Impact of dose on observer performance in breast tomosynthesis using breast specimens

The purpose of this study was to investigate the effect of dose on lesion detection and characterization in breast tomosynthesis (BT), using human breast specimens. Images of 27 lesions in breast specimens were acquired on a BT prototype based on a Mammomat Novation (Siemens) full-field digital mammography (FFDM) system. Two detector modes - binned (2x1 in the scan direction) and full resolution - and four BT exposure levels - approximately 2x, 1.5x, 1 x, and 0.5 x the total mAs at the same beam quality as used in a single FFDM view with a Mammomat Novation unit under automatic exposure control (AEC) conditions - were examined. The exposure for all BT scans was equally divided among 25 projections. An enhanced filtered back projection reconstruction method was applied with a constant filter setting. A human observer performance study was conducted in which the observers were forced to select the minimum (threshold) exposure level at which each lesion could be both detected and characterized for assessment of recall or not in a screening situation. The median threshold exposure level for all observers and all lesions corresponded to approximately 1x, which is half the exposure of what we currently use for BT. A substantial variation in exposure thresholds was noticed for different lesion types. For low contrast lesions with diffuse borders, an exposure threshold of approximately 2x was required, whereas for spiculated high contrast lesions and lesions with well defined borders, the exposure threshold was lower than 0.5x. The use of binned mode had no statistically significant impact on observer performance compared to full resolution mode. There was no substantial difference between the modes for the detection and characterization of the lesion types.