he sensitivity of digital mammography (DM) is lower in women with dense breasts than in those with lower breast density (1). Breast density is also associated with higher false-positive rates and recall rates (2) due to superposition of normal glandular tissue that can mimic cancer. The woman's age has an impact on mammography screening as breast density decreases (3) and cancer incidence increases. The distribution of cancers shifts toward less-aggressive slower-growing cancers with increasing age (4). It has been shown that mammography screening has a lower sensitivity (1) and higher false-positive rate (2) among younger women. Digital breast tomosynthesis (DBT) generates pseudo three-dimensional (3D) images where a single section of anatomy is in focus. The rest is blurred, with greater magnitude proportional to the distance from the focus plane. The screening performance of DBT for specific density and age groups may be different from that of DM, as DBT potentially can reduce masking and resolve superposition of breast tissue. Prospective (5-11) and retrospective (12-18) studies have shown that the integration of DBT improves the cancer detection or recall rates for both fatty and dense breasts and in age groups relevant for mammography screening. Data are limited in almost entirely fatty and extremely dense breasts. Two large studies compared DBT and DM in women with extremely dense breasts, with one study finding an increased cancer detection rate with DBT (5) and the other finding similar rates for DBT and DM (13). Therefore, there is a need for more data from large prospective trials.

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In computer tomography (CT) diagnostics, the measured Hounsfield units (HU) are used to characterize tissue and are in that respect compared to nominal HU values found in the radiological literature. Quality assurance (QA) phantoms are commercially available with a variety of tissue substitutes and materials to test the HU values in CT. It is however recognized from CT physics that the HU for a given material is energy dependent and may vary substantially between scanners. The aim of this study is to analyze the characteristics of a commonly used QA phantom, the Catphan 500/600 (The Phantom Laboratory, NY). Four CT phantoms were scanned on one CT scanner to examine possible interphantom variations in HU values. Secondly, one selected phantom was scanned at three kVp levels on eight different CT scanners. The interphantom variations in HU values were small, in the range 2-5 HU. The interscanner variations were however substantial, in the range 7-56 HU depending on energy and material. Varying the x-ray energy produced a shift in the measured HU of up to 79 HU on one scanner. Reference HU values for the eight sensitometric test materials in Catphan are provided for eight CT scanner models from four vendors. The reference HU values are provided for 80, 120 and 140 kVp. Our results suggest that scanner-independent threshold levels for HU should be used only with extreme caution. Tissue characterization can be used provided that a scanner-specific data set for normal and abnormal is determined.