Q: Why is the CT resolution of lung PET-CT lower than that of lung MDCT? A: In short, it is that in order to make the PET images and CT images match perfectly to form a high quality fusion image, the current feasible solution is to sacrifice part of the image quality of CT, and thus the CT intrapulmonary details of PET-CT are not as clear as MDCT (i.e., the resolution is lower). Usually, when PET images are fused with CT images, we use CT to attenuation correct the PET images, which may create artifacts and quantitative errors, which can then lead to misinterpretation of the image performance. Let’s say that a mismatch can occur between images acquired at different stages of the patient’s respiratory cycle, such as attenuation data from CT acquired during breath-holding and PET data acquired during static tidal breathing, leading to mislocalization of anatomical structures. In addition to localization errors, this misregistration can also lead to incorrect attenuation coefficients being applied to the PET data, affecting the SUV values (the most widely used parameter for quantifying FDG uptake intensity on PET). Poor alignment between CT data and PET data can lead to underestimation of SUV values and potentially to false positive results. To mitigate respiratory mismatch between CT and PET images, the main measure is to acquire CT images at the mid-expiratory segment that most closely approximates the static tidal respiratory lung volume during PET image acquisition. However, acquiring lung CT images in the mid-expiratory segment reduces the display of anatomical details within the lung, and small nodules at the base of the lung may also be missed. Recent studies have suggested a new method to improve the quantification of SUV values, respiratory averaging CT, which uses a 4-dimensional CT technique to acquire CT cine images at different times of the respiratory cycle. Compared to the standard method of CT attenuation data acquired in the mid-expiratory period, the application of breath-averaged CT for PET with attenuation correction revealed a difference of more than 50% in the maximum SUV values for some lesions. In addition, there are units where CT is scanned using low dose, and there are units where PET-CT reports give chest CT images that are reconstructed using standard algorithms (standard) or soft tissue algorithms (soft) and then displayed with lung windows, and these factors also reduce the detail shown in the lungs. The innovation and progress of imaging and nuclear medicine can be said to be rapidly changing, and we expect that with the rapid development of science and technology (especially computer technology) PET-CT will be more satisfactory and provide more useful information to patients.