Fluke Biomedical 8000 Victoreen - Manual (Page 19)

Operation
Using the NERO mAx
2
2-7
2.3.2 Measured Quantities
kV
The NERO mAx calculates kVp from the ratio between two differentially filtered detector channels (A&B).
When an x-ray exposure is made, the NERO mAx samples the two detector channels simultaneously at a
rate of 100,000 samples per second. The detector waveforms are digitized by a pair of 100 kHz 16 bit
A/D converters and stored in memory.
The NERO mAx has sufficient memory to store up to 480 milliseconds of waveform data. For
radiographic exposures that exceed 480 milliseconds in length, the NERO mAx stores the first 320
milliseconds of the waveform and the last 160 milliseconds. This method allows storage of both the rising
and falling edges of the x-ray waveform. In the fluoro and AMSE modes, the NERO mAx stores a 480-
millisecond sample of the x-ray waveform after the SAVE key is pressed.
A delay of up to 999 milliseconds may be used to delay the start of kV data acquisition in the Radio and
Mammo modes. This delay may be used to skip events that occur at the beginning of an exposure, such
as an overshoot or undershoot. When a delay is used, only the kV data acquisition is effected.
Exposure, mAs and time measurements are not delayed and are measured over the entire exposure.
After the exposure is complete, the NERO mAx searches the stored channel A and B waveforms for ratio
peaks, when a peak is found it is stored. These peaks are then averaged and the average kVp is
calculated. While the NERO mAx searches for ratio peaks, it also looks for the highest peak, when the
highest peak is found it is stored and peak kV is calculated. Effective kV is calculated from the ratio of the
integrated A and B waveforms which is analogous to the density ratio in the kVp film cassette.
When calculating kVp average in the radiographic mode, the NERO mAx only includes peaks that are
above the selected %kV in its kVp average calculation. When ZERO or 1ØPULSE are selected, the
NERO mAx includes all detected peaks in the kVp average calculation.
The NERO mAx calculates kV as a function (F) of the ratio between the two detector channels, A and B
(r). The calculations used to calculate kV peak, kV effective and kVp average are summarized below:
kVp Average = F (r), where r is the average peak ratio B/A
kV Peak = F (r), where r is the peak ratio B/A
kV Effective = F (r), where r is the ratio
∑
B/A
∑
The calibration information that the NERO mAx uses to calculate kV is stored in nonvolatile memory in the
NERO mAx control console.
Time
The NERO mAx measures exposure time by determining the time between the first and last passage
through a preset percentage of kVp average. To accomplish this, the NERO mAx calculates the detector
ratio that corresponds to the preset percentage of the exposure's kVp, then measures the time between
those points on the rising and falling edges of the ratio waveform. In the Radiographic mode, the
percentage of kVp average over which the NERO mAx measures radiographic exposure time may be
selected using the %kV key. In the Mammographic mode, the percentage is fixed at 90% of kVp
average.
In the radiographic mode, when 75%, 80%, or 90% kV is selected, the NERO mAx measures exposure
time between the 75%, 80%, or 90% points on the kV waveform. When zero crossing (ZERO) is
selected, the NERO mAx measures radiographic exposure time from the moment x-rays are detected
until they are no longer detected. When 1ØPULSE is selected, the NERO mAx counts the number of x-
ray pulses in a pulsed or single-phase radiographic exposure. This is primarily for use with single phase
full and half wave rectified generators.
In pulsed and single-phase applications when time is measured in pulses (1ØPULSE), use of a
measurement delay may introduce errors in the pulse count because the NERO mAx does not count
pulses during the delay time. In pulsed and single-phase applications when time is measured at a