Fluke Biomedical 8000 Victoreen - Manual (Page 44)

Victoreen 8000
Operators Manual
2-32
2.5.7 HVL Mode
NOTE
In the HVL mode, the HVL report is printed only
after the mode key is depressed or 10 exposures
occur.
In the HVL mode, the NERO mAx calculates half value layer based upon a series of exposure or rate
measurements made with varying thickness’ of aluminum absorbers placed in the x-ray beam. A
minimum of two measurements are required and up to ten may be used. The exposure or rate
measurements may be made using the NERO mAx’s internal ion chamber or an external ion chamber.
Exposure and rate are calculated using the selected ion chamber’s calibration factor and when using a
CT chamber, beam width. For more information about entry and storage of ion chamber calibration
factors, see CAL mode. For more information about selecting exposure/rate units, see SETUP mode.
The default absorber thickness is 3.30 mmAl except when the Victoreen mammo chamber has been
selected. The default absorber thickness when using the mammo chamber is 0.30 mmAl. The HVL filter
thickness can be incremented or decremented from the default value by pressing the up or down keys.
Exposure and rate measurements made with the NERO mAx’s internal ion chamber, where kV is not
measured, are calculated using the ion chamber’s 80 kV calibration factors. In measurement modes
where kV is measured, exposure and rate are calculated using calibration factors based upon the
measured kV. In the HVL mode, no kV measurements are made, so a constant is used to calculate
exposure and rate. The constant that is used is based upon the calibration factors corresponding to a
measured kVp average of 80 kVp. A user entered calibration factor (multiplier) allows the user to adjust
the calibration of the internal ion chamber. For more information about entry and storage of ion chamber
calibration factors, see CAL mode.
HVL is calculated using a linear regression of the natural log of the normalized exposure (normalized to
the first exposure) versus absorber thickness in millimeters of aluminum. In addition, the correlation
coefficient (“r”) of the exposure data is calculated to provide an indication of how well the data fits the
linear regression. A poor fit (r < 0.98) is indicated by a flashing “r” value. The first exposure of the series
is made with no HVL absorber in place. All subsequent exposures must be made with an aluminum
absorber in the x-ray beam. The absorbers do not need to be placed in the beam in any order of
thickness. Below is the formula used by the NERO mAx to calculate half value layer.
( )
B x A HVL
+ ∗ =
ln
Where: x = Normalized dose at Half Value Layer (0.5)
A = Slope of ln(x) vs. mmAl
B = Intercept of ln(x) vs. mmAl (Usually very close to zero)