  Objective: In clinical practice, small lung nodules can be easily overlooked
by radiologists. The paper aims to provide an efficient and accurate detection
system for small lung nodules while keeping good performance for large nodules.
Methods: We propose a multi-planar detection system using convolutional neural
networks. The 2-D convolutional neural network model, U-net++, was trained by
axial, coronal, and sagittal slices for the candidate detection task. All
possible nodule candidates from the three different planes are combined. For
false positive reduction, we apply 3-D multi-scale dense convolutional neural
networks to efficiently remove false positive candidates. We use the public
LIDC-IDRI dataset which includes 888 CT scans with 1186 nodules annotated by
four radiologists. Results: After ten-fold cross-validation, our proposed
system achieves a sensitivity of 94.2% with 1.0 false positive/scan and a
sensitivity of 96.0% with 2.0 false positives/scan. Although it is difficult to
detect small nodules (i.e. < 6 mm), our designed CAD system reaches a
sensitivity of 93.4% (95.0%) of these small nodules at an overall false
positive rate of 1.0 (2.0) false positives/scan. At the nodule candidate
detection stage, results show that a multi-planar method is capable to detect
more nodules compared to using a single plane. Conclusion: Our approach
achieves good performance not only for small nodules, but also for large
lesions on this dataset. This demonstrates the effectiveness and efficiency of
our developed CAD system for lung nodule detection. Significance: The proposed
system could provide support for radiologists on early detection of lung
cancer.
