Synchrotron split-beam method for clinical photon-counting computed tomography

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.

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.

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.

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.