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

Twelve-pulse generator

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