4-4.
ANODE STORAGE CAPACITY
To produce the desired contrast on a mammogram, it is necessary to use low
kVp. Low kVp in conjunction with high mAs (which is generally used in mammography)
produces a tremendous amount of heat in the tube. For example, suppose the
technique for a single projection is 26 kVp and 1,800 mAs. This would produce 46,800
heat units (HU) (26 X 1,800 = 46,800). Since three views of each breast are normally
included in a mammography study, the total would be 280,800 HU (6 X 46,800 =
280,800). The anode storage capacity of most x-ray tubes is considerably less than the
280,800 HU in the hypothetical case described above. Therefore, to prevent damage to
the anode, it would be necessary to allow the appropriate cooling time between
exposures. Cooling times is based on the anode-cooling curve found on the tube-rating
chart.
4-5.
FILTRATION
Radiographic demonstration of the differences in breast-tissue density requires a
soft, heterogeneous x-ray beam. Ideally, then, mammography should be accomplished
with absolutely no filtration. However, in the interest of keeping patient dosage to a
minimum, NCRP (National Council on Radiation Protection and Measurements) Reports
No. 33 requires certain amounts of filtration for all radiographic examinations. For
operating voltages up to and including 50 kVp, the report requires at least 0.5 mm
aluminum equivalent. Standard x-ray tubes may or may not have the required 0.5 mm
aluminum equivalent in the form of inherent filtration. If not, additional filtration to
achieve this standard must be added. Some special-purpose tubes usually have less
than 0.5 mm inherent filtration. For example, a beryllium-window tube only has from 0.1
to 0.3 mm. When filtration must be added, use clear plastic to avoid blocking the
filament flow since "light" from the filament helps in positioning the cone.
4-6.
BEAM-RESTRICTING DEVICES
SR (scatter radiation and secondary radiation) must be kept at a minimum
because it can cause film fogging that is degrading to radiographic detail. To reduce
SR and to keep the skin dose to the patient at a minimum, the primary beam should be
restricted so that it covers the breast. Extension cylinders, modified collimators, and
specially constructed cones are used to restrict the x-ray beam.
4-7.
FILMS
Standard, coarse-grained radiographic films are inadequate for the fine detail
required in mammography; thus, fine-grained or special mammography films are used.
Some radiologists prefer to use a "package" of two or more films. By using films of
various speeds, visualization of the deep fibroglandular structures as well as of the
peripheral anatomy is provided. Some mammography films can be processed
automatically while other films must be processed.
MD0959
4-5