|
Introduction
The topic of discussion in the previous
column was the generation and storage of a digital image based
on the sampling of an analog phenomenon, i.e., the computer
INPUT. Conventional monochrome output was assumed, i.e., the
absence of a signal is represented by black, a strong signal
(high amplitude) by white and intermediate strength signals
by shades of gray. We assumed that the mapping would be linear
in the digital domain and logarithmic in the sound domain
(decibels). In this issue, we concentrate on the visual display
or computer OUTPUT, with emphasis on color display and noting
that shades of gray are specific shades of color.
The image OUTPUT is the mapping or representation
of a number at each point in an image (pixel) to visible light
intended for our eye. The range of numbers stored in an image
matrix is typically limited to 2 to the 8th, 2 to the 16th,
and 2 to the 24th. The output or image display is produced
by coupling each of the possible numbers in a digital image
to a voltage applied to the acceleration grid of a single
gun Cathode Ray Tube (CRT). A conventional approach will map
the number 0 in an 8-bit image to 0 volts on a CRT and the
number 255 to the voltage required for maximum practical brightness
on the CRT. However, there is generally no limitation on how
this mapping is done, and we might instead suppress low signal
strength ultrasound noise by mapping many small numbers, instead
of just 0, to black.
If we use a three-gun (Red, Green, Blue)
color CRT instead of a single-gun grayscale model, we can
map the value of a pixel to all three guns equally and produce
the same grayscale image produced by the single gun system.
Or, we could be more creative and map numbers from the image
matrix to different intensities on the Red, Green and Blue
guns to produce a rainbow of colors to dazzle the eye and
justify the machine to the administration. This kind of mapping
has typically been met with enthusiasm similar to the colorization
of Ansel Adams' masterpieces. Color mapping without a sound
physical basis is generally viewed as extraneous noise confusing
the interpretation of an image. Apart from aesthetic considerations,
and potential distraction, use of color can in fact be deceptive.
When we map a grayscale image, we often do it linearly. When
color is used to display continuous data, mapping becomes
an issue. At some point along the continuum of data, increasing
the brightness of one color (representing one physical parameter)
transitions to display a different color, implying a second
physical parameter or a large change in intensity value i.e.,
a "border." At what point along the continuum of
data does red change to blue, and what "borders"
do these two colors represent? It is easy to see how this
could be confusing and used incorrectly.
However, one use of color that has significant
clinical value is the representation of a second parameter,
i.e., fluid flow, with overlaid on a traditional grayscale
representation of acoustic impedance. Fluid flow is measured
by its Doppler Effect or the change in frequency of a ultrasound
wave reflected from a moving object. The Doppler ultrasound
image is a separate image from the static echo ultrasound
image. A Power Doppler monochrome image, usually orange, is
combined with the acoustic impedance image, usually gray,
for display. This provides the visualization of two physical
parameters (acoustic impedance and fluid flow) in one image.
If the direction of the fluid flow were important, separate
colors could be assigned to blood flow toward or away from
the transducer. Being able to distinguish blood vessels from
ductal and cystic structures is valuable during ultrasound
directed aspirations of lesions as it helps the physician
avoid puncturing blood vessels and minimizes bleeding complications.
References
1. Moving Theory into Practice, Digital
Imaging Tutorial http://www.library.cornell.edu/preservation/tutorial/
Cornell University Library.
2. Kruenen Ben. Big Ben?s Digital Imaging Tutorial http://www.bigbenpublishing.com.au/digital/
3. Bouton, DG, Bouton, B, Kubicek, G, Nathanson, MZ, Rich
J, Ward A. Inside Adobe Photoshop 6, Limited Edition,
2001, Bouton, DG (editor) New Riders Publishing,
Indianapolis, IN.
|
