  Knowledge of x-ray attenuation is essential for developing and evaluating
x-ray imaging technologies. In mammography, measurement of breast density, dose
estimation, and differentiation between cysts and solid tumours are example
applications requiring accurate data on tissue attenuation. Published
attenuation data are, however, sparse and cover a relatively wide range. To
supplement available data we have previously measured the attenuation of cyst
fluid and solid lesions using photon-counting spectral mammography. The present
study aims to measure the attenuation of normal adipose and glandular tissue,
and to measure the effect of formalin fixation, a major uncertainty in
published data. A total of 27 tumour specimens, seven fibro-glandular tissue
specimens, and 15 adipose tissue specimens were included. Spectral
(energy-resolved) images of the samples were acquired and the image signal was
mapped to equivalent thicknesses of two known reference materials, from which
x-ray attenuation as a function of energy can be derived. The spread in
attenuation between samples was relatively large, partly because of natural
variation. The variation of malignant and glandular tissue was similar, whereas
that of adipose tissue was lower. Formalin fixation slightly altered the
attenuation of malignant and glandular tissue, whereas the attenuation of
adipose tissue was not significantly affected. The difference in attenuation
between fresh tumour tissue and cyst fluid was smaller than has previously been
measured for fixed tissue, but the difference was still significant and
discrimination of these two tissue types is still possible. The difference
between glandular and malignant tissue was close-to significant; it is
reasonable to expect a significant difference with a larger set of samples.
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