5-8 QUALITY ASSURANCE Biodex Medical Systems, Inc. © 2014
Additionally, the calibration calculates full width half max (FWHM). FWHM is determined by
interpolating between two immediate points above and below the 50% value of the spectrum
peak on both sides of the peak. The difference between the two interpolated channel numbers
are then divided by the channel of the peak itself and multiplied by 100% to arrive at FWHM.
The peak is simply the maximum value in the spectrum.
It is not required to calibrate both detectors each day unless you are going to use them. If you
do not calibrate the detector, you will get a message advising you to calibrate when that
detector is selected. It is recommended to do your calibrations in the mornings, daily, so that if
you need to use the system, it is ready to go. If desired, additional calibrations may be
performed on the same day.
While the daily calibration is counting, the ROI counts and CPM counts are displayed to the right
of the counting window, along with the elapsed time and the time remaining. The spectrum
increments as you count as the packets with the data are downloaded to the computer from the
MCA. The <Abort> icon at the bottom of the screen can be used at any time to stop the system
from counting and return to the Quality Assurance screen without completing the calibration.
When the system finishes counting, the Counting Daily Calibration screen displays the spectrum
graph. Select <Next> to advance to the Daily Calibration Information screen to view full width
half max (FWHM) and the energy in the peak channel. Staff may be selected or changed at this
point and a spectrum analysis can be performed. If the daily calibration information is
accept
able s
elect
<Accep
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Both daily calibration and Chi-Square tests are performed using a 10 µCi Cs-137 button source.
Once calibration is underway, the system's unique differential spectrometer automatically
measures the peak height and subtracts the base line - which makes zero adjustment obsolete.
It does this for 100,000 pulses and then plots a spectrum in internal memory. The peak
channel is located and equated to 662 keV, which is the Cs-137 gamma energy entered. This
gives the spectrometer the keV per channel which is used to calibrate all of the other isotope
ranges. Each clinical isotope has a ROI defined with lower and upper energy limits. These
energy limits are then converted to channels in the MCA when a particular isotope is counted.
Corrections are made for sodium iodide non-linearity. There is no need to view the spectrum of
calibration or any other isotope, although this is available on the display and in hard copy by
selecting the Spectrum Analysis icon.
The multi-channel analyzer in the Atomlab 960 has several fixed precision ranges and a
regulated high voltage supply. The pulse shapes are digitized and then processed by a high
speed digital signal processor. This processing results in a possible 1024 pulse heights which
has zero offset. A spectrum results when a histogram of these pulse heights (channels) is
plotted. Calibration of the spectrometer is defined as knowing the energy equivalence of each
channel. This is accomplished by determining the Cs-137 spectrum and then calculating the
ratio of the 662 keV/peak channel. After calibration, the report should always print a peak
value very close to 662 keV (some precision round-off may occur), and the change in calibration
will be reflected in the keV/channel slope value. The fine range can be thought of as a floating
point numerical range. The HV adjust will ultimately determine the maximum energy one can
measure on the range selected. It can be calculated by multiplying the keV/channel times 1024.
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