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Photoshop for Radiologists |
1 Department of Radiology, Children's Hospital, 300 Longwood Ave., Boston, MA 02115.
Received August 20, 2002;
accepted after revision September 10, 2002.
Address correspondence to G. A. Taylor.
Introduction
 Top Introduction Cloning ToolHistory BrushConclusionReferences |
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In the example provided, there are dust particles throughout the image, but they are best seen over the dilated ventricles (Figs. 1A, 1B, 1C). Dust in the localized area of the ventricle will be removed by cloning that area of the ventricle with an adjacent selection that contains no dust. Begin by choosing the "Clone Stamp Tool" in the "Toolbox" menu and selecting a brush size that is slightly larger than the piece of dust you want to remove. The "Brush" menu is located on the top left-hand side of the Photoshop marquee. Position the pointer of the clone tool over a part of the ventricle that is free from dust. Use "Alt" and click (Windows [Microsoft, Redmond, WA]) or "Option" and click (Macintosh OS, Apple Computer, Cupertino, CA) to sample the region. Click and drag the tool over the dust motes to remove them. As the dust is removed from different parts of the image, you may need to resample dustfree areas and the repeat the process until the appearance of image is acceptable. If an unwanted area of the image is inadvertently cloned (e.g., soft tissue outside the ventricle), simply undo the last step by finding "Undo Clone Stamp Tool" under the "Edit" menu of the marquee.
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Although we removed a few dust motes easily, the image shown in Figures 1A, 1B, 1C is replete with white dust particles scattered throughout the ventricles. The "Clone Stamp Tool" works well for small touch ups but is cumbersome for images that require more extensive cleaning. The "History Brush" is the preferred method for dealing with this situation effectively. Although it requires a few more steps, the results can be rewarding.
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Noise Dust & Scratches) that makes
the removal of unwanted spots easier. Unfortunately, it creates an image that
is soft and often unacceptable. By using the "History" palette and
"History Brush," the "Dust & Scratches" filter can
be selectively applied while maintaining the sharp image details. This
technique has been described by Tapp as "Back to the Future"
[4]. "Back to the
Future" involves three steps: a filter is applied, then a snapshot is
taken of the image in the filtered condition, and finally the filter is
undone, returning the image to its original state. Then the "History
Brush" and "History" state can be used to selectively paint
the "Dust & Scratches" filter only over the areas containing
unwanted spots (Figs. 2A,
2B,
2C,
2D,
2E).
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The "Dust & Scratches" filter can be found in the Photoshop
marquee under "Filter" (Filter Noise Dust &
Scratches). This sequence brings up a dialog box consisting of a preview box
and slider bars for "Radius" and "Threshold" settings.
The goal is to eliminate problem pixels while preserving pixel noise or grain
[3]. Begin by moving the
"Radius" slider to the right until the dust motes have disappeared
from the "Preview" box. Then, adjust the "Threshold"
bar until you see the image grain reappear without seeing dust spots. Click
"OK." The resulting image will likely be too soft to be acceptable
(Fig. 2B).
Next, select the "History" palette by clicking on the
"Window" menu bar (Window Show History). The
"History" palette depicts a record of all actions made on the
image since opening the file. It should have "Dust &
Scratches" as the last action recorded. A snapshot of the image in its
filtered state is created by clicking on the "bent page" icon on
the bottom of the "History" palette. The filtering is then removed
from the image by clicking "Undo Dust & Scratches" under the
"Edit" menu bar (Edit Undo Dust & Scratches). This
sequence returns the image to its original, dusty, unfiltered state. However,
Photoshop has saved a snapshot of how the image had looked while filtered.
This snapshot can be used to remember the filtered state and paint over the
dusty image with the same image in its dustfree, filtered state. The
"History Brush" is selected from the tool palette, and the
painting mode is changed to "Darken." By doing this, you make the
light spots darker, and they disappear. If the image you are retouching has
black or dark dust on a light background, choose "Lighten" as the
painting mode. The final step is to select a brush size appropriate for the
job and brush over the spots with the "History Brush." Brush size
may be varied as needed. The final image
(Fig. 2E) shows that most of
the dust has been successfully removed.
Images already in PowerPoint (Microsoft) files can be processed like any other electronic image (to remove dust or improve contrast) by copying the PowerPoint image you would like to improve and pasting it into a new Photoshop file. The new file will automatically open at the appropriate size to accommodate the pasted image. Before performing any modifications, you should resize the image to a larger size (1200 pixels wide) as suggested in the first Photoshop for Radiologists article [1]. Failure to resize will result in a poor resolution when the image is imported back into PowerPoint.
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In the next article, I will discuss techniques for sharpening an image before final electronic presentation.
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