GE Advantium CT9800, Технические публикации

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GE MEDICAL SYSTEMS CT 9800 QUICK SYSTEM
Rev. 19
Direction 18000
6-9-4
5. For proper alignment, the following conditions must be achieved:
•
To maintain uniform slice thickness across the reconstruction circle, the plane of rotation of the X-Ray fan
beam must be parallel with the plane of gantry rotation.
A system where the fan beam is not perpendicular to the gantry axis rotation will tend to exhibit cut off at
the sides of the beam in the Z direction and as a result, high standard deviation of both reference and
active channels.
•
For maximum sampling at the center of reconstruction, the focal spot must be aligned 1/4 channel above
the ”geometric center” of the detector. For instance, the geometric center of the detector is 368.5
channels. This number results from the fact that the detector contains 736 channels. Since the number of
channels is an “even” number, the center of the detector channel array must be 368.5 channels. Adding
the 1/4 channel displacement, results in 368.75 channels.
If the center of the detector is aligned to the focal spot, through the gantry isocenter, this detector will
see identical information on reciprocal views. The 1/4 channel displacement effectively doubles the
information sampling in the center of the image.
•
The edges of the fan beam must extend beyond the end reference channels so as not to introduce a
varying X-Ray intensity into the reference channels. This is Channel 1 and 736. For systems
manufactured before September 1985, the radiation beyond the ends of the reference channels must not
exceed 2% of the source-image distance (2% SID) to meet H.H.S. requirements.
•
The entire width of the fan beam at the detector must fall within the active area of the detector window in
order to meet H.H.S. requirements.
9-2 Adjustment of Components For X-ray Alignment on Xenon Detector Systems
This subsection describes how the tube, detector, and collimator are physically positioned in the gantry. Also
described is the pre-positioning of a component when it is replaced.
1. X-Ray Tube Unit
Refer to Illustration 6-9-3. An aluminum Tube support casting with a Z positioning lever is furnished as part of
the tube unit. Pins in this casting engage in slots in an intermediate steel plate to allow tube movement in the
Z direction.
The steel plate, in turn, rides in a transverse or theta machined groove in the front casting of the gantry
rotating structure. The plate is held in theta position by a threaded rod with a differential screw. A bracket is
located near the steel plate to which a dial indicator may be secured for measurement of tube theta shift. A
second bracket is located on the collimator assembly for attachment of a dial indicator to measure tube Z
shift.
Clearance holes in the steel plate and the tube support casting allow adjustment in both theta and Z
directions. The tube is held in place by two steel clamp plates and four studs with hex nuts.