  Purpose: Spectral imaging is a method in medical x-ray imaging to extract
information about the object constituents by the material-specific energy
dependence of x-ray attenuation. The authors have investigated a
photon-counting spectral imaging system with two energy bins for
contrast-enhanced mammography. System optimization and the potential benefit
compared to conventional non-energy-resolved absorption imaging was studied.
  Methods: A framework for system characterization was set up that included
quantum and anatomical noise and a theoretical model of the system was
benchmarked to phantom measurements.
  Results: Optimal combination of the energy-resolved images corresponded
approximately to minimization of the anatomical noise, which is commonly
referred to as energy subtraction. In that case, an ideal-observer
detectability index could be improved close to 50% compared to absorption
imaging in the phantom study. Optimization with respect to the
signal-to-quantum-noise ratio, commonly referred to as energy weighting,
yielded only a minute improvement. In a simulation of a clinically more
realistic case, spectral imaging was predicted to perform approximately 30%
better than absorption imaging for an average glandularity breast with an
average level of anatomical noise. For dense breast tissue and a high level of
anatomical noise, however, a rise in detectability by a factor of 6 was
predicted. Another ~70-90% improvement was found to be within reach for an
optimized system.
  Conclusions: Contrast-enhanced spectral mammography is feasible and
beneficial with the current system, and there is room for additional
improvements. Inclusion of anatomical noise is essential for optimizing
spectral imaging systems.
