The purpose of this study was to compare the spectral performance of three dual-energy CT (DECT) scanners and one photon-counting CT (PCCT) scanner on virtual monoenergetic images (VMIs) at low-energy levels and on iodine maps.

A spectral phantom was scanned using one PCCT scanner and three different DECT scanners that included a rapid kV-switching CT (R-KVSCT), an ultrafast kV-switching (U-KVSCT) and a dual-layer CT (DLCT) scanner. Acquisitions were obtained with each CT system using classical abdominal and pelvic examination parameters, as well as a volume CT dose index at 11 mGy. VMI at 40/50/60/70 keV and iodine maps were reconstructed for each scanner. Noise power spectrum (NPS) and task-based transfer function (TTF) were evaluated. Detectability indexes (d') were computed to model the detection task of two contrast-enhanced lesions.

Noise magnitude decreased from 40 to 70 keV for all DECT scanners and this decrease was greater for R-KVSCT (-80.0 ± 0.1 [standard deviation (SD)] %) and less pronounced for DLCT (-14.4 ± 0.8 [SD] %) scanners. The average NPS spatial frequency (f _av ) values decreased from 40 to 70 keV (0.26 to 0.17 mm ^-1 ) for R-KVSCT, increased for DLCT (0.18 to 0.25 mm ^-1 ) but were similar for U-KVSCT (0.19 ± 0.002 [SD] mm ^-1 ) and PCCT (0.21 ± [SD] 0.008 mm ^-1 ) scanner. For R-KVSCT and PCCT scanners, TTF at 50 % (f ₅₀ ) values increased from 40 to 70 keV for both inserts. For U-KVSCT and DLCT scanners, similar f ₅₀ values were found according to energy level for both inserts. For both contrast-enhanced lesions, d' values decreased from 40 to 70 keV for PCCT, DLCT and U-KVSCT scanners. For R-KVSCT scanner, d' values peaked at 60 keV. At 40 and 50 keV, the greatest d’ values were found with DLCT and PCCT scanners.

At 40 or 50 keV, the best combined results (objective and subjective assessments) are obtained with DLCT and PCCT scanners.