  Knowledge of x-ray attenuation is essential for developing and evaluating
x-ray imaging technologies. For instance, techniques to better characterize
cysts at mammography screening would be highly desirable to reduce recalls, but
the development is hampered by the lack of attenuation data for cysts. We have
developed a method to measure x-ray attenuation of tissue samples using a
prototype photon-counting spectral mammography unit. The method was applied to
measure the attenuation of 50 samples of breast cyst fluid and 50 samples of
water. Spectral (energy-resolved) images of the samples were acquired and the
image signal was mapped to equivalent thicknesses of two known reference
materials, which can be used to derive the x-ray attenuation as a function of
energy. The attenuation of cyst fluid was found to be significantly different
from water. There was a relatively large natural spread between different
samples of cyst fluid, whereas the homogeneity of each individual sample was
found to be good; the variation within samples did not reach above the quantum
noise floor. The spectral method proved stable between several measurements on
the same sample. Further, chemical analysis and elemental attenuation
calculation were used to validate the spectral measurement on a subset of the
samples. The two methods agreed within the precision of the elemental
attenuation calculation over the mammographic energy range.
