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There are two main acquisition modes for acquiring digital breast tomosynthesis (DBT) projection data: continuous mode and step-and-shoot mode. This work characterizes a new x-ray tube with flying focal spot (FFS) technology, designed to compensate for the continuous focus motion during exposure. The image sharpness of an established wide-angle DBT system with moving focus during exposure (the current standard) and a newly introduced FFS x-ray tube was assessed using the modulation transfer function (MTF). The spatial frequency for the 25% MTF value (f25%) was measured at 2, 4 and 6cm above the table. The impact on the visibility of calcification-like test objects was investigated with the CDMAM phantom that was positioned at various heights above the table. The threshold gold thickness (Tt) was calculated for the 0.1 mm discs. For the moving focus system, f25% measured at 2, 4 and 6cm in the 0° DBT projection fell by 35%, 49% and 59% in the tube-travel direction compared to figures of 7.6, 6.8, and 6.6 mm-1 for the FFS system. In the frontback direction, f25% was 7.4, 7.3 and 7.2 mm-1 at 2, 4 and 6cm, respectively for the moving focus unit while for the FFS system, these figures were 7.1, 6.7 and 5.9 mm-1. There was no significant difference in f25% for the front-back and tube-travel directions for the FFS unit (p<0.04). The Tt values at 4cm and 6cm above the table in DBT mode improved by 33% and 52%, respectively for the FFS system versus the moving focus system. The improvement in Tt increased as object height above the table increased. The FFS system has a reduced DBT scan time compared to the moving focus system (4.8s vs 21.7s) which is expected to reduce patient motion artefacts. The FFS system has in DBT mode an improved isotropic resolution, improved small detail detectability and faster total scan times. Therefore, this new DBT system has the potential for increased microcalcification detection and patient throughput compared to its predecessors.
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