GE MEDICAL SYSTEMS CT 9800 QUICK SYSTEM
Rev. 23
Direction 18000
6-9A-26
4) Review sag data.
Channel difference must be less than or equal to 1.1 channels.
The corresponding tube positions should be approximately 180
apart. They represent the gantry
positions where the extreme positions of sag were reported. Typically they
will be 280 to
to .
A
t the present time, there is no specification on this measurement.
The difference in first and last channels relates to the hysteresis in the system. At the present time, there
is no specification on this measurement.
5) Excessive sag amplitude
The sag readings of Step 5 are the sum of at least two different components.
b. Tube insert float within the tube casing-
This is a smaller component of total sag. This is a negative (-) factor as it is working against
gravity. A Tube unit with considerable insert float will have low sag, however, as the focal
spot location moves with respect to the center of the body attenuator a center artifact may be
produced.
c. Tube mount sag in the gantry-
This is also a minor component of total sag. This is a positive (+) factor and is generally of a
fixed amount which varies little from one system to another unless the tube is loose in its
mount, or the mount is loose on the gantry.
6) Excessive hysteresis will usually causes a TWAFFIE artifact. This is the artifact which shows two parallel
streaks tangent to the edge of a high contrast pin. The two streaks correspond to the first and last views of
the scan and the artifact results from an excessive displacement of these two views.
9A-12 The following records and data should be saved for H.H.S. purposes after completing x-ray
alignment
1. Beam on Window, Section 9A-5
The 10mm beam must be contained within the detector window.
2. Tube Output Check, Section 9A-6
Reference Channel output must be between 135,000 and 186,000.
3. ISO alignment, Section 9A-7
ISO = Ch 373.75 0.02
9A-13 Steps Following X-ray Alignment
1. If any components were replaced or adjustments made in the X-Ray alignment procedure, a new Q cal file
must be made before system calibration and image performance. The Qcal procedure is given in Chapter 4,
Section 10 of Direction 18000.
300 and 100
120
o
o o
o
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