Biomedical Engineering Reference
In-Depth Information
Figure 6.3-1 (a) Original image as seen on a poor-quality CRT-type display. This image has poor contrast, and details are difficult to
perceive d especially in the brighter parts of the image such as in areas with high tooth density or near filling material. (b) The nonlinearity of
the display is reversed by the transformation, and structural details become more visible. Details within the image such as the location of
amalgam, the cavity preparation liner, tooth structures, and bony structures are better visualized.
a linear intensity wedge is imaged, one can obtain a test
image that captures the complete intensity scale of the
image acquisition system. However, an intensity mea-
surement device that is linear is then required to assess
the output of the display system, in order to determine
its actual nonlinear characteristics.
A slightly exaggerated example of this type of
a transform is presented in Fig. 6.3-1 . Figure 6.3-1a
presents a simulated CRT display with a logarithmic
characteristic. This characteristic tends to suppress the
dynamic range of the image decreasing the contrast.
Figure 6.3-1b presents the same image after an inverse
transformation to correct for the display nonlinearity.
Although these operations do in principle correct for the
display, the primary mechanism for review and analysis
of image information is the human visual system, which
is fundamentally a nonlinear reception system and adapts
locally to the intensities presented.
known to define the intensity band of interest, a scaling
transformation may be defined as
e ¼ f 1 f f 2
0 otherwise
g ¼ e f 1
f 2 f 1
$
ðf max Þ;
where e is an intermediate image, g is the output image,
and f max is the maximum intensity of the display.
These operations may be seen through the images in
Fig. 6.3-2 . Figure 6.3-2a presents an image with detail in
the intensity band from 90 to 170 that may be of interest
to, for example a gum specialist. The image, however, is
displayed such that all gray levels in the range 0 to 255 are
seen. Figure 6.3-2b shows the histogram of the input
image and Fig. 6.3-2c presents the same image with the
90-to-170 intensity band stretched across the output
band of the display. Figure 6.3-2d shows the histogram of
the output image with the intensities that were initially
between 90 and 170, but are now stretched over the
range 0 to 255. The detail in the narrow band is now
easily perceived; however, details outside the band are
completely suppressed.
6.3.3.2 Intensity scaling
Intensity scaling is a method of image enhancement that
can be used when the dynamic range of the acquired
image data significantly exceeds the characteristics of the
display system, or vice versa. It may also be the case that
image information is present in specific narrow intensity
bands that may be of special interest to the observer.
Intensity scaling allows the observer to focus on specific
intensity bands in the image by modifying the image such
that the intensity band of interest spans the dynamic
range of the display [14, 16] . For example, if f 1 and f 2 are
6.3.3.3 Histogram equalization
Although intensity scaling can be very effective in en-
hancing image information present in specific intensity
bands, often information is not available a priori to
identify the useful intensity bands. In such cases, it may
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