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Photoshop for Radiologists |
1 Department of Radiology, Children's Hospital, 300 Longwood Ave., Boston, MA 02115.
Received July 10, 2002;
accepted after revision July 22, 2002.
Address correspondence to G. A. Taylor.
Introduction
 Top Introduction Contrast Improvement ToolsConclusionReferences |
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TopIntroductionContrast Improvement ToolsConclusionReferences |
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Levels
Two "Levels" tools—"Levels" and "Auto
Levels"—may be accessed via the pull-down menu under
"Image." After selecting "Image," select
"Adjust," and then select either "Levels" or
"Auto Levels." (Fig.
1A).
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The "Levels" dialogue box consists of a histogram showing the relative distribution of density values in the image. It allows the user to change the dynamic range of the image analogous to window and level controls on an imaging workstation. The outermost arrowhead markers denote the window width of the values displayed, and the center marker represents the midpoint of densities displayed (window level). The highest (brightest) pixel value depicted is controlled by the light gray arrowhead to the right of the histogram, and the lowest (darkest) value is controlled by the dark gray arrowhead to the left of the histogram. The effect of "Levels" correction can be seen in Figures 1A and 1B. Figure 1A shows the relatively low image contrast present in the original scanned image and its accompanying histogram. Note that the outermost markers are at a distance from the brightest and darkest pixels in the image, and the midpoint level is set slightly to the right of the histogram center. By bringing the outermost arrowhead markers to the edges of the histogram and the midpoint marker closer to the center of the histogram, the image contrast is markedly improved (Fig. 1B).
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"Auto Levels" is, as the name suggests, an automatic image-correction tool that is useful for quick and relatively straightforward corrections in contrast. The "Auto" function essentially makes the darkest pixels in the image become black and the brightest pixel become white, and then it spreads out the intervening density values evenly between them [2]. The effect of "Auto Levels" correction can be seen in Figure 1C. This figure is similar to Figure 1B, except that Figure 1C does not depict the reverberation artifact in the dilated third ventricle as well. The difference lies in the location of the midpoint arrowhead marker on the histogram. In Figure 1B, the midpoint was arbitrarily placed toward the denser pixels, whereas "Auto Levels" placed the midpoint in the center of the histogram.
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In general, I begin image contrast correction with "Auto Levels" and fine-tune the image with the "Levels" tool as needed.
Curves
The "Curves" tool may be accessed via the pull-down menu under
"Image." After selecting "Image," select
"Adjust," and then go to "Curves." The graphic
interface is based on an x-y axis. The y axis represents the
range of density values present in the image, and the x axis
represents how each of those values is displayed as an output
[3]. In the case of electronic
images, the graphic interface reflects the densities displayed on the monitor.
The default curve shows a straight linear correlation between density values
and their display. In gray-scale mode, darker pixels are represented by the
upper part of the curve and lighter pixels by the lower part. By shaping the
curve, one can affect a specific range of density values without affecting
others.
The first step in using the "Curves" tool is to sample the image to determine which pixel densities need to be changed. Select the "Eye Dropper" tool from the tool palette (Fig. 1D), place it over a light or dark area of the image, and click. The density of the selected pixel will be shown as a small circle along the curve. In Figure 1D, the "Eye Dropper" was placed over a relatively dark area, and the pixel density is represented by a circle on the upper third of the curve. The curve can be reshaped by clicking on a portion of the curve and dragging it up or down, depending on the change required. Figure 1E shows a reshaped curve and its effect on the image. The curve now has a sigmoid shape, in which the upper curve (representing darker pixels) is shifted upward, and the lower curve (representing lighter pixels) is shifted downward relative to the original curve. Note that the mid portion of the curve (representing middle gray pixels) has not been affected. The resulting image contrast is much closer to that of the analog original.
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When modifying a curve, it is important that the changes be smooth and gradual. A jagged or sharply angled curve will result in a degraded, unacceptable image.
Other Controls
Two other contrast adjustment tools are available under
"Adjust" in the "Image" menu
(Fig. 1D):
"Brightness/Contrast" and "Auto Contrast." Both may be
used as a quick fix for contrast problems. However, I rarely use them because
they tend to overadjust the image. "Levels" and
"Curves" give the user much finer control over the final
product.
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TopIntroductionContrast Improvement ToolsConclusionReferences |
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The next article in this series will cover methods of removing dust and scratches from the scanned image.
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TopIntroductionContrast Improvement ToolsConclusionReferences |
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