Task transfer function measurement of monoenergetic CT in energy-integrated detector CT and photon-counting detector...

When evaluating the task transfer function (TTF) of monoenergetic (MonoE) CT applying energy-domain noise reduction (EDNR), special caution should be exercised on the task size because of the low-pass filtering (LPF) process involved. The purpose of this study was to evaluate TTFs at MonoE at 40 keV compared with those at polyenergy (PolyE) at 120 kV.

A rod (diameter: 30 mm) or wire (diameter: 1 mm) phantom made of a material equivalent to 12-mg/ml diluted iodine (IROD or IW, respectively) was placed in a 210-mm water phantom and scanned at a radiation dose corresponding to a head CT angiography protocol, using an energy-integrated detector CT (EID-CT) system and a photon-counting detector CT (PCD-CT) system. TTFs for IROD and IW were measured using the circular edge (CE) method and a method typically used with the traditional wire method (IW method), respectively.

For EID-CT, TTF by the CE method was lower (25 % for 50 %TTF) for MonoE than for PolyE, whereas, by the IW method, the difference was minimal. The ratio of MonoE's CT number to PolyE's was, however, markedly lower with IW than with IROD, reflecting the CT number manipulation in EDNR. For PCD-CT, both the differences in TTF and in CT number were negligible between MonoE and PolyE.

With EDNR, PCD-CT was better than EID-CT because it avoids reduction in CT number at MonoE for small objects. When measuring TTF with EID-CT, the CT number reduction in low energy MonoE should be taken into consideration.