  Purpose: To evaluate a proposed method to reconstruct CT numbers that
accurately mimic conventional CT numbers from spectral CT data, as would have
been produced by a conventional system without effects of beam hardening.
  Approach: We implement the proposed method for simulated ideal and non-ideal
photon counting multi-bin systems, the latter based on a photon counting
Silicon detector, and compare them with a simulated ideal conventional energy
integrating system with a cupping correction algorithm. We compare the systems
using a mathematical phantom of a size and composition that produces severe
cupping and beam hardening artefacts when imaged with a conventional system
with no cupping correction.
  Results: The resulting images show CT numbers that are consistently accurate
for a varying range of tissues and are free of beam hardening artefacts.
  Conclusions: This method could facilitate use of established rules-of-thumb
regarding absolute CT numbers for various organs and conditions during the
transition from conventional CT to spectral CT.
