpolation methodology (Sections 10.1.1 and 10.1.2); usually only a limited repertoire of zooming and common distortion elimi- nation is available. The enhancement of images is the most fre-
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DK1212_C011 - 11 Image Enhancement Image enhancement is...

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