What Are the Different Types of PET Scans?

Chinese expression: Positron emission computed tomography

Chinese expression: Positron emission computed tomography
PET-CT perfectly integrates PET and CT. PET provides detailed molecular and other molecular information about the lesion, while CT provides precise anatomical positioning of the lesion. One-time imaging can obtain tomographic images of all parts of the body, which is sensitive and accurate. , Specific and accurate positioning, etc., can understand the overall status of the whole body at a glance, to achieve the purpose of early detection of lesions and diagnosis of diseases. The emergence of PET-CT is another revolution in medical imaging. It has been recognized and widely concerned by the medical community, and it can be called "the crown of high-tech in modern medicine".
PET-CT is the perfect integration of the most advanced PET scanner and advanced spiral CT equipment. It is mainly used in the early detection and diagnosis of major diseases in the fields of tumor, brain and heart.
Chinese name
Positron emission computed tomography
Foreign name
Oncology, brain and heart
The crown of high technology in modern medicine

PET-CT Features

PET-CT can diagnose tumors and other diseases early. Due to the active metabolism of tumor cells, the ability to ingest imaging agents is 2-10 times that of normal cells, forming obvious "light spots" on the image. Therefore, hidden microlesions (greater than 5mm).
2. Safety
Check safety is non-invasive. Most of the nuclides used in the examination are the basic elements that constitute human life or very similar nuclides, and the half-life is very short. The dose received is slightly higher than that of a chest CT scan. It is safe and efficient, and can be repeated in a short time.
3. Accurate
The inspection results are more accurate. Through qualitative and quantitative analysis, it can provide valuable functional and metabolic information, while providing accurate anatomical information, can help determine and find the precise location of tumors, and its test results have higher accuracy than PET or CT alone In particular, it significantly improves the ability to diagnose small lesions.
Perform a quick body check. Other imaging examinations scan some selected parts of the body, and a full-body PET-CT scan (neck, chest, abdomen, and pelvis) takes only about 20 minutes. PET, CT, and the two can be obtained separately. Cross-section, sagittal and coronal images of the whole body can visually see the affected parts and conditions of the whole body of the disease.
5.High cost performance
Can detect tumors early and determine the nature. The cost of treatment can be reduced by 1-5 times later, and the survival time can be increased by 1-5 times, or even 10 times. One inspection can accurately determine the benign and malignant tumors, whether there is metastasis, and avoid Various tests have been used to delay the diagnosis of the disease or to formulate the wrong treatment plan; can accurately stage the tumor, evaluate the treatment effect, and reduce unnecessary treatment methods and doses; can accurately determine the tumor recurrence after tumor treatment, although the cost of a single examination is slightly higher , But actually avoid unnecessary surgery, chemoradiotherapy and hospitalization, the overall cost performance is outstanding.

PET-CT adapts to the population

1. Social elites are under stress and have physical overdrafts
High-level civil servants, corporate executives, and entertaining stars are the pillars of society and families. They have great responsibilities, work pressure, and irregular lifestyles. In addition, they have more entertainment, and their bodies are in a state of overdraft for a long time. Some diseases are already in the incubation period, etc. It is too late to check for symptoms.
Medical experts suggest that social elites with high work pressures and responsibilities should regularly go to the hospital for physical examinations, and it is best to do anti-cancer screening once a year to eliminate major hidden diseases, maintain health, and create greater value. At the same time, strengthen physical exercise, improve the body's immune system, and regulate the rules of life.
2. People with a long history of illness
Due to a long-term illness, such as hepatitis B and chronic atrophic gastritis, most of them are controlled by drugs on weekdays. Such people need to pay special attention to physical examination, especially when the degree of symptoms gradually worsens.
Experts point out that people with a long history of chronic disease should have regular PET-CT examinations to rule out some exacerbations and complications, make early detection and avoid greater losses.
3.Family history of cancer
Cancer family history population means that the family has a history of tumors for several generations. After scientific research, cancer has a certain heredity, especially common malignant tumors such as esophageal cancer, lung cancer, breast cancer, gastric cancer, and intestinal cancer. If parents have such medical history, the probability of children suffering from the disease is several times higher. Therefore, early detection and monitoring is necessary for people with a family history of tumors.
Experts believe that family history of tumors is an important indicator for assessing the risk of genetic mutations and conducting reasonable tests. It is recommended that this group of people maintain a healthy lifestyle and have good habits of regular PET-CT anti-cancer screening.
4. People with bad living habits
Long-term restlessness, overeating, drinking and smoking, irregular bleeding in the lower body of middle-aged and elderly women, etc., no good health habits, etc .; Cough, sputum, chest pain, blood in the sputum, and breathing difficulties; , Blood in the stool, abdominal mass; progressive weight loss, significant weight loss, etc. These conditions need to be noticed by accurate examination and diagnosis to reduce the incidence of tumors, or early detection and early treatment.

PET-CT effect

The unique role of PET is based on metabolic imaging and quantitative analysis. It uses short-lived nuclides such as 11C, 13N, 15O, 18F and other positron nuclides, which are the main elements of the human body, as tracers. It can be used to study the physiological, biochemical, chemical transmitter, receptor and even genetic changes of the human body from the molecular level to dynamically observe the physiological and biochemical changes of metabolites or drugs at the molecular level. In recent years, PET has shown unique advantages in the diagnosis and guidance of the treatment of tumors, coronary heart disease and brain diseases.

PET-CT Features

PET / CT is an organic combination of PET and CT (Computed Tomography). Using the same examination table and the same image processing workstation, fusion of PET images and CT images can simultaneously show the pathophysiological changes and lesions of the lesion. Morphological structure significantly improves the accuracy of diagnosis.
I. PET-CT can make early diagnosis and differential diagnosis of tumors, identify tumors for recurrence, stage and restage tumors, find primary tumors and metastases, guide and determine tumor treatment programs, and evaluate curative effects. Among tumor patients, a considerable number of patients have undergone PET-CT examinations to change the treatment plan due to a clear diagnosis; PET-CT can accurately evaluate the efficacy, adjust the treatment plan in time, and avoid ineffective treatment. On the whole, it greatly saves medical expenses and strives for valuable treatment time.
2. PET-CT can accurately locate the epilepsy focus, and it is also a unique test method for diagnosing depression, Parkinson's disease, and senile dementia. The treatment of epilepsy is one of the top ten medical problems in the world. The difficulty lies in the accurate positioning of epileptic foci. PET-CT has solved this medical problem. Guided by PET-CT, using X-knife or -knife treatment, received good results.
3. PET-CT is also a means of health check-up. It can complete the whole body test with one imaging. It can detect tumors and heart and brain diseases that seriously endanger people's health at an early stage, and achieve the goal of early treatment and disease prevention.
Fourth, PET-CT can also perform a good effect evaluation: after certain treatments such as surgery, radiotherapy, chemotherapy, etc., petct can be used to determine whether the tumor has changed, whether the activity of cancer cells has decreased, and whether other parts of the body have expanded. , You can determine the effect of the previous treatment. [1]
Modern medicine believes that most diseases are the result of dysregulation of biochemical processes in vivo. PET-CT can dynamically and quantitatively observe biochemical changes in molecular levels in the body under physiological conditions. With the decryption of human genes, the emergence, development and treatment of tumors and heart and brain diseases and various genetic diseases that endanger human health will be fundamentally recognized, and it is expected that fundamentally effective treatment plan. PET-CT gene imaging is the "bridge" between clinical and basic genetic research.

PET-CT medical prospects

In recent years, the number of PET / CT instruments in China has increased rapidly. After investigation, as of the end of August 2006, 54 PET / CT instruments have been installed. The current problems are: uneven development in various regions and unreasonable preparation; lack of inspection on PET / CT Guidelines and diagnosis and treatment specifications; lack of scientific research innovation, lack of multi-center research results, lack of summary reports of large cases, and research on health economic evaluation has just begun; there is a clear shortage of talents in comprehensive imaging and radiopharmaceuticals, and urgent needs are addressed through continuing education .
PET / CT is mainly used for malignant tumors. In China, malignant tumors have become the main killer of people's health, and it is not uncommon for families to be impoverished by their loved ones. The reality is that although PET / CT is a high-end device that is very helpful for the diagnosis and treatment of patients, a considerable number of patients have to give up using it because they cannot afford expensive examination fees. For this reason, with the exception of the above-mentioned 54 centers, the inspection volume of one center throughout the year is difficult to exceed 1,500, so that most PET / CT and accelerators do not work. It can be expected that with the maturity of PET / CT applications, the clinical value of PET / CT will be recognized. Once the examination costs of some diseases (such as some malignant tumors) are included in medical insurance, the demand for PET / CT examinations The amount will increase significantly and will play a greater role in the clinic.

PET-CT principle

First, the basic principles of PET imaging
PET is the abbreviation of English Positron Emission Tomography. The clinical imaging process is: marking positron-emitting radionuclides (such as F-18, etc.) on compounds that can participate in blood flow or metabolic processes of human tissues, and injecting radionuclides labeled with positron-containing compounds into Subject's body. Let the subject perform PET imaging within the effective field of view of PET. The positrons emitted by the radionuclide move about 1mm in the body and combine with the negative electrons in the tissue to cause annihilation radiation. Produces two equal energies (511 KeV),
Gamma photons in opposite directions. Due to the different paths of the two photons in the body, the time to reach the two detectors is also different. If within a specified time window (usually 0-15 us), the probe system detects two 180 degrees (± 0) 25 degrees). That is to say, a coincidence event, the detector sends a time pulse respectively, the pulse processor changes the pulse into a square wave, and the coincidence circuit classifies the data and sends it to the workstation for image reconstruction. The images of the cross section, coronal section and sagittal section of each part of the human body are obtained.
The main components of a PET system include a frame, a ring detector, a compliance circuit, an examination table, and a workstation. The detection system is the main part of the entire positron emission imaging system, and the block-shaped detection structure it uses is beneficial to eliminate scattering and improve the counting rate. Many block structures form a ring, and the entire detector is composed of dozens of rings. Each block structure is composed of about 36 small bismuth germanate (BGO) crystals, followed by 2 pairs (4) of photomultiplier tubes (PMT) (see Figure 1). BGO crystals convert high-energy photons into visible light. The PMT converts the optical signal into an electrical signal, and the electrical signal is converted into a time pulse signal. The probe layer conforms to the line to test and determine the time coupling of each probe signal, eliminates interference from other sources of radiation, and gives positrons after operation Position, the computer uses scattering, accidental coincidence signal correction, and photon time-of-flight calculation to complete image reconstruction. The reconstructed image improves the overall resolution of PET to about 2 mm.
PET uses coincidence detection technology for electronic collimation correction, which greatly reduces random coincidence events and backgrounds. Electronic collimators have very high sensitivity (without the effect of lead shielding) and resolution. In addition. The size of BGO crystal is positively correlated with sensitivity. Block-shaped PET probe. Capable of 2D or 3D acquisition. 2D acquisition is to place lead or tungsten plates between the rings to reduce the impact of scattering on the image quality. When 2D image reconstruction, only the counts within a few rings (usually 2 to 3 rings) are calculated. High resolution and low counting rate; 3D data collection is different. The interval between the rings is cancelled, and the coincidence calculation is performed in all the rings, which obviously improves the counting rate, but the scattering is serious, the image resolution is also low, and a large number of data operations are performed during data recombination. Another important difference between the two acquisition methods is the different sensitivity. The sensitivity of 3D acquisition is highest in the center of the field of view.
Second, the working principle of multi-slice spiral CT
The basic principle of CT is image reconstruction. According to the characteristics of various human tissues (including normal and abnormal tissues) absorbing X-rays, a selected level of the human body is divided into many small cubes (also called voxels). X-rays After passing through a voxel, the measured density or gray value is called a pixel. The X-ray beam passes through the selected layer, and the detector receives the sum of the attenuation values of each voxel arranged in the direction of the X-ray beam after absorbing X-rays, which is a known value, and the X-ray attenuation value of each voxel forming the total is Unknown value, when the X-ray generator and the detector make relative circular movements around the human body in an arc or circle. Use iterative method
Calculate the X-ray attenuation value of each voxel and perform image reconstruction to obtain black and white images of different density tissues at this level.
Spiral CT breaks through the traditional CT design and uses slip ring technology to connect the power cable and some signal wires to different metal rings in the fixed frame and move the X-ray tube and detector sliding brushes to the metal ring leads. The tube and the detector are not limited by the cable length. They rotate continuously and uniformly along the long axis of the human body. The scanning bed advances synchronously and uniformly (the traditional CT scanning bed is stationary during scanning). The scanning trajectory advances spirally, which can be fast and uninterrupted. Complete the volume scan.
Multi-slice spiral CT is characterized by multiple layers of detectors. It is the best combination of high speed and high spatial resolution. The wide detector of multilayer spiral CT is made of high-efficiency solid rare-earth ceramic material. Each unit is only 0.5, 1, or 1.25 mm thick, and the photoelectric conversion efficiency of a thin-film scanning detector with a maximum thickness of only 5 mm is as high as 99%, which can continuously receive X-ray signals. The afterglow is extremely short and has good stability. Multi-slice spiral CT can complete a wide range of volume scanning at high speed, good image quality, fast imaging speed, high vertical resolution and good time resolution. Greatly broadened the application of CT
Use range, compared with single-layer spiral CT. Collecting the same volume of data, the scanning time is greatly shortened. Without increasing the X-ray dose, one site can be scanned every 15 seconds or so; the entire chest scan with a layer thickness of 3 mm can be completed within 5 seconds; a larger The P value of the pitch, holding the breath for 20 S at a time, can complete the body scan; the same layer thickness and the same time, the scanning range is increased by 4 times. The unit coverage of the scan is significantly improved, the radiation dose received by the patient is significantly reduced, and the service life of the x-ray tube is significantly extended. At the same time, the amount of contrast agent is saved, the low contrast resolution and spatial resolution are improved, and the noise is significantly reduced. Artifacts and hardening effects. In addition, the width of the X-ray cone beam can be automatically adjusted according to different layer thicknesses. The collimated X-ray beam is focused on a corresponding number of detectors. The detectors are connected to four data acquisition systems (DAS) through electronic switches. Each DAS can independently complete a set of images, according to the DAS and detector matching method. Electronic switching can be used to selectively obtain 1-layer, 2-layer, or 4-layer images, and the thickness of each layer can be freely selected (0.5, 1.0, 1.25 mm, or 5, 10 mm. The collected data can be used for conventional image display or in The workstation performs post-processing to complete 3D stereo reconstruction, multi-layer reconstruction, organ surface reconstruction, etc., and can display it in real time or near real time. In addition, rotation at different angles and different color markers make the image more three-dimensional, more intuitive and realistic. The technology of simulation endoscope and 3D CT angiography is more mature and faster.
3. PET-CT image fusion
PET and CT are combined on the same machine with two different imaging principles, not a simple addition of their functions. Instead, image fusion is performed on this basis. The fusion image has both fine anatomy and rich physiology. Biochemical function information can provide a basis for determining and finding the precise quantitative and qualitative diagnosis of tumors and other lesions. X-rays can be used to perform attenuation correction on nuclear medicine images.
The core of PET-CT is fusion. Image fusion means that the images of the same or different imaging methods are subjected to certain transformation processing to match their spatial positions and coordinates. The image fusion processing system uses the characteristics of the respective imaging methods to compare the two types. The images are spatially registered and combined, and the image data is registered into a single image. PET-CT same-machine fusion (also called hardware fusion, non-image alignment) has the same positioning coordinate system, and the patient can perform PET-CT same-machine acquisition without changing the position when scanning, which avoids the problem caused by patient displacement. error. After acquisition, the two images do not need to be aligned, converted and registered. Computer image fusion software can easily perform
The precise fusion of 2D and 3D, the fused image simultaneously shows the human anatomy and the metabolic activities of the organs, which greatly simplifies the technical difficulty in the entire image fusion process, avoids complicated labeling methods and a large number of operations after acquisition, and To some extent, the registration problem of time and space is solved, and the image reliability is greatly improved.
During the imaging process, PET is affected by attenuation factors such as the Compton effect, scattering, accidental coincidence events, and dead time. The collected data is inconsistent with the actual situation. The image quality is distorted. Effective measures must be used to correct it. Medical image. The resolution of the penetrating image system obtained by isotope correction is generally 12 mm, while the resolution of the penetrating image system of the X-ray method is about 1 mm, and the amount of image information is much larger than that of the isotope method. The attenuation correction of PET using CT images has greatly improved the sharpness of PET images. The image quality is significantly better than that of isotope penetration source correction (see Figure 2). The resolution has been increased by more than 25%, and the correction efficiency has been increased by 30. % And easy to operate. The corrected PET image and CT image are fused, and the information is complementary to obtain more information about the relationship between anatomical structure and physiological function, which has extremely important clinical significance for tumor patient surgery and radiotherapy localization.

Clinical application of PET-CT

PET-CT provides much more prediction and treatment processing information than PET and CT alone. It surpasses the existing fields of PET and CT alone, and can complete all the functions of ultra-high-end CT and PET. The whole body CT scan can be completed in 20 minutes, which is more than 60% more efficient than PET alone. It can also provide more accurate and faster images of myocardial and cerebral blood flow perfusion than CT. PET-CT fusion images can well describe the effects of diseases on biochemical processes, identify physiological and pathological uptake, detect early signs of disease before anatomical evidence is obtained, and even detect subclinical types less than 2 mm. Tumors provide the best treatment plan and screen the most effective therapeutic drugs for clinically determining the planned target area of radiotherapy (combination of clinical target area and biological target area), testing the drug and radiotherapy effect during treatment. Anatomical positioning plus functional imaging for the lesion.
1. Epilepsy localization: accurate localization of brain epilepsy lesions, providing a basis for surgical or gamma knife resection of epilepsy lesions;
2. Brain tumor qualitative and recurrence judgment: benign and malignant qualitative of brain tumor, determination of malignant glioma boundary, identification of radiation necrosis and recurrence after tumor treatment, selection of tumor biopsy site, etc.
3. Early diagnosis of dementia: Early diagnosis, staging and identification of dementia from other types of dementia, such as vascular dementia.
4. Brain receptor research: brain receptor analysis of Parkinson's disease, diagnosis of the disease and guided treatment.
5. Cerebrovascular disease: PET-CT can sensitively capture cerebral metabolic changes caused by ischemic attack, so it can be used for early diagnosis and localization of transient ischemic attack (TIA) and cerebral infarction, and to evaluate the efficacy And prognosis.
6. Drug research: Perform pharmacological evaluation and guidance of neuropsychiatric drugs, observe changes in brain glucose metabolism in patients with obsessive-compulsive disorder, and provide preoperative evidence and follow-up of postoperative curative effects for stereotactic surgery.
7. Advanced health checkup: Early tumors can be cured, but most tumors are already in the middle and advanced stages when they are discovered. Therefore, routine screening of tumors cannot be ignored. PET-CT is simple, safe, comprehensive and accurate. It is a health checkup for people. Best means. [1]
8. Lung cancer examination: 70% of lung cancers have reached the middle and advanced stages when they are diagnosed. The intermediate and advanced lung cancers have passed the optimal treatment period. The most advanced imaging instrument that can detect lung cancer lesions at an early stage is obviously PETCT. The high inspiration of PET-CT makes it possible to detect the trace functional metabolism of the human nervous system, which not only improves the sharpness and specificity of the lesions, but also greatly improves the detection ability and diagnosis rate of the small lesions, making the positioning more accurate. . [1]

PET-CT risk

In fact, Ding Yong, the director of the Department of Nuclear Medicine of the 307 Hospital of the People's Liberation Army, specifically explained the radiation dose of the contrast agent used in PET-CT on Weibo. He pointed out that the effective dose of whole body scanning radiation was about 10.5 mSv, One emergency exposure limit of 100 millisieverts in the Radiation Protection Regulations is much lower than the value of non-random radiation damage.
Researchers such as Professor Pek-Lan Khong, head of the Department of Radiological Diagnostics at the University of Hong Kong, used three current mainstream PET-CT instruments, and the results showed that the radiation dose was between 13-32 millisieverts. "Related cancer incidence" is 0.2% -0.8%, and the lower the age, the greater the risk. Therefore, the researchers suggest that PET-CT examinations should be performed only after there are good clinical reasons, and measures should be taken to reduce the dose.
Professor Robert E. Reiman of the Department of Radiation Safety at Duke University Medical Centre in the United Kingdom said in an interview with Southern Weekend reporters, "The scientific community recognizes that radiation damage to the body will accumulate as the number of times it is exposed to radiation increases. Therefore "Every PET-CT test will increase the risk, which will eventually lead to a significant radiation dose after a period of time." Robert E. Reiman emphasized that children are at greater risk.
The entire medical community attaches great importance to the issue of radiation dose in imaging studies. Whether it is X-ray, CT or PET / CT, doctors will control the radiation dose within the currently considered safety standards to avoid harm to the patient's body. PET / CT PET-CT is derived from the imaging agent 18F-FDG (full name fluorodeoxyglucose). The nuclide used is an ultra-short half-life isotope, which will automatically decay in a short time, and the injected dose is very low. Within the safe range acceptable to the human body, drinking more water is beneficial for rapid excretion.
Radiation in medical examinations will fully consider "practicability of practice" and summarize it with "minor harm to both." If PET / CT is used as a method to determine the benign and malignant tumors, and staging the diagnosis of malignant tumors, the positive significance of PET / CT examination will be far greater than the impact of radiation on the body, not to mention, the current PET / CT radiation dose is lower than the safety limit .
The effect of radiation on the body is related to the dose and frequency of radiation and the body's ability to repair itself. Large doses of radiation may cause tissue damage and necrosis. Radiotherapy uses the principle of radiation to kill cancer cells. After low doses of radiation are absorbed by the body, the body has a repair process. Medical workers engaged in radiology, interventional medicine, or nuclear medicine are exposed to radiation. They perform physical examinations every year. One of the indicators, the micronucleus rate of lymphocytes, reflects the extent of the body's impact after exposure. If the value rises, it means that the body is greatly affected by radiation. After a period of rest adjustment, this value usually returns to normal. Some people have undergone PET / CT, and have other radiation tests performed shortly after the interval, and the radiation dose will accumulate; if the human body has not been repaired in time after the radiation, the possibility of the body being affected by radiation will increase. [2-4]

PET-CT inspection process

I. Appointment registration: Make an appointment through the hotline or online reservation method;
2. Medical history collection: The subject is required to show all examination reports, X-rays, CT, MRI or ultrasound;
3. Measure body weight and blood sugar: To ensure the best results from the examination;
IV. Injection: Intravenous injection of imaging agents, different imaging agents are selected according to different inspection purposes;
V. Rest before examination: The purpose is to distribute the developer throughout the body. Generally, PET-CT will have a special rest room;
6. Scanning on the machine: Empty urine before PET-CT inspection; pack lightly without any accessories;
7. Image acquisition: The fusion and comparison of PET and CT images by computer;
Eight, expert reading: In order to be responsible for the patient's examination results, experts will meet to discuss the reading in the morning session;
Nine, report issuance: Generally, the report is obtained within three working days. If there are special circumstances, please explain to the director in detail. [3]


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