Isolated pulmonary nodule (SPN) is artificially defined as a confined round or ovoid dense shadow surrounded by lung tissue, less than 3 cm in diameter. large-scale radiological examinations have shown that the incidence of SPN is 0.09% to 0.20%, with the incidence of malignant nodules being 3% to 6%. Lung cancer is the leading cause of malignant SPN, followed by solitary metastases, which account for 10% to 30% of SPN, with carcinoid tumors and primary lymphomas being other rare causes. Despite the development of surgical techniques and new drug therapies, the five-year survival rate is only 14% to 20%. The prediction of early malignant SPN is very important. For example, the five-year survival rate for stage 1A (T1N0M0) is 67% to 83%, compared to 20.2%, 5.1%, and 7.9% for IIIA, IIIB, and IV, respectively. The majority of SPN seen clinically are benign, with granulomas formed after healing of tuberculosis or mycobacterial infection being the most common cause, and malformations being the second cause, accounting for 10% of surgically resected SPN. Age, history of smoking, history of occupational exposure (e.g. asbestos), history of malignancy, history of exposure in areas where tuberculosis or fungal infections (e.g. coccidioidomycosis and histoplasmosis) are endemic. under 30 years of age it is rarely lung cancer. over 40 years of age especially those with a history of smoking have a greatly increased chance of having primary lung cancer. The usefulness of sputum cytology screening tests for SPN has always been controversial. The positive rate for malignant SPN is very low, less than 20%. Low-dose spiral CT screening scans (7 or 10 mm layer thickness, 50 mA) are the method of choice for localizing SPN and detecting other nodules. The presence of other nodules is suggestive of metastases or infection, the latter mostly presenting as satellite foci. Nodule characteristics: size: the larger the nodule, the more malignant it tends to be. 80% of SPNs larger than 2 cm detected by CT are malignant. Marginal features: (1) smooth (2) more smooth or mildly lobulated (3) mildly burred or irregular (4) very irregular with burring. High-resolution CT scan studies show that in addition to 90% of lung cancers, an additional 4/5 of tuberculous lesions and 1/2 of inflammatory lesions can have burrs. These burrs are associated with fibroplasia due to a pro-connective tissue reaction. The pleural traction sign is an extension of this pro-connective tissue proliferative response and is also seen in inflammatory nodules, although it is more common in malignant nodules. The same study showed that the lobulated sign, representing uneven growth, is more valuable than the burr in determining malignant nodules. Two other marginal patterns are described: the polygonal sign and the halo sign. The former is mostly seen in benign cases and is associated with fibrosis, inflammatory cell infiltration, alveolar collapse and lobular septal mechanization. The halo sign on CT was previously described as a feature of Aspergillosis, but is now thought to be seen in many inflammatory lesions such as Candida, cytomegalovirus, herpesvirus pneumonia and tuberculosis, as well as in non-infectious lesions such as Kaposi’s sarcoma, metastatic angiosarcoma and lung cancer. The hairy glass shadow around a malignant nodule is due to the growth of the lesion along the surrounding alveolar wall. Internal features: calcification, fat, cavities, bronchial signs. A thin, spacerless high-resolution scan (1-3 mm) is preferred especially for small nodules (<1 cm). Uniform, central, stratified and popcorn-like calcifications suggest benign nodules such as sarcoidosis and malformations. Eccentric or speckled calcifications are suggestive of malignancy, with eccentric calcifications possibly surrounding tumor engulfment of pre-existing calcified granulomas and speckled calcifications possibly due to dystrophic calcification of necrotic tumor tissue. Fat is more suggestive of benign lesions such as malignancy and lipid pneumonitis than calcification. more than 50% of malignancies contain fat, and popcorn calcification is seen in 1/4 of CTs. The vacuolar sign and air bronchial sign suggest bronchoalveolar carcinoma and lymphoma. Cavity walls less than 4 mm suggest benign lesions while those larger than 16 mm are mostly malignant, with a large benign-malignant overlap between the two. Densitometry: First applied in 1980 to determine calcifications within the SPN, a pixel density above 164 HU is a sign of a benign nodule. This technique has been less successful because of the different technical conditions of the scanners and reconstruction procedures. We applied densitometry without body modelling and used pixel densities above 200 HU as a basis for the presence of calcification. Nodal enhancement: The peak enhancement within 4 minutes of dynamic enhancement scanning was assessed as a threshold of 15 HU higher than before enhancement with a sensitivity, specificity and accuracy of 98%, 58% and 77%, respectively. This method is more accurate and requires dynamic scanning of the nodule 1, 2, 3, and 4 minutes after enhancement; in addition, the ROI reaches approximately 70% of the short and long diameters of the nodule under the mediastinal window. Respiratory motility, cardiac motion artifacts, and lower specificity are shortcomings of this technique. Inflammatory lesions and malignant nodules can also be significantly enhanced, and the enhancement side of benign and malignant nodules is studied by dynamic enhancement scans using enhancement time density profiles. It is generally accepted that malignant SPNs gradually reach peak enhancement and eventually form a flat high line, whereas inflammatory SPNs intensify rapidly and decay rapidly. Growth rate: The growth rate of SPN is usually determined based on the doubling time. Because the SPN is a sphere, a 26% increase in diameter doubles its volume. The doubling time of malignant SPNs is highly variable, ranging from 30 to 1077 days and 52 to 1733 days in two studies, respectively. Benign nodules such as malignant nodules and sarcoidosis were more stable, with a doubling time of >500 days. Inflammatory SPNs grew more rapidly than malignant nodules, with a doubling time of less than 20 days. Typically, a doubling time of less than 1 month suggests infection, infarction, lymphoma, or rapidly growing metastases. Previously, failure to grow for more than 2 years was used as a sign of a benign nodule, but its predictive value was only 65%. It is now widely accepted that lung cancer has different times to ploidy, related to its morphology and histocytological type on CT. Adenocarcinoma has the longest time to ploidy (116.0-223.1 days), followed by squamous carcinoma (88.0-104.8 days), large cell carcinoma (71.0-100.0 days), and small cell carcinoma (30-80.9 days). A large-scale CT screening of small lung cancers showed that solid nodules took the shortest time to multiply (149 days) and the longest time to show localized hairy glass-like hyperintense shadows (813 days). The radiological pathway to study SPN includes clinical evaluation with a focus on malignant risk factors. Previous radiographs were reviewed to evaluate the stability of SPN. In the absence of previous films, CT is performed to characterize the lesion in detail, and follow-up is not required if the diagnosis is confidently benign (calcified, fatty and non-enhancing). SPN with malignant signs require further invasive testing depending on hospital facilities and patient preference.