Image enhancement techniques are used to refine a given image, so that desired image features become easier to perceive for the human visual system or more likely to be detected by automated image analysis systems [1,13]. Image enhancement allows the observer to see details in images that may not be immediately observable in the original image. This may be the case, for example, when the dynamic range of the data and that of the display are not commensurate, when the image has a high level of noise or when contrast is insufficient [4, 5, 8, 9].

Fundamentally, image enhancement is the transformation or mapping of one image to another [10, 14]. This transformation is not necessarily one-to-one, so that two different input images may transform into the same or similar output images after enhancement. More commonly, one may want to generate multiple enhanced versions of a given image. This aspect also means that enhancement techniques may be irreversible.

Often the enhancement of certain features in images is accompanied by undesirable effects. Valuable image information may be lost or the enhanced image may be a poor representation of the original. Furthermore, enhancement algorithms cannot be expected to provide information that is not present in the original image. If the image does not contain the feature to be enhanced, noise or other unwanted image components may be inadvertently enhanced without any benefit to the user.

In this chapter we present established image enhancement algorithms commonly used for medical images. Initial concepts and definitions are presented in Section 2. Pixel-based enhancement techniques described in Section 3 are transformations applied to each pixel without utilizing specifically the information in the neighborhood of the pixel. Section 4 presents enhancement with local operators that modify the value of each pixel using the pixels in a local neighborhood. Enhancement that can be achieved with multiple images of the same scene is outlined in Section 5. Spectral domain filters that can be used for enhancement are presented in Section 6. The techniques described in this chapter are applicable to dental and medical images as illustrated in the figures.

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