(2) The penetrating power or quality of a beam of x-rays is governed by
photon energy; in the three-phase beam, the average photon energy is much greater.
This, of course, is assuming that both systems are operated at equal peak kilovoltages.
Notice that the qualitative difference in the two beams is in average photon energy.
Both systems produce low-energy photons. The difference is in the proportion of low-
energy photons, which, as stated before, is greater in single-phase systems.
c. The intensity of an x-ray beam is greater with a three-phase system than with
a single-phase system for a given tube current. Therefore, the l2-pulse system will
produce a given amount of radiation in a much shorter period than required for the
2-pulse system. Figure 2-26 shows 2-pulse and 12-pulse waveforms. Image-forming
radiation is only produced at certain times with 2-pulse. At other times, either no
radiation at all is produced (when the sine wave is at zero value) or radiation is
produced that has insufficient energy to reach the film. The l2-pulse wave continuously
produces image-forming radiation because of its near-constant voltage level.
d. The average energy level of the beam of radiation produced by a three-phase
unit is higher than that produced by a single-phase unit when both are adjusted for the
same peak kV. Therefore, to produce radiographs with the same general scale of
contrast, it would be necessary to use a higher kVp with the single-phase unit. For a
given mA station, it would require approximately twice as much exposure time for a
single-phase unit as a three-phase (12-pulse). Therefore, it is probably more logical to
make the technique compensation with kVp rather than mAs. If the (single-phase) kVp
were increased by 15%, this would increase the average energy of the beam to a point
where the single-phase unit would produce radiographs of approximately the same
density and scale of contrast as those produced by the three-phase unit using the same
mA and time (mAs) factors. In addition, the increase in kVp would tend to keep the
absorbed dose of the patient to a minimum.
e. X-ray tube capacity is greater in a three-phase system for short exposures.
One reason for the increased tube capacity is because the heat is spread over a larger
area on the target. Figure 2-27 shows two rotating targets. Assume that each was
subjected to exposures of 1/60 second. Target A was exposed by a single-phase,
2-pulse system, while target B was exposed by a three-phase, 12-pulse system. Target
A shows two "hot spots" which would correspond to the peak of the two pulses
produced at 1/60 second. Therefore, the point heat buildup at these "hot spots" would
determine the maximum capacity of the tube on a short exposure. Target B, on the
other hand, shows no "hot spots" due to point heat buildup, which results in a more
even thermal load. In this matter, the anode disk is fully exploited for x-ray production.
The thermal capacity for x-ray tubes operated on three-phase is increased only for
exposures less than 1/2 second. From 1/2 to 1 second, the ratings are approximately
the same as for single-phase. Above 1 second, the ratings can be greater for x-ray
tubes operated on single phase.