In single-event particle imaging, removing nuclear interaction events is crucial for accurate relative stopping power (RSP) reconstruction. Additionally, for imaging with ions, secondary fragments have to be separated from the primary histories. We investigate the use of a ΔE-E filter in helium CT (HeCT) and proton CT (pCT) with the preclinical prototype pCT scanner developed by the US pCT collaboration.
Experimental HeCT scans were obtained with the scanner at the Heidelberg Ion-Beam Therapy Center. Simulated HeCT and pCT scans were acquired based on the prototype using the TOPAS Monte Carlo toolkit. To filter out secondary fragments and nuclear interactions that occurred within the  detector, the scanner 5-stage energy/range detector was adapted as ΔE-E telescope. The ΔE-E filter was compared to the data filters currently used as standard with the prototype.
Accurate HeCT was possible only when using the developed filter, as large systematic fluctuations were present in the reconstructions otherwise (Fig. 1). With the filter, both simulated and experimental HeCT yielded a mean RSP accuracy better than 0.5% (Fig. 2). For pCT, the filter removed similar, yet less pronounced systematic uncertainties and the achieved accuracy was close to
simulations for that nuclear interactions were disabled.
In conclusion, ΔE-E filtering proved to be effective in improving the visual quality and RSP accuracy of both HeCT and pCT. For the first time, accurate experimental single-event HeCT images could be presented.

Improving single-event particle imaging by ΔE-E filtering