GE Medical Systems PRODIGY Service Manual page 87

Spillover Stability test
Reference Value Test
Reference Value measurement
Reference value ratio test
Detector Pre-AMP test
Tissue Value Test
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PRODIGY Service Manual (Rev C - 2000)
Measures the effect of high energy photons registering as low energy
photons in the detector system. The beam passes through a brass piece that
stops all low energy photons. The detector then measures the number of low
energy photons detected as a measure of spillover. Spillover should be less
than 13%. Spillover with the CZT detector will degrade about 0.2% per year.
As part of the secondary calibration, the change in spillover must be
calculated on a per element basis and smoothed over the last 3 passing QA's.
Daily QA measures actual spillover and stores it in QA database. As part of
primary calibration, the spillover value used during that calibration is stored.
The delta spillover calculation using the spillover from primary calibration and
the running average of the last five spillover measurements. This information
is in the scan file.
The output of the x-ray tube is measured as part of a Reference Value (old
term was air) measurement. This is measured at various currents between
3mA and 0.150mA.
Note: Not all currents that are ramped to / tested are used to perform
patient scans.
The HE/LE counts are measured as the number of photons counted by
detector. This value is used in all bone density calculations. It must be stored
in the systems data file. Limits for low/high counts -
• Reference value measurements will be performed at various currents.
These reference values are stored separately for all detector elements.
• Reference Value measurements for all current settings are actually made
through lucite sections of the secondary calibration block.
The HE/LE ratio for each reference value is calculated for each tube current
tested in reference value measurement.
The detector Pre-amplifiers are tested by removing detector bias and verifying
that the signal out lines of the detector are not noisy.
Tissue secondary calibration
• The system measures two tissue eqivilent materials (lucite and Acetron).
System Overview 2-93