AJR 2003; 181:43-45
© American Roentgen Ray Society
Photoshop for Radiologists |
Sharpening the Image
George A. Taylor1
1 Department of Radiology, Children's Hospital, 300 Longwood Ave., Boston, MA
02115.
Received September 20, 2002;
accepted after revision October 3, 2002.
Address correspondence to G. A. Taylor.
Introduction
Conversion of an analog slide to an electronic file is a multistep process.
During this process, images often lose some of the detail held in the original
image. This loss of detail may be the result of less than optimal scanner
settings or compression settings used while saving the image. In this
installment of "Photoshop for Radiologists," I describe two
methods of sharpening an image using Photoshop (Adobe Systems, Mountain View,
CA) for insertion into an electronic presentation.
Unsharp Mask
The name of this tool is misleading and originates from predigital printing
house processes for contact negative printing that used frosted Mylar (DuPont,
Wilmington, DE) or film (an "unsharp mask") between the original
negative and the masking film to increase the sharpness of a contact
[1]. This Photoshop filter
looks for edges and large density differences and accentuates these findings,
which results in a sharper image. Although general guidelines for using this
filter are available, experimentation is needed to get it just right. For best
results, sharpen the image when viewing the image at 100% (1:1). This view
allows you to see the actual image data and achieve better control of the
filter settings [1].
Begin by accessing the "Unsharp Mask" tool from the
"Filter" menu of the Photoshop marquee (Filter 
Sharpen
Unsharp Mask) (Figs. 1A
and 1B). The "Unsharp
Mask" dialogue box consists of three slider bars that control
"Amount," "Radius," and "Threshold"
settings. If these sliders are set in different positions, move each slider
all the way to the left end of the bar. Begin to sharpen the image by moving
the "Amount" slider to the right to a value of between 50% and
130%. This value determines the amount of "sharpening" to be
applied to the image. Next move the "Radius" slider toward the
right to a value of between 0.1 and 2 pixels. This value controls the width of
the edge contrast effect. Pushing the "Radius" slider too far to
the left will result in noticeable dark and light halos along the edges of the
image. The "Threshold" scale tells the program to ignore adjacent
tones that are similar. For example, a setting of 5 tells Photoshop to ignore
all tones within 5 level values of each other
[1]. The
"Threshold" slider should generally be set at a value of 1–2
for medical images. Sharpening should be applied only in minimal amounts to be
effective. Oversharpening results in a coarse, noisy image
(Fig. 1C).
View larger version (102K):
[in this window]
[in a new window]
[as a PowerPoint slide]
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Fig. 1B. —Use of "Unsharp Mask" tool in Photoshop (Adobe
Systems, Mountain View, CA). Select the settings for "Amount,"
"Radius," and "Threshold" using sliders
(arrows). Move "Amount" slider to right to setting of
100%, move "Radius" slider to setting of 2 pixels, and move
"Threshold" slider to left at setting of 0 levels. Click
"OK." Resulting sharpened sonogram image is shown on right.
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View larger version (100K):
[in this window]
[in a new window]
[as a PowerPoint slide]
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Fig. 1C. —Use of "Unsharp Mask" tool in Photoshop (Adobe
Systems, Mountain View, CA). Example of oversharpened image.
"Amount" slider has been set at 150%, and "Radius"
slider (arrows) at 5 pixels. Resulting image on right is coarse and
noisy.
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High-Pass Filter
Even when applied sparingly, the "Unsharp Mask" tool tends to
sharpen and enhance noise in areas with little image content (e.g.,
gastrointestinal contrast on an abdominal CT scan). The
"High-Pass" filter, another sharpening tool, lessens this effect
by concentrating on enhancing larger, more visible edges.
The "High-Pass" filter works by turning all nonedge areas to
neutral gray and leaving the edges intact
[1]. To use this filter, open
the "Layers" window (Window Show Layers), and duplicate the
background layer by dragging the background layer to the "Bent
Page" icon at the bottom of the "Layers" window
(Fig. 2A). Select "High
Pass" from the "Filter" menu (Filter Other
High Pass), and use the "Radius" slider to bring out the image
edges. For medical images, a setting of between 1.5 and 2.5 generally works
well. Click "OK" to activate the filter. Change the blending mode
of the filtered layer from "Normal" to "Overlay" or
"Soft Light" (Fig.
2B) to make the neutral gray disappear, while preserving the edge
enhancement [1]. If the image
is too sharp, decrease the opacity of the filtered layer until you see the
desired effect. Once you finish fine-tuning this filter, merge the layers by
selecting "Flatten Image" (Fig.
2C). Save the final image as a JPEG (Joint Photographic Experts
Group) file, and import the file into PowerPoint (Microsoft, Redmond, WA) as
described previously
[2–4].
View larger version (70K):
[in this window]
[in a new window]
[as a PowerPoint slide]
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Fig. 2B. —Use of "High-Pass" filter in Photoshop (Adobe
Systems, Mountain View, CA). Baseline "High-Pass" filter dialogue
window is shown in left upper corner. Note "Radius" setting is at
minimal setting of 0.1 pixels (upper arrow). "Radius"
slider is moved to setting of 1.9 pixels (lower arrow). Resulting
background copy layer image on right shows all nonedge areas have turned to
neutral gray, and edges are barely visible.
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View larger version (101K):
[in this window]
[in a new window]
[as a PowerPoint slide]
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Fig. 2C. —Use of "High-Pass" filter in Photoshop (Adobe
Systems, Mountain View, CA). Blending mode in "Layers" window is
changed from "Normal" to either "Overlay" (black
arrow) or "Soft Light." Resulting image blended image is
shown on right. Image layers are merged by clicking on arrowhead on top
marquee of "Layers" window (white arrow) and choosing
"Flatten Image."
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Conclusions
It is important to apply these filters when the image is in its final file
size. Filtering done on a smaller image that is enlarged will look
unacceptably noisy and oversharpened. Conversely, reducing the size of the
final image can negate the sharpening effects.
During the preparation of this series of articles, I came across an
excellent primer on basic scanning and image editing techniques by Corl et al.
[5]. It is an excellent review
for anyone wishing to understand the basis for scanning, increasing image
resolution, resizing images, adding labels, and so on. Corl et al. address
what is and is not acceptable in manipulating images for scientific
presentation. I want to emphasize that the techniques discussed in this and
previous "Photoshop for Radiologists" articles should only be used
to return the digital image to its original appearance and not to
intentionally alter or distort the conspicuity of findings shown by a medical
image.
In the next article, I will discuss techniques for color correction of an
image before saving the final version for use in an electronic
presentation.
References
- Eismann M. Sharpen this: Photoshop layer blending.
Photo>Electronic Imaging2001; 44:26
–30
- Taylor GA. Initial steps in image preparation.
AJR 2002;179:1411
–1413[Free Full Text]
- Taylor GA. Improving image contrast. AJR2003; 180:329
–331[Free Full Text]
- Taylor GA. Removal of dust and scratches from electronic image
files. AJR2003; 180:1533
–1536[Free Full Text]
- Corl FM, Garland MR, Lawler LP, Fishman EK. A five-step approach to
digital image manipulation for the radiologist.
RadioGraphics2002; 22:981
–992[Abstract/Free Full Text]
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Introduction
Unsharp Mask
