Quantum Iterative Reconstruction for Low-Dose Ultra-High-Resolution Photon-Counting Detector CT of the Lung.

Thomas Sartoretti,Victor Mergen,Hatem Alkadhi,André Euler,Thomas G Flohr,Pascal Monnin,Katharina Martini,Lisa Jungblut,Thomas Frauenfelder,Damien Racine

doi:10.3390/diagnostics12020522

Abstract

The aim of this study was to characterize image quality and to determine the optimal strength levels of a novel iterative reconstruction algorithm (quantum iterative reconstruction, QIR) for low-dose, ultra-high-resolution (UHR) photon-counting detector CT (PCD-CT) of the lung. Images were acquired on a clinical dual-source PCD-CT in the UHR mode and reconstructed with a sharp lung reconstruction kernel at different strength levels of QIR (QIR-1 to QIR-4) and without QIR (QIR-off). Noise power spectrum (NPS) and target transfer function (TTF) were analyzed in a cylindrical phantom. 52 consecutive patients referred for low-dose UHR chest PCD-CT were included (CTDIvol: 1 ± 0.6 mGy). Quantitative image quality analysis was performed computationally which included the calculation of the global noise index (GNI) and the global signal-to-noise ratio index (GSNRI). The mean attenuation of the lung parenchyma was measured. Two readers graded images qualitatively in terms of overall image quality, image sharpness, and subjective image noise using 5-point Likert scales. In the phantom, an increase in the QIR level slightly decreased spatial resolution and considerably decreased noise amplitude without affecting the frequency content. In patients, GNI decreased from QIR-off (202 ± 34 HU) to QIR-4 (106 ± 18 HU) (p < 0.001) by 48%. GSNRI increased from QIR-off (4.4 ± 0.8) to QIR-4 (8.2 ± 1.6) (p < 0.001) by 87%. Attenuation of lung parenchyma was highly comparable among reconstructions (QIR-off: −849 ± 53 HU to QIR-4: −853 ± 52 HU, p < 0.001). Subjective noise was best in QIR-4 (p < 0.001), while QIR-3 was best for sharpness and overall image quality (p < 0.001). Thus, our phantom and patient study indicates that QIR-3 provides the optimal iterative reconstruction level for low-dose, UHR PCD-CT of the lungs.

Highlights

Photon-counting detector computed tomography (PCD-CT) is an emerging technology that enables the direct conversion of incident x-ray photons into an electrical signal
In our phantom and patient study, we found that high levels of quantum iterative reconstruction (QIR) enabled considerable noise reductions, increased Signal-to-noise ratio (SNR), and improved subjective image quality without affecting noise texture
Higher levels of QIR were associated with slightly decreased objective spatial resolution, subjective image sharpness was deemed best on QIR-2 and QIR-3 relative to all other reconstructions

Summary

Introduction

Photon-counting detector computed tomography (PCD-CT) is an emerging technology that enables the direct conversion of incident x-ray photons into an electrical signal. The first whole-body full field-of-view dual-source PCD-CT [13,14,15] has become available for clinical use This system overcomes the limitations of previous PCDCT prototype systems by offering a 50 cm scan FOV and 5.76 cm longitudinal detector coverage with automatic exposure control in both angular and longitudinal directions [14]. The PCD of the system offers a 0.15 × 0.176 mm detector element size (projected to the iso-center) that enables ultra-high spatial resolution (UHR) without the radiation dose penalty of other conventional CT systems [14]. In the context of lung imaging, the use of UHR imaging is of particular clinical interest, as fine parenchymal changes and disorders can be visualized in high quality at low noise levels [18,19]

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