Many conventional approaches, such as morphological methods, region-growing processes, energy-based optimization techniques, and machine learning methods, were proposed for lung nodule segmentation in the literature. In morphological methods, morphology-based operations and those based on shape hypothesis were applied to isolate lung nodules by selecting the connected region [6]. However, the isolation of lung nodules using morphological operations did not perform efficiently [7]. Following that, region-growing methods were proposed to improve lung nodule segmentation but it was observed that these methods were unable to segment juxta-vascular and juxtapleural nodules and were only well suited to isolate calcified nodules [3]. In this case, for region-growing methods, the convergence condition and irregular-shaped nodules create difficulty due to the breach of the shape hypothesis. Similarly, energy based-optimization methods were also proposed for lung nodule segmentation. These methods consist of a level set function for the characterization of the image, and an energy function to typically turn the segmentation task into an energy minimization problem [8]. However, juxtapleural nodules and low contrast nodules such as GGO nodules drastically affect the improvement of lung nodule segmentation.

In the past decade, many machine-learning–based methods were proposed for lung nodule segmentation [9]. An example of one such approach is the hybrid model for classifying the lung nodules with high-level feature maps proposed by Lu et al. [10]. Further, methods based on modified support vector machines were suggested to detect small lung nodules in 3D CT scans ^[11][12][11,12]. Down the line, researchers also developed 3D large-scale nodule segmentation techniques based on the Hessian strategy in combination with neural networks [13]. Similarly, segmentation approaches based on multi-phase models [14], unsupervised k-means clustering, and level set algorithms [15] were also proposed.

Among the recently developed deep learning-based approaches for segmentation, CNN is a multi-layered neural network that learns to map original image files and corresponding labels hierarchically, and to change the segmentation task into the classification of voxels ^[16][17][16,17]. For instance, Wang et al. introduced a multi-view CNN (MV-CNN) for nodule segmentation [18]. Further, Huang et al. developed a fast and fully automated detection and segmentation approach for pulmonary nodules in thoracic CT scans using deep convolutional neural networks [19]. Moreover, the authors Subham and Raman developed a lung nodule segmentation approach Using 3-dimensional convolutional neural networks [20]. A good review is also available on pulmonary nodule detection and diagnosis using deep learning applications in computed tomography images [21]. Moreover, Havaei et al. in [22] proposed a cascaded variant of CNN (Cascaded-CNN) for brain tumor segmentation. Further extending the use of CNN to lung nodule segmentation, Shen et al. proposed a multi-crop CNN (MC-CNN) to extract salient nodule features and classify malignancy [16]. Furthermore, Wang et al. in [18] introduced a multi-view deep CNN (MV-DCNN), and Sun et al. in [23] introduced a three multi-channel region of interest (RoI)-based CNN (MCROI-CNN) for lung nodule segmentation and displayed the effectiveness of CNN over existing computer-aided diagnosis systems. Further expanding work on CNN-based architectures, Zhao et al. have advocated an enhanced pyramid deconvolution network for increasing performance on lung nodule segmentation which blends low-level fine-grained characteristics with high-level functional characteristics [24].

On the other hand, fully convolutional networks [25] are a different approach for CT image segmentation. For example, the architectures 2D U-Net and 3D U-Net, proposed by Ronnerberger et al. and Ciccek et al., respectively, are better adapted to biomedical imaging ^[5][26][5,26]. The fully convolutional network U-Net (FCN-UNET) architecture is a convolutional network architecture used for fast and precise segmentation of images. Similarly, the central focused CNN (CF-CNN) proposed by Wang et al. is a data-driven method without involving the shape hypothesis. It showed strong performance for the segmentation of juxtapleural nodules [27]. Moreover, Cao et al. recently proposed incorporating intensity features into the CNN architecture by implementing a dual-branch residual network (DB-ResNet) [28].

Segmentation 2D convolutional U-Network (SegU-Net), a U-Net-based architecture, has also been suggested for lung nodule segmentation [29]. For pre-processing, Wang et al. and Cao et al. implemented weighted sampling of training data to deal with the comparatively smaller size of the Lung Image Database Consortium and Image Database Resource Initiative (LIDC-IDRI) dataset ^[27][28][27,28]. The input slice of a CT scan is cropped to a smaller size with a random weighted sampling strategy to increase the size of the training dataset in the abovementioned method. Recently, Singadkar et al. proposed another approach toward lung nodule segmentation based on a deep deconvolutional residual network (DDRN) [30]. In DDRN, the approach was based on an encoder–decoder architecture with one-directional long-skip connections. Moreover, Wen-Fan Chen et al. developed a residual-dense-attention (RDA) U-Net network architecture for hepatocellular carcinoma segmentation [31].