Photon-counting and spectral x-ray imaging

5 problems

• Optimal energy thresholds for material separation

  Partially resolved

  The detector sorts photons into a few energy bins. Where to place the bin edges for the best material separation at the lowest dose is unsolved in general.

• Sensor material choice for clinical count rates

  Contested

  The detector can be made of cadmium telluride, cadmium zinc telluride or silicon, each failing in a different way at high photon rates. Which is best overall is unsettled.

• Achievable dose reduction at fixed diagnostic performance

  Open

  Counting photons individually should allow the same image quality at a lower dose, but how much lower is debated.

• Detection limit for K-edge contrast agents

  Open

  Elements like iodine and gadolinium absorb x-rays sharply above one energy, which lets a spectral scanner tell them apart. How little of an element can be detected is not pinned down.

• Forward model for charge sharing and pulse pileup

  Partially resolved

  When a photon lands between pixels, its charge splits and is counted twice at the wrong energies; when photons arrive too fast they are counted as one. Both corrupt the measured spectrum.