Purpose: Free-space phase contrast propagation coupled with a photon-counting detector enables CT imaging with improved contrast in soft tissues at lower radiation dose. In addition, photon-counting detectors enable inherent spectral separation that can be used to capture tissue contrast at different energy levels. The objective of this study was to (i) develop a novel split-beam method for spectral synchrotron-based imaging considering limitations of photon-counting technology and clinical requirements, and (ii) propose a redefined mathematical model to calculate contrast-to-noise ratio in spectral imaging applications. Methods: Our novel approach was applied in a CT imaging setup using a custom-made breast phantom with tissue-equivalent inserts and compared to the more common setup utilizing monochromatic beams. To complement the traditional contrast-to-noise ratio metric, a new mathematical framework for spectral contrast-to-noise ratio was introduced as a composite metric that integrates the signal-to-noise performance across spectral channels. Results: The results show that the split-beam method proposed in this study obtains a comparable spectral contrast-to-noise ratio at the same radiation dose. Relative spectral contrast-to-noise differences were 0.12 (polyethylene), 1.92 (polyamide), 1.19 (polymethylmethacrylate), –0.29 (polyoxymethylene), and –0.17 (polytetrafluoroethylene) when comparing spectral imaging with two monochromatic beams at energies of 24 keV and 38 keV against the split-beam method. Conclusion: The potential advantages of the split-beam method for spectral CT imaging are numerous — it avoids non-rigid deformations, is fast to implement, and enables optimization in the post-processing step. The model for contrast-to-noise ratio redefined in this study applies to new generation spectral CT scanners beyond synchrotron setups.
