Precalibration Notes - CPI CMP 200 Series Service Manual

CPI Canada Inc.
3D.2.5 Precalibration Notes
Please note the following points before beginning AEC calibration.
CAUTION: THE PROCEDURES IN THESE SECTIONS REQUIRE X-RAY EXPOSURES. TAKE ALL
SAFETY PRECAUTIONS TO PROTECT PERSONNEL FROM X-RADIATION.
SHOULD AN IMPROPER TECHNIQUE BE SELECTED, OR AN AEC FAULT OCCUR CAUSING NO
AEC FEEDBACK SIGNAL TO THE GENERATOR, THE EXPOSURE WILL TERMINATE AND AN "AEC
DEVICE ERR / AEC FEEDBACK ERROR" MESSAGE WILL BE DISPLAYED IF THE RAMP VOLTAGE
FAILS TO REACH 4% OF THE EXPECTED RAMP VOLTAGE WHEN THE EXPOSURE TIME REACHES
20% OF THE SELECTED BACKUP TIME.
•
When using a photo diode for AEC, there is normally no need to iterate all the kV breakpoints during
AEC calibration. It will usually suffice to use the 75 kV breakpoint calibration value at all kV
breakpoints. If doing this, the calibration values should be confirmed at all kV breakpoints using the
acquired digital images.
•
For DR applications, the AEC calibration procedure uses the same potentiometers and menus as
described for film in the following sections. However, the imaging system manufacturers literature
should be consulted to determine the required dose / image brightness.
•
The generator must be known to be calibrated before beginning AEC calibration.
•
All components (X-ray tubes, collimators, AEC chambers, etc) used during AEC calibration must be
those that will be used during procedures, and must be positioned as they will be in actual use of
the X-ray room.
•
During AEC calibration, always ensure that the central ray is centered relative to the image receptor.
•
Before placing the absorbers, ensure that the collimator is opened sufficiently to irradiate ALL fields
on the AEC pickup device.
•
Ensure that the absorber is positioned to fully cover the X-ray field. The absorber must extend a
minimum of 3/8 in. (10 mm) beyond the X-ray field.
•
During AEC calibration, all AEC exposures should be done using mA values such that the
exposures are in the 30 to 100 ms range UNLESS STATED OTHERWISE.
•
The recommended absorber is water. This should be in a plastic container of uniform thickness.
Lexan of a similar thickness is also a suitable absorber.
•
Care must be exercised when using table Buckys with low kV values because most tabletops and
grids absorb considerable radiation in the 60 – 65 kV range. This will adversely affect AEC
operation.
•
During AEC calibration, if exposure times do not change if the mA is varied, it may be that
the input signal level to the AEC board is too high. If this is experienced, check the ramp
voltage at the output of the first gain stage (the first operational amplifier output) on the AEC
board for the subject AEC channel. This voltage must never exceed 10 V. If this voltage does
exceed 10 V, reduce the input signal level as required.
3D.2.6 Required Test Equipment
The following test equipment is required for AEC calibration.
•
Lexan or equivalent (or water) absorbers in various thicknesses. Water should be in a plastic container
of uniform thickness.
•
Radiation dosimeter for digital applications.
•
A supply of film in each film speed that will be calibrated.
•
A cassette with intensifying screen in each speed that will be calibrated.
Use and disclosure is subject to the restrictions on the title page of this CPI document.
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CMP 200 Service Manual Ch # 901476-05 Rev. A
3D
AEC Calibration
Page 3D-11