Contrast

Contrast can have many different meanings even in radiography.  I will be describing image contrast.

First let’s define image contrast:  Image contrast is the difference between adjacent densities.  The adjacent densities can range from white to black and various shades of grey in between.

When describing contrast of adjacent densities that are greatly different, such as a density of .25 (or a white density) and a density of 2.5 (a black density) these two densities adjacent to each other have a high contrast.  There are little or no shades of grey.

When describing contrast of adjacent densities that are similar in shade, such as a density of 1.0 ( a gray density) and a density of 1.25 ( another shade of gray) these two densities adjacent to each other have a low contrast.  They are similar in color, or have many shades of gray.

High contrast films have few shades of gray, Low kVp technique, short scale of contrast, have a short or narrow window width.

Low contrast films have many shades of gray, High kVp technique, long scale of contrast, have a large or wide window width.

The controlling factor has the most direct effect on the image.  Kilovoltage peak (kVp) is the controller of contrast.  As kVp increases contrast decreases.

Influencing factors can also effect contrast.  These include mAs, Focal spot size, anode heel effect, distance, filtration, beam restriction, anatomical part, image receptor, processing, and the use of grids.

The 15% rule.  In order to make a visible difference in contrast on a radiograph you must adjust the kVp between 4%-12% .  If the radiograph is outside acceptable limits at least 8%-15% change must be made.  This is how the 15% came about.  We tend to use higher kVp low mAs techniques, especially if you are using digital radiography.  When using higher kVp you must use the higher percentage for adjustment.

Physical contrast:  The total amount of densities recorded on the image.

Visible contrast:  The the total range of densities that can be seen by the human eye.

We must be careful not to under, or overexpose our film.  Having density values that are under exposed (in the toe of the D log E curve) or density values that are over exposed (in the shoulder of the D log E curve) will decrease the film density.

Anatomical Part:  If you are photographing a large anatomical part you will have more scatter to your film this will reduce the contrast on your film.  Part size is inversely proportional to film contrast.  Increased part size, decreased film contrast.

Film with low contrast                           Film with High contrast

Film with high contrast

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