iQMetrix-CT: New software for task-based image quality assessment of phantom CT images.

The purpose of this article was to explain how the new iQMetrix-CT software works, as well as to describe its current potential and discuss its future applications. iQMetrix-CT was developed by a working group from the French Society of Medical Physicists (SFPM). This software calculates three advanced metrics, which have been adapted to take into account the non-linear and non-stationary properties of iterative reconstruction algorithms. Noise power spectrum is calculated to assess noise texture and noise magnitude in the frequency domain. The task-based transfer function is computed to assess the spatial resolution under conditions of contrast and noise similar to the lesions encountered in clinical practice. Finally, the detectability index is used to estimate the radiologist's ability to perform a given task such as detecting a simulated lesion. These metrics are very useful to evaluate the performance of reconstruction algorithms in term of image quality and optimize the doses of a given protocol or to evaluate and compare a new technology.

Phantom task-based image quality assessment of three generations of rapid kV-switching dual-energy CT systems on virtual monoenergetic images.

PURPOSE: To compare the spectral performance of three rapid kV switching dual-energy CT (DECT) systems on virtual monoenergetic images (VMIs) at low-energy levels on abdominal imaging. METHODS: A multi-energy phantom was scanned on three DECT systems equipped with three different gemstone spectral imaging (GSI) platforms: GSI (1st generation, GSI-1st), GSI-Pro (2nd generation, GSI-2nd ), and GSI-Xtream (3rd generation, GSI-3rd). Acquisitions on the phantom were performed with a CTDIvol close to 11mGy. For all platforms, raw data were reconstructed using filtered-back projection (FBP) and a hybrid iterative reconstruction algorithm (ASIR-V at 50%; AV50). A deep-learning image reconstruction (DLR) algorithm (TrueFidelity) was used only for the GSI-3rd. Noise power spectrum (NPS) and task-based transfer function (TTF) were evaluated from 40 to 80 keV of VMIs. A detectability index (d') was computed to assess the detection of two contrast-enhanced lesions according to the keV level used. RESULTS: For all GSI platforms, the noise magnitude decreased from 40 to 70 keV, and using AV50 compared to FBP. The average NPS spatial frequency (fav ) and spatial resolution (TTF50% ) were similar from 40 to 70 keV and decreased with AV50 compared to FBP. Compared to AV50, using DLR reduced the noise magnitude (-27% ± 3%) and improved fav values (10% ± 0%) and altering spatial resolution (2% ± 5%). For the two lesions, d' values peaked at 70 keV for GSI-1st and GSI-2nd platforms and at 40/50 keV for GSI-3rd, for all reconstruction algorithms. The highest d' values were found for the GSI-3rd with DLR. CONCLUSION: Differences in image quality were found between the GSI platforms for VMIs at low keV. The new DLR algorithm on the GSI-3rd platform reduced noise and improved spatial resolution and detectability without changing the noise texture for VMIs at low keV. The choice of the best energy level in VMIs depends on the platform and the reconstruction algorithm.