Minimise filament boost (“prep”) time
Boost time will usually exceed the actual exposure time. High filament current
applied for too long will shorten filament life and will lead to unstable operation asevaporated tungsten from the filament is deposited onto the glass envelope. This is
especially the case at high mA stations.
Use lower tube current (mA)
The high filament current required to produce high tube current (mA) will shorten
filament life and will lead to unstable operation as evaporated tungsten from the
filament is deposited on to the glass envelope. Whenever possible, use a lower mA
station and a longer exposure time to arrive at the desired mAs.
Follow rating charts and anode heating/cooling curves
Operation beyond published ratings will result in premature focal track wear or
damage. Even moderate etching of the focal track will result in a fall-off in radiation
output, because electrons from the filament which strike in micro-crevices in the
target material produce radiation that is mostly absorbed in the surrounding target
material. More severe etching, or melting, results in the liberation of gasses from the
target material, which causes tube instability. Excessive heat transfer from the target
into the rotor body will cause bearing failure or slow rotation which will result in
melts on the focal track.
Limit operation to 80% of maximum single exposure ratings
although higher power levels are both possible and permitted, this reduction will help
assure long focal track life. Also, it will minimise the reduction in radiation output
associated with a roughened focal track.
Do not exceed anode thermal capacity or dissipation rate of the target
the greatest danger is to heat flow into the bearing structure, as discussed above. In addition, gasses may be emitted from the various metals within the tube if the
temperature reached during clinical use is appreciably higher than that used during the “outgassing” stage of manufacture. If outgassing occurs during clinical use, the tube will become unstable. (“Aging” the tube may reverse the process but this is not
assured.)
Do not make high mA exposures on a cold target
Uneven expansion caused by thermal stress from a high power exposure can result in
a cracked target. Do not assume that a “thermally relieved” target design provides
absolute protection. Always follow the recommended warm- up procedure. The procedure may need to be repeated between patients, if the “idle” time is long enough, in addition to being performed at the beginning of the workday.
Avoid long intervals between spot-films
Most systems provide for a “holdover” period of up to approximately 25 seconds
between spot- films, during which the rotor is kept at high speed before the rotor
brake/reboost cycle is allowed to occur. In some systems, the filament current remains at the exposure value during this period, thereby causing evaporation of filament material and resultant tube instability.
Limit rotor start/stop operations
Rotor start/stop operations especially to/from high speed (150/180 Hz) generate
considerable heat in the stator windings, which will lead to stator damage in extreme
cases. Generally there should be a minimum of 30 to 40 seconds between starts.
Tubes equipped with a heat exchanger will be less sensitive to this potential problem
because oil circulation will help prevent hot spots from occurring around the stator
windings.
