Question

Why do mammography systems use one type of filter, whereas chest x-ray or CT x-ray systems would use another type? What else is different about these x-ray systems?

Answer 1

Filters are the metal sheet placed in the X-ray beam between the window and the patient that are used to attenuate the low energy (soft) X-ray photons from the spectrum . filtering is the removal of these low energy x-rays from the beam spectrum which could otherwise not contribute to image quality but would add to patient dose and scatter . If unfiltered these low energy X-ray photons are generally absorbed by superficial structures of the body and contribute to the entrance surface dose (ESD) as they are absorbed by the superficial structures they contribute minimally to the imagge formation . USing the filter reduces the ESD and to lesser extent effective dose for teh patient .THe units of filteration are expressed in mm of aluminium equivalence (mm AI eq ).

There are two types of filteration :-

• Inherent filteration from components in the X-ray tube i.e window , housing , cooling oil ( equivalent to 0.5- 1.0 mm AI )
• Added filtration from interchangeable metal sheta ( Al , Cu , etc )

Beryllium is commonly used in mammography ( which use low - energy photons ) as it provide very little filteration . Other types of X-ray such as chest X- ray , CT x-ray , MRI x-ray , the generally use aluminium , copper or tin .

In a mammography , the filter with a k-edge of an energy just above the characteristics energies is used to remove the higher energy X-ray photons and make the beam as monoenergetic as possible . Molybedenum has a k-edge of 20 keV , it just high enough so that the large increase in attenuation ( k-edge) doesn't fall into the characteristcs energies produced at the molybdenum target .

Mostly MoMo ( molybdenum target , molybdenum filter) but this does not give high / enough energies for larger breasts