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a Chapter 6 Sonographic Color Flow Mapping: Basic Principles 73
achromatic colors and include black, gray, and white.
The chromatic colors are further classified into
groups according to their hue. All hues of red are
grouped together, all blues are together, and so on,
resulting in a continuous circle of overlapping hues.
The human eye is incapable of differentiating between
two superimposed primary hues, which led to
the discovery that it was possible to produce any given
color using a combination of three primary colors.
The selection of the three primary hues is arbitrary.
Red, blue, and green colors are used in color video
displays, including color flow mapping, whereas artists
use red, blue, and yellow pigments as their three
primary colors.
The next characteristic of light for color classification
is luminance, which is the brightness: Some of
the chromatic colors of a single hue are darker or
lighter than others, analogous to the degrees of gray
of the achromatic colors. This classification is known
as luminance or brightness.
The third property for color grouping is saturation,
which indicates the combination of a hue of particular
brilliance with an achromatic color of the
same brightness. The consequent light stimulus depends
on the relative amount of the chromatic and
achromatic components in which the latter has 0 saturation
and the former has a saturation value between
0 and 1.0.
Color Perception
Color is the perception generated by the stimulus of
light falling on the retina of the human eye. The eye
can distinguish a wide range of gradations of hue
and saturation. In contrast, the ability to differentiate
various grades of luminance is relatively limited.
Therefore refined appreciation of color maps is
achieved better with hue and saturation than with
luminance. Hues with higher luminance are better
perceived by older observers.
In regard to the impact of color blindness on a diagnostician's
ability to assess color flow mapping, it
should be recognized that about 8.0% of men and
fewer than 0.5% of women suffer from varying degrees
of deficiency of color perception. Total absence
of color vision is rare. Anomalopia is partial color
blindness, in which both red and green are poorly
recognized. Most color-blind persons find it easier to
recognize saturation characteristics of a color flow
map because saturation involves mixing of achromatic
colors with chromatic hues.
Color Encoding of Doppler Flow Signals
Hemodynamic attributes of the interrogated blood
flow are expressed by color encoding of the mean
Doppler shift signals. These attributes are the direction
of flow in relation to the transducer, the magnitude
of velocity, variance of the measured mean parameter
(mean Doppler shift), and the amount of
scattering power (the amplitude or energy or power
of the Doppler shift).
Color Mapping of Direction of Flow
The directionality of flow in relation to the transducer
is depicted in the primary colors of red and blue.
With most Doppler color flow systems, the default
mode depicts the flow toward the transducer as red
and the flow away as blue.
Color Mapping of Velocity
and the Color Bar
The magnitude of the flow velocity is qualitatively expressed
by assigning levels of luminance to the primary
hue. The highest velocity flow is depicted by
the most luminance and the lowest velocity by the
least luminance or black. The highest measurable
velocity toward the transducer is shown in the most
brilliant red, and the highest measurable velocity
away from the transducer in the most brilliant blue.
The highest limit of the velocity unambiguously measurable
by a Doppler color flow mapping system,
based on the pulse Doppler interrogation, is the
Nyquist limit. As noted before (see Chap. 3), this limit
depends on pulse repetition frequency and the depth
of Doppler interrogation. The lowest measurable
velocity in a color flow mapping device depends on a
multiplicity of factors and varies from device to device.
The calibration of the velocity magnitude as a
function of luminance gradation is displayed graphically
on the video screen as a color bar or circle
(Fig. 6.5). It is customary to display the magnitude of
the Doppler shift on the vertical axis and the variance
(see below) on the horizontal axis. The Doppler shift
may be displayed as either frequency or velocity. The
velocity information is merely an approximation, as
the Doppler angle of insonation is not measurable in
color flow mapping. For color flow mapping, it is
usually assumed that this angle is zero. The scales are
not quantitative, and a particular level of luminance
does not reflect a specific single value of Doppler
shift but, rather, a range of values. In addition, the
color bars of the devices do not have any uniformity
of scale for depicting the magnitude of Doppler shift,
as the corresponding levels of brightness are assigned
arbitrarily and on a nonlinear scale. The baseline,
which indicates zero frequency shift, divides the color
bar into positive and negative frequency shifts. The
baseline is indicated as a black band on the color bar,