Ultralow-dose CT with tin filtration for detection of solid and sub solid pulmonary nodules: a phantom study

Abstract

Objectives: To investigate the diagnostic performance of advanced modelled iterative reconstruction (ADMIRE) to filtered back projection (FBP) when using an ultralow-dose protocol for the detection of solid and subsolid pulmonary nodules.

Methods: Single-energy CT was performed at 100 kVp with tin filtration in an anthropomorphic chest phantom with solid and subsolid pulmonary nodules (2-10 mm, attenuation, 20 to -800 HU at 120 kVp). The mean volume CT dose index (CTDIvol) of the standard chest protocol was 2.2 mGy. Subsequent scans were obtained at 1/8 (0.28 mGy), 1/20 (0.10 mGy) and 1/70 (0.03 mGy) dose levels by lowering tube voltage and tube current. Images were reconstructed with FBP and ADMIRE. One reader measured image noise; two readers determined image quality and assessed nodule localization.

Results: Image noise was significantly reduced using ADMIRE compared with FBP (ADMIRE at a strength level of 5 : 70.4% for 1/20; 71.6% for 1/8; p < 0.001). Interobserver agreement for image quality was excellent (k = 0.88). Image quality was considered diagnostic for all images at 1/20 dose using ADMIRE. Sensitivity of nodule detection was 97.1% (100% for solid, 93.8% for subsolid nodules) at 1/20 dose and 100% for both nodule entities at 1/8 dose using ADMIRE 5. Images obtained with 1/70 dose had moderate sensitivity (overall 85.7%; solid 95%; subsolid 73.3%).

Conclusion: Our study suggests that with a combination of tin filtration and ADMIRE, the CTDIvol of chest CT can be lowered considerably, while sensitivity for nodule detection remains high. For solid nodules, CTDIvol was 0.10 mGy, while subsolid nodules required a slightly higher CTDIvol of 0.28 mGy.

Advances in knowledge: Detection of subsolid nodules is feasible with ultralow-dose protocols.

Figure 1.

Region of interest placement, size 449 mm². Avg, average; Dev, standard deviation; Perim, perimeter.

Figure 2.

Image noise stratified by dose protocol: filtered back projection (FBP), advanced modelled iterative reconstruction (ADMIRE). SD, standard deviation.

Figure 3.

Representative images of a solid nodule (8 mm diameter) obtained with different CT dose levels: (a) standard dose 120 kVp, 100 mAs; (b) 1/8 dose level 100 kVp, 80 mAs; (c) 1/20 dose level 100 kVp, 30 mAs; (d) 1/70 dose level 100 kVp, 10 mAs; all images were reconstructed with advanced modelled iterative reconstruction (ADMIRE) at a strength level of 5.

Figure 4.

Representative images of a subsolid nodule (6 mm diameter) obtained with different CT dose levels: (a) standard dose 120 kVp, 100 mAs; (b) 1/8 dose level 100 kVp, 80 mAs; (c) 1/20 dose level 100 kVp, 30 mAs; (d) 1/70 dose level 100 kVp, 10 mAs; all images were reconstructed with advanced modelled iterative reconstruction (ADMIRE) at a strength level of 5. Note the decreasing conspicuity of the nodule at increasing noise levels.