Subjective and objective comparisons of image quality between ultra-high-resolution CT and conventional area detector CT in phantoms and cadaveric human lungs

Abstract

Objectives: To compare the image quality of the lungs between ultra-high-resolution CT (U-HRCT) and conventional area detector CT (AD-CT) images.

Methods: Image data of slit phantoms (0.35, 0.30, and 0.15 mm) and 11 cadaveric human lungs were acquired by both U-HRCT and AD-CT devices. U-HRCT images were obtained with three acquisition modes: normal mode (U-HRCT_N: 896 channels, 0.5 mm × 80 rows; 512 matrix), super-high-resolution mode (U-HRCT_SHR: 1792 channels, 0.25 mm × 160 rows; 1024 matrix), and volume mode (U-HRCT_SHR-VOL: non-helical acquisition with U-HRCT_SHR). AD-CT images were obtained with the same conditions as U-HRCT_N. Three independent observers scored normal anatomical structures (vessels and bronchi), abnormal CT findings (faint nodules, solid nodules, ground-glass opacity, consolidation, emphysema, interlobular septal thickening, intralobular reticular opacities, bronchovascular bundle thickening, bronchiectasis, and honeycombing), noise, artifacts, and overall image quality on a 3-point scale (1 = worst, 2 = equal, 3 = best) compared with U-HRCT_N. Noise values were calculated quantitatively.

Results: U-HRCT could depict a 0.15-mm slit. Both U-HRCT_SHR and U-HRCT_SHR-VOL significantly improved visualization of normal anatomical structures and abnormal CT findings, except for intralobular reticular opacities and reduced artifacts, compared with AD-CT (p < 0.014). Visually, U-HRCT_SHR-VOL has less noise than U-HRCT_SHR and AD-CT (p < 0.00001). Quantitative noise values were significantly higher in the following order: U-HRCT_SHR (mean, 30.41), U-HRCT_SHR-VOL (26.84), AD-CT (16.03), and U-HRCT_N (15.14) (p < 0.0001). U-HRCT_SHR and U-HRCT_SHR-VOL resulted in significantly higher overall image quality than AD-CT and were almost equal to U-HRCT_N (p < 0.0001).

Conclusions: Both U-HRCT_SHR and U-HRCT_SHR-VOL can provide higher image quality than AD-CT, while U-HRCT_SHR-VOL was less noisy than U-HRCT_SHR.

Key points: • Ultra-high-resolution CT (U-HRCT) can improve spatial resolution. • U-HRCT can reduce streak and dark band artifacts. • U-HRCT can provide higher image quality than conventional area detector CT. • In U-HRCT, the volume mode is less noisy than the super-high-resolution mode. • U-HRCT may provide more detailed information about the lung anatomy and pathology.

Keywords: Artifacts; Diagnostic imaging; Image enhancement; Lung diseases; Multidetector computed tomography.

Fig. 1

Evaluation items on reference images. On each reference image (U-HRCT_N), each visual evaluation item to be scored is indicated using colored marks. This U-HRCT_N image of diffuse alveolar damage shows six evaluation items: 1, bronchi; 2, vessels; 3, ground-glass opacity; 4-6, interlobular septal thickening. Streak (arrow) and dark band artifacts (arrowhead) can also be seen. U-HRCT_N: ultra-high-resolution CT with normal mode

Fig. 2

Slit phantom images. Entire picture and layout of the phantom and slit phantoms of stainless steel are shown (a). There are four installation sites of slit phantoms. Four stainless steel slit phantoms can be inserted into one installation site at a time (i.e., maximum 16 slit phantoms). In the present study, 0.35-, 0.30-, and 0.15-mm slits were used. AD-CT image with a 20-mm field of view (b, c, and d) and U-HRCT_SHR image with a 20-mm field of view (e). The 0.35-mm slit can be seen in the AD-CT image (b) but not clearly with the 0.30-mm slit (c). The 0.15-mm slit cannot be seen in the AD-CT image (d). However, the 0.15-mm slit can be seen in the U-HRCT_SHR image (e). AD-CT: area detector CT. U-HRCT_SHR: ultra-high-resolution CT with super-high-resolution mode

Fig. 3

CT images of a cadaveric lung with diffuse panbronchiolitis. Whole and zoomed CT images of AD-CT (a, b, and c), U-HRCT_SHR (d, e, and f), and U-HRCT_SHR-VOL (g, h, and i). Tiny (2-mm-diameter) nodules show ill-defined margins and unclear internal structure (b). Dark band artifacts (arrowheads) can be seen (c). Tiny nodules show well-defined margins and clear internal structure (air bronchiologram) (e and h). There are almost no dark band artifacts (f and i). Both U-HRCT_SHR (d) and U-HRCT_SHR-VOL (g) produced significantly better overall image quality than AD-CT (a). AD-CT: area detector CT. U-HRCT_SHR: ultra-high-resolution CT with super-high-resolution mode U-HRCT_SHR-VOL: ultra-high-resolution CT with volume mode

Fig. 4

CT images of a cadaveric lung with pulmonary hemorrhage. AD-CT (area detector CT) image of a cadaveric lung with pulmonary hemorrhage (a). Zoomed CT images corresponding to a dashed rectangle in (a) are shown (b, c, and d). Intralobular reticular opacities can be detected more easily on AD-CT (b) than on U-HRCT_SHR (c) and U-HRCT_SHR-VOL (d). On the other hand, normal anatomical structures such as bronchi and vessels (arrow) can be seen more clearly on U-HRCT_SHR (c) and U-HRCT_SHR-VOL (d) than on AD-CT (b). U-HRCT_SHR (c) and U-HRCT_SHR-VOL (d) seem to also show normal anatomical structures such as small bronchi and vessels as low attenuation areas. While dark band artifacts (arrowheads) can be seen in (b), there are almost no dark band artifacts in (c and d).