Fluke Biomedical 8000 Victoreen - Manual (Page 20)

Victoreen 8000
Operators Manual
2-8
percentage of kV, use of a measurement delay can also introduce errors. If the delay terminates between
pulses, when no x-rays are present, the NERO mAx waits until the beginning of the next pulse to begin
timing. If the delay terminates during a pulse, when x-rays are present, the NERO mAx begins timing
immediately.
When a delay is used in making measurements in the Radio and Mammo Modes, the NERO mAx
calculates exposure time differently. If a positive measurement delay is used in making a measurement,
the NERO mAx includes the delay time in its calculated exposure time. In addition, the exposure time
may be slightly longer than the exposure time between the selected %kV points on the kV waveform.
This is because the Model 8000 does not store the leading edge of the x-ray output waveform; it waits for
the specified delay time before acquiring kVp data. Because of this, the %kV point on the rising edge of
the kV waveform is not stored and the rise time of the waveform appears to be instantaneous. The
difference between the actual and measured exposure times is the rise time between zero and the
specified %kV. When a negative measurement delay is specified, the NERO mAx does not include the
delay time in the measured exposure time. The measured exposure time is from the beginning of data
acquisition or the %kV point on the first rising edge after the delay until passage through the %kV point on
the falling edge of the kV waveform.
More information on using the %kV function or using a measurement delay may be found at the end of
the Radio Mode section under "Using %kV and Exposure Time Measurements" or "Using a Measurement
Delay".
Exposure and Exposure Rate
The NERO mAx measures exposure by integrating the signal from an ion chamber (either internal or
external) and applying the proper conversion factor(s) to calculate exposure (R or Gy). Exposure rate is
calculated by sampling the integrated charge from the ion chamber at one-second intervals and dividing
the integrated charge by the sample period and applying the proper conversion factor(s). In the Pulsed
Fluoro and AMSE modes, the NERO mAx calculates exposure per pulse or frame by sampling the
integrated charge from the ion chamber at one second intervals and dividing by the number of pulses or
frames that it counts in a one second interval and applying the proper conversion factor(s). The NERO
mAx's ion chamber is internally vented and all exposure and rate measurements are corrected for air
density based upon user entered temperature and pressure.
The NERO mAx’s internal ion chamber is factory calibrated to provide accurate exposure and rate
measurements over the entire kVp range of the NERO mAx. These exposure and rate measurements
are adjusted by applying energy dependent correction factors that are determined by comparison to
applicable N.I.S.T. techniques.
In addition to the factory calibration, a user entered multiplier is available for exposure and rate
measurements made with the NERO mAx’s internal ion chamber.
When using external ion chambers, exposure and rate are calculated using the user entered calibration
factor (R/nC or Gy/nC) for the selected chamber. When making measurements in the CT Exposure mode
with a CT chamber, user entered beam width is also used in calculating exposure and rate.
mAs and mA
The NERO mAx measures mAs by integrating the signal from the mAs input during an x-ray exposure.
To calculate mA, the integrated signal from the mAs input is sampled at one-second intervals and divided
by the sample period. In the Pulsed Fluoro and AMSE modes, the NERO mAx calculates mAs per pulse
or frame by sampling the integrated signal from the mAs input at one second intervals and dividing by the
number of pulses or frames that it counts in a one second interval. The mA(s) circuitry is factory
calibrated using a calibrated current source and is not user adjustable.
HVL
In the HVL mode, the NERO mAx calculates half value layer based upon a series of exposure or rate
measurements made with varying thicknesses of aluminum absorbers placed in the x-ray beam. The
exposure or rate measurements may be made using the NERO mAx’s internal ion chamber or an external