Tissue Value; Bone Mineral Of The Standard Chambers - GE Medical Systems PRODIGY Service Manual

The values recorded for these tests should remain fairly constant over time.
Variations between QA's of under 25 steps should not be a cause for concern
as 1 transverse step = 0.05 mm and 1 longitudinal step = 0.1 mm, so the
actual variation is only a few millimeters.
If the number of steps continually increases from QA to QA, this could indicate
an impediment to the scanner's motion and should be rectified (see 4.1
(Transverse) or 4.2 (Longitudinal)).

4.8.7 Tissue Value

The tissue value should not be the sole failing parameter. The failure of this
test is usually accompanied by problems with Bone Mineral values (see
below) or Reference Counts Ratio trends (see above). If this is failing alone it
is probably a problem with the counting system or high or low kV (4.8.9).

4.8.8 Bone Mineral of the Standard Chambers

The software contains values which it expects the scanner to measure for the
scan of each chamber. If the mean of the BM measurements made for the
chamber does not fall within the predefined software limits the QA will fail. If
the percent coefficient of variance is over 3%, the QA will fail.
These failures will always occur if the Reference Counts or Reference Ratio
test have deviated severely from normal results. However if the Reference
Count results look normal, and the values are approximately equal to the
numbers obtained during the scanner installation, then the counts may be
unstable.
Another cause of failing on the bone chamber measuremnts can be arcing
during the QA. This can be detected by examining the QA History graphically
and looking for variation of the Large BM values. Note the few data points that
vary. These are the early signs of arcing.
This will also be apparent (but not obvious) on the QA Results printout. The
arc occurred in the third standard scan and elevated the BM values. If a
customer reports a failing QA because of a bone chmaber measurement
being too high, be aware that this could be an early warning of arcing. Obtain
from the site the QA history file and error log for analysis. Look for variation of
the Large BM values.
It is very important to notice these early warning signs of arcing so that the
system can be re-greased before any damage is done to the high voltage
cable connectors or the tube head.
4-130Troubleshooting
• Incorrect Limit Switch positioning
• Defective Limit Switch
• A mechanical constraint
• A defective Motor
• A defective Centent Motor Controller
DPX-NT Service Manual (Rev A- 1999)