Simulation of photon-counting detectors for conversion of dual-energy-subtracted computed tomography number to electron density

  • Masatoshi SaitoEmail author


For accurate tissue-inhomogeneity correction in radiotherapy treatment planning, the author previously proposed a conversion of the energy-subtracted computed tomography (CT) number to electron density (ΔHU–ρe conversion). The purpose of the present study was to provide a method for investigating the accuracy of a photon-counting detector (PCD) used in the ΔHU–ρe conversion by performing dual-energy CT image simulations of a PCD system with two energy bins. To optimize the tube voltage and threshold energy, the image noise and errors in ρe calibration were evaluated using three types of virtual phantoms: a 35-cm-diameter pure water phantom, 33-cm-diameter solid water surrogate phantom equipped with 16 inserts, and another solid water surrogate phantom with a 25-cm diameter. The third phantom was used to investigate the effect of the object’s size on the ρe-calibration accuracy of PCDs. Two different scenarios for the PCD energy response were considered, corresponding to the ideal and realistic cases. In addition, a simple correction method for improving the spectral separation of the dual energies in a realistic PCD was proposed to compensate for its performance loss. In the realistic PCD case, there exists a trade-off between the image noise and ρe-calibration errors. Furthermore, the weakest image noise was nearly twice that for the ideal case, and the ρe-calibration error did not reach practical levels for any threshold energy. Nevertheless, the proposed correction method is likely to decrease the ρe-calibration errors of a realistic PCD to the level of the ideal case, yielding more accurate ρe values that are less affected by object size variation.


Photon-counting detector Electron density Dual-energy CT Energy-selective CT 



This work was supported in part by JSPS KAKENHI Grant number 16K09011.

Compliance with ethical standards

Conflict of interest

The author has no conflicts of interest to disclose.

Ethical statement

This article does not contain any studies with human participants or animals performed.

Informed consent

Informed consent is not required.


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Copyright information

© Japanese Society of Radiological Technology and Japan Society of Medical Physics 2019

Authors and Affiliations

  1. 1.Department of Radiological Technology, School of Health Sciences, Faculty of MedicineNiigata UniversityNiigataJapan

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