Use Factor, Occupancy Factor, Workload

Use Factor

If radiation, whether primary or secondary, is never directed at a particular wall or structure, then ordinary or existing construction is sufficient. Most structures in a diagnostic x-ray suite, however, are struck by radiation to some degree for some fraction of the weekly beam-on time. The use factor (U) is a quantity that was introduced to select this fractional contact time.

For primary radiation, the use factor represents the portion of beam-on time that the x-ray beam is directed at a primary barrier during the week. Consider a typical radiographic suite with a wall Bucky unit. If 50% of the x-ray examinations involve this device, the wall behind the Bucky unit has a U (primary) = 1/2 .

Because scatter and leakage radiation emerge in all directions in the x-ray room, every wall, door, viewing window, and other surface will always be struck by some quantity of radiation. Therefore U (secondary) = 1 for all radiation-accessible structures.

(Statkiewicz Sherer, Mary Alice. Radiation Protection in Medical Radiography, 6th Edition. Mosby, 032010. p. 300).

Occupancy Factor

Radiation barriers are installed to protect personnel and the general public from radiation that otherwise would reach them uninhibited. If no one will ever be present beyond an existing wall in a particular area while the x-ray unit is being operated, the addition of supplementary shielding to that wall is unnecessary. The opposite situation is an area in which someone is always present. When planning radiation protection shielding for a diagnostic x-ray suite, the designer must consider not only zero and full occupancy cases but also the more common partial occupancy situation. The occupancy factor (T) is used to modify the shielding requirement for a particular barrier by taking into account the fraction of the workweek during which the space beyond the barrier is occupied.

(Statkiewicz Sherer, Mary Alice. Radiation Protection in Medical Radiography, 6th Edition. Mosby, 032010. p. 301).

Workload

Because a diagnostic x-ray unit does not produce radiation 24 hours per day, 7 days per week, a parameter must be used to reflect the unit’s radiation-on time in the determination of barrier shielding requirements. The quantity that best describes the weekly radiation use of a diagnostic x-ray unit is called its workload (W).The workload is essentially the radiation output weighted time that the unit is actually delivering radiation during the week. Workloads are specified either in units of milliampere-seconds (mAs) per week or milliampere-minutes (mA-min) per week. The following example illustrates this concept.

(Statkiewicz Sherer, Mary Alice. Radiation Protection in Medical Radiography, 6th Edition. Mosby, 032010. p. 300).