Deep Learning Reconstruction for Ultra-High-Resolution Photon-Counting CT of the Lung: Image Quality and Texture Characterization
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Background
Deep learning reconstruction (DLR), in contrast to hybrid iterative reconstruction (IR), offers an alternative approach to mitigate noise and improve contrast in ultra-high-resolution (UHR) photon-counting CT (PCCT).
Purpose
To characterize image noise, lesion contrast, spatial resolution, and attenuation distribution for DLR using a patient-derived 3D-printed phantom in UHR PCCT, and to apply the same analytical framework to patient examinations acquired on the same system.
Methods
A patient CT scan-derived lung phantom containing three digitally inserted lesions of differing morphology was fabricated using PixelPrint 3D printing and imaged on a CZT-based PCCT prototype at four dose levels (CTDI vol 6, 4, 2, and 1 mGy). Images were reconstructed with hybrid IR at normal resolution (512 × 512) and UHR (1024 × 1024), and with DLR at UHR. Image noise, lesion contrast, and contrast-to-noise ratio (CNR) were measured within prespecified regions of interest replicated across reconstructions by coordinate transfer. Spatial resolution was assessed by the modulation transfer function (MTF) from a high-contrast edge at eight locations. Attenuation distribution and local texture were characterized using histograms and gray-level co-occurrence matrices (GLCMs) within a structured parenchymal region. Two patient examinations with part solid lung nodules acquired on the same prototype were analyzed as illustrative examples, with solid and ground-glass components segmented separately and their attenuation distributions compared.
Results
In the phantom, DLR reduced image noise relative to UHR hybrid IR (7.8% at 6 mGy and 15.2% at 1 mGy). Lesion contrast was constant across reconstructions and dose levels, so CNR differences reflected differences in noise alone. MTF curves for DLR lay above those for hybrid IR across the evaluated spatial frequency range at both 6 and 1 mGy, averaged +0.4 lp/mm in MTF50. In a structured parenchymal region, the attenuation histogram mode became progressively narrower from hybrid IR at 1 mGy to DLR at 6 mGy (averaged −23%), while mode position and mean regional attenuation were preserved, consistent with reduced partial-volume mixing rather than altered attenuation. In the patient example, the attenuation distributions of solid and ground-glass components were more widely separated with UHR DLR (2.0) than with either hybrid IR(1.6).
Conclusions
In UHR photon-counting CT of the lung, DLR reduced image noise while preserving lesion contrast and regional attenuation, increased measured MTF at a high-contrast edge, and produced attenuation distributions consistent with reduced partial-volume mixing at material boundaries. The same effect was observed in the separation of solid and ground-glass components in two patient examples.
