Recently, patients always ask whether PET-CT can clarify whether a small nodule or ground glass shadow in the lung is benign or malignant. First of all, what is PET-CT? PET is also known as Positron Emission Computed Tomography (PET-CT), which perfectly integrates PET and CT, providing detailed molecular information on the function and metabolism of the lesion, while CT provides precise anatomical positioning of the lesion. It can be used for early detection of lesions and diagnosis of diseases. But can PET-CT of small nodules less than 1 cm determine the benignity and malignancy? In fact, recent domestic and international experience has confirmed that PET-CT is very difficult to determine the nature of nodules under 1 cm, especially for the diagnosis of ground glass shadows of limited value. In other words, the false-positive and false-negative rates of PET-CT for nodules smaller than 1 cm are too high and inaccurate, which may lead to misjudgment. Moreover, PET-CT is expensive. Therefore, for close follow-up of small lung nodules or ground glass images, conventional CT is sufficient.