Dual-source photon-counting computed tomography (PCCT) enables a novel ultra-high-resolution (UHR) scanning mode that can provide UHR conventional images (0.2 mm) as well as spectral results (0.4 mm). To evaluate the spatial resolution and quantitative capabilities of the UHR mode, with a focus on thoracic imaging, a PixelPrint lung phantom mimicking interstitial lung disease with honeycombing and iodine rods of different diameters and concentrations directly attached to the phantom were scanned at doses from 1.0 to 7.5 mGy. Virtual monoenergetic images at 50 keV, virtual non-contrast, and iodine density maps at 0.4 and 1 mm slice thickness were reconstructed as well as conventional images at 0.2, 0.4, and 1 mm slice thickness, all with standard lung and quantitative reconstruction kernels (matrix size 512x512). Iodine quantification was performed for the attached rods, and clinically relevant features in the lung phantom were utilized to evaluate spatial resolution. Overall, iodine quantification was stable across radiation dose, reconstruction kernels, and slice thickness with errors of 0.25, 0.20, and 0.40 mg/mL for 1, 2, and 5 mg/mL iodine, respectively. Even the smallest iodine core rod was detected in the extended CT phantom for a higher dose. For the diseased lung region, images at 0.2 mm slice thickness appeared sharper and depicted smaller structures better, even with increased noise in comparison to thicker slices. In conclusion, UHR mode demonstrated high spatial resolution with detection of small features and accurate iodine quantification, which may provide diagnostic advantage to thoracic imaging with more precise and accurate information.