A discrepancy of measurement could indicate that the SID indicator is in
error. More measurements should be taken to confirm this indication.
4-17. PREVENTIVE MAINTENANCE
You should perform the focal spot test routinely at three-month intervals and
when an x-ray tube is replaced.
4-18. CORRECTIVE MAINTENANCE
The focal spot test is used only to indicate approximate focal spot sizes and to
subjectively indicate the gradual erosion of a focal spot. The Department of Health and
Human Services Bureau of Radiological Health maintains that a given nominal focal
spot size should be able to resolve the same number of groups over its useful life to
within plus or minus one group. If a great change in focal spot resolution occurs,
replace the x-ray tube.
Section IV. COLLIMATOR QUALITY CONTROL TEST
This collimator test verifies the centering alignment of the beam restriction
system, the accuracy of the numerically indicated field size, and the congruence of the
light and radiation fields. There are several tests used to check the collimator, and this
is one of the simpler ones.
a. One 10- x 12-inch cassette.
b. Nine pennies.
c. Lead markers "A" and "F" may be used rather than the ninth penny.
d. Lead letters for the room number and date.
a. Place the cassette on the x-ray table with the cassette's long dimension
parallel to the table's long dimension.
b. Place the light field to the center of the cassette at 40 SID.
c. Collimate the x-ray beam at 6- by 8-inch field size.