Ultra low-dose chest CT using filtered back projection: Comparison of 80-, 100- and 120 kVp protocols in a prospective randomized study

• Filtered back projection technique enables acceptable image quality for chest CT examinations at 0.9 mGy (estimated effective dose of 0.5 mSv) for selected sizes of patients.
• Lesion detection (such as solid non-calcified lung nodules) in lung parenchyma is optimal at 0.9 mGy, with limited visualization of thyroid nodules in FBP images.
• Further dose reduction down to 0.4 mGy is possible for most patients undergoing follow-up chest CT for evaluation of larger lung nodules and GGOs.
• Our results may help set the reference ALARA dose for chest CT examinations reconstructed with filtered back projection technique using the minimum possible radiation dose with acceptable image quality and lesion detection.

PurposeTo assess lesion detection and diagnostic image quality of filtered back projection (FBP) reconstruction technique in ultra low-dose chest CT examinations.Methods and materialsIn this IRB-approved ongoing prospective clinical study, 116 CT-image-series at four different radiation-doses were performed for 29 patients (age, 57–87 years; F:M – 15:12; BMI 16–32 kg/m2). All patients provided written-informed-consent for the acquisitions of additional ultra low-dose (ULD) series on a 256-slice MDCT (iCT, Philips Healthcare). In-addition to their clinical standard-dose chest CT (SD, 120 kV mean CTDIvol, 6 ± 1 mGy), ULD-CT was subsequently performed at three-dose-levels (0.9 mGy [120 kV]; 0.5 mGy [100 kV] and 0.2 mGy [80 kV]). Images were reconstructed with FBP (2.5 mm * 1.25 mm) resulting into four-stacks: SD-FBP (reference-standard), FBP0.9, FBP0.5, and FBP0.2. Four thoracic-radiologists from two-teaching-hospitals independently-evaluated data for lesion-detection and visibility-of-small-structures. Friedman's-non-parametric-test with post hoc Dunn's-test was used for data-analysis.ResultsInterobserver-agreement was substantial between radiologists (k = 0.6–0.8). With pooled analysis, 146-pulmonary (27-groundglass-opacities, 64-solid-lung-nodules, 7-consolidations, 27-emphysema) and 347-mediastinal/soft tissue lesions (87-mediastinal, 46-hilar, 62-axillary-lymph-nodes, and 11-mediastinal-masses) were evaluated. Compared to the SD-FBP, 100% pulmonary-lesions were seen with FBP0.9, up to 81% with FBP0.5 (missed: 4), and up to 30% with FBP0.2 images (missed:16). Compared to SD-FBP, all enlarged mediastinal-lymph-nodes were seen with FBP0.9 images. All mediastinal-masses (>2 cm, 11/11) were seen equivalent to SD-FBP images at 0.9 mGy. Across all sizes of patients, FBP0.9 images had optimal visualization for lung findings. They were optimal for mediastinal soft tissues for only non-obese patients.ConclusionFiltered-back-projection technique allows optimal lesion detection and acceptable image quality for chest-CT examinations at CDTIvol of 0.9 mGy for lung and mediastinal findings in selected sizes of patients.