a. The technical factors used in mammography depend upon several variables.
Such things as film speed, output characteristics of the x-ray tube, tube capacity, and
filtration must be considered. The kVp should be 20 to 35 (except for the axillary
projection) because, in this range, the small differences in photon absorption are
enough to provide adequate contrast. SID can be from 18 to 40 inches. As a general
rule, the mAs can be as much as 1800, depending upon the kVp, SID, and the speed of
the film. For example, using type M film with 26 kVp and 36-inch SID, the mAs would
be about 1800. On the other hand, if the same kVp and SID were used with type AA
film, less mAs would be required because type AA is faster than type M.
Check with the manufacturer for specific technique ranges.
b. Normally, when establishing a technique, it is only necessary to determine the
factors for the craniocaudad projection of a medium-sized mature breast. Variations in
technique because of breast size, breast density, and projection can be determined
from the initial factors.
c. One method for establishing a technique is to make trail exposures of an
aluminum step wedge with the central ray directed to the 15 mm step. Using
predetermined mAs and SID, depending upon the film speed; the kVp is varied until the
15 mm step is faintly penetrated. This setting is used for the craniocaudad projection.
d. For the mediolateral projection, 2 kVp are added to the craniocaudad kVp,
with the remaining factors unchanged. The axillary projection requires about 50 kVp.
The kVp cannot exceed 50 if only 0.5 mm aluminum-equivalent filtration is
e. The density of the breast also requires variation in technique. Usually, a
change of about 2 kVp is enough to compensate from one density category to the next.
For example, if 28 kVp is used for a mature breast, 26 kVp would suffice for a
f. As a general rule, the requirement for altering the technique to compensate
for the different breast sizes is just the opposite from most other examinations. For
example, a larger breast usually contains a greater proportion of fatty tissue and lies
flatter on the film holder. Consequently, less technique is required. Smaller breasts
usually contain a greater proportion of fibroglandular tissue and require more technique.
One good way to compensate for these differences is to take advantage of the inverse
square lay by altering the SID. This method also allows precise restriction of the
primary beam, which (as previously mentioned) is important. To put this in perspective,
consider the following comparison between a medium-sized breast and a large breast.