Acquisition Speed versus Spatial Resolution in Ultra-High-Resolution Photon-Counting CT: Phantom Study to Guide Protocol Development
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Background
Faster ultrahigh-resolution (UHR) photon-counting CT acquisitions facilitate breath-hold imaging, but identical reconstruction settings may conceal differences in spatial resolution.
Purpose
To assess spatial resolution and depiction of small airways and venous stents across helical UHR acquisition configurations.
Materials and Methods
Stationary wire, airway, and venous-stent phantoms underwent one acquisition per configuration using five configurations on a dual-source scanner at 120 kVp. Reconstructions used 0.2- and 0.4-mm thicknesses, the same sharp kernel, and iterative reconstruction strength 1. We measured the full-width at half-maximum of the slice sensitivity profile (SSP) and spatial frequency at 20% of the modulation transfer function (MTF20). Rank-based F tests with Holm adjustment assessed configuration differences in resolution and variability across slices. Airway detection used a profile contrast-to-noise criterion; stent assessment was qualitative.
Results
Calculated acquisition time for 400-mm coverage decreased from 9.80 to 1.30 seconds between the slowest and fastest configurations. At 0.2-mm reconstruction thickness, median SSP increased from 0.316 to 0.496 mm, and median MTF20 decreased from 2.870 to 2.059 lp/mm (both P<.001). Interquartile ranges increased from 0.047 to 0.197 mm for SSP and from 0.256 to 0.654 lp/mm for MTF20 (both variability P<.001). Increasing reconstruction thickness to 0.4 mm increased SSP by a median 0.221 mm across 2459 matched slice-wire measurements (P<.001). At 0.2-mm reconstruction thickness, 0.4- and 0.6-mm lumens with 0.2-mm walls were detected only in the slowest configuration. Faster configurations showed less distinct stent struts and greater adjacent-wall distortion.
Conclusion
Faster helical UHR photon-counting CT configurations had poorer spatial resolution despite identical nominal slice thickness. Consistent acquisition and reconstruction settings are important for longitudinal comparisons.
Original Research
Summary Statement
Faster helical ultrahigh-resolution photon-counting CT configurations had poorer spatial resolution and small-structure visibility despite identical reconstruction settings, supporting task-specific protocols and consistent techniques for longitudinal imaging.
Key Results
At 0.2-mm reconstruction thickness in this phantom study, median slice sensitivity profile was 0.496 versus 0.316 mm for the fastest versus slowest configurations (calculated 400-mm scan times, 1.30 vs 9.80 seconds; P<.001).
In-plane spatial frequency at 20% modulation decreased from 2.870 to 2.059 lp/mm (P<.001), with greater variability across slices.
At 0.2-mm reconstruction thickness, only the slowest configuration depicted 0.4- and 0.6-mm lumens with 0.2-mm walls.
