1.The concept of PET/CT
Nuclear medicine imaging is an imaging technique that uses radionuclides to calibrate specific molecules and study their corresponding metabolic or pathophysiological processes in the body, a feature that puts it at the forefront of molecular imaging. Nuclear medicine imaging can be divided into two types, depending on the type of radionuclide used.
The first is nuclides that emit single photons (i.e. gamma photons), such as 99mTc, which is known as ECT imaging (common tests such as renal blood flow imaging, whole body bone scan, etc.).
The second is a nuclide that emits positrons (positron drugs emit positrons that combine with negative electrons contained in the surrounding tissue to undergo annihilation reactions, producing a pair of γ photons with opposite directions and the same energy), such as 18F, which is the so-called PET imaging.
The combination of PET and CT (as shown below) becomes PET/CT technology, which realizes the complementary of function and anatomy, and the combination of molecular and structural imaging.
2. Types and sources of commonly used radionuclides
The positronium nuclides commonly used in PET include 11C, 13N, 15O, 18F, etc. These positrons must be generated by cyclotron (shown on the right side of the figure below). The molecules labeled with these nuclides, such as sugar and amino acids, can be used for PET imaging, which can theoretically observe all physiological and biochemical processes in the human body.
3.The imaging principle of 18F-FDG
18F-FDG is an analog of glucose, which is obtained by replacing a hydroxyl group in the glucose molecule with the positronucleophile 18F. It enters the cell through the glucose transporter-1 (GLUT-1) on the cell membrane and is phosphorylated to 6-P-18F-FDG by the action of hexokinase, but is not able to further participate in glycolysis to produce ATP like natural glucose. therefore, the amount of intracellular 6-P-18F-FDG reflects the level of glucose metabolism in tissue cells.
Glucose is the most important source of energy in the body, and tissues with high growth and metabolism such as.
1. brain (brain can only use glucose as a metabolic substrate)
2, heart (heart can use glucose and fatty acids as metabolic substrates, the use of glucose depends on the body’s glucose load, insulin status and whether the myocardium is diseased, so the preparation work needed to do myocardial metabolic imaging with FDG is different from that of FDG tumor imaging)
3. tumors, etc.
The use of glucose in these tissues is significantly higher than in others, which is particularly important to be familiar with the normal distribution of 18F-FDG in the body and is the basis for the use of 18F-FDG for PET tumor imaging and myocardial metabolic imaging. In addition, the kidney and bladder are the main excretory organs of FDG, which are also visualized under normal conditions.
The normal distribution of FDG is shown below.
4.The clinical indications of FDG PET/CT
(mainly include the following but not limited to)
1.Tumor: 90% of its clinical applications
Assist in diagnosis
1.For people with high risk of tumor, early detection or exclusion of tumor.
2.Patients with high tumor markers or paraneoplastic syndrome, to find where the tumor lesions are located.
3.For those who have found suspicious lesions, to identify benign and malignant.
4.Patients with suspected tumor, guidance in selecting the site of biopsy.
5.For those who have found tumor metastasis, search for the primary foci.
Guidance on treatment
1.For patients with malignant tumors, more accurate staging and comprehensive understanding of systemic lesions.
2.Grading of tumor malignancy and prognosis judgment.
3.Guiding the setting of target area of radiotherapy plan.
Therapeutic efficacy assessment and recurrence monitoring
1.Evaluation of tumor treatment effect.
2.Differentiation of tumor residual lesions from necrotic and fibrotic tissues.
3.The markers are still high or re-elevated after tumor treatment, looking for residual, recurrent or metastatic lesions.
2. Heart disease.
Stenosis and calcification of blood vessels can be combined with blood flow and metabolic changes in the blood supply area for analysis and diagnosis to better guide clinical treatment.
3.Brain diseases.
Better localization and analysis of brain lesions, such as epilepsy localization, Parkinson’s, differential diagnosis of dementia.
Note: 18F-FDG-PET is advantageous for the diagnosis and evaluation of most tumors, but has limitations in the diagnosis of tumors such as primary tumors of the liver, clear cell carcinoma of the kidney, prostate cancer, and gastric indolent cell carcinoma, which has given rise to many other types of PET imaging agents that are generally unique, such as 11C-Acetate for low-grade liver malignant tumors and cystic kidney cancer, 68Ga-TATE for neuroendocrine tumors, and Exendin-4 for pancreatic islet cell tumors all have excellent diagnostic efficacy.
5 PET/CT examination procedure
(excluding FDG myocardial metabolic imaging)
The whole PET/CT examination process generally takes about 1,0 to 1,5 hours.
1.Before the examination
Avoid strenuous activities (including exercise, prolonged walking and carrying heavy objects), cold stimulation and mental tension for several days before the examination.
Patients who have recently used barium for gastrointestinal imaging need to postpone the examination, otherwise it will seriously interfere with the CT image, and laxatives can be used to speed up the discharge of contrast agent if necessary.
Water is allowed during the period, but no sugary drinks; infusion of sugary fluids or nutritional solutions (as well as the use of insulin) is not allowed for those who are infused with fluids. For body part examination, you also need to prepare 500ml of mineral water or 500ml of milk or soy milk (one is sufficient) (for patients with lactose intolerance or soy allergy, plain water is sufficient). Patients with diabetes need to adjust their blood sugar in advance (for diabetic patients with poor blood sugar control, further measures need to be taken after communication with the PET center doctor).
PET/CT examinations are not recommended for those who may be pregnant. For breastfeeding women, pro-feeding of recipient breast milk may be performed before the injection of tracer on the day of the examination, and no pro-feeding for 12 hours after the injection of tracer, but breast milk may be placed in a bottle before breastfeeding. Avoid close contact with pregnant women and infants and young children.
2. Injection of 18F-FDG
Blood glucose will be measured first, and those who meet the requirements will then be administered intravenously according to their body weight. After drug injection, the patient rests for about 1 hour in a quiet, warm, dimly lit room with eyes closed or lying down. During this procedure, some patients require slow oral administration of an intestinal contrast agent.
For those who perform torso imaging, empty the bladder as much as possible before the scan and drink milk or soy milk to prop up the stomach (to increase the contrast, otherwise the tumor and the wrinkled stomach wall are not easily distinguished).
3.Scanning
The process takes about 10 to 20 minutes. After positioning, try to keep still and breathe calmly. After the scan is finished, wait to confirm that the image display is clear, the information is kept intact and no additional examination is needed before leaving.
6 Quantitative parameters in the PET/CT report
Finally, the quantitative parameters in PET/CT reports are introduced: In order to provide a uniform quantitative evaluation of tissue uptake of 18F-FDG, the standardized uptake value (SUV) is often used as a parameter, which represents the concentration of the tracer in the tissue after the weight and the injected dose have been standardized. It is important to note that SUV is not an absolute quantitative index, but is related to many uncontrollable factors, including the performance of the machine crystal, reconstruction method, etc. Therefore, SUV is only a semi-quantitative index.