In 1971, the world’s first CT machine was successfully developed in the UK and applied in clinical practice with extremely satisfactory results. Since then, CT technology has continued to develop, and after the world’s first 4-layer spiral CT machine was introduced in 1998, multi-layer CT technology was developed rapidly, and 16 layers were developed in 2001, and 64-layer CT started to enter clinical practice in 2003. As the number of scanned layers increased, the ability of simultaneous scanning became stronger, the scanning speed became faster (0.5s – 0.42s – 0.37s – 0.33s), and the image resolution became higher (in terms of Z-axis resolution: 1mm – 0.75mm – 0.6mm – 0.33mm). Today, the speed and resolution of CT scans have reached unprecedented levels, greatly satisfying the needs of various clinical specialties, and in recent years, multi-layer spiral CT has continued to develop to 128 and 256 layers. 2005, the annual meeting of the Radiological Society of North America (RSNA) officially launched dual-source CT technology, which is known as a revolutionary innovation in the history of CT development, beyond the simple number of scanning layers ( or detector rows) (which was the main competitive point of CT technology in the past few years), redefined and reinterpreted the concept of CT, comprehensively expanded the clinical applications of CT, and pushed the field of diagnostic imaging to a surprising height! With the commissioning of single source/double source multilayer CT, the ultra-thin layer scans previously expected by clinicians have been realized, and high-resolution CT images with layer thickness of 0.6mm are now easily available clinically, with sub-millimeter scans for fast scanning at large coverage, i.e., shorter scan time, higher accuracy, and lower radiation dose to the examinees. For respiratory system diseases, it can clearly show peripheral bronchi, pulmonary vessels, intrapulmonary lesions and related adjoining relationships, etc. It is uniquely valuable for the diagnosis of bronchiectasis, emphysema, lung infection and lung cancer. Isolated small pulmonary nodules (SPN), which were previously difficult to detect by conventional radiography, can be detected early with the widespread use of CT thin-section scanning technology, and small nodules ≤1.0 cm or even <0.5 cm in diameter can now be easily observed. The size of pulmonary nodules is extremely closely related to malignancy, with a probability of malignancy of 93%-97% for >3cm in diameter, 64%-82% for >2cm; 6%-28% for 0.5-1.0cm; and 0-1% for <0.5cm. Small pulmonary nodules ≤1.0 cm in diameter, also known as subcentimeter nodules, of which 80% are nodules without calcification, are the most clinically valuable and their diagnosis is the most challenging. Positron emission computed tomography (PET) is one of the most advanced medical imaging techniques available. PET technology is the only technique that uses anatomical morphology for functional, metabolic and receptor imaging in a non-invasive manner. It is currently one of the best clinical tools used to diagnose and guide the treatment of tumors. However, PET-CT examination has significant false negatives for the diagnosis of microscopic lung cancer ≤1.0 cm, or bronchoalveolar carcinoma, as well as combined alveolar carcinoma components within adenocarcinoma, and is therefore considered to be of limited use for sub-centimeter nodules ≤1.0 cm. The location of small pulmonary nodules is also an independent predictor of lung malignancy, with the right lung and upper lobe being more likely to be malignant. Nodules located in the lung parenchyma are more likely to be malignant than nodules located in the pleura, lobar fissures, or paravalvular. Benign lesions can generally be distributed throughout the lung lobes and do not correlate significantly with location. The number of nodules (single or multiple) is not necessarily related to their benignity or malignancy, and any one nodule may be malignant. In the case of small nodules in the lung that are not yet clear on CT scan, the diagnosis can be made by dynamic observation of the nodules' changes. We know that malignant tumors are derived from the malignant transformation of normal cells, and the rate of multiplication of tumor cells is medically expressed by the time of multiplication, i.e., the time when one becomes two, two becomes four, and four becomes eight. Clinically, the multiplication is generally calculated by the diameter of the mass, and a 1/3 increase in diameter means a 1-fold increase in volume. The doubling time for malignant lesions is usually between 20 and 400 days, with doubling times <20 days mostly for infectious diseases and >400 days generally for benign tumors. Small pulmonary nodules are characteristic of malignant lesions when CT imaging shows burrs at the margins, lobulation and thick-walled cavities or ground glass-like changes inside them, but it is sometimes very difficult to analyze the internal features, marginal features, CT enhancement and other imaging features of nodules ≤1.0 cm, so clinicians need to carefully observe the size, location, growth rate and other indicators of nodules. At present, there are several reasons for the difficulty in diagnosis and treatment of sub-centimeter nodules in the lung: (1) they are usually clinically asymptomatic and are mostly found during physical examination, so they are often not given sufficient attention by patients and their families; (2) patients are afraid of thoracoscopic or open-heart surgery and worry about the high cost of surgery; (3) it is difficult to determine the benign and malignant nature of nodules by various imaging examinations including X-ray chest films, CT, PET-CT, etc. (4) invasive methods such as fiberoptic bronchoscopy, TBNA and percutaneous lung puncture are difficult to obtain sufficient tissue from the focal area for pathological confirmation of nodules that are too small in size; (5) clinicians have a biased understanding of subcentimeter nodules in the lung. Therefore, clinicians should change the viewpoint of paying too much attention to the radiation dose during CT examination and ignoring the thickness of CT scan layers, and actively carry out low-dose millimeter/submillimeter CT scan, which is not only beneficial to the early identification of benign and malignant lung subcentimeter nodules, but also can avoid unnecessary trauma and complications and relieve patients’ psychological and physical burdens as early as possible through the adoption of reasonable and standardized clinical treatment strategies.