Imaging Problems; White, Or Grey In The First Or Second Scan Line; Femur Scan Problems - GE Medical Systems PRODIGY Service Manual

If the transverse mechanics of the scanner become imprecise, it may cause a
Limit Switch to be tripped during a scan. This will usually only happen during a
total body scan or during an Alignment Test test since both of these scans
involve full-width scanning. Once the switch is closed, the motors stop, the
End-of-Exposure Alarm sounds, the amber SHUTTER OPEN lamp turns off
and after a few seconds a Diagnostic Failure appears on the screen.
A cause of this problem is a loosening of the first drive Reduction Belt which
connects the Transverse Motor to the first Reduction Pulley. This loosening
causes the belt to "walk" on the pulleys causing enough imprecision in the
motion to trip a switch. To tighten the belt, first remove the Pulley Shroud and
loosen all four nuts that hold the motor in place. Then, while holding the motor
such that the belt is pulled taught, tighten the nuts to secure the motor in
place. Replace the shroud and test the scanner (an Alignment Test test works
well).
If the Tube Head cables come into contact with the cable bundle entering the
cable track, it may be impossible for the scanner to complete all of the
necessary transverse steps away from the operator. Consequently, on each
scan line the detector will move closer to the front transverse limit switch, and
the switch may be eventually closed. Form the cable bundle exiting from the
cable track into an arch such that the Tube Head cables will move under the
arch rather than running into the bundle. This arch must not be too high. If it is,
there will not be sufficient slack in the Tube Head cable bundle to allow the
Tube Head to move to the front transverse limit switch. Also, if the arch is too
small, it may cause too much slack when the Tube Head is at the forward side
of the table. This causes the Tube Head cable bundle to rub against the inside
of the front panel causing a scraping noise to be heard. If the above does not
solve the problem then check the other mechanical components. Electrical
components that could cause such a failure are the Centent Motor Controller,
the Transverse Motor, the FOINK board or the SBC board.

4.11 Imaging Problems

4.11.1 White, or Grey in the first or second scan line:

The software is not perfect in its ability to determine the correct grey level of
the entire scan based on the first line of data. Sometimes the grey levels will
be set incorrectly during acquisition. During analysis the grey level is easily
adjusted to give a good image, and the results are not affected by this imaging
problem. This problem is most often found on very thin patients. Adding extra
tissue equivalent material (rice bag or saline solution) to a very thin patient
may be all that is necessary to cure the symptom.

4.11.2 Femur Scan Problems

4-136Troubleshooting
• During full-width scanning, the Detector Carriage comes very close to the
Limit Switches, so any imprecision will cause a Limit Switch to be tripped
and the scan will be aborted with a Diagnostic Failure message for
Transverse Motion Failure.
DPX-NT Service Manual (Rev A- 1999)