Table 3. Mean attenuation, image noise and CNR measured on different anatomical structures on ULD and STD chest CT acquisitions.
| Objective image quality parameters | STD | ULD | Mean difference (%) | P value |
|---|---|---|---|---|
| Mean attenuation (NCT, HU) | ||||
| Trachea | –926.5 (–939.0; –914.0) | –940.0 (–948.8; –925.5) | 1 | P<0.001 |
| Normal lung | –861.0 (–879.0; –843.0) | –865.5 (–882.8; –842.8) | 1 | P<0.01 |
| GGO | –479.0 (–543.0; –425.3) | –475.5 (–527.8; –413.5) | –1 | 0.26 |
| Condensation | –69.8 (–104.1; –23.3) | –44.4 (–120.8; –18.5) | 8 | 0.31 |
| Image noise (HU) | ||||
| Trachea | 71.5 (64.5; 81.9) | 54.3 (48.9; 59.3) | –25 | P<0.001 |
| Normal lung | 94.9 (85.5; 110.8) | 63.9 (58.3; 73.7) | –33 | P<0.001 |
| GGO | 156.0 (143.3; 176.5) | 101.5 (87.3; 115.0) | –35 | P<0.001 |
| Condensation | 157.0 (130.8; 179.5) | 87.9 (76.6; 106.5) | –43 | P<0.001 |
| CNR | ||||
| Normal lung | 0.71 (0.49; 0.93) | 1.07 (0.76; 1.34) | 58 | P<0.001 |
| GGO | 3.16 (2.78; 3.76) | 4.80 (4.11; 5.89) | 52 | P<0.001 |
| Condensation | 6.20 (5.36; 7.39) | 10.35 (8.78; 11.71) | 57 | P<0.001 |
Outcomes of mean difference of each metric between both chest CT protocols and its respective statistical results are depicted. Values are expressed as medians (1st quartile; 3rd quartile). A P value lower than 0.05 was considered significant. CNR, contrast-to-noise ratio; ULD, ultra-low-dose; STD, standard; CT, computed tomography; NCT, mean attenuation of pixel; HU, Hounsfield unit; GGO, ground-glass opacity.