Investigating the Small Pixel Effect in Ultra-High Resolution Photon-Counting CT of the Lung

Abstract

Objectives The aim of this study was to investigate potential benefits of ultra-high resolution (UHR) over standard resolution scan mode in ultra-low dose photon-counting detector CT (PCD-CT) of the lung. Materials and Methods Six cadaveric specimens were examined with 5 dose settings using tin prefiltration, each in UHR (120 × 0.2 mm) and standard mode (144 × 0.4 mm), on a first-generation PCD-CT scanner. Image quality was evaluated quantitatively by noise comparisons in the trachea and both main bronchi. In addition, 16 readers (14 radiologists and 2 internal medicine physicians) independently completed a browser-based pairwise forced-choice comparison task for assessment of subjective image quality. The Kendall rank coefficient (W) was calculated to assess interrater agreement, and Pearson's correlation coefficient (r) was used to analyze the relationship between noise measurements and image quality rankings. Results Across all dose levels, image noise in UHR mode was lower than in standard mode for scan protocols matched by CTDIvol (P < 0.001). UHR examinations exhibited noise levels comparable to the next higher dose setting in standard mode (P ≥ 0.275). Subjective ranking of protocols based on 5760 pairwise tests showed high interrater agreement (W = 0.99; P ≤ 0.001) with UHR images being preferred by readers in the majority of comparisons. Irrespective of scan mode, a substantial indirect correlation was observed between image noise and subjective image quality ranking (r = −0.97; P ≤ 0.001). Conclusions In PCD-CT of the lung, UHR scan mode reduces image noise considerably over standard resolution acquisition. Originating from the smaller detector element size in fan direction, the small pixel effect allows for superior image quality in ultra-low dose examinations with considerable potential for radiation dose reduction.

Influence of helical pitch and gantry rotation time on image quality and file size in ultrahigh-resolution photon-counting detector CT

“…Simultaneously, as a result of the interdependent relationship between effective mAs, tube current, rotation time and pitch , merely altering the latter two will not affect the CTDI vol or SSDE, as the effective mAs remains unchanged 18 . Specifically, with a small pitch and a high rotation time, the tube current can be kept relatively low, which prolongs the scanning process, but burden on the anode is reduced as there is no need to operate at its maximum settings 7 . This modification of settings is advantageous when higher doses are required, e.g., to realize higher image quality in adipose patients.…”

“…Although the maximum resolution of the investigated scanner is even higher than 0.6 mm, the so-called "small pixel effect" allows for considerable noise reduction advantages in the reconstruction of UHR data below the raw system MTF. Depending on the applied radiation dose, the use of thinner slices could lead to considerable disadvantages in terms of image noise 7 , 13 . Raw data was reconstructed using a dedicated ultrasharp bone kernel (Br80; ρ 50 = 19.31 line pairs/cm; ρ 10 = 24.88 lp/cm) with iterative reconstruction (Quantum Iterative Reconstruction [QIR], Siemens Healthcare) set to strength level 2 14 .…”

“…With the increasing availability of photon-counting CT (PCCT) and its inherent potential for ultrahigh-resolution (UHR) imaging without additional dose penalty, this type of examination is likely to become even more prevalent. Recent studies have shown that improved image quality and reduced radiation exposure can be achieved by using UHR mode with PCCT systems 4 – 7 . While dose modulation based on topograms has become a standard in modern CT scanners, certain adjustments such as gantry rotation time, helical pitch factor, and tube voltage level must still be specifically selected.…”

Optimization of the Reconstruction Settings for Low-Dose Ultra-High-Resolution Photon-Counting Detector CT of the Lungs

“…The radiation doses used in our low-dose UHR-PCD-CT study were within the same range. As recently published, the UHR mode (120 × 0.2 mm) of the PCD-CT scanner allows for dose reduction compared to the standard mode (140 × 0.4 mm) if images are reconstructed with the same slice thickness, which is based on the small pixel effect [28]. Because the detector elements are smaller in the fan direction, the small pixel effect enables superior image quality in ultra-low-dose examinations.…”

Comparison of ultrahigh and standard resolution photon-counting CT angiography of the femoral arteries in a continuously perfused in vitro model

“…On one side, this phenomenon can be exploited to produce scans with a lower radiation dose while maintaining the same noise level compared to SR data. On the other side, the small pixel effect allows for noise reduction with the same radiation dose [ 7 , 38 ]. Due to the predominantly elderly population receiving pelvic-leg CTA, better image quality may be preferred over dose reduction in these patients.…”