Impact of acquisition parameters in photon-counting computed tomography on the stability of virtual monoenergetic...

Photon-counting CT (PCCT) can generate virtual monoenergetic images (VMI) and effective atomic number (Zeff), demonstrating considerable potential for tissue quantification. However, the influence of key acquisition parameters on the stability of these quantitative metrics has not been fully elucidated. This study aimed to evaluate the reliability of VMI CT numbers and Zeff in abdominal organs under varying tube voltage, image quality (IQ) level, and pitch.

An adult male anthropomorphic abdominal phantom (Kyoto Kagaku CTU-41) was scanned using PCCT. The reference protocol was 120 kVp, image quality level (IQ level) 145, and pitch 1. Eight additional protocols were acquired by independently altering tube voltage (140 kVp, 90 kVp, Sn100 kVp), IQ level (115, 85, 55, 25), and pitch (3). VMI were reconstructed at 40, 60, 70, and 100 keV, and CT numbers and Zeff were measured in multiple organs. Differences from the reference protocol were calculated to assess the reliability of these quantitative indices under modified scan conditions.

Across all protocols, VMI attenuation showed excellent correlation with the reference protocol (r ≥ 0.996). For IQ levels of 115, 85, and 55, all absolute CT number deviations were below 5 HU; at IQ level 25, only two measurements fell within the 5-10 HU range. At 140 kVp, all absolute deviations were below 5 HU. For the 90 kVp protocol, the prostate deviation at 40 keV was -11.33 HU, and five other measurements ranged between 5 and 10 HU. With the Sn100 kVp protocol, absolute deviations of the L1 vertebra and prostate exceeded 10 HU at 60 keV (L1 reached 19.33 HU), and three additional measurements were between 5 and 10 HU. Pitch alteration caused the greatest variability: only 75% of measurements had absolute deviations within 10 HU and 67.5% within 5 HU; deviations were prominent on 40 keV images, with the L1 vertebra reaching -41 HU. The intraclass correlation coefficient for Zeff was ≥0.999 under all conditions. Zeff deviations were within 0.2 for 140 kVp and IQ levels of 115, 85, and 55; the pancreas deviation was 0.34 at IQ level 25; at pitch 3, the maximum deviation (L1) was 0.37, and several organs showed deviations close to or exceeding 0.2.

Pitch exerts the greatest influence on the stability of VMI CT numbers, whereas tube voltage and IQ level have relatively minor effects. VMI at medium-to-high energy levels (≥70 keV) exhibit relative stability across different parameters and are recommended as the first choice for quantitative analysis. Zeff measurements remain highly stable under various acquisition conditions, supporting their reliability in quantitative PCCT applications.