Fluke Biomedical 8000 Victoreen - Manual (Page 68)

Victoreen 8000
Operators Manual
4-6
If the NERO mAx reports “Low” kV or if the measured kV is much lower than the actual kV, one of its
solid-state detectors may have failed. To easily diagnose this condition, perform the calibration check as
described in Section 3.2 - Calibration Check. The ratios normally displayed after an exposure are
approximately 2 and if the ratios displayed are near 1, one of the NERO mAx solid-state detectors has
failed and must be replaced. Please call Fluke Biomedical at 440.248.9300 for assistance.
In the pulsed fluoro mode, overshoots can occur on the leading edges of the pulses. This is usually
indicated by a significant difference between the peak or average kVp and the effective kV. To determine
if this is the case, use the NERO mAx Excel Add-In to extract the kV waveform from the NERO mAx for
examination.
Blank screen after exposure starts in Pulsed Fluoro mode
This happens when pulsed fluoro mode is incorrectly used on a continuous fluoro generator. In the
pulsed fluoro mode, the NERO mAx calculates exposure and mAs per pulse; if no pulses are detected,
exposure and mAs per pulse cannot be calculated. This may also happen in high sensitivity if the x-ray
intensity is excessive and drives the detector into saturation. To cure this, switch to low sensitivity.
4.2.4 AMSE Mode
For an explanation of error messages, see Section 4.3--Error Messages.
All of the explanations of perceived anomalies that may occur in the Fluoro mode also apply to the AMSE
mode. Please refer to those listed for the Fluoro mode for assistance.
4.2.5 Exposure Mode & CT Exposure Mode
The measured exposure is affected by the user entered ion chamber calibration factor and air density
correction factors. Make sure that the selected ion chamber is the same as the chamber that is in use.
Also make sure that the ion chamber’s correction factor has been entered accurately and has not been
changed by accident (see Cal Mode). In addition, verify that temperature and pressure have been
correctly entered (defaults are 22°C, 760 mmHg., see Setup Mode). For more information regarding
NERO mAx calibration see Section 3--Calibration.
4.2.6 HVL Mode
The reproducibility of the x-ray machine can directly affect the accuracy of HVL measurements made with
the NERO mAx. If an x-ray machine has poor reproducibility of time, mA or kV, the calculated HVL may
be in error.
Another possible source of error in HVL measurements may be incorrect absorber thickness. Make sure
that the aluminum absorber thickness is accurate and that the thickness is entered correctly into the
NERO mAx.
The NERO mAx calculates HVL via linear regression of the natural log of the normalized exposure
(normalized to the first exposure) versus absorber thickness in millimeters of aluminum. To provide an
indication of how well the exposure data fits the natural log of the normalized exposure the NERO mAx
calculates the correlation coefficient (“r”) of the data whenever it calculates HVL. A poor fit (r < 0.98) is
indicated by a flashing “r” value. Poor reproducibility in the x-ray exposures or errors in absorber
thickness will result in a low correlation coefficient.
Also, the closer that the added filtration is to the calculated HVL, the more accurate the HVL calculation
will be. To assist in establishing the optimum average HVL filter thickness, the NERO mAx displays either
a “HIGH” or “LOW” message above the HVL if the average of the added filtration for each exposure is not
within 10% of the calculated HVL. If no messages are displayed, the average added filtration is within
10% of the calculated HVL and no more exposures need to be made.
If the calculated HVL is displayed as a negative number, the second exposure is greater than the first
exposure. Remove all HVL plates and start over, making sure that no HVL filters are in place for the first
HVL exposure. The first HVL exposure must be made with no absorbers in the x-ray beam.