NIRStar 14.1 - User Manual
Page 53 of 124
Gain Map
The gain-settings results are displayed, color-coded, as a Gain Map. Recall that each step represents a 10-
fold increase in amplification. There is an inverse relationship between gain and the intensity of the
received light: the weaker the light signal, the more it must be amplified. For typical source-detector
separations (2.5 – 3.0 cm), gains of 4 and higher are expected. Gain stages lower than 4 occur when the
level of light attenuation is relatively low. This can occur when with short source-detector separations,
measures performed on the static phantom supplied with the NIRx imagers, or with measures performed
on the forehead. Channels for which the maximum gain level of 7 does not provide sufficient signal
strength to reach the recommended signal level are marked red. In such cases, it is advised that you
check and possibly improve the optode-skin contact. The gain settings for a chosen source-detector
layout generate a characteristic pattern, which should be qualitatively reproducible across different
subjects.
Signal Level
While the gain for a particular channel is the same at both wavelengths, signal levels and noise ratios can
be inspected for each wavelength separately. The user can choose the respective quantity by selecting
the appropriate radio buttons.
The Signal Levels are mapped, on a logarithmic scale, in units of voltage. For each channel, the displayed
quantity is the average signal value obtained during the short scan that follows the gain setup.
Noise
The quantity used as a measure of each channel’s Noise is the Coefficient of Variation (CV = Standard
Deviation / Mean * 100) of the data values recorded used to evaluate the signal levels. Because the
Quality Scale operates on unfiltered data, a SNR of around 1% (orange on the old-style scale) is typical
for physiological setups, owing to light-signal fluctuations associated with the cardiac pulsation.
Color scheme
By default, the color scheme is displayed as ‘Quality Scale’, which is graded into the ‘traffic light’ scheme
(Excellent, Acceptable, Critical and Lost) outlined on section 8.1 so the user can examine whether a
channel reaches a specific quality threshold for that metric. By deactivating the ‘Quality Scale’ selector,
the old color scheme, as present on older versions of NIRStar, will be displayed as shown in Figure 36.
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