Q1: What is the difference between PET and CT and MRI? A1: PET has been more and more widely used in the diagnosis, staging and follow-up of various malignant tumors in recent years, such as Solitary Pulmonary Nodules (SPNs), non-small cell lung cancers, lymphomas, melanomas, breast cancers, esophageal cancers, colorectal cancers, and head and neck tumors, etc. The function of PET is to show abnormal metabolic activities in the organs at the molecular level, but cannot show abnormal manifestations in morphology. metabolic activity, but cannot show morphological manifestations of abnormalities. In contrast to PET, CT and MRI are mainly used to diagnose, stage and follow up tumors through morphological changes. However, in recent years, new techniques such as perfusion imaging (PWI) of CT, quantitative material analysis of dual-source CT, and perfusion imaging (PWI), diffusion imaging (DWI), diffusion tensor imaging (DTI), magnetic susceptibility imaging (SWI), spectral imaging (SI), and BOLD imaging of MRI have been under intensive study.PET helps to identify benign or malignant tumors as well as to stage malignant tumors. malignant tumors. For most patients after chemotherapy or surgical resection of tumors due to postoperative changes or scar tissue, CT or MRI has a more complex presentation, and PET is also useful in the follow-up of these patients. Note: CT examination is the patient lying on the examination bed, the machine sends out X-rays through the human body and then the machine detects and collects data to form a CT image of the human body; MRI examination is similar to CT examination, except that MRI examination is the machine sends out pulsed electromagnetic waves (not X-rays, no radiation) and then the machine detects and collects data to form an MRI image of the human body; PET is the reverse of the former two, by the body to send out isotopes injected into the body to form an MRI image. PET, on the other hand, is the opposite of the previous two, in which the isotope injected into the body, such as 18F, is emitted from the human body and then the machine detects and collects data to form a PET image of the human body, and PET-CT is actually a fusion of the two examinations, PET and CT, to form an image. Q2: Can you briefly introduce the principle of PET imaging? A2: Let’s take the isotope 18F as an example to briefly introduce FDG PET imaging. 18F is an unstable radioisotope with a half-life of 109 minutes, and the principle of FDG PET imaging is to detect the positron emitted by the radioisotope 18F and the pair of photons (γ) that appear in the process of electron annihilation. Positron-emitting radioisotope 18F can be obtained by bombarding the target material with protons after cyclotron acceleration, which is then used to synthesize FDG, a radiopharmaceutical involved in biochemical pathways (i.e., glucose metabolism) in the body. in short, the principle of FDG PET imaging is to diagnose a disease based on the detection of the difference in the uptake of FDG by the body’s tissues and organs. Q3: Why can FDG PET imaging detect such differences in FDG uptake in the body? A3: Let’s take a tumor as an example to briefly analyze this difference in FDG uptake. First let’s review tumor cells. Malignant tumor cells are characterized by rapid proliferation, increased size, local invasion and distant metastasis. Tumor formation results from large amounts of peptide growth factors (platelet-derived growth factor-PDGF and insulin growth factor-IGF) and tumor-promoting angiogenic factors (vascular endothelial growth factor-VEGF and basic fibroblast growth factor-bFGF). Rapidly proliferating large tumors have a growth rate that significantly exceeds the rate of blood supply, which leads to ischemia and necrosis of the tumor. 1 to 2 mm is the limit of the blood supply required for tumor growth to appear as the tumor diameter increases. 2 mm size limit is exactly what represents the maximum distance capacity of oxygen supply and nutrients that can be diffused from the vasculature. Then we come to the metabolism of tumor cells. Malignant tumor cells upregulate hexokinase activity and show an increase in glucose utilization. Malignant tumor cells take up glucose via auxiliary transport (cell membrane glucose transporter protein GLUT) and then undergo glycolysis. In the presence of oxygen glucose forms pyruvate; whereas in the presence of hypoxia (e.g., when the tumor is necrotic), this leads to an increase in tumor lactate levels. FDG is a radiolabeled glucose analog, a radiopharmaceutical that can be taken up like glucose by metabolically active tumor cells via auxiliary transport. The rate of FDG uptake by tumor cells is proportional to their own metabolic activity; FDG can be phosphorylated like glucose to form FDG-6-P, but cannot be further metabolized, and ultimately the FDG is confined to highly metabolized live tumor cells. With FDG PET imaging, it is possible to detect tumors with high FDG activity. Q4: Is there anything a patient should be aware of when having PET-CT? A4: Patients need to fast for 4 to 6 hours before the PET-CT examination, which enhances the uptake of FDG by the tumor and minimizes the uptake of FDG by the heart. Alcohol and caffeinated beverages are not allowed during this time, but water is allowed. Prior to FDG injection, patients should ideally have a blood glucose level below 150 mg/dL. Good blood glucose control is necessary because cellular uptake of FDG can be competitively inhibited by glucose. There is no consensus on whether or not to inject insulin for glucose level control in diabetic patients. Insulin aids in the transport of glucose into muscle, fat, and other tissues via GLUT (the brain and liver do not require insulin for adequate glucose uptake), but physiologic uptake by muscle can be exaggerated in diabetic patients. Patients are prohibited from any strenuous exercise prior to the examination and after injection of radioisotopes to avoid the effects of muscle physiologic uptake of FDG. There is no current industry consensus on the need for bowel cleansing, bladder intubation, and oral intestinal contrast.There are no current contraindications to FDG. Q5: What is the SUV value for PET? A5: The SUV (Standardized Uptake Value) value is an important parameter commonly used on PET-CT, which is a semi-quantitative parameter for assessing the extent of radiotracer uptake, measured instantaneously from a single point on a static PET image. The SUV value in a given tissue is obtained by calculating the following formula: SUV value = (activity of the radiotracer in the tissue)/(dose of radiotracer injected into the body/body weight of the patient). The activity of the radiotracer in the tissue is expressed in microcuries/g, the dose of radiotracer injected into the body is expressed in millicuries, and the patient’s body weight is expressed in kilograms. The SUV value within a tissue can also be expressed as a maximum, minimum, and average value within a region of interest (ROI), just like the CT value. The mean SUV value is the arithmetic mean of all pixels within the ROI, while the maximum and minimum SUV values are the highest and lowest SUV values of all pixels within the ROI. Evaluation of suspicious lesions and follow-up of masses with FDG activity was mainly by visual inspection and SUV values. Generally malignant tumors have SUV values greater than 2.5 to 3, whereas normal liver, lung, and bone marrow tissues have SUV values of 0.5 to 2.5. Knowing the SUV value of a tumor before starting treatment is helpful in evaluating tumor grading and post-treatment effects of radiotherapy. It is important to standardize the time interval between two injections of the radiotracer, and the SUV variability of the PET examination over time should be carefully documented and controlled. Q6: What are the major confounding factors in the use of PET-CT? A6: The first is the obvious artifacts formed by the patient’s movement during the PET-CT examination, which causes difficulty in localizing the lesion during image fusion. Therefore, the patient should be braked during the examination, which means that the patient is not allowed to move between the CT examination and the PET examination. Measures such as positioning the patient more comfortably before the examination, confirming that the patient is not on diuretics before the examination (to prevent urination during the examination), and asking the patient to urinate or have a bladder tube inserted before the start of the examination are used to avoid motion artifacts; however, respiratory, cardiac, and bowel movement artifacts cannot be avoided. Second, when patients have artificial hips, pacemakers, dentures, and contrast-enhanced blood vessels, significant ray-hardening artifacts can occur, and care should be taken to exclude them from the hypermetabolic range during image fusion. Thirdly, it is forbidden to exercise before and after FDG injection as mentioned in the preparation of the patient before the examination, because after exercise, a large amount of radiotracer concentration can appear in normal muscles in the PET image, which is usually symmetrical, and it can be easily identified by reconfirmation of CT, but occasionally, asymmetrical radiotracer concentration may appear due to paralysis of a group of muscles. Q7: What are the advantages of PET-CT? A7: First, PET-CT can localize small slices of radiotracer uptake, which can help identify normal hypermetabolic structures from abnormally elevated metabolic areas. Second, PET-CT provides both the very good functional information of PET and the high spatial and contrast resolution of CT. CT examinations can also reveal other important lesions of clinical interest. Third, CT data can still be evaluated quantitatively and semi-quantitatively after attenuation correction. Q8: What’s wrong with the low SUV value of PET-CT for tumor detection? A8: Due to the low spatial resolution of PET, tumors smaller than 1 cm are often undetectable and can be detected on CT, resulting in a low SUV value of the tumor, but it cannot be ruled out that it is a malignant tumor.The detection rate of FDG PET for different grades of malignant tumors does vary, and the specific data have to be determined according to the study of different tumors. In addition, certain sites with relatively high physiological uptake can also affect tumor detection. Q9: Can children undergo PET-CT? A9: 18F FDG PET has been gradually applied to the staging of various malignant tumors in children, evaluation of treatment effects, restaging, assessment of residual tumors after treatment, development of puncture protocols and development of radiotherapy protocols. The malignant tumors evaluated include lymphoma, soft tissue sarcoma, osteosarcoma, Ewing’s sarcoma, neuroblastoma, spermatogonia, hepatoblastoma, Wilms’ tumor, and plexiform fibroma malignancy, among others. Normal 18F FDG physiologic uptake areas in children are the brain, heart, liver, spleen, gastrointestinal tract, urine collection system (including the bladder), bone marrow, pharyngeal lymphatic ring, salivary glands, thymus gland, and diaphragm, diaphragm foot, and intercostal muscles in hyperventilation. It is important to note that the normal distribution area of 18F FDG in children is very different from that of adults. Q10: What do I need to pay attention to when evaluating the effectiveness of tumor treatment with PET-CT? A10：(1) FDG PET examination should preferably be performed after 8 weeks of tumor surgery to reduce the effect of postoperative changes in FDG activity. (2) Both testes and ovaries may show physiologic FDG activity, and familiarity with gonadal translocation may prevent misidentification of malignancy on examination. (3) Pleural lesions observed to have FDG activity on PET-CT show a corresponding hyperdensity on CT, and one should find out if there is a history of pleural fixation, because less than 10% of pleural malignancies can form calcifications. (4) FDG PET examination should be performed 8 to 12 weeks after radiotherapy so that the radiation effect of the desired FDG activity is minimized. (5) New FDG-active bone disease in the radiation field may be a recurrence of malignancy, a new radiation-induced malignancy, or a benign radionecrotic lesion of bone.