Spectral performance of two split-filter dual-energy CT systems: A phantom study

Background: Recently, a second generation of split filter dual-energy CT (SFCT) platform has been developed. The thicknesses of the gold and tin filters used to obtain both low- and high-energy spectra have been changed. These differences in filter thickness may affect the spectral separation between the two spectra and thus the quality of spectral images.

Purpose: To compare the spectral performance of two Split-Filter Dual-Energy CT systems (SFCT-1^st and SFCT-2^nd ) on virtual monoenergetic images (VMIs) and iodine map.

Methods: A Multi-Energy CT phantom was scanned on two SFCT with a tube voltage of 120 kVp for both systems (SFCT-1^st -120 and SFCT-2^nd -120) and 140 kVp only for the second generation (SFCT-2^nd -140). Acquisitions were performed on the phantom with a CTDI_vol close to 11 mGy. Noise power spectrum (NPS) and task-based transfer function (TTF) were evaluated on VMIs from 40 to 70 keV. A detectability index (d') was computed to assess the detection of two contrast-enhanced lesions on VMIs. Hounsfield Unit (HU) accuracy was assessed on VMIs and the accuracy of iodine concentration was assessed on iodine maps.

Results: For all keV, noise magnitude values were lower with the SFCT-2^nd -120 than with the SFCT-1^st -120 (on average: -22.5 ± 2.9%) and higher with the SFCT-2^nd -140 than with the SFCT-2^nd -120 (on average: 25.0 ± 6.2%). Average NPS spatial frequencies (f_av ) were lower with the SFCT-1^st -120 than with the SFCT-2^nd -120 (-6.0 ± 0.5%) and the SFCT-2^nd -140 (-3.6 ± 1.6%). Similar TTF_50% values were found for both systems and both kVp for blood and iodine inserts at 2 mg/mL (0.29 ± 0.01 mm^-1 ) and at 4 mg/mL (0.31 ± 0.01 mm^-1 ). d' values peaked at 40 keV for the SFCT-2^nd and at 70 keV for the SFCT-1^st . Highest d' values were found for the SFCT-2^nd -120 for both simulated lesions. Accuracy of HU values and iodine concentration was higher with the SFCT-2^nd than with the SFCT 1^st .

Conclusion: Compared to the SFCT-1^st , with similar spatial resolution and noise texture values, the SFCT-2^nd -120 exhibited the lowest values for noise magnitude, the highest detectability index values, and more accurate HU values and iodine concentrations.