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Enhancing Radiograph Images

A raw image, such as a radiograph, may need to be adjusted to make it easier to see a specific feature such as caries or an abscess. You can not do this with a film radiograph, and this is a major advantage of using digital radiographs. 

This page discusses some of the terminology used when scanning or viewing digital radiographs. To set default scanning values in Open Dental, see Scanning Defaults / Imaging Setup. To scan radiographs, see the Images module.

Pixels
An image is made up of many pixels, or dots. Each pixel can have a value between black (0) and white. The numeric value of white depends on the situation. If you are using 8 bits, white is 256, or for 12 bits, white is 4096. In the examples below, we use 1 as the value for white, with the various gray levels between 0 and 1.

Below is a graph representing no enhancement. The x axis represents the color (gray level) of each pixel input from the raw image, and the y axis represents the resulting output in the enhanced image. As you can see, for any value that is input, the exact same value will be output. The red shows how an input of .5 results in an output of .5. So this graph is the basis against which all enhancements will be compared.

Contrast and Brightness
Contrast is the steepness of the line; the steeper the line, the more contrast. Brightness is represented by a horizontal shift in the line without changing the contrast.

Gamma
Gamma is the curve in the line. It is input to the power of the gamma value, or f(x)=x^gamma. Enhancing an image with gamma results in a pleasing appearance because the human eye perceives color in a non linear manner. A gamma of 2.2 is shown in the graph below. An input pixel value of 0.5 results in an output of 0.22. So overall, the image is a little darker, and in the lighter range, there is more contrast as shown by the steeper part of the curve. CRT monitors usually have a little too much gamma (2.5) due to technical limitations, and the images need to have the gamma adjusted down, either through hardware or software. LCD monitors have a somewhat irregular output curve. Gamma is more useful in color photos than in black and white images like radiographs.

Windowing
Windowing is an alternative to contrast and brightness that is very useful for images like radiographs.

Windowing

This is the Open Dental slider control that lets the user set the windowing values (Imaging Setup, Images module):

The line is steeper (more contrast) over a specific range of input values. This results in a smaller window of values which are spread into the output so they can be seen more easily. A window has a lower bound and an upper bound. All input values below the lower bound are output as black, and all values above the upper bound are output as white. This is a wonderful way to enhance a radiograph because you are usually interested in very subtle changes in a small range of gray values. Moving the window left and right isolates the gray levels you are interested in. The slider should affect the entire image in real time for it to be useful.

Contrast Limited Adaptive Histogram Equalization
This is an automated way of enhancing the contrast and brightness. A histogram is simply a count of how many pixels are in the image for each grayscale level. If the image is too dark or too light, then this technique will shift the pixels towards a more neutral shade. And if there is not enough contrast, then it will spread out the shades similarly to windowing. But it's all automated and based on localized regions of the image rather than just treating the image as a whole. This technique is used by Schick and a few others. It is not used by Open Dental.

 

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