Ge revolution

The phantom was scanned on a 16-cm multidetector CT scanner (GE Revolution; GE Healthcare; Milwaukee, WI), with scan parameters listed in Table 1, applying a range of tube currents considered clinically relevant [26] . To increase data for statistical analysis, the phantom was scanned three times for each dose level where inserts were interchanged and rotated freely about the z-axis between these scans to avoid lesion location recall bias among readers. All series were reconstructed with FBP, 50% hybrid IR (IR50; ASiR-V 50%), and DLIR of low strength (DLL; TrueFidelity Low), medium strength (DLM; TrueFidelity Medium) and high strength (DLH; TrueFidelity High) with 2.5 mm slice thickness. For reconstruction of the DLIR images, raw data were sent to the DLIR vendor (GE Healthcare) as the study was initiated prior to clinical implementation of the DLIR algorithm which is now commercially available.Table 1

Scan parameters applied in study.

Table 1

Scan parameter	Data
Detector collimation (mm)	80 (128 ×0.625 mm)
Tube potential (kVp)	120
Pitch	0.5
Rotation speed (seconds)	0.5
Tube current-time product (mAs)	75, 150, 225, 300 and 375
CT dose index (mGy)	5, 10, 15, 20 and 25
Matrix	512 × 512
Scan field of view	Large body
Display field of view (mm)	350
Reconstruction kernel	Standard kernel

1 Tube potential of 120 kVp chosen based on phantom size and density.

All chest CT examinations were acquired using 256-slice multidetector CT (GE Revolution; General Electric Healthcare, Milwaukee, USA). We first used a usual high-resolution chest CT (CCT) protocol to obtain reference images: 0.5 s gantry rotation time, 120 kVp, 0.984 : 1 beam pitch, 40 mm table feed for gantry rotation, and z-axis tube current modulation.
To obtain LD and ULD acquisitions, the automatic tube current modulation was replaced by manually chosen values of mA, while the other parameters were maintained. For the ULD protocol, kVp was lowered to 80 and mA was set to 30. We decided to experiment with different low-dose protocols to verify how reductions in kV and mA affect the dose and nodule detection, since previous studies show that CT performed for other reasons, like pulmonary embolism, can be performed with lower doses and similar detection rates when those parameters are modified [10 (link)]. For LD protocol A, (lower mA) mA was lowered to 20 and, for LD protocol B (lower kVp), kVp was lowered to 100. LD protocol C (conventional LDCT) followed the AAPM recommendations for lung cancer screening [11 ]. Table 1 specifies the protocols used.
Images were reconstructed at 0.625 mm thickness, 512 × 512 matrix, using iterative reconstruction (ASiR-V™ General Electric Healthcare, Milwaukee, USA).