5: Intraoral Digital Imaging

Chapter 5 Intraoral Digital Imaging

BASIC CONCEPTS

Gray scale resolution is how many shades of gray are in the image; also known as contrast or bit depth. The imaging system is capable of capturing and separating literally thousands of shades of gray. Contrast is expressed in bits. A 1-bit image has only 2 shades (pure black and white—the darkest and lightest shades of gray in the imaging scale) and is expressed as “1 to the power of 1.” A 2-bit image is expressed as “1 to the power of 2,” or 1 × 2 = 2 × 2 = 4 shades of gray. A 3-bit image has 8 shades of gray, or 1 × 2 = 2 × 2 = 4 × 2 = 8. A 4-bit image has 16 shades of gray, and so on. In an 8-bit image, there are 256 shades of gray and this is the standard. However, systems capable of up to 12 bits or 4098 shades of gray presently exist. The more bits in the image, the greater are the storage needs for the images. The human eye of the person in the street can commonly separate 16 shades of gray, a photographer or radiologist can separate about 25 shades of gray, and under laboratory conditions the maximum for the unaided eye to separate is somewhere around 64 shades of gray. The image itself usually does not occupy the entire gray scale as can be seen by viewing the histogram. The image may be confined to about 30 shades of gray. For best results it is desirable to have a system capable of at least 256 shades of gray. This way there is space on the scale to lighten or darken the image (histogram shift) or spread the shades of gray over a bigger part of the scale (histogram stretch). Remember, 8 bits or 256 shades of gray represents the limit of most monitors.

Jan 12, 2015 | Posted by in Oral and Maxillofacial Radiology | Comments Off on 5: Intraoral Digital Imaging
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