3-14. RATING CHARTS
a. Tube Rating Charts. X-ray tubes must be operated within the limitations set
by the manufacturer. If not, they can be permanently damaged by excessive heat that
results when improper exposure factors are used. To prevent damage to tubes,
radiologic specialists must be aware of tube rating charts and how to apply them in the
daily operation of the x-ray unit. With each tube, the manufacturer supplies a series of
rating charts to be used only with that particular tube.
(1) The amount of heat generated within a tube depends upon the electrical
energy expended in the tube and is measured in heat units. Heat units (HU) are the
product of kVp X mA X exposure time in single-phase, full-wave equipment. The
formula is written HU = kVp X mA X sec. For example, how many heat units would be
generated in a tube by the following exposure factors?
75 kVp
100 mA
1/2 second
Solution: HU = kVp X mA X set
HU = 75 X 100 X 1/2
HU = 7,500 X 1/2
HU = 3,750
Answer: 3,750 HU
(2) Heat units are an arbitrary measure and are the product of kVp X mA X
second only in single-phase, full-wave equipment. The product of these factors in a
three-phase, full-wave generator does not accurately reflect the heat units. To
determine heat units in three-phase equipment, multiply the product of the exposure
factors by 1.35 for 6-pulse and by 1.41 for 12-pu1se. In other words, HU in three-phase
equipment are determined by the following formulas:
Six-pulse generator
HU = kVp X mA X second X 1.35
HU = kVp X mA X second X 1.41
For example, how many Hu would be generated in a three-phase, 6-pulse unit using the
following exposure factors?
70 kVp
100 mA
1/2 second
Solution: HU = kVp X mA X second X 1.35
HU = 70 X 100 X 1/2 X 1.35
HU = 3,500 X 1.35
HU = 4,725
Answer: 4,725 HU
MD0950
3-21